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Feasibility study on applying the mist-spraying cooling to improve the capacity of ultra-large container ships for loading reefers
Ocean Engineering 163 (2018) 377–390
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Feasibility study on applying the mist-spraying cooling to improve the capacity of ultra-large container ships for loading reefers
T
Zili Yanga,b, Zhiwei Liana,∗, Jing Xionga, Zhixin Miaoa, Yuhui Anc, Anyang Chenc a
State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University, Shanghai, 200240, PR China Department of Civil and Energy Engineering, College of Environmental Science and Engineering, Donghua University, Shanghai, 201620, PR China c No. 708 Research Institute of China State Shipbuilding Corporation, Shanghai, 200011, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Loading capacity improvement Refrigerated containers Ultra-large container ship Mist spray
This work proposed a novel hybrid cooling system that applies the mist-spraying technology to improve the thermal conditions and loading capacity of refrigerated containers (reefers) in the cargo holds of ultra-large container ships. Thorough studies were carried out via both experiments and simulations. First, effects of the air supply rate and the mist spray rate on the reefer's exhaust temperature and the ambient temperature of the reefer Lab. were investigated in comparison with the simulation models. Then the possible operating conditions of the air and mist flow rates were clarified and the cargo hold from a practical container ship was adopted as the example to verify their feasibility. Finally, the possible capacity improvement for loading reefers was discussed while the method of determining the operating range of mist spray rates and air supply rates was also established. It was found that great benefits were generated by the proposed system in lowering the reefer's exhaust temperature and improving the thermal conditions inside the cargo hold. In addition, a substantial capacity improvement for loading reefers in the cargo hold was also achieved in the present study. The study will help in enhancing the efficiency and economic benefits for the reefer transportation.
1. Introduction Container shipping is playing a vital role in the global trade since it can realize the loading and unloading of goods in a swift and safe way (Levinson, 2016). In particular, transportation of refrigerated containers (i.e. reefers), which are self-equipped with the refrigeration systems, has become increasingly popular and important. This is because they can keep the temperature-sensitive goods, such as fruits, vegetables and meat, chilled and fresh after long-distance transport. In addition, the costs are usually lower than other transportation methods (Levinson, 2016). All of these advantages together with the increasing demand of global shipping of the fresh goods have raised an urgent requirement of enhancing the loading capacity of reefers on container ships. (Kremer, 2013). In general, the reefers can be placed either on the deck or inside the cargo hold. However, considering the limited number of sockets and the possible poor weather conditions that may occur on the deck, such as the strong sunshine and significant temperature variance between day and night, it is preferred to load as many reefers as possible inside the cargo. As a result, better cooling performance and longer service life of reefers can be achieved (Rodríguez-Bermejo et al., 2007). Nevertheless,
∗
Corresponding author. E-mail address: [email protected] (Z. Lian).
https://doi.org/10.1016/j.oceaneng.2018.06.009 Received 25 November 2016; Received in revised form 5 April 2018; Accepted 4 June 2018 0029-8018/ © 2018 Elsevier Ltd. All rights reserved.
this also raises another serious challenge for the container ship designers and builders, i.e. how to effectively remove the heat released by the reefers from the cargo holds. The current popular strategy is to install the ventilation systems that are mainly composed of blowers, air ducts and vents, in the cargo hold to exhaust the heat. However, as the number of reefers loaded inside the cargo is growing significantly, increasingly powerful ventilation systems would be required, especially in the ultra-large container ships which are able to hold more than 14,500 twenty-foot equivalent units (TEU) (Evangelos, 2006). This will not only lead to an enormous rise of power consumption (Cao et al., 2016) and construction cost of the ventilation system but also take up more space which is already severely limited inside the cargo hold. The size growth of the ductwork could also make it even narrower and harder for the personnel in the cargo hold to operate or walk. Therefore, it is critical to come up with ideas to improve the cooling capacity of the ventilation systems inside the cargo holds without significant rise of space occupation or retrofitting costs. One potential approach to achieve this goal could be the application of mist-spraying cooling technology. In this condition, a great amount of heat can be absorbed during the evaporation process of the water
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The field test chamber (Chamber 1) is the most important part of the whole system. In this chamber, a standard 20-foot Thermo King reefer was employed for the experimental tests and positioned at the middle of the test site. A hot air machine (with the heating power up to 10 kW) was placed inside the reefer to simulate the cooling load generated by the goods (such as fruits) during the shipping. The hybrid cooling system, composed of air-supplying system and the mist-spraying system, was equipped in the chamber with the vent directly facing the front face of the reefer at 70 cm. Meanwhile, spray nozzles were installed evenly along the edge of the vent. After being pressurized to 5.5 MPa by the pump, the supplied water was atomized into numerous tiny mist droplets by the mist nozzles, with the diameter around 150 μm (according to the manufacture). These droplets mixed well and flew together with the airstream towards the inlet of the reefer condenser. With the help of a background air conditioning system and other auxiliary facilities such as the fan, the duct, the water pipe and valve, Chamber 2 was employed as the transit site to handle the air and water to the desired conditions. Meanwhile, an intelligent electronic control system was set up in Chamber 3 to realize the effective control of the hot air machine inside the reefer. Additionally, the control system in Chamber 3 was also taking charge of the start and stop of the reefer's refrigeration module and the initial temperatures inside Chamber 1 and Chamber 2. Detailed operating process of the experiments can be seen in Section 3.2.
mist. The technology has been widely applied to various systems and achieved promising cooling effects. For example, Yang et al. (2012) investigated the performance improvement of air-cooled chillers after employing the water mist system. They reported a significant improvement (51.5%) in the chillers' coefficient of performance (COP) while the annual electricity consumption was found reduced by 14.1% (Yang et al., 2012). Meanwhile, application study of mist cooling system into the railway tunnels was conducted by Barrow and Pope etc. (Barrow and Pope, 1995, 2007) to lower both the local and overall average tunnel air temperatures. They pointed out that the costs of installation, maintenance and operation of the mist-spraying system were relatively smaller. Furthermore, Log, Torgrim, Cannon-Brookes and Peter etc. made use of the water mist cooling system to the fire protection of historic buildings and museums (Log and CannonBrookes, 1995) while the cooling effects of mist flow in a heated horizontal tube was thoroughly studied by Guo, Wang and Gaddis etc.(Guo et al., 1999) In addition, small droplets with the micron-level diameter could facilitate the heat and mass transfer effectively (Yang et al., 2014, 2015, 2016a, 2016b). This will also guarantee a fast and adequate evaporation for the mist-spraying technology. However, few studies have been carried out to investigate the feasibility of applying the mist evaporation cooling technology to improve the loading capacity for reefers inside the cargo holds. To address this under-explored topic, this study aims to answer the following technical questions:
3. Performance study of the hybrid system for single reefer 1. Is it possible to lower the airflow rates in the cargo hold with the application of the mist-spraying cooling system? How much air supply can be reduced while the cooling effects remain the same? 2. How much mist should be sprayed for each reefer? What is the relationship between the mist spray rates and the cooling effects inside the cargo hold? 3. What are the feasible flow rate combinations of airflow and mist? Are there any other combinations? How to determine the optimal one? 4. How many improvements in the loading capacity of cargo holds for reefers can be achieved?
As a first step, simulation tests via the computational fluid dynamics (CFD) were conducted prior to the experimental study to guide the design of the system's operating conditions. Then experiments were carried out under the operating conditions predicted from the simulations within the test setup shown in Fig. 1. The simulation results were then compared with the experimental data to validate the model's reliability. 3.1. Simulation study 3.1.1. Model prototype and meshing In this work, the simulation model was built based on the practical experimental setup (Fig. 2) in the ratio of 1:1. The geometric parameters of the reefer Lab. were 10 m in length, 6 m in width and 3.5 m in height. Other parts of the model geometry were also strictly built based on the practical measurements or the manufacture data of the reefer. Once the model's geometry well built, it was carefully meshed with the unstructured hexagonal grids, as shown in Fig. 3(a). In addition, the mesh was further refined for the critical zones, such as the space between the outlet of the duct and the inlet of the reefer's condenser where significant heat (cooling process) and mass (mist evaporation) transfer would occur.
To address the above issues, a novel hybrid cooling system, in which the existing air-supplying system was reinforced with the water mistspraying for each reefer, was proposed in this work. Thorough studies on the cooling performance of the proposed hybrid system were conducted via both simulation and experimental tests. To begin with, the simulation study was carried out to predict the cooling performance and guide the design of the possible operating conditions of the system. Then, numerous experimental runs were conducted to validate the simulation model. Furthermore, the above results were adopted to a practical cargo hold and the method to figure out the potential flow rate combinations of mist and airflow was established. This work verified the feasibility of applying the mist-spraying cooling technology to improve the loading capacity of reefer ships and may generate significant economic benefits for container ship transportation.
3.1.2. Boundary conditions Boundary conditions for the present simulation were set up as shown in Table 1 & Fig. 3(b), according to the practical situation of the reefer Lab. where the experimental tests would be conducted. As suggested by the operation manual of the reefer, the cooling load, produced by the hot air machine inside the reefer, was set as 7 kW for present study. Airflow rates tested under different opening degrees of the control valve were employed in the simulation while the mist spray rates were ranging from 5 kg h−1 to 25 kg h−1 with the interval of 5 kg h−1. Since the latent heat from the mist evaporation process takes up the predominant part of the cooling effects, the influence of normal changes of water temperature can be negligible (Yuan et al., 2009). In view of this, the tap water from the municipal pipe, with temperature around 28 °C, was adopted as the water source for the mist-spraying system. Meanwhile, the initial temperature inside the reefer Lab. was set as 43 °C based on the study results of reference (An et al., 2008) and
2. Prototype of the experimental system To validate the cooling effects of the proposed system, a novel experimental setup (see Fig. 1) was built in the Comprehensive Laboratory of Refrigerated Containers in Shanghai Maritime University. As shown in Fig. 1, apart from the conventional air-supplying system, a mist-spraying system that mainly consist of the water supply, the pressure pump, the high-pressure hose, the nozzles and regulating valves, was added to build up the hybrid system. Besides, a mist and air handling system as well as a reefer controlling system was also included in the current study for handling the conditions of the fluids and the reefer. These components were distributed in three different chambers (Fig. 1) which can be introduced as follows. 378
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Fig. 1. Schematic of the experimental setup.
the diameter of mist droplets was set as 150 μm in the model, in accordance with the nozzle manufacture. Since the wall of the reefer Lab. was well covered with the thermal insulation material, it was set as adiabatic in present simulation.
Wang, 2012). Owing to the above features, the DPM model, which tended to be more eligible (Bhattacharya et al., 2009), was adopted in this paper. Simulation runs were performed under different flow rate combinations of airflow and mist (Table 1) to reveal the cooling effects of the hybrid system. The simulation results will be presented in comparison with the experiment data, as shown in Section 4.
