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A detachable plate falling film generator and condenser coupling using lithium bromide and water as working fluids
Accepted Manuscript
A Detachable Plate Falling Film Generator and Condenser Coupling Using Lithium Bromide and Water as Working Fluids Tianle Hu , Xiaoyun Xie , Yi Jiang PII: DOI: Reference:
S0140-7007(18)30391-8 https://doi.org/10.1016/j.ijrefrig.2018.10.007 JIJR 4128
To appear in:
International Journal of Refrigeration
Received date: Revised date: Accepted date:
3 July 2018 26 September 2018 27 October 2018
Please cite this article as: Tianle Hu , Xiaoyun Xie , Yi Jiang , A Detachable Plate Falling Film Generator and Condenser Coupling Using Lithium Bromide and Water as Working Fluids, International Journal of Refrigeration (2018), doi: https://doi.org/10.1016/j.ijrefrig.2018.10.007
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Highlights
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A detachable plate falling film heat and mass exchanger coupling is proposed. The coupling can be either generator-condenser or absorber-evaporator. Heat transfer coefficient of prototype generator varied between 0.345 ~ 0.660 kW/ (m2·℃). Heat transfer coefficient of prototype condenser varied between 0.627 ~ 0.731 kW/ (m2·℃)
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A Detachable Plate Falling Film Generator and Condenser Coupling Using Lithium Bromide and Water as Working Fluids Tianle Hu, Xiaoyun Xie*, Yi Jiang
Department of Building Science, Tsinghua University, Beijing 100084, China
Abstract
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A novel detachable plate falling film heat and mass exchanger (HMX) coupling using lithium bromide and water as working fluids is proposed, with both maintainability and compactness taken into account and a prototype built. The HMX coupling can be applied as either a generator and condenser coupling, or an absorber and evaporator coupling in an absorption heat pump using water as refrigerant. The prototype, working as generator and condenser coupling, was evaluated and analyzed experimentally. In a sensitivity study of the operating variables of generator, heat transfer coefficient of the generator varied between 0.345 kW/ (m2·K) and 0.660 kW/ (m2·K), while the mass transfer coefficient of the generator was between 2.7×10-5 m/s to 7.8×10-5 m/s. The heat transfer coefficient of the condenser varied from 0.627 kW/ (m2·K) to 0.731 kW/ (m2·K), and the minimum generation rate of the designed condenser to maximize its heat transfer capability is 3.85 g/s.
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Keywords:
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Absorption; Heat and mass transfer; Plate heat exchanger; Falling film; Detachability
Nomenclature
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Kh Km Q h m P Re t x
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cp
area, m2 specific heat at constant pressure, kJ/(kg·K) heat transfer coefficient, kW/(m2·K) mass transfer coefficient, m/s heat rate, kW specific enthalpy, J/kg mass flow rate, kg/s pressure, kPa Reynolds number temperature, ℃ solution mass fraction, %
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Greek symbols Δtlm logarithmic mean temperature difference, ℃
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Introduction
Δm ρ
generation rate, g/s density
Subscripts c g r s v w in out eq
condenser generator refrigerant water solution refrigerant vapor water inlet of a component outlet of a component equilibrium
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The application of absorption heat pump (AHP), which plays a role in the conversion of different grades of thermal energy, and absorption heat exchanger (AHE), which realizes heat exchange between two fluids with mismatched flow rate, is expanding these years. AHE has shown great energy saving potential when applied in long distance heat transportation(Xie & Jiang, 2017). AHP and AHE are capable of improving energy efficiency and reducing pollutants emission when applied in coal-fired cogeneration plants (Hu, Xie, & Jiang, 2017b; Sun, Fu, Sun, & Zhang, 2014). AHP can recover a large part of waste heat of flue gas in natural gas cogeneration plants (Zhao et al., 2017). However, massiveness and cost have been long-standing problems of AHP systems, which have impeded the promotion of this technology.
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Many researchers attempted to scale down the size of AHP using ammonia as refrigerant through adopting some compact heat exchanger configurations. Boudéhenn et al. (2012) developed an ammonia absorption chiller with 5 kW cooling capacity using commercially available brazed plate heat exchanger as generator, rectifier and absorber, and the resulting prototype reduced the solution and refrigerant quantities. Zacarías, Ventas, Venegas, and Lecuona (2010) experimentally investigated the boiling heat transfer and pressure drop of ammonia-lithium nitrate solution in a plate generator, and obtained the correlations of them. Delahanty, Garimella, and Garrabrant (2015) presented two concepts for compact desorber-analyzer-rectifiers utilizing microchannel features. Among them, the plate configuration showed better feasibility due to the mature technology of plate heat exchangers.
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Lithium bromide and water was used as working fluids in most of the above-mentioned AHP and AHE application cases. However, the size of components in an AHP using water as refrigerant is naturally larger compared with one using ammonia due to the difference in thermodynamic properties between the two fluids; besides, vacuum condition is necessary in flow zones of working pairs of the former. These make it more challenging to develop a compact AHP using water than ammonia as refrigerant. Only a few researchers focused on this area. Mortazavi, Isfahani, Bigham, and Moghaddam (2015) introduced an absorber design suitable for the plate-and-frame absorber configuration. The design utilizes a fin structure installed on a vertical flat plate to produce a uniform solution film and minimize the film thickness, and to interrupt the boundary layer continuously. Michel, Le Pierrès, and Stutz (2017) developed a vertical grooved falling film absorber using a new plate-type falling film absorber design. A 1-D stationary model of water vapor absorption in a laminar vertical falling film was introduced and validated. In our previous study (Hu, Xie, & Jiang, 2017a), a plate falling film generator, which includes a directly connected overflow solution distributor, was presented. This more compact novel structure, with ensured stable falling-film flow, provides better feasibility to regulate capacity by plate assembly compared with conventional falling film distributors.
