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Experimental evaluation of an ejector as liquid re-circulator in a falling-film water chiller
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 4 0 ( 2 0 1 4 ) 3 0 9 e3 1 6
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Experimental evaluation of an ejector as liquid re-circulator in a falling-film water chiller YingLin Li a,b,*, Laizai Tan c, Xiaosong Zhang a, Kai Du a a
Southeast University, Nanjing 210096, China Nanjing Normal University, Nanjing 210042, China c Nanjing Wuzhou Refrigeration Group Co., Ltd., Nanjing 211100, China b
article info
abstract
Article history:
To experimentally evaluate the performance of an ejector working as a liquid re-circulator
Received 15 August 2013
in a horizontal-tube falling-film evaporator with R134a, experimental tests are performed
Received in revised form
using a horizontal-tube falling-film water chiller prototype. Experimental observations on
5 October 2013
intertube liquid flow pattern of tube bundle validate the feasibility of the liquid re-
Accepted 2 November 2013
circulation system using a liquideliquid ejector. The analysis results show that the influ-
Available online 10 November 2013
ence of the motive flow rate on the entrainment ratio of the ejector is small, and the average entrainment ratio of the ejector is about 2.03. With the increase of the valve
Keywords:
opening of the regulating valve, the evaporating capacity of the falling-film water chiller
Falling film
rises 4.8%, from 940.2 kW with the re-circulation ratio of one, to 985.5 kW with the re-
Liquideliquid ejector
circulation ratio of 1.135. The COP of the falling-film water chiller reaches a maximum
Re-circulation ratio
and then drops down with the increase of the re-circulation ratio, and the optimal re-
Entrainment ratio
circulation ratio is 1.135. ª 2013 Elsevier Ltd and IIR. All rights reserved.
Evaluation expe´rimentale d’un e´jecteur comme recirculateur liquid dans un refroidisseur d’eau a` film tombant Mots cle´s : Film tombant ; e´jecteur liquide/liquide ; Taux de re-circulation ; Taux d’entraıˆnement
1.
Introduction
Refrigeration evaporators can be classified according to the liquid feed method employed, as direct-expansion evaporators, flooded evaporators and overfeed evaporators. Direct expansion evaporators are usually fed by using an expansion valve that regulates the liquid flow, the refrigerant which leaves the evaporator is superheated, and only vapor flows to
the compressor. Flooded evaporators are completely filled with liquid refrigerant, so that the entire inner surface of evaporator is wet thus improving the heat transfer coefficient. For overfeed evaporators, some liquid boils in the evaporator and the remainder floods out of the outlet, and the refrigerant leaving the evaporator is always saturated. The mass flow rate flowing through the evaporator is higher than through the compressor or condenser.
* Corresponding author. Nanjing Normal University, Nanjing 210042, China. Tel.: þ86 (0)25 85481140. E-mail address: [email protected] (YingLin Li). 0140-7007/$ e see front matter ª 2013 Elsevier Ltd and IIR. All rights reserved. http://dx.doi.org/10.1016/j.ijrefrig.2013.11.001
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Nomenclature COP CP EJ EMV EV FFE FFWC FM GS h m OS R RRS RV SHE u V
coefficient of performance compressor ejector electromagnetic valve expansion valve falling-film heat exchanger falling-film water chiller flow meter gaseliquid separator specific enthalpy, kJ kg1 refrigerant mass flow rate, kg s1 oil separator re-circulation ratio refrigerant re-circulation system regulative valve shell-and-tube heat exchanger entrainment ratio of the ejector volumetric flow rate, m3 s1
Greek symbols r density, kg m3 subscripts 1 point 1 2 point 2 3 point 3 4 point 4 5 point 5 7 point 7 cir re-circulation cond condenser mot motive port of the ejector suc suction port of the ejector
Horizontal-tube falling-film evaporators belong to typical overfeed evaporators. In air conditioning and refrigeration applications, compared with flooded evaporators, falling-film evaporators have the advantages of higher cycle efficiency, lower costs and a smaller environmental impact due to its reduced charge of refrigerant. The advantages of falling-film evaporators drive researchers to do experimental and numerical studies on overfeed liquid flow rate, the liquid distribution, the flow pattern, liquid subcooling, tube surface phenomena, tube spacing and heat flux, etc. Structured surfaces promote nucleate boiling in the film at modest temperature differences, enhance convection within the film and provide an increase in heat transfer area. The parameters that influence the enhancement are mainly the shape, geometry and surface area of the cavities. Chien and Webb (1998a, b) investigated enhanced surfaces with R-11 and R-123. They observed that, at low heat flux, the tubes with smaller total open areas had higher heat transfer coefficients; and that at higher heat fluxes, tubes with larger total open areas yielded higher heat transfer performance. Moeykens et al. (1995) found that enhanced boiling surfaces obtained higher performance than finned tubes but lower performance than enhanced condensing surfaces used for evaporation. They illustrated an increase of heat transfer coefficient with