3.1.3. Simulation model Apparently, with the mist involved in the airflow, the current study belonged to the two-phase flow and there exist some computational models for such problems, e.g. the volume of fluid (VOF) model and the discrete phase model (DPM). As to the present work, the mist was dispersed in the air (the continuous phase) after being sprayed. Strong interaction between the gas phase and the droplets phase would occur, such as the possible mist evaporation together with the heat and mass transfer between the mist and the airflow. In addition, as inferred from the operating conditions shown in Table 1, the volume ratio of the mist to the airflow was smaller than 10%. This indicates the mist was slightly loaded in the air and can be treated as the discrete phase (Ragab and
3.2. Experimental study A novel hybrid cooling system for reefers was constructed (see Fig. 4) in this work. Extensive experimental tests were carried out to investigate the cooling performance of the hybrid system and compared with the simulation results. 3.2.1. Measuring instruments During the experimental runs, various instruments were adopted to test the system conditions. The major components and their
Fig. 2. Photograph of the reefer front face (a) and partial space of the reefer Lab. (b). 379
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Fig. 3. Computational grid (a) and boundary conditions (b) of the reefer Lab. model.
air were monitored while slight adjustments can be made by the background air conditioning system or the humidifiers equipped in Chamber 2. The power of the reefer as well as the hot air machine installed inside can be adjusted by the controlling system in Chamber 3. A PC-based data acquisition system (Keithley 2700 & 7700) was adopted to record the tested data during the whole experimental process. Multiple tests were conducted for each operating condition and the average value was obtained for the result analysis.
Table 1 Boundary conditions of mist and air flow rates. No.
Vair, sply (m3/h)
mmist (kg·h−1)
tair,sply (°C)
dair,sply (g/kg)
tmist (°C)
Mist diameter (μm)
1 2 3 4
4200 3650 2900 1950
5 10 15 25
35a
19.5a
28a
150b
a Based on the typical conditions of outdoor air and city water in late July of Nanhui District, Shanghai. b According to the nozzle manufacture.
3.2.2. Test conditions and procedures To validate the simulation model in Section 3.1, experiments were carried out under the initial conditions same with the simulation runs (see Table 1). Considering the required massive airflow rates (up to 4200 m3 h−1), it can hardly make significant changes to airflow's temperature and humidity ratio. Therefore, experimental runs were performed from 1:00 p.m. to 4:30 p.m. in the sunny days of late July when the outdoor conditions of the experimental site were stable and almost same to the designed operating conditions (35 °C &20 g·kg−1 dry air). With the slight adjustments by the background heating or air conditioning system equipped in Chamber 2, the desired inlet temperature and the humidity ratio for the air-supplying system can be achieved. In operation, the basic procedure of the experiments was as follows:
specifications are list in Table 2. Among the tested parameters, the airflow rates under different opening degrees of the fan valve were measured in advance with a hot-wire anemometer according to reference (Caré et al., 2014). Four air temperature sensors were installed at the outlet of condenser vent to obtain the temperatures of the air exhausted from the reefer's condenser. The ambient temperature and humidity inside the reefer Lab. were simultaneously measured via five ROTRONIC probes (Model HC2-S) distributed at the location shown in Fig. 1. In addition, the temperature and humidity ratio of the supplied 380
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Fig. 4. Photographs of the experimental setup: a) supplying blower and Agilent 13970 A; b) reefer front face and the air duct; c) temperature and humidity sensors being installed; d) mist sprayed from the outlet edge of air duct.
the reefer Lab. remained constant. And the experimental data was saved by the DAQ system for the upcoming results analysis. After one experimental run was finished, parameters of the airflow and water mist were reverted to the initial values, with one parameter changed within Table 1, for the next round of the experiments.
Table 2 Specification of main instruments. Parameters
Devices
Accuracy
Operational range
Water flow rate
Glass rotor flow meters PT100 RTD PT100 RTD ROTRONIC HYGROMER IN-1 Hot-wire anemometer
± 2.5%
0.01–0.1 L/min & 0.1–1 L/min 0–100 °C 0–100 °C 0–100%
Water temperature Air temperature Cargo air humidity Air flow rate
± 0.2 °C ± 0.2 °C 0.8% 0.03 m/s+5% of measured value
3.3. Performance indices To evaluate the cooling performance of the proposed hybrid system, three indicators, namely the reefer's exhaust temperature, the ambient temperature inside reefer Lab. and the loading capacity improvement, were adopted in this work.
0–20 m/s
3.3.1. Reefer's exhaust temperature The exhaust air temperature from the reefer's condenser, i.e. ta, ex, was directly determined by the cooling performance of the ventilation system. Besides, with a lower air temperature exhausted from the reefer's condenser, which indicates a lower running temperature for the condenser, a better working condition can be realized for the reefer's refrigeration system. Therefore, it was used as one of the performance indices.
Beforehand, the air-supplying system and mist-spraying system were adjusted to the initial conditions shown in Table 1, with the background heating or air conditioning system in Chamber 2. After this, the heater in Chamber 1 was turned on to warm up the ambient environment of the reefer Lab. and the door of reefer Lab. was closed to keep the Lab. isolated. When the ambient temperature of the reefer Lab. reached 43 °C, turn off the ambient heater in Chamber 1 and switch on the reefer's refrigeration system. As soon as the temperature inside the reefer decreased to the design value, switch on the hot air machine inside the reefer to simulate the cooling load from chilled goods. Just then, switch on the hybrid cooling system, i.e. turn on the air-supplying and the mist-spraying system, to start the cooling process for the reefer's condenser. Finally, the steady state was realized when the air temperature exhausted from the reefer's condenser and the thermal conditions inside
3.3.2. Ambient temperature of reefer lab To evaluate the effects of the proposed hybrid cooling system in removing the heat from the ambient space, the ambient thermal conditions inside the reefer Lab., which can be represented by the tested ambient temperatures (ab. ta, amb), was adopted as one of the performance indices. Apparently, with the lower ta, amb, better cooling effects of the hybrid system will be achieved. 381
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4.2. Effect of flow rate combinations of airstream and mist
value of the exhaust temperature obtained under the conventional operating condition, namely Vair, sply = 4200 m3 h−1 without spraying mist, is employed as the baseline. Obviously, applied with the mistspraying cooling technology, the air exhaust temperature dropped remarkably. Furthermore, the necessary air supply rate was found decreasing simultaneously, as shown in Fig. 6 (a). This manifests that owing to the benefits of the mist evaporation process, a great deal of airflow can be saved for each reefer. To figure out how much air flow on earth can be reduced, another two groups of experiments were conducted with the further decrease of the air supply rate, namely Vair, 3 −1 and Vair, sply = 1950 m3 h−1. The results were dissply = 2850 m h played in Fig. 6(b) and (c), respectively. As shown in Fig. 6(b) and (c), the reefer's exhaust air temperature was falling continuously with the mass growth of the sprayed mist. In particular, the exhaust air temperature became even lower than the baseline condition (i.e. Vair, 3 −1 without mist) when the sprayed mist was about sply = 4200 m h more than 5 kg h−1 in the case of Vair, sply = 2850 m3 h−1 (Fig. 6(b)). Similar results can also be observed in the case of Vair, 3 −1 , as shown Fig. 6 (c), when mist flow rate was higher sply = 1950 m h than 22.5 kg h−1. Compared to the baseline condition, the necessary air supply rate was reduced substantially from 4200 m3 h−1 to 1950 m3 h−1 with the reduction amplitude reached 53.57%. As the consequence, it seems to be highly possible and promising to increase the capacity of container ships for loading reefers with just minor reforms, which can be realized via supplementing the mistcooling system inside the cargo hold with few changes in the air-supplying system. This will be validated and discussed in Section 5. Meanwhile, with the growth of the mist spray rate, the air exhaust temperature was dropping rapidly at the beginning and leveled off at the end. The decreasing trends were well fitted as exponential with the R2 > 0.92 under experimental conditions (see Table 3). This manifests that significant cooling effect can be achieved when the mist-spraying system was applied. However, with the further growth of the mist spray rate, a continuing but weakening decrease trend of reefer's exhaust temperature was observed under all the operating conditions. The reefer's exhaust temperature was lowered effectively with consuming a small amount of the mist, which also contributed to the feasibility of the proposed hybrid system in the real ultra-large container ships. From another perspective of view, the relationship shown in Fig. 6 also indicates that the mist spray rate can be designed within a reasonable range to achieve an optimal cooling effects. This will be further discussed with a case study for the entire cargo hold in Section 5.2.
4.2.1. In terms of reefer's exhaust temperature Fig. 6 demonstrates the effects of mist spray rates on the reefer's exhaust temperature under different air supply rates. The experimental
4.2.2. In terms of reefer Lab's ambient temperature Fig. 7 illustrates the relationships between the mist spray rate and
3.3.3. Loading capacity improvement of container ships To assess the benefits of the proposed hybrid cooling system in improving the loading capacity of the container ship for reefers, the indicator, loading capacity improvement (ab. LCI), is employed in present work. It can be calculated as follows.
LCI =
LCcrnt − LCprev LCprev
× 100%
(1)
Evidently, with better cooling performance of the hybrid system, less airflow would be required for each reefer to meet the cooling demand of the reefer's condensers. Thus, more reefers can, ideally, be loaded in the container cargo holds without enlarging the existing ventilation system. In addition, the average absolute difference (ab. AAD) between the predicted values and the experimental results was employed in this work to evaluate the reliability and accuracy of the simulation model under different conditions. It was defined as Eq. (2) shows. N
AAD (X ) =
1 ⋅ ∑ Xexpt − Xpred N i=1
(2)
where X refers to the system performance indices, such as ta, ex and ta, amb; N stands for the number of the simulation or experimental tests under each condition 4. Results and discussion 4.1. Comparison between the experimental performance and the simulation results The predicted cooling performance of the hybrid system by the simulation model is compared with the experimental results in Fig. 5. As shown in Fig. 5 (a), the predicted values of the reefer's exhaust temperature are in good agreement with the experimental finding, with the clear majority of the results fall into a deviation range of 3%. Meanwhile, with a maximum discrepancy less than 8%, good accuracy was also realized in predicting the ambient temperature of the cargo hold (Fig. 5 (b)). Therefore, it can be concluded that the present simulation model is reliable to predict the cooling effects of the hybrid system on reefers.
Fig. 5. Comparison between the experimental results and the predicted values of the reefer's exhaust air temperature (a) and the interior ambient temperature of the reefer Lab. (b). 382
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Fig. 6. Effects of mist mass flow rate on the reefer's exhaust temperature under different airflow rates: (a) Vair, Vair, sply = 1950 m3 h−1.
sply =
3650 m3 h−1; (b) Vair,
sply = 2850
m3 h−1; (c)
would arise after introducing the mist-spraying system.
the ambient temperature inside the reefer's Lab under various air supply rates. The experimental ambient temperatures obtained under the baseline condition (Vair, sply = 4200 m3 h−1 without spraying mist), was adopted for comparison. As can be seen in Fig. 7, the ambient temperatures declined significantly, which were well fit as exponential as shown in Table 3, and much less air supply rate was required after applying the mist-cooling technology. For instance, with the mist spray rate reached 5 kg h−1, the necessary air supply rate was reduced significantly from 4200 kg h−1 in the baseline condition to 3650 m3 h−1, as shown in Fig. 7 (a). When the mist spray rate kept increasing to 22.5 kg h−1, it was found that the necessary air supply rate fell dramatically to only 1950 m3 h−1 to achieve the same cooling effect. All the relationships shown in Fig. 7 prove that the ambient temperature of the reefer Lab. can be lowered effectively with the application of the mist-spraying system, while the necessary air supply rate decreased remarkably. At the same time, it is noticed in Fig. 7 (c) that the predicted ambient temperature was somewhat higher than the experimental values. The reason for this may be due to the slight difference on thermal property of the Lab.’ wall which was set adequate in simulation models but hardly to be fully achieved in experimental sites though it was well covered with thermal isolation materials. However, with the maximum deviation less than 8%, the difference was negligible.