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When trying to transform an AHP with conventional shell and tube structure to a more compact one, some essential features must be taken into account to ensure reliability. First, maintenance and descaling of heat medium channels in AHP and AHE are usually necessary in actual application cases. Therefore, the detachability of heat medium channels, without damaging vacuum condition of working fluids channels, is essential for a compact AHP. However, there is no literature focused on this area. Besides, the falling-film configuration has shown great advantages over the flooded-type ones due to that falling-film generators/evaporators do not suffer from static pressure loss and feature a lower temperature difference between the working fluid and the wall(Gonda et al., 2014; Hu et al., 2017a). Taking all the essential features above into account, a novel detachable plate falling film heat and mass exchanger (HMX) coupling using lithium bromide and water as working fluids is proposed. A prototype used as generator and condenser coupling is designed, experimentally evaluated and analyzed. 2
Description of the HMX Coupling
The directly connected liquid distributor presented in our previous study(Hu et al., 2017a) is adopted in the novel HMX coupling design. As shown in Fig.1, a stream of working liquid (solution or refrigerant water) flows into
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liquid inlet chambers (1), and is then divided and introduced to each liquid storage zone (5).With the liquid level increasing in the liquid storage zone, it overflows the overflow weir (4) and flows into the working fluids channel (6) in the form of a falling film adhering to the plate surface. Liquids after experiencing a heat and mass transfer process (absorption, generation, evaporation, or condensation) in different working fluids channels of a module (11 or 12) are streamed into a liquid outlet chamber (8). Vapor generated (introduced) flows out of (in) a working fluids channel though an injected vapor channel (3), and the vapor channel also functions as a liquid baffle. The vapor is collected or divided in a vapor chamber (2). One module (11 or 12) can include multiple (although each module consists only two units in Fig.1) projecting heat and mass transfer units. Working fluids of the multiple units of each module are collected or divided in the chambers (1, 2, and 8) respectively. Especially, all the chambers of a unit are settled on the same side. Therefore, the solution distributor is asymmetric because the inlet solution enters into the distributor from only one side of it rather than both sides in our previous study mentioned above. Two modules are entwined together and accordingly heat medium channels (7) are developed within the spaces between the two entwined modules. Chambers used to collect or divide the working fluids are placed on the same side of each module, on the opposite side of the two entwined modules.
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The HMX presented above, which consists of two entwined modules, is detachable since its heat medium channels can be exposed by extracting the two entwined modules from the opposite direction. As show in Fig. 1, the attaching and detaching processes are similar to the entwining and loosing of fingers.
Fig. 1. Conceptual sketch of a detachable heat and mass exchanger
Two HMXs are coupled to reduce vacuum connection components. HMX coupling is realized by connecting two corresponding vapor chambers with two vapor pipes on the same side of two modules from different HMX. The two pipe-connected modules can be extracted as a whole in a detaching process. The HMX coupling can be functioned as both a generator and condenser coupling, and an absorber and evaporator coupling in an AHP which uses water as refrigerant.
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Fig. 2. shows a HMX coupling prototype based on the concept demonstrated above. Each HMX in the prototype contains two modules, and each module consists of five heat and mass transfer units. The main body of the HMX coupling is 950mm high, 450mm wide, and 880mm long. Thermal performance of this HMX coupling prototype when applied as a generator and condenser coupling was analyzed in this paper.
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Fig. 2. Rendering of a heat and mass exchanger coupling
Experiment description 3.1 Experiment apparatus
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A kind of specially designed bubble corrugated plate, which can bear the pressure difference without a significant increase of flow resistance loss of refrigerant vapor, was adopted in the prototype. The depth of the heat medium channel and the working pairs channel is 6mm and 16mm, respectively. Each plate is 200mm wide and 600mm long. The total effective heat and mass transfer areas of the generator and the condenser Ag and Ac are 1.94m2 since part of each plate is covered by the distributor. The design heat transfer capacity of each heat and mass exchanger is 10kW.
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The experiments were carried out on a test bench for vacuum heat and mass transfer process(Li, Xie, & Jiang, 2015; Zheng, Xie, & Jiang, 2016). The heat source of the generator was a stream of circulating hot water heated by electrical heaters. The heat sink of the condenser was a stream of circulating cooling water produced by an aircooled chiller. A PID controller was used to ensure that temperature of the inlet cooling water fluctuates within ±1℃. Strong solution from the generator mixes with the refrigerant water from the condenser in the solution tank. Weak solution flows out of a huge solution tank and into the generator. There were auxiliary electrical heaters to pre-heat the solutions when needed. The solution tank volume is 2 m3, and thus is large enough to ensure the stable solute mass concentration of the weak solution out of the tank. The LiBr/H2O working pairs formed a closed cycle. The electrical heaters for the hot water circuit and the solution circuit were controlled by changing the number of working heaters, and input heat capacity of the generator was then accordingly controlled.