heat flux up to a maximum, then, with the increase of heat flux, the heat transfer coefficient decreased. Roques and Thome (2007a, b) studied three different enhanced surfaces of the Gewa-B, Turbo-Bii and High-Flux. Similar trends for each surface were found, as well as a strong dependence of heat transfer on the heat flux. The performance of High-Flux tube achieved up to three times better than that of the other tubes. Habert and Thome (2010) investigated three enhanced surfaces of the Gewa-B4, Turbo-EDE2 and the condensing Gewa-C. The tests were performed using R-134a and R-236fa. Christians and Thome (2012) presented falling-film evaporation experiments using a single tube, a vertical row of ten horizontal tubes and a small tube bundle with three rows of 10 tubes each, and the Wolverine Turbo-B5 and the Wieland Gewa-B5 were tested using R-134a and R-236fa. Horizontal-tube falling-film evaporation on tube bundle is more complex than on a single tube or several tubes. In tube bundle, partial dryout of the bottom tubes is a key problem in practical applications. The bottom tubes may suffer from dryout, because the liquid flow rates decrease due to evaporation while flowing downwards. How to select operation conditions is concernful for falling-film bundles. Lorenz and Yung (1982) identified the critical Reynolds number of 300, below which the falling film evaporation coefficients on tube arrays were less than those on a single tube. Moreover, when the Reynolds number was small, the bottom tubes of the array would suffer more from partial dryout than those on higher layers. Since the dry areas transferred the heat only by natural convection, a sudden drop of heat transfer coefficients was observed both on smooth tube arrays by Fujita and Tsutsui (1998) and Ribatski and Thome (2007), and enhanced ones by Roques and Thome (2007b). The device used for liquid distribution can greatly affect the evaporator performance. Chien and Tsai (2011) tested the heat transfer performance on horizontal copper tubes with refrigerant R-245fa. The refrigerant in the falling-film heat exchanger was pure liquid, and the liquid film distributor consisted of a 6.35 mm outer diameter copper tube, having 15 holes of 1 mm diameter each, at 5 mm pitch above the heated section of the test tube. Li et al. (2011) investigated the mean heat transfer coefficients of water falling film using different enhanced tubes. The liquid distributor was a horizontal perforated integral-fin tube with 0.8 mm holes at the top and a fin density of 26 fins per inch. Hou et al. (2012) tested a liquid film falling around a horizontal tube to determine the distribution characteristics of the film thickness. The working fluid included fresh water and seawater, and the liquid distributor was 200 mm length, with the flow entering at the top through a hole and leaving at the bottom through a semicircle with 2 mm diameter holes spaced 3 mm apart. Lee et al. (2012) designed a new drip tray which was placed above the tube bundles to distribute dilute solution on the tubes; the tray had 4 rows of 75 holes for the insertion of 1.5 mm OD capillary tubes, which distributed the dilute solution on the first row of the tube bundle. Up to now, the falling-film technology has been studied by many investigators; however, the focus is primarily on fallingfilm evaporators, only a few researchers have carried out investigations on the system performance of refrigerant units using falling-film evaporators. Yang and Wang (2011)
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(3) (7)
GS FFE
Vsuc (5)
CP
EMV
(6)
FM
Vmot SHE one-way valve
FM
Outlet port Suction port
Tube bundle tray
EV
Flo meter
FM
Primary distributor
RV
Vcond (1)
(2)
Low-pressure refrigerant from expansion valve
Motive port
EJ
(4)
OS
High-pressure working fluid from dry filter Ejector
Dry filter
Fig. 1 e Schematic diagram of the falling-film water chiller (FFWC).
Low-pressure entraining fluid from the bottom of FFE
Top array Bottom array
Fig. 3 e Diagram of the refrigerant re-circulation system of the FFWC. conducted various system loads of a large refrigerant unit using a falling-film evaporator. HCFC-22 was used as the working fluid, and the experimental cooling capacity was about 421.0 kW, the maximum coefficient of performance was 5.32. Additionally, in the liquid overfeed systems, to feed the evaporator with saturated liquid re-circulated from the separator, a common practice is to use a pump as a liquid recirculator component, described by Bivens et al. (1997) and Giuliani et al. (1999). However, the use of pumps increases the initial investment of the facility and the operation and maintenance costs. On the other hand, an alternative to the use of pumps in liquid overfeed systems is the use of ejectors. The principal advantage of this option is its simplicity and avoidance of mobile parts. In addition, ejectors are more economical compared to pumps. In recent years, ejectors and their applications in refrigeration cycles have been widely studied. The reviews performed by Sarkar (2012), Sumeru et al. (2012) and Chen et al. (2013) can be cited. One refrigerant recirculation system using R-22, in which an ejector was used to feed a plate freezer of eight freezing stations was studied by Radchenko (1985), his results included data of the liquid collected in the liquid/vapor separator. Dopazo and Seara (2011) dealt with the experimental performance evaluation of an ejector in an overfeed NH3 plate evaporator, a Phillips ejector with a 0.5 mm diameter throat and 1.4 mm diameter nozzle was used. The evaporator was tested in a cascade refrigeration system prototype, and the experimental result
showed the entrainment ratio of the NH3 liquideliquid ejector was between 2.1 and 2.67, and the evaporating capacity varied from 9.48 kW to 18.37 kW. The purpose of this work is to experimentally evaluate the performance of a liquideliquid ejector working as a liquid recirculator component in a horizontal-tube falling-film evaporator with R134a. The experimental tests are performed using a falling-film water chiller (FFWC) prototype. In the present work, the experimental rig is described, the data reduction detailed, and the results shown and discussed.
2.
System description and data reduction
2.1.
System description
The schematic diagram of the FFWC is shown in Fig. 1. The FFWC mainly consists of the compressor (CP), falling-film heat exchanger (FFE), water-cooled, shell-and-tube heat exchanger
p 4
1
5
7 2
3
6 h Fig. 2 e Ph diagram of the FFWC system.