5. Validation of loading capacity improvement for reefer ship The results shown in Section 4 confirmed that the application of mist-spraying cooling can make a great contribution to lowering the exhaust air temperature and the ambient temperatures of the single reefer Lab., without raising risks of the excessive humidity or the undesired condensation. However, since the experiments were conducted on one reefer in the laboratory, whereas there would be many reefers loaded in a practical cargo hold, a validation study was performed as follows.
5.1. Validation background To validate the cooling effects of the proposed hybrid system for the reefer cargo hold, the layout of a real cargo hold (see Fig. 9) of a practical ultra-large container ship designed by the China State Shipbuilding Corporation was employed as the study case. As shown in Fig. 9, the cargo hold, which consist of three bays and was symmetrical in the B-B direction, was 44.74 m long, 43.48 m wide and 24.4 m high. Initially, it was designed to load 453 of 40-foot standard containers, among which only 150 containers were reefers due to the over-sized ventilation ductworks and the corresponding space conflicts, while 303 containers were non-refrigerated. Evidently, this led to a severe limitation to the ability of the container ship in carrying reefers. To clarify the cooling performance of the proposed hybrid system for the cargo hold, a simulation model was built as displayed in Fig. 9. The model geometry and the meshing result is shown in Fig. 10(a) and (b), respectively. Operating conditions that were potentially satisfying in the single reefer Lab (as investigated in Section 4.3). were adopted here to validate their theoretical feasibility for the cargo hold.
4.2.3. In terms of ambient humidity Fig. 8 displays the humidity conditions (represented by the relative humidity and the humidity ratio) in the reefer Lab. As shown in Fig. 8, both the relative humidity and the corresponding humidity ratio were growing continuously (in exponential trends as shown in Table 4) with the rise of mist spray rate. However, the highest relative humidity and humidity ratio in present study was lower than 60% and 30 g·kg−1 dry air, respectively, which is far from its saturation level and possible condensation problems can be avoided. Therefore, it can be inferred that few issues on the undesired water condensation inside the cargo hold
Table 3 Relationship of mist-spraying rate on the reefer's air exhaust temperature & Lab.’s ambient temperature. Vair, supply (m3·h−1)
Regression Formulas (based on the experimental results) Reefer's exhaust air temperature (oC)
Ambient temperature (oC)
3650
ta,exhaust = 44.691 + 8.409⋅exp(−0.0475⋅mmist ) , R2 = 0.927
2850
ta,exhaust = 48.809 + 6.337⋅exp(−0.104⋅mmist ) , R2 = 0.914
tamb = 40.292 + 3.488⋅exp(−0.1082⋅mmist ) , R2 = 0.944 tamb = 40.746 + 4.418⋅exp(−0.1408⋅mmist ) , R2 = 0.919
1950
ta,exhaust = 52.228 + 8.966⋅exp(−0.1471⋅mmist ) , R2 = 0.969
tamb = 40.511 + 5.138⋅exp(−0.1043⋅mmist ) , R2 = 0.979
383
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Fig. 7. Effects of mist mass flow rate on the reefer Lab.’s ambient temperature under different airflow rates: (a) Vair, sply = 3650 m3 h−1; (b) Vair, sply = 2850 m3 h−1; (c) Vair, sply = 1950 m3 h−1.
5.2.2. Validation for the cargo hold During the running process of the refrigerated containers, it is suggested that the inlet temperature of the cooling air sucked by the reefer's condenser should be lower than 45 °C. This temperature requirement, consequently, was adopted as the criterion to verify the feasibility of the operating options for the cargo hold. In the practical cargo hold, reefers are usually placed one above another. Owing to this packed loading pattern and the buoyant effect of hot air, the inlet temperature of cooling air for the upper reefers as well as the thermal condition at the top of the cargo hold can be significantly affected by the lower and the nearby reefers. Therefore, feasibility of the potential operating conditions was carefully verified from different aspects of the cargo hold, which can be introduced as follows.
5.2. Feasibility of the potential mist and air combinations for the cargo hold After clarifying the effects of the mist spray rate on the reefer's exhaust temperature in Section 4.3.1, the operating conditions of the hybrid system that could achieve the same (or better) cooling effects with the baseline condition (4200 m3 h−1 without spraying mist) were selected as the potential options for the cargo hold. Then their feasibility for cooling the whole cargo hold was verified. At the end, possible improvement of the ship's loading capacity was figured out. 5.2.1. The potential design for cargo hold Since the hot air exhausted from the reefer's condensers was heat source in the cargo hold, the thermal conditions inside the cargo hold may be improved when the reefer's exhaust air temperature is equal to or lower than the temperature of the baseline condition (52.5 °C in present study). Thus, according to the results shown in Section 4.3, nine operating conditions of air and mist flow rate were selected as the potential options. These airflow-mist flow rate combinations to be validated were 1950 m3 h−1-25 kg h−1, 3650 m3 h−1-5 kg h−1, 3 −1 −1 3 −1 3650 m ·h - 10 kg h , 3650 m h -25 kg h−1, 2900 m3 h−15 kg h−1, 2900 m3 h−1-10 kg h−1, 2900 m3·h−115 kg h−1, 2900 m3·h−1- 25 kg h−1, 2250 m3·h−1- 25 kg h−1. Simulations were carried out under the above operating conditions to validate their cooling effects for the hold cargo while the initial ventilation design, 4200 m3 h−1 without spraying mist, was employed as the baseline. The validation results can be shown and discussed as follows.
a) In terms of the air let temperature for the reefers Table 5 summarizes the air inlet temperatures for all the 150 reefer condensers in the cargo hold under the aforementioned potential air and mist supply rate designs. As shown in the table, most of the cooling air's inlet temperatures, except for the case 1950 m3 h−1-25 kg h−1, were lower than 45 °C, which seems to meet the set criterion. Nevertheless, with the low airflow rate, for instance Vair = 1950 m3 h−1, the hot air cannot be circulated sufficiently and excessive heat may be accumulated in the cargo hold, which could lead the inlet temperature of the upper reefers to be much higher. The detailed thermal condition inside the cargo hold can be seen in the following section.
Fig. 8. Effects of mist mass flow rate on the reefer Lab.’s ambient humidity under different airflow rates: (a) Vair, sply = 3650 m3 h−1; (b) Vair, sply = 2850 m3 h−1; (c) Vair, sply = 1950 m3 h−1. 384
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Table 4 Relationship of mist-spraying rate on the reefer Lab.’s ambient relative humidity and humidity ratio. Vair, supply (m3·h−1)
3650 2850 1950
Regression Formulas (based on the simulation results) Relative humidity inside the reefer Lab. (%)
Humidity ratio inside the reefer Lab. (g·kg−1 dry air)
RHamb = 75.829 − 37.037⋅exp( −0.0267⋅mmist ) , R2 = 0.997 RHamb = 62.561 − 27.683⋅exp( −0.0495⋅mmist ) , R2 = 0.998 RHamb = 60.134 − 31.817⋅exp( −0.0474⋅mmist ) , R2 = 0.994
damb = 32.608 − 10.603⋅exp( −0.01703⋅mmist ) , R2 = 0.973 damb = 27.183 − 5.206⋅exp(−0.0565⋅mmist ) , R2 = 0.993 damb = 29.101 − 7.064⋅exp(−0.0529⋅mmist ) , R2 = 0.997
The reason for this was due to effect of heat accumulation at the upper layers in the cargo hold. As can displayed in Fig. 11, the hot air exhausted from the lower layer reefers flew upwards and led the massive hot air to accumulate at the upper position of the cargo hold. With less fresh cooling air supplied, a bigger amount of hot air from the lower reefers would be involved into the airflow supplied to the upper reefers, resulting in the poor thermal conditions inside the cargo hold. The different thermal conditions shown in Fig. 11 also manifests that spraying mist did help a lot in reducing the necessary air supply rates. However, it cannot completely replace the effects of airflow. In other words, there should be a balanced range during the design process of air supply rates and the mist spray rates. This will be discussed in Section 5.3.
b) Thermal conditions inside the cargo hold During the laboratory research on the single reefer (Section 4.3), it was found that the reefer's exhaust temperatures under the operating conditions (namely 1950 m3 h−1-25 kg h−1 and 3650 m3 h−1-5 kg h−1) were almost same with the baseline condition (4200 m3 h−1 without spraying mist). As a comparison, their temperature distributions for the whole cargo hold were illustrated in Figs. 11 and12. Three featured cross sections (see Fig. 9) were selected to demonstrate the contours. Namely, 1) the view perspective perpendicular to the front face of middle reefers (cross section A-A, x = 16750 mm); 2) the view perpendicular to the middle reefers (cross section B-B, z = 11647 mm); 3) the horizontal perspective crossing through the top exhaust vents of the cargo hold (cross section C-C, y = 24400 mm). However, considering the space limitation of the manuscript, contours detail of air temperature distribution from the view of cross section A-A can be found in the supplemental materials. As shown in Figs. 11 and12, though similar exhaust temperature was achieved during the single reefer research, their cooling effects for the cargo hold differed remarkably. In particular, the cooling effect shown in Fig. 11(c) was apparently unfavorable, even though the mean inlet air temperature for reefers was around 45.52 °C.
c) Effects of the mist spray rate on the inlet air temperatures for reefers in cargo hold Fig. 13 demonstrates the effects of the mist spray rate on the inlet temperatures of the air sucked by reefer condensers, taking the air supply rate of 2900 m3 h−1 as the example. As shown in Fig. 13, the air inlet temperatures (the maximum value, the minimum value and the facet average) for the condensers decreased continuously with the
Fig. 9. Top view a) and front view b) of the validation reefer cargo hold. 385
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Fig. 10. Geometric model (a) and meshing result (b) for the cargo hold simulation.
may exist an optimal operating range for the hybrid system to achieve the balance. This will be further discussed in Section 5.3.