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T Generator
Condenser
T Thermal Resistance
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C Coriolis P Pressure Sensor
F Flow meter C
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Cooling Water Tank
Heater
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T F
Chiller
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Heater
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Solution Tank
Fig. 3. Piping and instrumentation diagram of the experiment apparatus
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The solution densities ρg,s,in and ρg,s,out were measured by corresponding Coriolis sensors. The temperatures of the working pairs and heat mediums tg,s,in, tg,s,out, tg,w,in, tg,w,out, tc,w,in, tc,w,out, and tv were measured by platinum thermal resistances. The inlet solution mass flow rate mg,s,in was measured by a local Coriolis sensor. The flow rate of the cooling water mc,w was measured by a magnetic flow meter. A pressure sensor was used to measure the generation pressure P, and the thermal load of the generator Qg was obtained through directly measuring the electrical heating rate in the hot water circuit. All the parameters, including pressure, were measured after they were stabilized. Under the experimental conditions, the condensing pressure fluctuates as the inlet temperature of cooling water fluctuates around the set temperature of the cooling water tank (the amplitude is usually within 20 Pa) and so are the other parameters influenced by the condensing pressure. It would take 3~6 hours for all the parameters to reach a stable state. The mean average values within 10 minutes after all the parameters reached stable state were taken as outcomes. Table 1 shows all the instruments mentioned above and their accuracies. Piping and instrumentation diagram of the experiment apparatus is shown in Fig.3. Table 1 Instruments and accuracy
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Instruments Coriolis flow/density meter PT1000 Pressure sensor(Setra) Magnetic flow meter Wattmeter (Hioki 3169-20 Clamp On Power Hitester)
Accuracy 0.20% 0.1℃ 0.50% 0.50% 0.50%
To get an overall evaluation of the designed HMX coupling prototype, a sensitivity study on the heat and mass transfer in the generator and a thermal performance test on the condenser was conducted. In the sensitivity study, effects of hot water flow rate, solution flow rate, and heat flux on the heat and mass transfer in generator were studied. Thermal performance of the condenser in different generation rates was tested. The variation range of key parameters of the experiments are shown in Table 2. Table 2 Variation range of key parameters
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kg/(m·s) kW/m2 ℃ wt.% ℃ kPa
Range of variation 0.027~0.055 963~2762 3.95~10.68 21.5 46.8~54.9 51.5~83.9 2.4~5.1
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Parameter Inlet solution mass flow rate per perimeter Re of hot water in generator Heat flux in generator Inlet temperature of cooling water Solute mass fraction Inlet temperature of hot water Pressure
Data reduction
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A series of parameters were calculated using the values of the variables measured to analyze the operation of the generator and condenser. During the data reduction process, the following assumptions were made to simplify the calculation: 1) Steady-state condition. 2) The interfacial mass transfer resistance is neglected and the interface is in an equilibrium state. 3) The pressure and temperature of vapor is assumed to be constant in the entire generator and condenser coupling. 4) Condensed refrigerant water at the outlet of the condenser is saturated. 5) Gas leakage is neglected and non-condensable gas is not existed in the HMXs. 6) Perfect heat insulation is assumed. The thermodynamic properties of the water and lithium bromide mixtures were obtained using the computational formulations from Patek and Klomfar(Pátek & Klomfar, 2006). Through the computational formulations, xs,in and xs,out can be obtained based on ρg,s,in and tg,s,in, and ρg,s,out and tg,s,out respectively. Equilibrium concentrations of solution xeq,in and xeq,out can be obtained based on tg,s,in and tg,s,out for a given(measured) generation pressure P respectively. Condensed water is assumed to be saturated, and then its temperature can be obtained based on the generation pressure P. The enthalpy of solution was obtained based on solution temperature measured and solution concentration calculated, and the enthalpy of water was obtained through local temperature and generation pressure P. Programs were coded using EES (V10.113)(Klein & Alvarado, 2009).
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Thermal load of the condenser was calculated as follows:
Qc c p , w mw,c tc , w,out tc , w ,in
(1)
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The logarithmic mean temperature difference in the condenser was calculated as follows:
tlm,c
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tc , w,in tr tc , w,out t t ln r c , w,in tr tc , w,out
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The calculated thermal load of the condenser and the logarithmic temperature difference can be used to calculate the overall heat transfer coefficient in the condenser:
K h ,c
Qc Ac tlm.c
(3)
In a falling film generation process, energy conservation equation of heating water is as follows:
c p,wmg ,wdtw, g Kh, g (tw, g ts )dAg
(4)
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Assuming the equilibrium film exists at the interface between the gas phase and the liquid phase, thus the mass transfer equation between solution and vapor is as follows: dms Km s ( xeq x)dA (5) Energy conservation equation of solution is as follows:
d (ms hs ) hevap dms c p,wmw, g dtw, g
(6)
d (ms x) 0
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Conservation equation of solute mass is as follows:
(7)
With the measured parameters ρg,s,in, tg,s,in, tg,s,out, tg,w,in, tg,w,out, and Qg as boundary conditions, Kh,g and Km of each experiment condition can be attained by solving above differential equations(4)~(7) using finite-difference method.
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Outlet solution mass flow rate of the generator can be calculated through the solute mass conservation equation. Accordingly, the generation rate is calculated as follows:
m1 ms ,in ms ,out 1000
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An error analysis on energy and mass balance was conducted to check the validity of experiment results. The input heat rate of the test system is defined as follows:
Qin Qg
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(9)
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The input heat rate of the test system is used to heat the cooling water, to cause solution enthalpy difference between outlet and inlet, and to produce the condensing water. Therefore, the output heat rate of the test system is defined as follows: (10)
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Qout Qc ms ,out hs ,out ms ,in hs ,in ms ,in ms ,out hc ,r ,in
Based on the input and output heat rates calculated above, an energy balance graph is constructed and shown in Fig. 4. The energy balance errors of all selected experiment results are within [-20%, +20%].
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The generation rate can also be calculated by dividing condensing heat rate by vaporization enthalpy:
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Qc 1000 hc ,r ,in hc ,r ,out
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Based on the generation rates calculated by two methods, mass balance errors are obtained and shown in Fig. 5. It is obvious that the mass balance errors of all selected experiment results are within [-20%, +20%].
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Fig. 5. Mass balance checking
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Fig. 4. Energy balance checking
Results and Discussion 4.1 Sensitivity study on heat and mass transfer in generator
For the experiment apparatus described above, the four variable and controllable decisive parameters are: the hot water flow rate in generator, the inlet solution mass flow rate ms,in, the heat flux of generator (through controlling electrical heating rate), and the inlet cooling water temperature tc,w,in. When the sensitivity study was performed on one of the parameters, the other three were set at a given value. 4.1.1 Effect of the hot water flow rate
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Sensitivity study on the hot water flow rate in the Reynold number range of 963~2762 was conducted when the inlet solution mass flow rate ms,in, the heat flux of generator, and the inlet cooling water temperature tc,w,in were set at 0.03 kg/(m·s), 6.0 kW/m2 and 21.5℃, respectively. Fig. 6 shows the effect of the hot water flow rate on heat and mass transfer, taking the experimental heat and mass transfer coefficients as functions of Reynold number of hot water. As a general tendency, the heat transfer coefficient increases from 0.345 kW/ (m2·K) to 0.660 kW/ (m2·K) with the increase of Reynold number of hot water. However, there is no notable tendency on the changes of the mass transfer coefficient, which fluctuates randomly from 2.9×10-5 m/s to 7.2×10-5 m/s. It is safe to say that the hot water flow has a large effect on the overall heat transfer coefficient, but little effect on the mass transfer coefficient.