Fig. 4 e Photos of the primary distributor and second distributing tray.
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Fig. 5 e Photos of the enhanced tubes: a) horizontal tube array; and b) partial zoom of enhanced tube.
(SHE), ejector (EJ), gaseliquid separator (GS), oil separator (OS), electromagnetic valve (EMV), expansion valve (EV), flow meter (FM) and manual regulating valve (RV), etc. The compressor used in this work is a screw compressor, the FFE is a horizontal-tube falling-film heat exchanger, and the liquideliquid ejector employs a converging nozzle. The FFWC differs from conventional water chillers due to the refrigerant re-circulation system (RRS) and the FFE employed. Fig. 2 shows the Ph diagram of the FFWC system, and Fig. 3 depicts the diagram of the refrigerant re-circulation system. As shown in Figs. 1 and 2, the refrigerant at point 1 flows to the dry filter, and then divides into two, one branch goes through the expansion valve, the refrigerant is throttled to point 2; the other flows through the RV, then goes into the motive port of the EJ to be regarded as high-pressure working flow. The residual low-pressure liquid refrigerant point 5 from the bottom of the FFE flows into the suction port of the EJ and expands to point 6. The high-pressure motive fluid point 1 induces the low-pressure suction fluid point 6 to mix together in the EJ. Lastly the mixed refrigerant point 7 at the outlet port of the ejector combines the refrigerant point 2 to run into the inlets of FFE. The FFE consists of 1 horizontal-tube bundle, 2 distributors, 2 top inlets, 1 top outlets, and 1 bottom receivers, etc. As shown in Fig. 3, the tube bundle, which comprises 6 sub arrays, is housed in a 3730 mm long 716 mm diameter outer shell with 3 additional 64 mm sight windows for viewing at different positions. The wall thickness of outer shell is 8 mm. The total number of tubes in the bundle is 247. All tubes in
each sub array are installed by equilateral triangle arrangement with a vertical center-to-center pitch of 24 mm. Fig. 4 depictures the structure of the distributor which comprises a primary distributor and a second distributing tray. Special care has been taken in the distributor design in order to achieve uniform liquid distribution on the top tubes. The primary distributor is a self-designed component which utilizes a long horizontal tube with 2 additional parallel short tubes, and there are many small holes which are perforated at the bottom of horizontal tubes. The second distributing tray is placed above the tube bundle and designed to distribute refrigerant liquid on the tubes. The tray has 25 rows of 96 holes (on a longitudinal 18 mm pitch transverse 20.8 mm pitch) with perforated 2 mm diameter holes (a total of 2400). The second distributing tray is located 10 mm above the top of the first row of tube bundle. The designed height of the partition baffles in the tray is revised as 10 mm. The use of enhanced surfaces can enhance heat transfer coefficients in comparison with those obtained on plain surfaces, and improve the liquid refrigerant distribution. Enhanced copper tubes employed in the FFE consist of plain
Table 1 e Parameters of the enhanced copper tubes. Tube section Plain section
Item
Outside diameter Wall thickness Effective length of both tube ends Finned section Outside diameter Wall thickness Minimum wall thickness Ridge height Actual outside surface area Actual inside surface area Effective length Tube number of tube bundle
Parameters 19.05 mm 1.04 mm 100 mm 18.85 mm 0.63 mm 0.56 mm 0.35 mm 0.2297 m2 m1 0.0751 m2 m1 3590 mm 247
Fig. 6 e Photos of the liquideliquid ejector: a) structure; and b) processing part.
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Table 2 e The designed parameters of the EJ. Item The The The The The The The The The The The
Parameters
nozzle outlet diameter nozzle inlet diameter nozzle length mixing section diameter mixing section length diffuser outlet diameter diffuser length entraining inlet diameter suction chamber diameter spacing between the nozzle and mixing section total length of ejector
2 mm 10 mm 45.7 mm 14 mm 45 mm 24 mm 57 mm 16 mm 35 mm 10 mm 177.7 mm
measured and the densities rsuc and rcond, are obtained as a function of the saturated evaporating pressure of the FFE, and the pressure and temperature measurements of the SHE, respectively. msuc ¼ m5 ¼ Vsuc rsuc
(5)
mmot ¼ m1 m2 ¼ Vmot rcond
(6)
The ratio of the total refrigerant mass flow rate in the inlets of FFE compared to that of the condenser (SHE) is defined as the refrigerant re-circulation ratio R, and R is determined according to Eqs. (7) and (8). The volumetric flow rates Vcond is obtained by experimental tests.
section and finned section, the effective length of plain section and finned section is 100 mm and 3590 mm, respectively. The enhanced external surface of finned section is shown in Fig. 5, and the internal surface of finned section is the structure with spiral groove. The main structural parameters of the enhanced tubes are listed in Table 1. Fig. 6 is the photograph of the EJ which mainly consists of the nozzle, suction chamber, mixing section and diffuser. The inner diameter of the nozzle’s outlet is limited as 2 mm, and the detailed structural parameters have been listed in Table 2.
2.2.