Table 5 Summary of inlet air temperatures for the reefer condensers. Air and mist compositions
Air inlet temperature for all reefers inside the cargo hold under different operating conditions
Aira (m3·h−1)
Water mist (kg·h−1)
Maximum
Minimum
Facet Average
1950 2250 2850 2850 2850 2850 3650 3650 3650
25 25 5 10 15 25 5 10 25
48.87 48.45 47.89 46.78 45.82 45.57 45.71 44.61 42.82
43.05 42.33 41.46 40.79 40.45 40.33 39.57 39.13 38.37
45.52 44.76 43.79 43.03 42.41 42.24 42.24 41.53 40.35
a
5.3. Method for determining the mist and airflow rates range Based on the study results shown in Section 5.2, the effects of mist spray rate as well as the air supply rate on the maximum inlet temperature of the cooling air for reefer condensers can be regressed as Eq. (3) shows.
t a,inlet,max = 37.205 + 28.127⋅exp( −0.010728m mist − 0.000492⋅Va,supplied ) (3)
R2
= 0.930 ; with The accuracy of Eq. (3) was validated with the experimental data. As shown in Table 6, the predicted values by Eq. (3) were found parallel to the tested results. Thus, when different inlet temperatures of cooling air, ta, i, max, were set for the reefer condensers in present study, the corresponding operating conditions of the proposed hybrid system can be obtained by Eq. (3). For instance, if the maximum temperature of cooling air (ta, i, max) was set as 45 °C and the maximum ability of the water-supplying system was 25 kg h−1 for each reefer, then the necessary design range of mist and airflow rates can be obtained as the green section shown in Fig. 15 (a). Similarly, if lower temperature of the cooling air (ta, i, max) was required, for example 43 °C, the corresponding flow rate combinations of the mist and airflow would then fall into the green section displayed in Fig. 15 (b). With the same method, the operating conditions of the proposed hybrid cooling system for other design targets can be easily determined. In the present study, the maximum temperature of the cooling air for each reefer's condenser was lower than 45 °C. Efforts were made to achieve a balance between reducing the air supply rate and consumption of the water mist. According to the practical water-supplying ability of the container ship studied in this work, which could provide 10 kg h−1 pressured water for each reefer, the corresponding air supply rate was found to be only 2400 m3 h−1 via Eq. (3). As the result, great capacity improvement of the container ships for loading reefers can be achieved. This is to be discussed in the following section.
Temperatures and humidity of the supplied air are shown in Table 1.
increase of the mist flow rate. When the mist flow rate was enhanced from 5 kg h−1 to 10 kg h−1, the mean intake temperature of the condensers dropped significantly by 0.76 °C. This proves that significant cooling effect can be realized by raising the mist flow rate at its lower level. However, with more mist sprayed, weakening cooling effects were observed. For instance, with the further increase of mist flow rate from 15 kg h−1 to 25 kg h−1, temperature drop (0.2 °C) of the facet average value was limited. As the consequence, it can be inferred that there should exist an optimal design range of mist spray rate in which balance between the mist consumption rate and cooling performance can be achieved. Further discussion is conducted in Section 5.2. d) Effects of air supply rates on the inlet air temperatures for reefers in cargo hold Fig. 14 shows the effects of the air supply rate on the air inlet temperatures for the reefer condensers in the cargo hold with the mist flow rate of 25 kg h−1 set as the example. As can be seen in the figure, the air temperatures for cooling the reefer's condensers decrease significantly with the growth of airflow rates. In particular, when the air supply rates increased from 1950 m3 h−1 to 2900 m3 h−1, a remarkable temperature drop (3.2 °C) of the cooling air that flows into the condensers was observed. With the further increase of the air supply rate, though the significant temperature drop of the cooling air can still be observed, its decreasing trends became moderate. However, it should be noted that the enhanced air supply rate could also raise the electricity consumption and more space would be occupied by the enlarged ventilation system. Considering the cooling effects via spraying mist (Fig. 13) or enhancing the air supply rates (Fig. 14), it seems that there
5.4. Potential improvement on the reefer loading capacity of the container ship Within the optimal operating conditions (i.e. 2400 m3·h−110 kg h−1 for each reefer) obtained in Section 5.3, the airflow rate was reduced dramatically from the original 4200 m3 h−1 to only 2400 m3 h−1 while the mist consumption (10 kg h−1) tends to be 386
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(caption on next page) 387
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Fig. 11. Contours of air temperature from the view of cross section B-B (z = 11647 mm) under various combinations of air and mist flow rate: (a) 4200 m3·h−10 kg h−1; (b) 3650 m3·h−1- 5 kg h−1; (c) 1950 m3·h−1- 25 kg h−1.
Fig. 13. Effects of the mist-spraying rate on the intake air temperatures of reefers' condensers.
Fig. 14. Effects of the air-supplying rate on the intake air temperatures of reefers' condensers. Table 6 Validation of the necessary airflow rates predicted by Eq. (3). Target temperature for validationa ta,i, max (oC)
45.52 43.79 43.03 42.41 42.24 42.24 40.35 41.53 44.76
Fig. 12. Contours of air temperature from the top view of the cargo (cross section C-C, y = 24400 mm) under various combinations of air and the mist flow rate: (a) 4200 m3·h−1- 0 kg h−1; (b) 3650 m3·h−1- 5 kg h−1; (c) 1950 m3·h−1- 25 kg h−1.
Required air flow rate (m3·h−1)
Designed mist spray flow rate, mmist, designed (kg·h−1)
Designed
Predicted
25 5 10 15 25 5 25 10 25
1950 2850 2850 2850 2850 3650 3650 3650 2250
1939 2837 2975 3086 2957 3383 3913 3580 2134
Relative error (%)
0.56 0.46 4.39 8.28 3.75 7.32 7.21 1.92 5.16
a Obtained from the practical tested results of the whole cargo situation based on the feasible operating conditions.
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Fig. 15. Predicted range of airflow and mist-spraying rates for various targets: a) tinlet,max ≤ 45oC and b) tinlet,max ≤ 43oC .
dramatically with application of the mist-spraying technology. In the case study of current work, the loading capacity was enhanced from 100 reefers to 175 reefers per cargo hold, theoretically, without enlarging the ventilation system. 4) It is feasible and reliable to adopt the DPM model to simulate the cooling performance of the proposed hybrid system. Good agreement was achieved between the predicted performance and the experimental results with the maximum deviation less than 3% for reefer's exhaust temperature and 8% for the ambient temperature. 5) A method for determining the operating conditions of the mist spray rates and air supply rates at different cooling demands was established. Instead of the scattered operating conditions, a continuous design range of air supply rate and mist spray rate was obtained.
moderate and affordable for most of the reefer ships. As the consequence, much less energy would be cost by the blowers for supplying the fresh air into the cargo hold and the necessary space taken up by the bulky ventilation system can be reduced simultaneously. From another point of view, the loading capacity for reefers can be improved significantly without expanding the size of the existing air-supplying system. For the cargo hold studied in this paper (see Fig. 9), the capacity of the air-supplying system, which was mainly limited by the space conflicting of the bulky ductwork and the excessive energy consumption by blowers, was designed as 420,000 m3 h−1 for each hold. In this condition, the loading capacity in each cargo hold was set to be 100 reefers while the rest of the space was filled with the non-refrigerated containers. However, after applying the proposed hybrid cooling system that added the mist-spraying system, the necessary air supply rate was reduced significantly to only 2400 m3 h−1 for each reefer. As the consequence, compared with the original 100 boxes, the number of reefers that can be loaded inside each cargo hold is significantly raised (to as many as 175 reefers in theory) based on the same air-supplying system. This indicates that the loading capacity of the container ships for reefers could be dramatically improved. Considering the significantly improved capacity for loading extra reefers and minor build or reform costs of the system, it can be inferred that enormous economic benefits will also be produced by the proposed hybrid cooling system for the container shipping industry. Detailed study on this issue should be carried out in the future work.
It should, however, be noted that the loading capacity improvement (LCI) was figured out, theoretically. This should be further validated via thorough field tests in a real container ship in future work. Acknowledgement This work is financially supported by the Fundamental Research Funds for the Central Universities of China (No. 2232018D3-36), China Postdoctoral Science Foundation (No. 2018M630385) and the National Natural Science Foundation of China (No. 51176107). Besides, the authors want to express their sincere gratitude to Dr. Hongmin Liu and Dr. Ankang Kan in the Comprehensive Laboratory of Refrigerated Containers of Shanghai Maritime University for the great help in providing the experimental site.
6. Conclusions
Nomenclature
In this work, a novel hybrid cooling system that applied the mistspraying cooling system to the reefers cargo hold was proposed and its feasibility in improving the capacity of container ships to load reefers was carefully validated. The main conclusions can be summarized as follows:
ULCS m V
t d RH LC LCI AAD Subscripts
1) The cooling effects for the reefer condensers can be improved significantly with the application of the mist-spraying technology. In the present study case, it was found that the air temperature exhausted from the reefer condenser was lowered from the original 52.5 °C (the baseline condition) to only 47.4 °C (3650 m3·h−125 kg h−1), with the temperature drop of 5.1 °C. 2) The airflow rates for cooling the reefers in the cargo hold can be reduced remarkably after applying the mist-spraying technology. In this work, the necessary demand of airflow for each reefer decreased from the original volume of 4200 m3 h−1 to only 2400 m3 h−1. 3) The loading capacity of the reefer ships can be enhanced
a amb crnt ex 389
Ultra-large container ship mass flow rate, [kg·h−1] volume flow rate, [m3·h−1] temperature, [°C] humidity ratio, [g·kg−1 dry air] relative humidity, [%] loading capacity, [−] loading capacity improvement, [%] average absolute difference
air ambient current exhaust
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i max pred prev sply
inlet maximum predicted previous supply
Guo, T., Wang, T., Gaddis, J.L., 1999. Mist/steam cooling in a heated horizontal tube: Part 2—results and modeling. In: ASME 1999 International Gas Turbine and Aeroengine Congress and Exhibition. American Society of Mechanical Engineers pp. V003T001A036–V003T001A036. Kremer, W., 2013. How Much Bigger Can Container Ships Get, vol. 19 BBC World Service. Levinson, M., 2016. The Box: How the Shipping Container Made the World Smaller and the World Economy Bigger. Princeton University Press. Log, T., Cannon-Brookes, P., 1995. Water mist for fire protection of historic buildings and museums. Mus. Manag. Curatorsh. 14 (3), 283–298. Ragab, R., Wang, T., 2012. An investigation of applicability of transporting water mist for cooling turbine vanes. In: ASME Turbo Expo 2012: Turbine Technical Conference and Exposition. American Society of Mechanical Engineers, pp. 1809–1821. Rodríguez-Bermejo, J., Barreiro, P., Robla, J.I., Ruiz-García, L., 2007. Thermal study of a transport container. J. Food Eng. 80 (2), 517–527. Yang, J., Chan, K., Wu, X., Yu, F., Yang, X., 2012. An analysis on the energy efficiency of air-cooled chillers with water mist system. Energy Build. 55, 273–284. Yang, Z., Lin, B., Zhang, K., Lian, Z., 2015. Experimental study on mass transfer performances of the ultrasonic atomization liquid desiccant dehumidification system. Energy Build. 93 (0), 126–136. Yang, Z., Zhang, K., Hwang, Y., Lian, Z., 2016a. Performance investigation on the ultrasonic atomization liquid desiccant regeneration system. Appl. Energy 171, 12–25. Yang, Z., Zhang, K., Lian, Z., Zhang, H., 2016b. Sensitivity and stability analysis on the performance of ultrasonic atomization liquid desiccant dehumidification system. Energy 112, 1169–1183. Yang, Z., Zhang, K., Yang, M., Lian, Z., 2014. Improvement of the ultrasonic atomization liquid desiccant dehumidification system. Energy Build. 85 (0), 145–154. Yuan, J., Jin, R., Yang, L., Zhang, J., 2009. CFD simulation of water spray downstream in gas cooling of marine engine. J. Wuhan Univ. Technol. (Transp. Sci. Eng.) 33 (2), 314–317.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.oceaneng.2018.06.009. References An, Y., Lian, Z., Shi, D., 2008. Simulation and evaluation of air distribution in reefer container cargo hold. Shipbuild. China 49 (3), 146–153. Barrow, H., Pope, C., 1995. Cooling methods for railway tunnels. Proc. Inst. Mech. Eng. Part F J. Rail Rapid Transit 209 (2), 105–112. Barrow, H., Pope, C., 2007. Droplet evaporation with reference to the effectiveness of water-mist cooling. Appl. Energy 84 (4), 404–412. Bhattacharya, P., Samanta, A.N., Chakraborty, S., 2009. Spray evaporative cooling to achieve ultra fast cooling in runout table. Int. J. Therm. Sci. 48 (9), 1741–1747. Cao, T., Lee, H., Hwang, Y., Radermacher, R., Chun, H.H., 2016. Modeling of waste heat powered energy system for container ships. Energy 106, 408–421. Caré, I., Bonthoux, F., Fontaine, J.-R., 2014. Measurement of air flow in duct by velocity measurements. In: Epj Web of Conferences. EDP Sciences. Evangelos, F., 2006. Ultra Large Container Ships. World Maritime University.