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(a) Effect of hot water flow rate on heat transfer in generator
(b) Effect of hot water flow rate on mass transfer in generator Fig. 6. Effect of hot water flow rate in generator
4.1.2 Effect of the solution flow rate
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Sensitivity study on the solution flow rate, in the range of 0.027~0.055 kg/(m·s), was conducted when the Reynold number of hot water in generator Rew, the heat flux of generator, and the inlet cooling water temperature tc,w,in were set at 2750, 6.0 kW/m2, and 21.5℃ respectively. Fig. 7 shows the experiment results, taking heat and mass transfer coefficients as functions of the solution flow rate. The heat transfer coefficient fluctuates randomly from 0.481 kW/(m2·K) to 0.660 kW/(m2·K), and the mass transfer coefficient fluctuates randomly from 2.7×10-5 m/s to 5.1×10-5 m/s. It is shown that the solution mass flow has few effects on the heat and mass transfer coefficients within the experiment range.
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Typically, high flow rates of cooling water are used in falling film absorbers or generators to ensure a fully turbulent flow and high heat transfer coefficients. This guarantees that the hot water side does not limit the absorption or generation process. However, in this study, the hot water flow velocity and Reynold number in the heat medium channel of the prototype could not achieve the higher optimal value, since its depth is too large due to the manufacture process limit. Since solution mass flow rate shows few effects on heat and mass transfer coefficient while hot water flow rate shows significantly effects in Fig.6, convection heat transfer resistance on the heat medium side is the main thermal resistance in the overall heat transfer process of generator. Actually, the overall heat and mass transfer coefficient can be highly improved when a more complex corrugation geometry is applied, which is mature in plate heat exchanger industry, to avoid the low turbulence problem mentioned above.
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(a) Effect of solution flow rate on heat transfer in generator
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(b) Effect of solution flow rate on mass transfer in generator
4.1.3 Effect of the heat flux
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Fig. 7. Effect of solution flow rate on mass transfer in generator
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Sensitivity study of heat flux in the range of 3.93~10.68 kW/m2 was conducted when Reynold number of hot water in generator Rew, inlet solution mass flow rate ms,in, and inlet cooling water temperature tc,w,in were set at 2750, 0.03 kg/(m·s), and 21.5℃ respectively. As shown in Fig. 8, heat flux has no obvious effect on heat mass transfer coefficient, since it fluctuates from 0.562 kW/(m2·K) to 0.632 kW/(m2·K). While mass transfer coefficient almost linearly increases from 2.9×10-5 m/s to 7.8×10-5 m/s when heat flux increases from 3.93 kW/m2 to 10.68 kW/m2.
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(a) Effect of heat flux on heat transfer in generator
(b) Effect of heat flux on mass transfer in generator Fig. 8. Effect of heat flux in generator
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Under the all conditions experimented and discussed above, the heat transfer coefficient of the generator varies between 0.345 kW/ (m2·K) and 0.660 kW/ (m2·K). The heat transfer coefficient level is slightly lower than our preliminary work of a one-working-fluids-channel generator/condenser couple with symmetric solution distribution, which varies between 0.735 kW/ (m2·K) and 0.856 kW/ (m2·K), due to the decrease of wetting rate caused by the asymmetric solution distributor adopted in this paper to realize the detachability of heat medium channel. Thermal performance of condenser
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The condenser of the prototype operated well in a wide range of generation rate from 2.15 g/s to 6.96 g/s as Fig.9 shows. In general, the heat transfer coefficient of the condenser increases from 0.415 kW/ (m2·K) to 0.698 kW/ (m2·K), with the generation rate increases from 2.1 g/s to 3.8 g/s. When the generation rate is larger than 4.0 g/s, the heat transfer coefficient of the condenser shows no notably increase but varies from 0.627 kW/ (m2·K) to 0.731 kW/ (m2·K). The results indicate the minimum generation rate of the designed condenser required to maximize its heat transfer capability is 3.85 g/s.
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Fig. 9. Thermal performance of condenser under difference generation rates
Conclusions
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A detachable plate falling film HMX coupling using lithium bromide and water as working fluids is proposed in this paper. The HMX coupling is capable of functioning as either a generator and condenser coupling, or an absorber and evaporator coupling in an AHP using water as refrigerant. Experiments on the designed HMX coupling prototype when applied as generator and condenser coupling were conducted, with performance evaluated and analyzed. Some conclusions can be made as follows:
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1) A detachable HMX is developed by entwining two asymmetrical modules. The attaching and detaching processes of the HMX are very similar to entwining and loosing fingers. Two HMXs with their vapor chamber connected form a HMX coupling. The tested prototype consists two connected HMXs, each HMX includes two entwined modules, and each module contains five projecting heat and mass units. 2) From the sensitivity study, it can be seen that hot water flow has significant effect on the overall heat transfer coefficient, but few effect on the mass transfer coefficient. Solution mass flow shows a few effects on the heat and mass transfer coefficients in the experiment cases due to the large water channel width, which is restricted by manufactural process. Heat flux has greater effect on the mass transfer coefficient than the overall heat transfer coefficient. Under the conditions experimented, the heat transfer coefficient of the generator varies between 0.345 kW/ (m2·K) and 0.660 kW/ (m2·K), while the mass transfer coefficient of the generator between 2.7×10-5 m/s and 7.8×10-5 m/s, within the hot water Reynold number range of 963~2762, solution flow rate per perimeter range of 0.027~0.055 kg/(m·s), and heat flux range of 3.93~10.68 kW/m2. 3) The condenser in the prototype operated well within a large range of generation rate, its overall heat transfer coefficient varies between 0.415~0.731 kW/m2 within a generation rate range of 2.15 g/s~ 6.96 g/s. The minimum generation rate of the designed condenser required to maximize its heat transfer capability is 3.85 g/s.
There is still a great performance enhancement potential of the prototype presented in this paper. Improving the wetting behavior of the asymmetric solution distributor, which was adopted in this paper to realize the detachability of the heat medium channels, will be the most critical topic in future work to enhance the heat and mass transfer performance of the detachable falling film HMX.
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Acknowledgments The authors gratefully acknowledge the support from the National Natural Science Foundation of China (grant numbers 51306098, 51138005), the innovative Research Groups of National Natural Science Foundation of China (grant number 51521005), and Tsinghua University Initiative Scientific Research Program (No. 20151080470).