313
mcond ¼ m1 ¼ Vcond rcond
(7)
R ¼ mcir =mcond ¼ 1 þ msuc =mcond
(8)
Substituting Eqs (5) and (7) into Eq. (8) gives R¼1þ
rsuc Vsuc rcond Vcond
(9)
The entrainment ratio of the ejector is defined by u ¼ msuc =mmot ¼ Vsuc rsuc =ðVmot rcond Þ
(10)
Data reduction
3. The mass and energy balances on the expansion valve and on the ejector are given by Eqs. (1)e(4), respectively. m4 ¼ m3 ¼ m1
(1)
h1 ¼ h2
(2)
m7 ¼ mmot þ msuc
(3)
m7 ¼ h1 mmot =h7 þ h5 msuc =h7
(4)
The motive and suction mass flow rates at both inlets of the ejector are given by Eqs. (5) and (6). In these equations the R134a volumetric flow rates Vsuc and Vmot are experimentally
Fig. 7 e Overview of the test facility.
Experimental tests
In order to analyze the effect of liquid refrigerant recirculation system on the performance of a horizontal-tube falling-film water chiller, the test rig is set up. Fig. 7 shows the arrangement of the test facility. The test facility is used to test FFWC performance at the cooling working condition. Once all temperature, pressure and flow measurements are stabilized, the experimental data are collected during a time frame of about 10 min, at intervals of approximately 120 s. The data in the following figures are the average values of the three groups of data for each stable working condition. As shown in Fig. 8, there are totally 3 sight glasses (sight windows) which are installed outside the cylinder of FFE. During the following experimental tests, these sight windows will be used to observe the dynamic state of refrigerant distribution. The top sight window 7 is used to visualize refrigerant distribution of the second distributing tray; the middle sight window 8 is utilized to inspect the intertube flow pattern of tube arrays; the bottom sight window 9 is in charge of overseeing the liquid surface of the residual low-pressure refrigerant in the FFE. All the experimental process was finished in an artificial environment chamber. The performance of FFWC prototype is tested at the inlet temperature of cooling water of 30 C, cooling water flow rate of 249.4 m3 h1, outlet temperature of chilled water of 7 C, and chilled water flow rate of 199.5 m3 h1. The number of passes in chilled water side is 4, and the inlet and outlet of chilled water were on the same side, as shown in Figs. 7 and 8. K-type thermocouples are used to measure the inlet and outlet temperatures of cooling and chilled water, whose accuracy is 0.1 K. The pressure is measured by a transducer with an accuracy of 1% of the span 0e20 bar. The R134a
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1
3
2
6
7 8 9 4
5
Fig. 8 e Structure of the FFE: 1. top inlet; 2. top outlet; 3. intlet of chilled water; 4. outlet of chilled water; 5. bottom receiver; 6. primary distributor and second distributing tray; 7e9. sight window.
volumetric flow rates Vsuc and Vmot are measured by using electromagnetic flow meters with an accuracy of 0.5% of the span 0.2e6 m3 h1; and the volumetric flow rate Vcond is measured by using an electromagnetic flow meter with an accuracy of 1% of the span 2.5e50 m3 h1. The volumetric flows of cooling and chilled water are measured by using electromagnetic flow meters with a repeatability of within 1.0% of the span 25e400 m3 h1.
4.
Results and discussion
4.1.
The validation of RRS
While adjusting the valve opening of RV at the motive port of the EJ, the refrigerant flow rate induced by the high-pressure motive fluid varies gradually, hence the total refrigerant spraying flow rate in the inlets of FFE changes. Fig. 9 depicts the intertube refrigerant distribution of the bundle observed by the sight window 7. Fig. 9a is obtained while the regulative valve is closed. From the Fig. 9a, we can see that the flow pattern between tubes is a discrete droplet. Similarly, Fig. 9b is inspected at the half valve opening of the regulative valve. Fig. 9b indicates that the flow pattern between tubes is then a column liquid while adjusting the RV at half valve opening. In addition, while the flow pattern between tubes is a discrete droplet, the dryout may potentially emerge on the external
surface of tubes. In other words, if the RRS is closed, the external surface of bottom tubes of the bundle would easily suffer from dryout. Consequently, the RRS is necessary.
4.2.
Experimental results
Fig. 10 shows the variations of the evaporating and condensing temperatures with the motive flow rate of the ejector. From the Fig. 10, it can be seen that the influence of the motive flow rate on the evaporating and condensing temperature is small; and the evaporating temperature slightly increases first and then drops as the motive flow rate rises. On the contrary, the condensing temperature slightly descends at first and ascends at last as the motive flow rate increases. The influence of the motive flow rate on the suction flow rate and entrainment ratio is illustrated in Fig. 11, both the suction flow rate and the entrainment ratio are monotonically increasing with the motive flow rate. The suction flow rate of the ejector enhances sharply with the increase of motive flow rate. Somewhat differently, the impact from the motive flow rate on the entrainment ratio is rather limited, and the average value of the entrainment ratio is about 2.03. The experimental entrainment ratio of this liquideliquid ejector is less than the average value of 2.41 obtained by Dopazo and Seara (2011); they utilized a commercial nozzle with a nozzle outlet diameter of 1.2e1.6 mm. In the present work, the nozzle
Fig. 9 e Intertube refrigerant distribution observed by the sight window 7: a) at zero valve opening; and b) at half valve opening.
315
°C
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Fig. 10 e Variations of the evaporating and condensing temperatures with the motive flow rate.