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Feasibility study on applying the mist-spraying cooling to improve the capacity of ultra-large container ships for loading reefers
T
Zili Yanga,b, Zhiwei Liana,∗, Jing Xionga, Zhixin Miaoa, Yuhui Anc, Anyang Chenc a
State Key Laboratory of Ocean Engineering, Shanghai Jiao Tong University, Shanghai, 200240, PR China Department of Civil and Energy Engineering, College of Environmental Science and Engineering, Donghua University, Shanghai, 201620, PR China c No. 708 Research Institute of China State Shipbuilding Corporation, Shanghai, 200011, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Loading capacity improvement Refrigerated containers Ultra-large container ship Mist spray
This work proposed a novel hybrid cooling system that applies the mist-spraying technology to improve the thermal conditions and loading capacity of refrigerated containers (reefers) in the cargo holds of ultra-large container ships. Thorough studies were carried out via both experiments and simulations. First, effects of the air supply rate and the mist spray rate on the reefer's exhaust temperature and the ambient temperature of the reefer Lab. were investigated in comparison with the simulation models. Then the possible operating conditions of the air and mist flow rates were clarified and the cargo hold from a practical container ship was adopted as the example to verify their feasibility. Finally, the possible capacity improvement for loading reefers was discussed while the method of determining the operating range of mist spray rates and air supply rates was also established. It was found that great benefits were generated by the proposed system in lowering the reefer's exhaust temperature and improving the thermal conditions inside the cargo hold. In addition, a substantial capacity improvement for loading reefers in the cargo hold was also achieved in the present study. The study will help in enhancing the efficiency and economic benefits for the reefer transportation.
1. Introduction Container shipping is playing a vital role in the global trade since it can realize the loading and unloading of goods in a swift and safe way (Levinson, 2016). In particular, transportation of refrigerated containers (i.e. reefers), which are self-equipped with the refrigeration systems, has become increasingly popular and important. This is because they can keep the temperature-sensitive goods, such as fruits, vegetables and meat, chilled and fresh after long-distance transport. In addition, the costs are usually lower than other transportation methods (Levinson, 2016). All of these advantages together with the increasing demand of global shipping of the fresh goods have raised an urgent requirement of enhancing the loading capacity of reefers on container ships. (Kremer, 2013). In general, the reefers can be placed either on the deck or inside the cargo hold. However, considering the limited number of sockets and the possible poor weather conditions that may occur on the deck, such as the strong sunshine and significant temperature variance between day and night, it is preferred to load as many reefers as possible inside the cargo. As a result, better cooling performance and longer service life of reefers can be achieved (Rodríguez-Bermejo et al., 2007). Nevertheless,
∗
Corresponding author. E-mail address: [email protected] (Z. Lian).
https://doi.org/10.1016/j.oceaneng.2018.06.009 Received 25 November 2016; Received in revised form 5 April 2018; Accepted 4 June 2018 0029-8018/ © 2018 Elsevier Ltd. All rights reserved.
this also raises another serious challenge for the container ship designers and builders, i.e. how to effectively remove the heat released by the reefers from the cargo holds. The current popular strategy is to install the ventilation systems that are mainly composed of blowers, air ducts and vents, in the cargo hold to exhaust the heat. However, as the number of reefers loaded inside the cargo is growing significantly, increasingly powerful ventilation systems would be required, especially in the ultra-large container ships which are able to hold more than 14,500 twenty-foot equivalent units (TEU) (Evangelos, 2006). This will not only lead to an enormous rise of power consumption (Cao et al., 2016) and construction cost of the ventilation system but also take up more space which is already severely limited inside the cargo hold. The size growth of the ductwork could also make it even narrower and harder for the personnel in the cargo hold to operate or walk. Therefore, it is critical to come up with ideas to improve the cooling capacity of the ventilation systems inside the cargo holds without significant rise of space occupation or retrofitting costs. One potential approach to achieve this goal could be the application of mist-spraying cooling technology. In this condition, a great amount of heat can be absorbed during the evaporation process of the water
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The field test chamber (Chamber 1) is the most important part of the whole system. In this chamber, a standard 20-foot Thermo King reefer was employed for the experimental tests and positioned at the middle of the test site. A hot air machine (with the heating power up to 10 kW) was placed inside the reefer to simulate the cooling load generated by the goods (such as fruits) during the shipping. The hybrid cooling system, composed of air-supplying system and the mist-spraying system, was equipped in the chamber with the vent directly facing the front face of the reefer at 70 cm. Meanwhile, spray nozzles were installed evenly along the edge of the vent. After being pressurized to 5.5 MPa by the pump, the supplied water was atomized into numerous tiny mist droplets by the mist nozzles, with the diameter around 150 μm (according to the manufacture). These droplets mixed well and flew together with the airstream towards the inlet of the reefer condenser. With the help of a background air conditioning system and other auxiliary facilities such as the fan, the duct, the water pipe and valve, Chamber 2 was employed as the transit site to handle the air and water to the desired conditions. Meanwhile, an intelligent electronic control system was set up in Chamber 3 to realize the effective control of the hot air machine inside the reefer. Additionally, the control system in Chamber 3 was also taking charge of the start and stop of the reefer's refrigeration module and the initial temperatures inside Chamber 1 and Chamber 2. Detailed operating process of the experiments can be seen in Section 3.2.
mist. The technology has been widely applied to various systems and achieved promising cooling effects. For example, Yang et al. (2012) investigated the performance improvement of air-cooled chillers after employing the water mist system. They reported a significant improvement (51.5%) in the chillers' coefficient of performance (COP) while the annual electricity consumption was found reduced by 14.1% (Yang et al., 2012). Meanwhile, application study of mist cooling system into the railway tunnels was conducted by Barrow and Pope etc. (Barrow and Pope, 1995, 2007) to lower both the local and overall average tunnel air temperatures. They pointed out that the costs of installation, maintenance and operation of the mist-spraying system were relatively smaller. Furthermore, Log, Torgrim, Cannon-Brookes and Peter etc. made use of the water mist cooling system to the fire protection of historic buildings and museums (Log and CannonBrookes, 1995) while the cooling effects of mist flow in a heated horizontal tube was thoroughly studied by Guo, Wang and Gaddis etc.(Guo et al., 1999) In addition, small droplets with the micron-level diameter could facilitate the heat and mass transfer effectively (Yang et al., 2014, 2015, 2016a, 2016b). This will also guarantee a fast and adequate evaporation for the mist-spraying technology. However, few studies have been carried out to investigate the feasibility of applying the mist evaporation cooling technology to improve the loading capacity for reefers inside the cargo holds. To address this under-explored topic, this study aims to answer the following technical questions:
3. Performance study of the hybrid system for single reefer 1. Is it possible to lower the airflow rates in the cargo hold with the application of the mist-spraying cooling system? How much air supply can be reduced while the cooling effects remain the same? 2. How much mist should be sprayed for each reefer? What is the relationship between the mist spray rates and the cooling effects inside the cargo hold? 3. What are the feasible flow rate combinations of airflow and mist? Are there any other combinations? How to determine the optimal one? 4. How many improvements in the loading capacity of cargo holds for reefers can be achieved?
As a first step, simulation tests via the computational fluid dynamics (CFD) were conducted prior to the experimental study to guide the design of the system's operating conditions. Then experiments were carried out under the operating conditions predicted from the simulations within the test setup shown in Fig. 1. The simulation results were then compared with the experimental data to validate the model's reliability. 3.1. Simulation study 3.1.1. Model prototype and meshing In this work, the simulation model was built based on the practical experimental setup (Fig. 2) in the ratio of 1:1. The geometric parameters of the reefer Lab. were 10 m in length, 6 m in width and 3.5 m in height. Other parts of the model geometry were also strictly built based on the practical measurements or the manufacture data of the reefer. Once the model's geometry well built, it was carefully meshed with the unstructured hexagonal grids, as shown in Fig. 3(a). In addition, the mesh was further refined for the critical zones, such as the space between the outlet of the duct and the inlet of the reefer's condenser where significant heat (cooling process) and mass (mist evaporation) transfer would occur.
To address the above issues, a novel hybrid cooling system, in which the existing air-supplying system was reinforced with the water mistspraying for each reefer, was proposed in this work. Thorough studies on the cooling performance of the proposed hybrid system were conducted via both simulation and experimental tests. To begin with, the simulation study was carried out to predict the cooling performance and guide the design of the possible operating conditions of the system. Then, numerous experimental runs were conducted to validate the simulation model. Furthermore, the above results were adopted to a practical cargo hold and the method to figure out the potential flow rate combinations of mist and airflow was established. This work verified the feasibility of applying the mist-spraying cooling technology to improve the loading capacity of reefer ships and may generate significant economic benefits for container ship transportation.