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Zheng, S., Xie, X., & Jiang, Y. (2016). Experimental study on the flash evaporation process of LiBrH2O solution in an absorption heat pump. International Journal of Refrigeration, 61, 117-126. doi:10.1016/j.ijrefrig.2015.09.006
A Detachable Plate Falling Film Generator and Condenser Coupling Using Lithium Bromide and Water as Working Fluids Tianle Hu , Xiaoyun Xie , Yi Jiang PII: DOI: Reference:
S0140-7007(18)30391-8 https://doi.org/10.1016/j.ijrefrig.2018.10.007 JIJR 4128
To appear in:
International Journal of Refrigeration
Received date: Revised date: Accepted date:
3 July 2018 26 September 2018 27 October 2018
Please cite this article as: Tianle Hu , Xiaoyun Xie , Yi Jiang , A Detachable Plate Falling Film Generator and Condenser Coupling Using Lithium Bromide and Water as Working Fluids, International Journal of Refrigeration (2018), doi: https://doi.org/10.1016/j.ijrefrig.2018.10.007
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Highlights
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A detachable plate falling film heat and mass exchanger coupling is proposed. The coupling can be either generator-condenser or absorber-evaporator. Heat transfer coefficient of prototype generator varied between 0.345 ~ 0.660 kW/ (m2·℃). Heat transfer coefficient of prototype condenser varied between 0.627 ~ 0.731 kW/ (m2·℃)
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A Detachable Plate Falling Film Generator and Condenser Coupling Using Lithium Bromide and Water as Working Fluids Tianle Hu, Xiaoyun Xie*, Yi Jiang
Department of Building Science, Tsinghua University, Beijing 100084, China
Abstract
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A novel detachable plate falling film heat and mass exchanger (HMX) coupling using lithium bromide and water as working fluids is proposed, with both maintainability and compactness taken into account and a prototype built. The HMX coupling can be applied as either a generator and condenser coupling, or an absorber and evaporator coupling in an absorption heat pump using water as refrigerant. The prototype, working as generator and condenser coupling, was evaluated and analyzed experimentally. In a sensitivity study of the operating variables of generator, heat transfer coefficient of the generator varied between 0.345 kW/ (m2·K) and 0.660 kW/ (m2·K), while the mass transfer coefficient of the generator was between 2.7×10-5 m/s to 7.8×10-5 m/s. The heat transfer coefficient of the condenser varied from 0.627 kW/ (m2·K) to 0.731 kW/ (m2·K), and the minimum generation rate of the designed condenser to maximize its heat transfer capability is 3.85 g/s.
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Keywords:
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Absorption; Heat and mass transfer; Plate heat exchanger; Falling film; Detachability
Nomenclature
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Kh Km Q h m P Re t x
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cp
area, m2 specific heat at constant pressure, kJ/(kg·K) heat transfer coefficient, kW/(m2·K) mass transfer coefficient, m/s heat rate, kW specific enthalpy, J/kg mass flow rate, kg/s pressure, kPa Reynolds number temperature, ℃ solution mass fraction, %
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Greek symbols Δtlm logarithmic mean temperature difference, ℃
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Introduction
Δm ρ
generation rate, g/s density
Subscripts c g r s v w in out eq
condenser generator refrigerant water solution refrigerant vapor water inlet of a component outlet of a component equilibrium
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The application of absorption heat pump (AHP), which plays a role in the conversion of different grades of thermal energy, and absorption heat exchanger (AHE), which realizes heat exchange between two fluids with mismatched flow rate, is expanding these years. AHE has shown great energy saving potential when applied in long distance heat transportation(Xie & Jiang, 2017). AHP and AHE are capable of improving energy efficiency and reducing pollutants emission when applied in coal-fired cogeneration plants (Hu, Xie, & Jiang, 2017b; Sun, Fu, Sun, & Zhang, 2014). AHP can recover a large part of waste heat of flue gas in natural gas cogeneration plants (Zhao et al., 2017). However, massiveness and cost have been long-standing problems of AHP systems, which have impeded the promotion of this technology.
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Many researchers attempted to scale down the size of AHP using ammonia as refrigerant through adopting some compact heat exchanger configurations. Boudéhenn et al. (2012) developed an ammonia absorption chiller with 5 kW cooling capacity using commercially available brazed plate heat exchanger as generator, rectifier and absorber, and the resulting prototype reduced the solution and refrigerant quantities. Zacarías, Ventas, Venegas, and Lecuona (2010) experimentally investigated the boiling heat transfer and pressure drop of ammonia-lithium nitrate solution in a plate generator, and obtained the correlations of them. Delahanty, Garimella, and Garrabrant (2015) presented two concepts for compact desorber-analyzer-rectifiers utilizing microchannel features. Among them, the plate configuration showed better feasibility due to the mature technology of plate heat exchangers.
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Lithium bromide and water was used as working fluids in most of the above-mentioned AHP and AHE application cases. However, the size of components in an AHP using water as refrigerant is naturally larger compared with one using ammonia due to the difference in thermodynamic properties between the two fluids; besides, vacuum condition is necessary in flow zones of working pairs of the former. These make it more challenging to develop a compact AHP using water than ammonia as refrigerant. Only a few researchers focused on this area. Mortazavi, Isfahani, Bigham, and Moghaddam (2015) introduced an absorber design suitable for the plate-and-frame absorber configuration. The design utilizes a fin structure installed on a vertical flat plate to produce a uniform solution film and minimize the film thickness, and to interrupt the boundary layer continuously. Michel, Le Pierrès, and Stutz (2017) developed a vertical grooved falling film absorber using a new plate-type falling film absorber design. A 1-D stationary model of water vapor absorption in a laminar vertical falling film was introduced and validated. In our previous study (Hu, Xie, & Jiang, 2017a), a plate falling film generator, which includes a directly connected overflow solution distributor, was presented. This more compact novel structure, with ensured stable falling-film flow, provides better feasibility to regulate capacity by plate assembly compared with conventional falling film distributors.