Fig. 12 e Effects of the motive flow rate on the recirculation spraying flow rate and re-circulation ratio.
outlet diameter of the ejector is designed as 2 mm. Generally speaking, the smaller the nozzle outlet diameter, the bigger the entrainment ratio of the ejector; however, the smaller the nozzle outlet diameter, the higher the manufacturing costs. Fig. 12 shows the experimental re-circulation spraying flow rate of the FFE and the re-circulation ratio calculated as a function of the motive flow rate of the ejector. Increasing trends are observed in both cases when the motive flow rate rises. The recirculation spraying flow rate rises 18.4%, from 6.32 kg s1 with motive flow rate of zero, to 7.64 kg s1 with motive flow rate of 0.56 kg s1, while the increment observed in the re-circulated ratio is up 18%, from 1.0 to 1.18. In Fig. 13 the experimental results of the evaporating capacity of the FFWC and the COP are represented as a function of the motive flow rate. Similar increasing-and-decreasing trends in both cases can be appreciated. It is worth pointing out the special behavior observed when the measurement value of motive flow rate is 0.43 kg s1. At this point, the obtained peak value of the evaporating capacity is 985.5 kW as well as the maximum COP 5.133. The extremum obtained at this point can be linked with the intertube flow pattern
observed by the sight window (depicted in Fig. 9). As previously indicated, by adjusting the valve opening of the RV, the RRS can control the inter-tube flow pattern of tube arrays to involve all three flow modes of droplet, column and sheet. When the valve opening is bigger, the larger re-circulation spraying flow rate of the FFE will lead to intertube continuous sheet, the effect of the accumulated film outside of tubes on heat transfer will decreases the evaporating capacity of the FFWC. On the other hand, while the valve opening is smaller, the less spraying flow rate of the FFE will result in intertube discrete droplet, the dryout emerged outside of tubes will deteriorate the heat transfer to reduce the evaporating capacity of the FFWC. Fig. 14 shows the variation of the performance coefficient with the refrigerant re-circulation ratio. It can be claimed that, the performance coefficient reaches a maximum and then drops down with the increase of the re-circulation ratio, and the performance coefficient rises 2.3%, from 5.017 with the recirculation ratio of one, to 5.133 with the re-circulation ratio of 1.135. Fig. 14 points out that the optimal re-circulation ratio of the FFWC is about 1.135, and the corresponding cooling COP is about 5.133.
Fig. 11 e Influences of the motive flow rate on the suction flow rate and entrainment ratio.
Fig. 13 e Influences of the motive flow rate on the evaporating load and performance coefficient.
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Fig. 14 e Variation of the performance coefficient with the refrigerant re-circulation ratio.
5.
Conclusions
A falling-film water chiller is presented, the experimental analysis about the effect of the refrigerant re-circulation system on the performance of the FFWC has been proposed, and the following conclusions can be drawn: By adjusting the valve opening of the RV, the RRS can control the inter-tube flow pattern of tube arrays to involve all three flow modes of droplet, column and sheet, and the RRS is feasible and necessary. The influence of the motive flow rate on the entrainment ratio of the ejector is small, and the average value of the entrainment ratio is about 2.03 at average evaporating and condensing pressure of 3.436 bar (4.5 C) and 9.656 bar (38.1 C). Increasing gradually the valve opening of the regulating valve, the evaporating capacity of FFWC rises 4.8%, from 940.2 kW with the motive flow rate of zero, to 985.5 kW with the motive flow rate at 0.43 kg s1. The performance coefficient reaches a maximum and then drops down with the increase of the re-circulation ratio.
Acknowledgments The authors gratefully acknowledge supports through the Financial Grant from 12th Five-year National Science Project of China (2011BAJ03B05) and the General Financial Grant from the China Postdoctoral Science Foundation (2012M520970).
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journal homepage: www.elsevier.com/locate/ijrefrig
Experimental evaluation of an ejector as liquid re-circulator in a falling-film water chiller YingLin Li a,b,*, Laizai Tan c, Xiaosong Zhang a, Kai Du a a
Southeast University, Nanjing 210096, China Nanjing Normal University, Nanjing 210042, China c Nanjing Wuzhou Refrigeration Group Co., Ltd., Nanjing 211100, China b
article info
abstract
Article history:
To experimentally evaluate the performance of an ejector working as a liquid re-circulator
Received 15 August 2013
in a horizontal-tube falling-film evaporator with R134a, experimental tests are performed
Received in revised form
using a horizontal-tube falling-film water chiller prototype. Experimental observations on
5 October 2013
intertube liquid flow pattern of tube bundle validate the feasibility of the liquid re-
Accepted 2 November 2013
circulation system using a liquideliquid ejector. The analysis results show that the influ-
Available online 10 November 2013
ence of the motive flow rate on the entrainment ratio of the ejector is small, and the average entrainment ratio of the ejector is about 2.03. With the increase of the valve
Keywords:
opening of the regulating valve, the evaporating capacity of the falling-film water chiller
Falling film
rises 4.8%, from 940.2 kW with the re-circulation ratio of one, to 985.5 kW with the re-
Liquideliquid ejector
circulation ratio of 1.135. The COP of the falling-film water chiller reaches a maximum
Re-circulation ratio
and then drops down with the increase of the re-circulation ratio, and the optimal re-
Entrainment ratio
circulation ratio is 1.135. ª 2013 Elsevier Ltd and IIR. All rights reserved.
Evaluation expe´rimentale d’un e´jecteur comme recirculateur liquid dans un refroidisseur d’eau a` film tombant Mots cle´s : Film tombant ; e´jecteur liquide/liquide ; Taux de re-circulation ; Taux d’entraıˆnement
1.