3.1.2. Boundary conditions Boundary conditions for the present simulation were set up as shown in Table 1 & Fig. 3(b), according to the practical situation of the reefer Lab. where the experimental tests would be conducted. As suggested by the operation manual of the reefer, the cooling load, produced by the hot air machine inside the reefer, was set as 7 kW for present study. Airflow rates tested under different opening degrees of the control valve were employed in the simulation while the mist spray rates were ranging from 5 kg h−1 to 25 kg h−1 with the interval of 5 kg h−1. Since the latent heat from the mist evaporation process takes up the predominant part of the cooling effects, the influence of normal changes of water temperature can be negligible (Yuan et al., 2009). In view of this, the tap water from the municipal pipe, with temperature around 28 °C, was adopted as the water source for the mist-spraying system. Meanwhile, the initial temperature inside the reefer Lab. was set as 43 °C based on the study results of reference (An et al., 2008) and
2. Prototype of the experimental system To validate the cooling effects of the proposed system, a novel experimental setup (see Fig. 1) was built in the Comprehensive Laboratory of Refrigerated Containers in Shanghai Maritime University. As shown in Fig. 1, apart from the conventional air-supplying system, a mist-spraying system that mainly consist of the water supply, the pressure pump, the high-pressure hose, the nozzles and regulating valves, was added to build up the hybrid system. Besides, a mist and air handling system as well as a reefer controlling system was also included in the current study for handling the conditions of the fluids and the reefer. These components were distributed in three different chambers (Fig. 1) which can be introduced as follows. 378
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Fig. 1. Schematic of the experimental setup.
the diameter of mist droplets was set as 150 μm in the model, in accordance with the nozzle manufacture. Since the wall of the reefer Lab. was well covered with the thermal insulation material, it was set as adiabatic in present simulation.
Wang, 2012). Owing to the above features, the DPM model, which tended to be more eligible (Bhattacharya et al., 2009), was adopted in this paper. Simulation runs were performed under different flow rate combinations of airflow and mist (Table 1) to reveal the cooling effects of the hybrid system. The simulation results will be presented in comparison with the experiment data, as shown in Section 4.
3.1.3. Simulation model Apparently, with the mist involved in the airflow, the current study belonged to the two-phase flow and there exist some computational models for such problems, e.g. the volume of fluid (VOF) model and the discrete phase model (DPM). As to the present work, the mist was dispersed in the air (the continuous phase) after being sprayed. Strong interaction between the gas phase and the droplets phase would occur, such as the possible mist evaporation together with the heat and mass transfer between the mist and the airflow. In addition, as inferred from the operating conditions shown in Table 1, the volume ratio of the mist to the airflow was smaller than 10%. This indicates the mist was slightly loaded in the air and can be treated as the discrete phase (Ragab and
3.2. Experimental study A novel hybrid cooling system for reefers was constructed (see Fig. 4) in this work. Extensive experimental tests were carried out to investigate the cooling performance of the hybrid system and compared with the simulation results. 3.2.1. Measuring instruments During the experimental runs, various instruments were adopted to test the system conditions. The major components and their
Fig. 2. Photograph of the reefer front face (a) and partial space of the reefer Lab. (b). 379
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Fig. 3. Computational grid (a) and boundary conditions (b) of the reefer Lab. model.
air were monitored while slight adjustments can be made by the background air conditioning system or the humidifiers equipped in Chamber 2. The power of the reefer as well as the hot air machine installed inside can be adjusted by the controlling system in Chamber 3. A PC-based data acquisition system (Keithley 2700 & 7700) was adopted to record the tested data during the whole experimental process. Multiple tests were conducted for each operating condition and the average value was obtained for the result analysis.
Table 1 Boundary conditions of mist and air flow rates. No.
Vair, sply (m3/h)
mmist (kg·h−1)
tair,sply (°C)
dair,sply (g/kg)
tmist (°C)
Mist diameter (μm)
1 2 3 4
4200 3650 2900 1950
5 10 15 25
35a
19.5a
28a
150b
a Based on the typical conditions of outdoor air and city water in late July of Nanhui District, Shanghai. b According to the nozzle manufacture.
3.2.2. Test conditions and procedures To validate the simulation model in Section 3.1, experiments were carried out under the initial conditions same with the simulation runs (see Table 1). Considering the required massive airflow rates (up to 4200 m3 h−1), it can hardly make significant changes to airflow's temperature and humidity ratio. Therefore, experimental runs were performed from 1:00 p.m. to 4:30 p.m. in the sunny days of late July when the outdoor conditions of the experimental site were stable and almost same to the designed operating conditions (35 °C &20 g·kg−1 dry air). With the slight adjustments by the background heating or air conditioning system equipped in Chamber 2, the desired inlet temperature and the humidity ratio for the air-supplying system can be achieved. In operation, the basic procedure of the experiments was as follows:
specifications are list in Table 2. Among the tested parameters, the airflow rates under different opening degrees of the fan valve were measured in advance with a hot-wire anemometer according to reference (Caré et al., 2014). Four air temperature sensors were installed at the outlet of condenser vent to obtain the temperatures of the air exhausted from the reefer's condenser. The ambient temperature and humidity inside the reefer Lab. were simultaneously measured via five ROTRONIC probes (Model HC2-S) distributed at the location shown in Fig. 1. In addition, the temperature and humidity ratio of the supplied 380
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Fig. 4. Photographs of the experimental setup: a) supplying blower and Agilent 13970 A; b) reefer front face and the air duct; c) temperature and humidity sensors being installed; d) mist sprayed from the outlet edge of air duct.
the reefer Lab. remained constant. And the experimental data was saved by the DAQ system for the upcoming results analysis. After one experimental run was finished, parameters of the airflow and water mist were reverted to the initial values, with one parameter changed within Table 1, for the next round of the experiments.
Table 2 Specification of main instruments. Parameters
Devices
Accuracy
Operational range
Water flow rate
Glass rotor flow meters PT100 RTD PT100 RTD ROTRONIC HYGROMER IN-1 Hot-wire anemometer
± 2.5%
0.01–0.1 L/min & 0.1–1 L/min 0–100 °C 0–100 °C 0–100%
Water temperature Air temperature Cargo air humidity Air flow rate
± 0.2 °C ± 0.2 °C 0.8% 0.03 m/s+5% of measured value
3.3. Performance indices To evaluate the cooling performance of the proposed hybrid system, three indicators, namely the reefer's exhaust temperature, the ambient temperature inside reefer Lab. and the loading capacity improvement, were adopted in this work.
0–20 m/s
3.3.1. Reefer's exhaust temperature The exhaust air temperature from the reefer's condenser, i.e. ta, ex, was directly determined by the cooling performance of the ventilation system. Besides, with a lower air temperature exhausted from the reefer's condenser, which indicates a lower running temperature for the condenser, a better working condition can be realized for the reefer's refrigeration system. Therefore, it was used as one of the performance indices.
Beforehand, the air-supplying system and mist-spraying system were adjusted to the initial conditions shown in Table 1, with the background heating or air conditioning system in Chamber 2. After this, the heater in Chamber 1 was turned on to warm up the ambient environment of the reefer Lab. and the door of reefer Lab. was closed to keep the Lab. isolated. When the ambient temperature of the reefer Lab. reached 43 °C, turn off the ambient heater in Chamber 1 and switch on the reefer's refrigeration system. As soon as the temperature inside the reefer decreased to the design value, switch on the hot air machine inside the reefer to simulate the cooling load from chilled goods. Just then, switch on the hybrid cooling system, i.e. turn on the air-supplying and the mist-spraying system, to start the cooling process for the reefer's condenser. Finally, the steady state was realized when the air temperature exhausted from the reefer's condenser and the thermal conditions inside
3.3.2. Ambient temperature of reefer lab To evaluate the effects of the proposed hybrid cooling system in removing the heat from the ambient space, the ambient thermal conditions inside the reefer Lab., which can be represented by the tested ambient temperatures (ab. ta, amb), was adopted as one of the performance indices. Apparently, with the lower ta, amb, better cooling effects of the hybrid system will be achieved. 381
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4.2. Effect of flow rate combinations of airstream and mist
value of the exhaust temperature obtained under the conventional operating condition, namely Vair, sply = 4200 m3 h−1 without spraying mist, is employed as the baseline. Obviously, applied with the mistspraying cooling technology, the air exhaust temperature dropped remarkably. Furthermore, the necessary air supply rate was found decreasing simultaneously, as shown in Fig. 6 (a). This manifests that owing to the benefits of the mist evaporation process, a great deal of airflow can be saved for each reefer. To figure out how much air flow on earth can be reduced, another two groups of experiments were conducted with the further decrease of the air supply rate, namely Vair, 3 −1 and Vair, sply = 1950 m3 h−1. The results were dissply = 2850 m h played in Fig. 6(b) and (c), respectively. As shown in Fig. 6(b) and (c), the reefer's exhaust air temperature was falling continuously with the mass growth of the sprayed mist. In particular, the exhaust air temperature became even lower than the baseline condition (i.e. Vair, 3 −1 without mist) when the sprayed mist was about sply = 4200 m h more than 5 kg h−1 in the case of Vair, sply = 2850 m3 h−1 (Fig. 6(b)). Similar results can also be observed in the case of Vair, 3 −1 , as shown Fig. 6 (c), when mist flow rate was higher sply = 1950 m h than 22.5 kg h−1. Compared to the baseline condition, the necessary air supply rate was reduced substantially from 4200 m3 h−1 to 1950 m3 h−1 with the reduction amplitude reached 53.57%. As the consequence, it seems to be highly possible and promising to increase the capacity of container ships for loading reefers with just minor reforms, which can be realized via supplementing the mistcooling system inside the cargo hold with few changes in the air-supplying system. This will be validated and discussed in Section 5. Meanwhile, with the growth of the mist spray rate, the air exhaust temperature was dropping rapidly at the beginning and leveled off at the end. The decreasing trends were well fitted as exponential with the R2 > 0.92 under experimental conditions (see Table 3). This manifests that significant cooling effect can be achieved when the mist-spraying system was applied. However, with the further growth of the mist spray rate, a continuing but weakening decrease trend of reefer's exhaust temperature was observed under all the operating conditions. The reefer's exhaust temperature was lowered effectively with consuming a small amount of the mist, which also contributed to the feasibility of the proposed hybrid system in the real ultra-large container ships. From another perspective of view, the relationship shown in Fig. 6 also indicates that the mist spray rate can be designed within a reasonable range to achieve an optimal cooling effects. This will be further discussed with a case study for the entire cargo hold in Section 5.2.
4.2.1. In terms of reefer's exhaust temperature Fig. 6 demonstrates the effects of mist spray rates on the reefer's exhaust temperature under different air supply rates. The experimental
4.2.2. In terms of reefer Lab's ambient temperature Fig. 7 illustrates the relationships between the mist spray rate and
3.3.3. Loading capacity improvement of container ships To assess the benefits of the proposed hybrid cooling system in improving the loading capacity of the container ship for reefers, the indicator, loading capacity improvement (ab. LCI), is employed in present work. It can be calculated as follows.