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When trying to transform an AHP with conventional shell and tube structure to a more compact one, some essential features must be taken into account to ensure reliability. First, maintenance and descaling of heat medium channels in AHP and AHE are usually necessary in actual application cases. Therefore, the detachability of heat medium channels, without damaging vacuum condition of working fluids channels, is essential for a compact AHP. However, there is no literature focused on this area. Besides, the falling-film configuration has shown great advantages over the flooded-type ones due to that falling-film generators/evaporators do not suffer from static pressure loss and feature a lower temperature difference between the working fluid and the wall(Gonda et al., 2014; Hu et al., 2017a). Taking all the essential features above into account, a novel detachable plate falling film heat and mass exchanger (HMX) coupling using lithium bromide and water as working fluids is proposed. A prototype used as generator and condenser coupling is designed, experimentally evaluated and analyzed. 2
Description of the HMX Coupling
The directly connected liquid distributor presented in our previous study(Hu et al., 2017a) is adopted in the novel HMX coupling design. As shown in Fig.1, a stream of working liquid (solution or refrigerant water) flows into
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liquid inlet chambers (1), and is then divided and introduced to each liquid storage zone (5).With the liquid level increasing in the liquid storage zone, it overflows the overflow weir (4) and flows into the working fluids channel (6) in the form of a falling film adhering to the plate surface. Liquids after experiencing a heat and mass transfer process (absorption, generation, evaporation, or condensation) in different working fluids channels of a module (11 or 12) are streamed into a liquid outlet chamber (8). Vapor generated (introduced) flows out of (in) a working fluids channel though an injected vapor channel (3), and the vapor channel also functions as a liquid baffle. The vapor is collected or divided in a vapor chamber (2). One module (11 or 12) can include multiple (although each module consists only two units in Fig.1) projecting heat and mass transfer units. Working fluids of the multiple units of each module are collected or divided in the chambers (1, 2, and 8) respectively. Especially, all the chambers of a unit are settled on the same side. Therefore, the solution distributor is asymmetric because the inlet solution enters into the distributor from only one side of it rather than both sides in our previous study mentioned above. Two modules are entwined together and accordingly heat medium channels (7) are developed within the spaces between the two entwined modules. Chambers used to collect or divide the working fluids are placed on the same side of each module, on the opposite side of the two entwined modules.
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The HMX presented above, which consists of two entwined modules, is detachable since its heat medium channels can be exposed by extracting the two entwined modules from the opposite direction. As show in Fig. 1, the attaching and detaching processes are similar to the entwining and loosing of fingers.
Fig. 1. Conceptual sketch of a detachable heat and mass exchanger
Two HMXs are coupled to reduce vacuum connection components. HMX coupling is realized by connecting two corresponding vapor chambers with two vapor pipes on the same side of two modules from different HMX. The two pipe-connected modules can be extracted as a whole in a detaching process. The HMX coupling can be functioned as both a generator and condenser coupling, and an absorber and evaporator coupling in an AHP which uses water as refrigerant.
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Fig. 2. shows a HMX coupling prototype based on the concept demonstrated above. Each HMX in the prototype contains two modules, and each module consists of five heat and mass transfer units. The main body of the HMX coupling is 950mm high, 450mm wide, and 880mm long. Thermal performance of this HMX coupling prototype when applied as a generator and condenser coupling was analyzed in this paper.
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Fig. 2. Rendering of a heat and mass exchanger coupling
Experiment description 3.1 Experiment apparatus
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A kind of specially designed bubble corrugated plate, which can bear the pressure difference without a significant increase of flow resistance loss of refrigerant vapor, was adopted in the prototype. The depth of the heat medium channel and the working pairs channel is 6mm and 16mm, respectively. Each plate is 200mm wide and 600mm long. The total effective heat and mass transfer areas of the generator and the condenser Ag and Ac are 1.94m2 since part of each plate is covered by the distributor. The design heat transfer capacity of each heat and mass exchanger is 10kW.
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The experiments were carried out on a test bench for vacuum heat and mass transfer process(Li, Xie, & Jiang, 2015; Zheng, Xie, & Jiang, 2016). The heat source of the generator was a stream of circulating hot water heated by electrical heaters. The heat sink of the condenser was a stream of circulating cooling water produced by an aircooled chiller. A PID controller was used to ensure that temperature of the inlet cooling water fluctuates within ±1℃. Strong solution from the generator mixes with the refrigerant water from the condenser in the solution tank. Weak solution flows out of a huge solution tank and into the generator. There were auxiliary electrical heaters to pre-heat the solutions when needed. The solution tank volume is 2 m3, and thus is large enough to ensure the stable solute mass concentration of the weak solution out of the tank. The LiBr/H2O working pairs formed a closed cycle. The electrical heaters for the hot water circuit and the solution circuit were controlled by changing the number of working heaters, and input heat capacity of the generator was then accordingly controlled.