Introduction
Refrigeration evaporators can be classified according to the liquid feed method employed, as direct-expansion evaporators, flooded evaporators and overfeed evaporators. Direct expansion evaporators are usually fed by using an expansion valve that regulates the liquid flow, the refrigerant which leaves the evaporator is superheated, and only vapor flows to
the compressor. Flooded evaporators are completely filled with liquid refrigerant, so that the entire inner surface of evaporator is wet thus improving the heat transfer coefficient. For overfeed evaporators, some liquid boils in the evaporator and the remainder floods out of the outlet, and the refrigerant leaving the evaporator is always saturated. The mass flow rate flowing through the evaporator is higher than through the compressor or condenser.
* Corresponding author. Nanjing Normal University, Nanjing 210042, China. Tel.: þ86 (0)25 85481140. E-mail address: [email protected] (YingLin Li). 0140-7007/$ e see front matter ª 2013 Elsevier Ltd and IIR. All rights reserved. http://dx.doi.org/10.1016/j.ijrefrig.2013.11.001
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Nomenclature COP CP EJ EMV EV FFE FFWC FM GS h m OS R RRS RV SHE u V
coefficient of performance compressor ejector electromagnetic valve expansion valve falling-film heat exchanger falling-film water chiller flow meter gaseliquid separator specific enthalpy, kJ kg1 refrigerant mass flow rate, kg s1 oil separator re-circulation ratio refrigerant re-circulation system regulative valve shell-and-tube heat exchanger entrainment ratio of the ejector volumetric flow rate, m3 s1
Greek symbols r density, kg m3 subscripts 1 point 1 2 point 2 3 point 3 4 point 4 5 point 5 7 point 7 cir re-circulation cond condenser mot motive port of the ejector suc suction port of the ejector
Horizontal-tube falling-film evaporators belong to typical overfeed evaporators. In air conditioning and refrigeration applications, compared with flooded evaporators, falling-film evaporators have the advantages of higher cycle efficiency, lower costs and a smaller environmental impact due to its reduced charge of refrigerant. The advantages of falling-film evaporators drive researchers to do experimental and numerical studies on overfeed liquid flow rate, the liquid distribution, the flow pattern, liquid subcooling, tube surface phenomena, tube spacing and heat flux, etc. Structured surfaces promote nucleate boiling in the film at modest temperature differences, enhance convection within the film and provide an increase in heat transfer area. The parameters that influence the enhancement are mainly the shape, geometry and surface area of the cavities. Chien and Webb (1998a, b) investigated enhanced surfaces with R-11 and R-123. They observed that, at low heat flux, the tubes with smaller total open areas had higher heat transfer coefficients; and that at higher heat fluxes, tubes with larger total open areas yielded higher heat transfer performance. Moeykens et al. (1995) found that enhanced boiling surfaces obtained higher performance than finned tubes but lower performance than enhanced condensing surfaces used for evaporation. They illustrated an increase of heat transfer coefficient with
heat flux up to a maximum, then, with the increase of heat flux, the heat transfer coefficient decreased. Roques and Thome (2007a, b) studied three different enhanced surfaces of the Gewa-B, Turbo-Bii and High-Flux. Similar trends for each surface were found, as well as a strong dependence of heat transfer on the heat flux. The performance of High-Flux tube achieved up to three times better than that of the other tubes. Habert and Thome (2010) investigated three enhanced surfaces of the Gewa-B4, Turbo-EDE2 and the condensing Gewa-C. The tests were performed using R-134a and R-236fa. Christians and Thome (2012) presented falling-film evaporation experiments using a single tube, a vertical row of ten horizontal tubes and a small tube bundle with three rows of 10 tubes each, and the Wolverine Turbo-B5 and the Wieland Gewa-B5 were tested using R-134a and R-236fa. Horizontal-tube falling-film evaporation on tube bundle is more complex than on a single tube or several tubes. In tube bundle, partial dryout of the bottom tubes is a key problem in practical applications. The bottom tubes may suffer from dryout, because the liquid flow rates decrease due to evaporation while flowing downwards. How to select operation conditions is concernful for falling-film bundles. Lorenz and Yung (1982) identified the critical Reynolds number of 300, below which the falling film evaporation coefficients on tube arrays were less than those on a single tube. Moreover, when the Reynolds number was small, the bottom tubes of the array would suffer more from partial dryout than those on higher layers. Since the dry areas transferred the heat only by natural convection, a sudden drop of heat transfer coefficients was observed both on smooth tube arrays by Fujita and Tsutsui (1998) and Ribatski and Thome (2007), and enhanced ones by Roques and Thome (2007b). The device used for liquid distribution can greatly affect the evaporator performance. Chien and Tsai (2011) tested the heat transfer performance on horizontal copper tubes with refrigerant R-245fa. The refrigerant in the falling-film heat exchanger was pure liquid, and the liquid film distributor consisted of a 6.35 mm outer diameter copper tube, having 15 holes of 1 mm diameter each, at 5 mm pitch above the heated section of the test tube. Li et al. (2011) investigated the mean heat transfer coefficients of water falling film using different enhanced tubes. The liquid distributor was a horizontal perforated integral-fin tube with 0.8 mm holes at the top and a fin density of 26 fins per inch. Hou et al. (2012) tested a liquid film falling around a horizontal tube to determine the distribution characteristics of the film thickness. The working fluid included fresh water and seawater, and the liquid distributor was 200 mm length, with the flow entering at the top through a hole and leaving at the bottom through a semicircle with 2 mm diameter holes spaced 3 mm apart. Lee et al. (2012) designed a new drip tray which was placed above the tube bundles to distribute dilute solution on the tubes; the tray had 4 rows of 75 holes for the insertion of 1.5 mm OD capillary tubes, which distributed the dilute solution on the first row of the tube bundle. Up to now, the falling-film technology has been studied by many investigators; however, the focus is primarily on fallingfilm evaporators, only a few researchers have carried out investigations on the system performance of refrigerant units using falling-film evaporators. Yang and Wang (2011)
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(3) (7)
GS FFE
Vsuc (5)
CP
EMV
(6)
FM
Vmot SHE one-way valve
FM
Outlet port Suction port
Tube bundle tray
EV
Flo meter
FM
Primary distributor
RV
Vcond (1)
(2)
Low-pressure refrigerant from expansion valve
Motive port
EJ
(4)
OS
High-pressure working fluid from dry filter Ejector
Dry filter
Fig. 1 e Schematic diagram of the falling-film water chiller (FFWC).