LCI =
LCcrnt − LCprev LCprev
× 100%
(1)
Evidently, with better cooling performance of the hybrid system, less airflow would be required for each reefer to meet the cooling demand of the reefer's condensers. Thus, more reefers can, ideally, be loaded in the container cargo holds without enlarging the existing ventilation system. In addition, the average absolute difference (ab. AAD) between the predicted values and the experimental results was employed in this work to evaluate the reliability and accuracy of the simulation model under different conditions. It was defined as Eq. (2) shows. N
AAD (X ) =
1 ⋅ ∑ Xexpt − Xpred N i=1
(2)
where X refers to the system performance indices, such as ta, ex and ta, amb; N stands for the number of the simulation or experimental tests under each condition 4. Results and discussion 4.1. Comparison between the experimental performance and the simulation results The predicted cooling performance of the hybrid system by the simulation model is compared with the experimental results in Fig. 5. As shown in Fig. 5 (a), the predicted values of the reefer's exhaust temperature are in good agreement with the experimental finding, with the clear majority of the results fall into a deviation range of 3%. Meanwhile, with a maximum discrepancy less than 8%, good accuracy was also realized in predicting the ambient temperature of the cargo hold (Fig. 5 (b)). Therefore, it can be concluded that the present simulation model is reliable to predict the cooling effects of the hybrid system on reefers.
Fig. 5. Comparison between the experimental results and the predicted values of the reefer's exhaust air temperature (a) and the interior ambient temperature of the reefer Lab. (b). 382
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Fig. 6. Effects of mist mass flow rate on the reefer's exhaust temperature under different airflow rates: (a) Vair, Vair, sply = 1950 m3 h−1.
sply =
3650 m3 h−1; (b) Vair,
sply = 2850
m3 h−1; (c)
would arise after introducing the mist-spraying system.
the ambient temperature inside the reefer's Lab under various air supply rates. The experimental ambient temperatures obtained under the baseline condition (Vair, sply = 4200 m3 h−1 without spraying mist), was adopted for comparison. As can be seen in Fig. 7, the ambient temperatures declined significantly, which were well fit as exponential as shown in Table 3, and much less air supply rate was required after applying the mist-cooling technology. For instance, with the mist spray rate reached 5 kg h−1, the necessary air supply rate was reduced significantly from 4200 kg h−1 in the baseline condition to 3650 m3 h−1, as shown in Fig. 7 (a). When the mist spray rate kept increasing to 22.5 kg h−1, it was found that the necessary air supply rate fell dramatically to only 1950 m3 h−1 to achieve the same cooling effect. All the relationships shown in Fig. 7 prove that the ambient temperature of the reefer Lab. can be lowered effectively with the application of the mist-spraying system, while the necessary air supply rate decreased remarkably. At the same time, it is noticed in Fig. 7 (c) that the predicted ambient temperature was somewhat higher than the experimental values. The reason for this may be due to the slight difference on thermal property of the Lab.’ wall which was set adequate in simulation models but hardly to be fully achieved in experimental sites though it was well covered with thermal isolation materials. However, with the maximum deviation less than 8%, the difference was negligible.
5. Validation of loading capacity improvement for reefer ship The results shown in Section 4 confirmed that the application of mist-spraying cooling can make a great contribution to lowering the exhaust air temperature and the ambient temperatures of the single reefer Lab., without raising risks of the excessive humidity or the undesired condensation. However, since the experiments were conducted on one reefer in the laboratory, whereas there would be many reefers loaded in a practical cargo hold, a validation study was performed as follows.
5.1. Validation background To validate the cooling effects of the proposed hybrid system for the reefer cargo hold, the layout of a real cargo hold (see Fig. 9) of a practical ultra-large container ship designed by the China State Shipbuilding Corporation was employed as the study case. As shown in Fig. 9, the cargo hold, which consist of three bays and was symmetrical in the B-B direction, was 44.74 m long, 43.48 m wide and 24.4 m high. Initially, it was designed to load 453 of 40-foot standard containers, among which only 150 containers were reefers due to the over-sized ventilation ductworks and the corresponding space conflicts, while 303 containers were non-refrigerated. Evidently, this led to a severe limitation to the ability of the container ship in carrying reefers. To clarify the cooling performance of the proposed hybrid system for the cargo hold, a simulation model was built as displayed in Fig. 9. The model geometry and the meshing result is shown in Fig. 10(a) and (b), respectively. Operating conditions that were potentially satisfying in the single reefer Lab (as investigated in Section 4.3). were adopted here to validate their theoretical feasibility for the cargo hold.
4.2.3. In terms of ambient humidity Fig. 8 displays the humidity conditions (represented by the relative humidity and the humidity ratio) in the reefer Lab. As shown in Fig. 8, both the relative humidity and the corresponding humidity ratio were growing continuously (in exponential trends as shown in Table 4) with the rise of mist spray rate. However, the highest relative humidity and humidity ratio in present study was lower than 60% and 30 g·kg−1 dry air, respectively, which is far from its saturation level and possible condensation problems can be avoided. Therefore, it can be inferred that few issues on the undesired water condensation inside the cargo hold
Table 3 Relationship of mist-spraying rate on the reefer's air exhaust temperature & Lab.’s ambient temperature. Vair, supply (m3·h−1)
Regression Formulas (based on the experimental results) Reefer's exhaust air temperature (oC)
Ambient temperature (oC)
3650
ta,exhaust = 44.691 + 8.409⋅exp(−0.0475⋅mmist ) , R2 = 0.927
2850
ta,exhaust = 48.809 + 6.337⋅exp(−0.104⋅mmist ) , R2 = 0.914
tamb = 40.292 + 3.488⋅exp(−0.1082⋅mmist ) , R2 = 0.944 tamb = 40.746 + 4.418⋅exp(−0.1408⋅mmist ) , R2 = 0.919
1950
ta,exhaust = 52.228 + 8.966⋅exp(−0.1471⋅mmist ) , R2 = 0.969
tamb = 40.511 + 5.138⋅exp(−0.1043⋅mmist ) , R2 = 0.979
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Fig. 7. Effects of mist mass flow rate on the reefer Lab.’s ambient temperature under different airflow rates: (a) Vair, sply = 3650 m3 h−1; (b) Vair, sply = 2850 m3 h−1; (c) Vair, sply = 1950 m3 h−1.
5.2.2. Validation for the cargo hold During the running process of the refrigerated containers, it is suggested that the inlet temperature of the cooling air sucked by the reefer's condenser should be lower than 45 °C. This temperature requirement, consequently, was adopted as the criterion to verify the feasibility of the operating options for the cargo hold. In the practical cargo hold, reefers are usually placed one above another. Owing to this packed loading pattern and the buoyant effect of hot air, the inlet temperature of cooling air for the upper reefers as well as the thermal condition at the top of the cargo hold can be significantly affected by the lower and the nearby reefers. Therefore, feasibility of the potential operating conditions was carefully verified from different aspects of the cargo hold, which can be introduced as follows.
5.2. Feasibility of the potential mist and air combinations for the cargo hold After clarifying the effects of the mist spray rate on the reefer's exhaust temperature in Section 4.3.1, the operating conditions of the hybrid system that could achieve the same (or better) cooling effects with the baseline condition (4200 m3 h−1 without spraying mist) were selected as the potential options for the cargo hold. Then their feasibility for cooling the whole cargo hold was verified. At the end, possible improvement of the ship's loading capacity was figured out. 5.2.1. The potential design for cargo hold Since the hot air exhausted from the reefer's condensers was heat source in the cargo hold, the thermal conditions inside the cargo hold may be improved when the reefer's exhaust air temperature is equal to or lower than the temperature of the baseline condition (52.5 °C in present study). Thus, according to the results shown in Section 4.3, nine operating conditions of air and mist flow rate were selected as the potential options. These airflow-mist flow rate combinations to be validated were 1950 m3 h−1-25 kg h−1, 3650 m3 h−1-5 kg h−1, 3 −1 −1 3 −1 3650 m ·h - 10 kg h , 3650 m h -25 kg h−1, 2900 m3 h−15 kg h−1, 2900 m3 h−1-10 kg h−1, 2900 m3·h−115 kg h−1, 2900 m3·h−1- 25 kg h−1, 2250 m3·h−1- 25 kg h−1. Simulations were carried out under the above operating conditions to validate their cooling effects for the hold cargo while the initial ventilation design, 4200 m3 h−1 without spraying mist, was employed as the baseline. The validation results can be shown and discussed as follows.
a) In terms of the air let temperature for the reefers Table 5 summarizes the air inlet temperatures for all the 150 reefer condensers in the cargo hold under the aforementioned potential air and mist supply rate designs. As shown in the table, most of the cooling air's inlet temperatures, except for the case 1950 m3 h−1-25 kg h−1, were lower than 45 °C, which seems to meet the set criterion. Nevertheless, with the low airflow rate, for instance Vair = 1950 m3 h−1, the hot air cannot be circulated sufficiently and excessive heat may be accumulated in the cargo hold, which could lead the inlet temperature of the upper reefers to be much higher. The detailed thermal condition inside the cargo hold can be seen in the following section.
Fig. 8. Effects of mist mass flow rate on the reefer Lab.’s ambient humidity under different airflow rates: (a) Vair, sply = 3650 m3 h−1; (b) Vair, sply = 2850 m3 h−1; (c) Vair, sply = 1950 m3 h−1. 384
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Table 4 Relationship of mist-spraying rate on the reefer Lab.’s ambient relative humidity and humidity ratio. Vair, supply (m3·h−1)
3650 2850 1950
Regression Formulas (based on the simulation results) Relative humidity inside the reefer Lab. (%)
Humidity ratio inside the reefer Lab. (g·kg−1 dry air)
RHamb = 75.829 − 37.037⋅exp( −0.0267⋅mmist ) , R2 = 0.997 RHamb = 62.561 − 27.683⋅exp( −0.0495⋅mmist ) , R2 = 0.998 RHamb = 60.134 − 31.817⋅exp( −0.0474⋅mmist ) , R2 = 0.994
damb = 32.608 − 10.603⋅exp( −0.01703⋅mmist ) , R2 = 0.973 damb = 27.183 − 5.206⋅exp(−0.0565⋅mmist ) , R2 = 0.993 damb = 29.101 − 7.064⋅exp(−0.0529⋅mmist ) , R2 = 0.997
The reason for this was due to effect of heat accumulation at the upper layers in the cargo hold. As can displayed in Fig. 11, the hot air exhausted from the lower layer reefers flew upwards and led the massive hot air to accumulate at the upper position of the cargo hold. With less fresh cooling air supplied, a bigger amount of hot air from the lower reefers would be involved into the airflow supplied to the upper reefers, resulting in the poor thermal conditions inside the cargo hold. The different thermal conditions shown in Fig. 11 also manifests that spraying mist did help a lot in reducing the necessary air supply rates. However, it cannot completely replace the effects of airflow. In other words, there should be a balanced range during the design process of air supply rates and the mist spray rates. This will be discussed in Section 5.3.
b) Thermal conditions inside the cargo hold During the laboratory research on the single reefer (Section 4.3), it was found that the reefer's exhaust temperatures under the operating conditions (namely 1950 m3 h−1-25 kg h−1 and 3650 m3 h−1-5 kg h−1) were almost same with the baseline condition (4200 m3 h−1 without spraying mist). As a comparison, their temperature distributions for the whole cargo hold were illustrated in Figs. 11 and12. Three featured cross sections (see Fig. 9) were selected to demonstrate the contours. Namely, 1) the view perspective perpendicular to the front face of middle reefers (cross section A-A, x = 16750 mm); 2) the view perpendicular to the middle reefers (cross section B-B, z = 11647 mm); 3) the horizontal perspective crossing through the top exhaust vents of the cargo hold (cross section C-C, y = 24400 mm). However, considering the space limitation of the manuscript, contours detail of air temperature distribution from the view of cross section A-A can be found in the supplemental materials. As shown in Figs. 11 and12, though similar exhaust temperature was achieved during the single reefer research, their cooling effects for the cargo hold differed remarkably. In particular, the cooling effect shown in Fig. 11(c) was apparently unfavorable, even though the mean inlet air temperature for reefers was around 45.52 °C.
c) Effects of the mist spray rate on the inlet air temperatures for reefers in cargo hold Fig. 13 demonstrates the effects of the mist spray rate on the inlet temperatures of the air sucked by reefer condensers, taking the air supply rate of 2900 m3 h−1 as the example. As shown in Fig. 13, the air inlet temperatures (the maximum value, the minimum value and the facet average) for the condensers decreased continuously with the
Fig. 9. Top view a) and front view b) of the validation reefer cargo hold. 385
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Fig. 10. Geometric model (a) and meshing result (b) for the cargo hold simulation.
may exist an optimal operating range for the hybrid system to achieve the balance. This will be further discussed in Section 5.3.