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T Generator
Condenser
T Thermal Resistance
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C Coriolis P Pressure Sensor
F Flow meter C
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Cooling Water Tank
Heater
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Chiller
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Heater
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Solution Tank
Fig. 3. Piping and instrumentation diagram of the experiment apparatus
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The solution densities ρg,s,in and ρg,s,out were measured by corresponding Coriolis sensors. The temperatures of the working pairs and heat mediums tg,s,in, tg,s,out, tg,w,in, tg,w,out, tc,w,in, tc,w,out, and tv were measured by platinum thermal resistances. The inlet solution mass flow rate mg,s,in was measured by a local Coriolis sensor. The flow rate of the cooling water mc,w was measured by a magnetic flow meter. A pressure sensor was used to measure the generation pressure P, and the thermal load of the generator Qg was obtained through directly measuring the electrical heating rate in the hot water circuit. All the parameters, including pressure, were measured after they were stabilized. Under the experimental conditions, the condensing pressure fluctuates as the inlet temperature of cooling water fluctuates around the set temperature of the cooling water tank (the amplitude is usually within 20 Pa) and so are the other parameters influenced by the condensing pressure. It would take 3~6 hours for all the parameters to reach a stable state. The mean average values within 10 minutes after all the parameters reached stable state were taken as outcomes. Table 1 shows all the instruments mentioned above and their accuracies. Piping and instrumentation diagram of the experiment apparatus is shown in Fig.3. Table 1 Instruments and accuracy
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Instruments Coriolis flow/density meter PT1000 Pressure sensor(Setra) Magnetic flow meter Wattmeter (Hioki 3169-20 Clamp On Power Hitester)
Accuracy 0.20% 0.1℃ 0.50% 0.50% 0.50%
To get an overall evaluation of the designed HMX coupling prototype, a sensitivity study on the heat and mass transfer in the generator and a thermal performance test on the condenser was conducted. In the sensitivity study, effects of hot water flow rate, solution flow rate, and heat flux on the heat and mass transfer in generator were studied. Thermal performance of the condenser in different generation rates was tested. The variation range of key parameters of the experiments are shown in Table 2. Table 2 Variation range of key parameters
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kg/(m·s) kW/m2 ℃ wt.% ℃ kPa
Range of variation 0.027~0.055 963~2762 3.95~10.68 21.5 46.8~54.9 51.5~83.9 2.4~5.1
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Parameter Inlet solution mass flow rate per perimeter Re of hot water in generator Heat flux in generator Inlet temperature of cooling water Solute mass fraction Inlet temperature of hot water Pressure
Data reduction
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A series of parameters were calculated using the values of the variables measured to analyze the operation of the generator and condenser. During the data reduction process, the following assumptions were made to simplify the calculation: 1) Steady-state condition. 2) The interfacial mass transfer resistance is neglected and the interface is in an equilibrium state. 3) The pressure and temperature of vapor is assumed to be constant in the entire generator and condenser coupling. 4) Condensed refrigerant water at the outlet of the condenser is saturated. 5) Gas leakage is neglected and non-condensable gas is not existed in the HMXs. 6) Perfect heat insulation is assumed. The thermodynamic properties of the water and lithium bromide mixtures were obtained using the computational formulations from Patek and Klomfar(Pátek & Klomfar, 2006). Through the computational formulations, xs,in and xs,out can be obtained based on ρg,s,in and tg,s,in, and ρg,s,out and tg,s,out respectively. Equilibrium concentrations of solution xeq,in and xeq,out can be obtained based on tg,s,in and tg,s,out for a given(measured) generation pressure P respectively. Condensed water is assumed to be saturated, and then its temperature can be obtained based on the generation pressure P. The enthalpy of solution was obtained based on solution temperature measured and solution concentration calculated, and the enthalpy of water was obtained through local temperature and generation pressure P. Programs were coded using EES (V10.113)(Klein & Alvarado, 2009).
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Thermal load of the condenser was calculated as follows:
Qc c p , w mw,c tc , w,out tc , w ,in
(1)
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The logarithmic mean temperature difference in the condenser was calculated as follows:
tlm,c
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tc , w,in tr tc , w,out t t ln r c , w,in tr tc , w,out
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The calculated thermal load of the condenser and the logarithmic temperature difference can be used to calculate the overall heat transfer coefficient in the condenser:
K h ,c
Qc Ac tlm.c
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In a falling film generation process, energy conservation equation of heating water is as follows:
c p,wmg ,wdtw, g Kh, g (tw, g ts )dAg
(4)
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Assuming the equilibrium film exists at the interface between the gas phase and the liquid phase, thus the mass transfer equation between solution and vapor is as follows: dms Km s ( xeq x)dA (5) Energy conservation equation of solution is as follows:
d (ms hs ) hevap dms c p,wmw, g dtw, g
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d (ms x) 0
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Conservation equation of solute mass is as follows:
(7)
With the measured parameters ρg,s,in, tg,s,in, tg,s,out, tg,w,in, tg,w,out, and Qg as boundary conditions, Kh,g and Km of each experiment condition can be attained by solving above differential equations(4)~(7) using finite-difference method.
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Outlet solution mass flow rate of the generator can be calculated through the solute mass conservation equation. Accordingly, the generation rate is calculated as follows:
m1 ms ,in ms ,out 1000
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An error analysis on energy and mass balance was conducted to check the validity of experiment results. The input heat rate of the test system is defined as follows:
Qin Qg
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The input heat rate of the test system is used to heat the cooling water, to cause solution enthalpy difference between outlet and inlet, and to produce the condensing water. Therefore, the output heat rate of the test system is defined as follows: (10)
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Qout Qc ms ,out hs ,out ms ,in hs ,in ms ,in ms ,out hc ,r ,in
Based on the input and output heat rates calculated above, an energy balance graph is constructed and shown in Fig. 4. The energy balance errors of all selected experiment results are within [-20%, +20%].
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The generation rate can also be calculated by dividing condensing heat rate by vaporization enthalpy:
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Qc 1000 hc ,r ,in hc ,r ,out
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Based on the generation rates calculated by two methods, mass balance errors are obtained and shown in Fig. 5. It is obvious that the mass balance errors of all selected experiment results are within [-20%, +20%].
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Fig. 5. Mass balance checking
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Fig. 4. Energy balance checking
Results and Discussion 4.1 Sensitivity study on heat and mass transfer in generator
For the experiment apparatus described above, the four variable and controllable decisive parameters are: the hot water flow rate in generator, the inlet solution mass flow rate ms,in, the heat flux of generator (through controlling electrical heating rate), and the inlet cooling water temperature tc,w,in. When the sensitivity study was performed on one of the parameters, the other three were set at a given value. 4.1.1 Effect of the hot water flow rate
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Sensitivity study on the hot water flow rate in the Reynold number range of 963~2762 was conducted when the inlet solution mass flow rate ms,in, the heat flux of generator, and the inlet cooling water temperature tc,w,in were set at 0.03 kg/(m·s), 6.0 kW/m2 and 21.5℃, respectively. Fig. 6 shows the effect of the hot water flow rate on heat and mass transfer, taking the experimental heat and mass transfer coefficients as functions of Reynold number of hot water. As a general tendency, the heat transfer coefficient increases from 0.345 kW/ (m2·K) to 0.660 kW/ (m2·K) with the increase of Reynold number of hot water. However, there is no notable tendency on the changes of the mass transfer coefficient, which fluctuates randomly from 2.9×10-5 m/s to 7.2×10-5 m/s. It is safe to say that the hot water flow has a large effect on the overall heat transfer coefficient, but little effect on the mass transfer coefficient.