Low-pressure entraining fluid from the bottom of FFE
Top array Bottom array
Fig. 3 e Diagram of the refrigerant re-circulation system of the FFWC. conducted various system loads of a large refrigerant unit using a falling-film evaporator. HCFC-22 was used as the working fluid, and the experimental cooling capacity was about 421.0 kW, the maximum coefficient of performance was 5.32. Additionally, in the liquid overfeed systems, to feed the evaporator with saturated liquid re-circulated from the separator, a common practice is to use a pump as a liquid recirculator component, described by Bivens et al. (1997) and Giuliani et al. (1999). However, the use of pumps increases the initial investment of the facility and the operation and maintenance costs. On the other hand, an alternative to the use of pumps in liquid overfeed systems is the use of ejectors. The principal advantage of this option is its simplicity and avoidance of mobile parts. In addition, ejectors are more economical compared to pumps. In recent years, ejectors and their applications in refrigeration cycles have been widely studied. The reviews performed by Sarkar (2012), Sumeru et al. (2012) and Chen et al. (2013) can be cited. One refrigerant recirculation system using R-22, in which an ejector was used to feed a plate freezer of eight freezing stations was studied by Radchenko (1985), his results included data of the liquid collected in the liquid/vapor separator. Dopazo and Seara (2011) dealt with the experimental performance evaluation of an ejector in an overfeed NH3 plate evaporator, a Phillips ejector with a 0.5 mm diameter throat and 1.4 mm diameter nozzle was used. The evaporator was tested in a cascade refrigeration system prototype, and the experimental result
showed the entrainment ratio of the NH3 liquideliquid ejector was between 2.1 and 2.67, and the evaporating capacity varied from 9.48 kW to 18.37 kW. The purpose of this work is to experimentally evaluate the performance of a liquideliquid ejector working as a liquid recirculator component in a horizontal-tube falling-film evaporator with R134a. The experimental tests are performed using a falling-film water chiller (FFWC) prototype. In the present work, the experimental rig is described, the data reduction detailed, and the results shown and discussed.
2.
System description and data reduction
2.1.
System description
The schematic diagram of the FFWC is shown in Fig. 1. The FFWC mainly consists of the compressor (CP), falling-film heat exchanger (FFE), water-cooled, shell-and-tube heat exchanger
p 4
1
5
7 2
3
6 h Fig. 2 e Ph diagram of the FFWC system.
Fig. 4 e Photos of the primary distributor and second distributing tray.
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Fig. 5 e Photos of the enhanced tubes: a) horizontal tube array; and b) partial zoom of enhanced tube.
(SHE), ejector (EJ), gaseliquid separator (GS), oil separator (OS), electromagnetic valve (EMV), expansion valve (EV), flow meter (FM) and manual regulating valve (RV), etc. The compressor used in this work is a screw compressor, the FFE is a horizontal-tube falling-film heat exchanger, and the liquideliquid ejector employs a converging nozzle. The FFWC differs from conventional water chillers due to the refrigerant re-circulation system (RRS) and the FFE employed. Fig. 2 shows the Ph diagram of the FFWC system, and Fig. 3 depicts the diagram of the refrigerant re-circulation system. As shown in Figs. 1 and 2, the refrigerant at point 1 flows to the dry filter, and then divides into two, one branch goes through the expansion valve, the refrigerant is throttled to point 2; the other flows through the RV, then goes into the motive port of the EJ to be regarded as high-pressure working flow. The residual low-pressure liquid refrigerant point 5 from the bottom of the FFE flows into the suction port of the EJ and expands to point 6. The high-pressure motive fluid point 1 induces the low-pressure suction fluid point 6 to mix together in the EJ. Lastly the mixed refrigerant point 7 at the outlet port of the ejector combines the refrigerant point 2 to run into the inlets of FFE. The FFE consists of 1 horizontal-tube bundle, 2 distributors, 2 top inlets, 1 top outlets, and 1 bottom receivers, etc. As shown in Fig. 3, the tube bundle, which comprises 6 sub arrays, is housed in a 3730 mm long 716 mm diameter outer shell with 3 additional 64 mm sight windows for viewing at different positions. The wall thickness of outer shell is 8 mm. The total number of tubes in the bundle is 247. All tubes in
each sub array are installed by equilateral triangle arrangement with a vertical center-to-center pitch of 24 mm. Fig. 4 depictures the structure of the distributor which comprises a primary distributor and a second distributing tray. Special care has been taken in the distributor design in order to achieve uniform liquid distribution on the top tubes. The primary distributor is a self-designed component which utilizes a long horizontal tube with 2 additional parallel short tubes, and there are many small holes which are perforated at the bottom of horizontal tubes. The second distributing tray is placed above the tube bundle and designed to distribute refrigerant liquid on the tubes. The tray has 25 rows of 96 holes (on a longitudinal 18 mm pitch transverse 20.8 mm pitch) with perforated 2 mm diameter holes (a total of 2400). The second distributing tray is located 10 mm above the top of the first row of tube bundle. The designed height of the partition baffles in the tray is revised as 10 mm. The use of enhanced surfaces can enhance heat transfer coefficients in comparison with those obtained on plain surfaces, and improve the liquid refrigerant distribution. Enhanced copper tubes employed in the FFE consist of plain
Table 1 e Parameters of the enhanced copper tubes. Tube section Plain section
Item
Outside diameter Wall thickness Effective length of both tube ends Finned section Outside diameter Wall thickness Minimum wall thickness Ridge height Actual outside surface area Actual inside surface area Effective length Tube number of tube bundle
Parameters 19.05 mm 1.04 mm 100 mm 18.85 mm 0.63 mm 0.56 mm 0.35 mm 0.2297 m2 m1 0.0751 m2 m1 3590 mm 247
Fig. 6 e Photos of the liquideliquid ejector: a) structure; and b) processing part.