Table 5 Summary of inlet air temperatures for the reefer condensers. Air and mist compositions
Air inlet temperature for all reefers inside the cargo hold under different operating conditions
Aira (m3·h−1)
Water mist (kg·h−1)
Maximum
Minimum
Facet Average
1950 2250 2850 2850 2850 2850 3650 3650 3650
25 25 5 10 15 25 5 10 25
48.87 48.45 47.89 46.78 45.82 45.57 45.71 44.61 42.82
43.05 42.33 41.46 40.79 40.45 40.33 39.57 39.13 38.37
45.52 44.76 43.79 43.03 42.41 42.24 42.24 41.53 40.35
a
5.3. Method for determining the mist and airflow rates range Based on the study results shown in Section 5.2, the effects of mist spray rate as well as the air supply rate on the maximum inlet temperature of the cooling air for reefer condensers can be regressed as Eq. (3) shows.
t a,inlet,max = 37.205 + 28.127⋅exp( −0.010728m mist − 0.000492⋅Va,supplied ) (3)
R2
= 0.930 ; with The accuracy of Eq. (3) was validated with the experimental data. As shown in Table 6, the predicted values by Eq. (3) were found parallel to the tested results. Thus, when different inlet temperatures of cooling air, ta, i, max, were set for the reefer condensers in present study, the corresponding operating conditions of the proposed hybrid system can be obtained by Eq. (3). For instance, if the maximum temperature of cooling air (ta, i, max) was set as 45 °C and the maximum ability of the water-supplying system was 25 kg h−1 for each reefer, then the necessary design range of mist and airflow rates can be obtained as the green section shown in Fig. 15 (a). Similarly, if lower temperature of the cooling air (ta, i, max) was required, for example 43 °C, the corresponding flow rate combinations of the mist and airflow would then fall into the green section displayed in Fig. 15 (b). With the same method, the operating conditions of the proposed hybrid cooling system for other design targets can be easily determined. In the present study, the maximum temperature of the cooling air for each reefer's condenser was lower than 45 °C. Efforts were made to achieve a balance between reducing the air supply rate and consumption of the water mist. According to the practical water-supplying ability of the container ship studied in this work, which could provide 10 kg h−1 pressured water for each reefer, the corresponding air supply rate was found to be only 2400 m3 h−1 via Eq. (3). As the result, great capacity improvement of the container ships for loading reefers can be achieved. This is to be discussed in the following section.
Temperatures and humidity of the supplied air are shown in Table 1.
increase of the mist flow rate. When the mist flow rate was enhanced from 5 kg h−1 to 10 kg h−1, the mean intake temperature of the condensers dropped significantly by 0.76 °C. This proves that significant cooling effect can be realized by raising the mist flow rate at its lower level. However, with more mist sprayed, weakening cooling effects were observed. For instance, with the further increase of mist flow rate from 15 kg h−1 to 25 kg h−1, temperature drop (0.2 °C) of the facet average value was limited. As the consequence, it can be inferred that there should exist an optimal design range of mist spray rate in which balance between the mist consumption rate and cooling performance can be achieved. Further discussion is conducted in Section 5.2. d) Effects of air supply rates on the inlet air temperatures for reefers in cargo hold Fig. 14 shows the effects of the air supply rate on the air inlet temperatures for the reefer condensers in the cargo hold with the mist flow rate of 25 kg h−1 set as the example. As can be seen in the figure, the air temperatures for cooling the reefer's condensers decrease significantly with the growth of airflow rates. In particular, when the air supply rates increased from 1950 m3 h−1 to 2900 m3 h−1, a remarkable temperature drop (3.2 °C) of the cooling air that flows into the condensers was observed. With the further increase of the air supply rate, though the significant temperature drop of the cooling air can still be observed, its decreasing trends became moderate. However, it should be noted that the enhanced air supply rate could also raise the electricity consumption and more space would be occupied by the enlarged ventilation system. Considering the cooling effects via spraying mist (Fig. 13) or enhancing the air supply rates (Fig. 14), it seems that there
5.4. Potential improvement on the reefer loading capacity of the container ship Within the optimal operating conditions (i.e. 2400 m3·h−110 kg h−1 for each reefer) obtained in Section 5.3, the airflow rate was reduced dramatically from the original 4200 m3 h−1 to only 2400 m3 h−1 while the mist consumption (10 kg h−1) tends to be 386
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(caption on next page) 387
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Fig. 11. Contours of air temperature from the view of cross section B-B (z = 11647 mm) under various combinations of air and mist flow rate: (a) 4200 m3·h−10 kg h−1; (b) 3650 m3·h−1- 5 kg h−1; (c) 1950 m3·h−1- 25 kg h−1.
Fig. 13. Effects of the mist-spraying rate on the intake air temperatures of reefers' condensers.
Fig. 14. Effects of the air-supplying rate on the intake air temperatures of reefers' condensers. Table 6 Validation of the necessary airflow rates predicted by Eq. (3). Target temperature for validationa ta,i, max (oC)
45.52 43.79 43.03 42.41 42.24 42.24 40.35 41.53 44.76
Fig. 12. Contours of air temperature from the top view of the cargo (cross section C-C, y = 24400 mm) under various combinations of air and the mist flow rate: (a) 4200 m3·h−1- 0 kg h−1; (b) 3650 m3·h−1- 5 kg h−1; (c) 1950 m3·h−1- 25 kg h−1.
Required air flow rate (m3·h−1)
Designed mist spray flow rate, mmist, designed (kg·h−1)
Designed
Predicted
25 5 10 15 25 5 25 10 25
1950 2850 2850 2850 2850 3650 3650 3650 2250
1939 2837 2975 3086 2957 3383 3913 3580 2134
Relative error (%)
0.56 0.46 4.39 8.28 3.75 7.32 7.21 1.92 5.16
a Obtained from the practical tested results of the whole cargo situation based on the feasible operating conditions.
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Fig. 15. Predicted range of airflow and mist-spraying rates for various targets: a) tinlet,max ≤ 45oC and b) tinlet,max ≤ 43oC .
dramatically with application of the mist-spraying technology. In the case study of current work, the loading capacity was enhanced from 100 reefers to 175 reefers per cargo hold, theoretically, without enlarging the ventilation system. 4) It is feasible and reliable to adopt the DPM model to simulate the cooling performance of the proposed hybrid system. Good agreement was achieved between the predicted performance and the experimental results with the maximum deviation less than 3% for reefer's exhaust temperature and 8% for the ambient temperature. 5) A method for determining the operating conditions of the mist spray rates and air supply rates at different cooling demands was established. Instead of the scattered operating conditions, a continuous design range of air supply rate and mist spray rate was obtained.
moderate and affordable for most of the reefer ships. As the consequence, much less energy would be cost by the blowers for supplying the fresh air into the cargo hold and the necessary space taken up by the bulky ventilation system can be reduced simultaneously. From another point of view, the loading capacity for reefers can be improved significantly without expanding the size of the existing air-supplying system. For the cargo hold studied in this paper (see Fig. 9), the capacity of the air-supplying system, which was mainly limited by the space conflicting of the bulky ductwork and the excessive energy consumption by blowers, was designed as 420,000 m3 h−1 for each hold. In this condition, the loading capacity in each cargo hold was set to be 100 reefers while the rest of the space was filled with the non-refrigerated containers. However, after applying the proposed hybrid cooling system that added the mist-spraying system, the necessary air supply rate was reduced significantly to only 2400 m3 h−1 for each reefer. As the consequence, compared with the original 100 boxes, the number of reefers that can be loaded inside each cargo hold is significantly raised (to as many as 175 reefers in theory) based on the same air-supplying system. This indicates that the loading capacity of the container ships for reefers could be dramatically improved. Considering the significantly improved capacity for loading extra reefers and minor build or reform costs of the system, it can be inferred that enormous economic benefits will also be produced by the proposed hybrid cooling system for the container shipping industry. Detailed study on this issue should be carried out in the future work.
It should, however, be noted that the loading capacity improvement (LCI) was figured out, theoretically. This should be further validated via thorough field tests in a real container ship in future work. Acknowledgement This work is financially supported by the Fundamental Research Funds for the Central Universities of China (No. 2232018D3-36), China Postdoctoral Science Foundation (No. 2018M630385) and the National Natural Science Foundation of China (No. 51176107). Besides, the authors want to express their sincere gratitude to Dr. Hongmin Liu and Dr. Ankang Kan in the Comprehensive Laboratory of Refrigerated Containers of Shanghai Maritime University for the great help in providing the experimental site.
6. Conclusions
Nomenclature
In this work, a novel hybrid cooling system that applied the mistspraying cooling system to the reefers cargo hold was proposed and its feasibility in improving the capacity of container ships to load reefers was carefully validated. The main conclusions can be summarized as follows:
ULCS m V
t d RH LC LCI AAD Subscripts
1) The cooling effects for the reefer condensers can be improved significantly with the application of the mist-spraying technology. In the present study case, it was found that the air temperature exhausted from the reefer condenser was lowered from the original 52.5 °C (the baseline condition) to only 47.4 °C (3650 m3·h−125 kg h−1), with the temperature drop of 5.1 °C. 2) The airflow rates for cooling the reefers in the cargo hold can be reduced remarkably after applying the mist-spraying technology. In this work, the necessary demand of airflow for each reefer decreased from the original volume of 4200 m3 h−1 to only 2400 m3 h−1. 3) The loading capacity of the reefer ships can be enhanced
a amb crnt ex 389
Ultra-large container ship mass flow rate, [kg·h−1] volume flow rate, [m3·h−1] temperature, [°C] humidity ratio, [g·kg−1 dry air] relative humidity, [%] loading capacity, [−] loading capacity improvement, [%] average absolute difference
air ambient current exhaust
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i max pred prev sply
inlet maximum predicted previous supply
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