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(a) Effect of hot water flow rate on heat transfer in generator
(b) Effect of hot water flow rate on mass transfer in generator Fig. 6. Effect of hot water flow rate in generator
4.1.2 Effect of the solution flow rate
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Sensitivity study on the solution flow rate, in the range of 0.027~0.055 kg/(m·s), was conducted when the Reynold number of hot water in generator Rew, the heat flux of generator, and the inlet cooling water temperature tc,w,in were set at 2750, 6.0 kW/m2, and 21.5℃ respectively. Fig. 7 shows the experiment results, taking heat and mass transfer coefficients as functions of the solution flow rate. The heat transfer coefficient fluctuates randomly from 0.481 kW/(m2·K) to 0.660 kW/(m2·K), and the mass transfer coefficient fluctuates randomly from 2.7×10-5 m/s to 5.1×10-5 m/s. It is shown that the solution mass flow has few effects on the heat and mass transfer coefficients within the experiment range.
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Typically, high flow rates of cooling water are used in falling film absorbers or generators to ensure a fully turbulent flow and high heat transfer coefficients. This guarantees that the hot water side does not limit the absorption or generation process. However, in this study, the hot water flow velocity and Reynold number in the heat medium channel of the prototype could not achieve the higher optimal value, since its depth is too large due to the manufacture process limit. Since solution mass flow rate shows few effects on heat and mass transfer coefficient while hot water flow rate shows significantly effects in Fig.6, convection heat transfer resistance on the heat medium side is the main thermal resistance in the overall heat transfer process of generator. Actually, the overall heat and mass transfer coefficient can be highly improved when a more complex corrugation geometry is applied, which is mature in plate heat exchanger industry, to avoid the low turbulence problem mentioned above.
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(a) Effect of solution flow rate on heat transfer in generator
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(b) Effect of solution flow rate on mass transfer in generator
4.1.3 Effect of the heat flux
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Fig. 7. Effect of solution flow rate on mass transfer in generator
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Sensitivity study of heat flux in the range of 3.93~10.68 kW/m2 was conducted when Reynold number of hot water in generator Rew, inlet solution mass flow rate ms,in, and inlet cooling water temperature tc,w,in were set at 2750, 0.03 kg/(m·s), and 21.5℃ respectively. As shown in Fig. 8, heat flux has no obvious effect on heat mass transfer coefficient, since it fluctuates from 0.562 kW/(m2·K) to 0.632 kW/(m2·K). While mass transfer coefficient almost linearly increases from 2.9×10-5 m/s to 7.8×10-5 m/s when heat flux increases from 3.93 kW/m2 to 10.68 kW/m2.
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(a) Effect of heat flux on heat transfer in generator
(b) Effect of heat flux on mass transfer in generator Fig. 8. Effect of heat flux in generator
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Under the all conditions experimented and discussed above, the heat transfer coefficient of the generator varies between 0.345 kW/ (m2·K) and 0.660 kW/ (m2·K). The heat transfer coefficient level is slightly lower than our preliminary work of a one-working-fluids-channel generator/condenser couple with symmetric solution distribution, which varies between 0.735 kW/ (m2·K) and 0.856 kW/ (m2·K), due to the decrease of wetting rate caused by the asymmetric solution distributor adopted in this paper to realize the detachability of heat medium channel. Thermal performance of condenser
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The condenser of the prototype operated well in a wide range of generation rate from 2.15 g/s to 6.96 g/s as Fig.9 shows. In general, the heat transfer coefficient of the condenser increases from 0.415 kW/ (m2·K) to 0.698 kW/ (m2·K), with the generation rate increases from 2.1 g/s to 3.8 g/s. When the generation rate is larger than 4.0 g/s, the heat transfer coefficient of the condenser shows no notably increase but varies from 0.627 kW/ (m2·K) to 0.731 kW/ (m2·K). The results indicate the minimum generation rate of the designed condenser required to maximize its heat transfer capability is 3.85 g/s.
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Fig. 9. Thermal performance of condenser under difference generation rates
Conclusions
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A detachable plate falling film HMX coupling using lithium bromide and water as working fluids is proposed in this paper. The HMX coupling is capable of functioning as either a generator and condenser coupling, or an absorber and evaporator coupling in an AHP using water as refrigerant. Experiments on the designed HMX coupling prototype when applied as generator and condenser coupling were conducted, with performance evaluated and analyzed. Some conclusions can be made as follows:
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1) A detachable HMX is developed by entwining two asymmetrical modules. The attaching and detaching processes of the HMX are very similar to entwining and loosing fingers. Two HMXs with their vapor chamber connected form a HMX coupling. The tested prototype consists two connected HMXs, each HMX includes two entwined modules, and each module contains five projecting heat and mass units. 2) From the sensitivity study, it can be seen that hot water flow has significant effect on the overall heat transfer coefficient, but few effect on the mass transfer coefficient. Solution mass flow shows a few effects on the heat and mass transfer coefficients in the experiment cases due to the large water channel width, which is restricted by manufactural process. Heat flux has greater effect on the mass transfer coefficient than the overall heat transfer coefficient. Under the conditions experimented, the heat transfer coefficient of the generator varies between 0.345 kW/ (m2·K) and 0.660 kW/ (m2·K), while the mass transfer coefficient of the generator between 2.7×10-5 m/s and 7.8×10-5 m/s, within the hot water Reynold number range of 963~2762, solution flow rate per perimeter range of 0.027~0.055 kg/(m·s), and heat flux range of 3.93~10.68 kW/m2. 3) The condenser in the prototype operated well within a large range of generation rate, its overall heat transfer coefficient varies between 0.415~0.731 kW/m2 within a generation rate range of 2.15 g/s~ 6.96 g/s. The minimum generation rate of the designed condenser required to maximize its heat transfer capability is 3.85 g/s.
There is still a great performance enhancement potential of the prototype presented in this paper. Improving the wetting behavior of the asymmetric solution distributor, which was adopted in this paper to realize the detachability of the heat medium channels, will be the most critical topic in future work to enhance the heat and mass transfer performance of the detachable falling film HMX.
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Acknowledgments The authors gratefully acknowledge the support from the National Natural Science Foundation of China (grant numbers 51306098, 51138005), the innovative Research Groups of National Natural Science Foundation of China (grant number 51521005), and Tsinghua University Initiative Scientific Research Program (No. 20151080470).
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