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 4 0 ( 2 0 1 4 ) 3 0 9 e3 1 6
Table 2 e The designed parameters of the EJ. Item The The The The The The The The The The The
Parameters
nozzle outlet diameter nozzle inlet diameter nozzle length mixing section diameter mixing section length diffuser outlet diameter diffuser length entraining inlet diameter suction chamber diameter spacing between the nozzle and mixing section total length of ejector
2 mm 10 mm 45.7 mm 14 mm 45 mm 24 mm 57 mm 16 mm 35 mm 10 mm 177.7 mm
measured and the densities rsuc and rcond, are obtained as a function of the saturated evaporating pressure of the FFE, and the pressure and temperature measurements of the SHE, respectively. msuc ¼ m5 ¼ Vsuc rsuc
(5)
mmot ¼ m1 m2 ¼ Vmot rcond
(6)
The ratio of the total refrigerant mass flow rate in the inlets of FFE compared to that of the condenser (SHE) is defined as the refrigerant re-circulation ratio R, and R is determined according to Eqs. (7) and (8). The volumetric flow rates Vcond is obtained by experimental tests.
section and finned section, the effective length of plain section and finned section is 100 mm and 3590 mm, respectively. The enhanced external surface of finned section is shown in Fig. 5, and the internal surface of finned section is the structure with spiral groove. The main structural parameters of the enhanced tubes are listed in Table 1. Fig. 6 is the photograph of the EJ which mainly consists of the nozzle, suction chamber, mixing section and diffuser. The inner diameter of the nozzle’s outlet is limited as 2 mm, and the detailed structural parameters have been listed in Table 2.
2.2.
313
mcond ¼ m1 ¼ Vcond rcond
(7)
R ¼ mcir =mcond ¼ 1 þ msuc =mcond
(8)
Substituting Eqs (5) and (7) into Eq. (8) gives R¼1þ
rsuc Vsuc rcond Vcond
(9)
The entrainment ratio of the ejector is defined by u ¼ msuc =mmot ¼ Vsuc rsuc =ðVmot rcond Þ
(10)
Data reduction
3. The mass and energy balances on the expansion valve and on the ejector are given by Eqs. (1)e(4), respectively. m4 ¼ m3 ¼ m1
(1)
h1 ¼ h2
(2)
m7 ¼ mmot þ msuc
(3)
m7 ¼ h1 mmot =h7 þ h5 msuc =h7
(4)
The motive and suction mass flow rates at both inlets of the ejector are given by Eqs. (5) and (6). In these equations the R134a volumetric flow rates Vsuc and Vmot are experimentally
Fig. 7 e Overview of the test facility.
Experimental tests
In order to analyze the effect of liquid refrigerant recirculation system on the performance of a horizontal-tube falling-film water chiller, the test rig is set up. Fig. 7 shows the arrangement of the test facility. The test facility is used to test FFWC performance at the cooling working condition. Once all temperature, pressure and flow measurements are stabilized, the experimental data are collected during a time frame of about 10 min, at intervals of approximately 120 s. The data in the following figures are the average values of the three groups of data for each stable working condition. As shown in Fig. 8, there are totally 3 sight glasses (sight windows) which are installed outside the cylinder of FFE. During the following experimental tests, these sight windows will be used to observe the dynamic state of refrigerant distribution. The top sight window 7 is used to visualize refrigerant distribution of the second distributing tray; the middle sight window 8 is utilized to inspect the intertube flow pattern of tube arrays; the bottom sight window 9 is in charge of overseeing the liquid surface of the residual low-pressure refrigerant in the FFE. All the experimental process was finished in an artificial environment chamber. The performance of FFWC prototype is tested at the inlet temperature of cooling water of 30 C, cooling water flow rate of 249.4 m3 h1, outlet temperature of chilled water of 7 C, and chilled water flow rate of 199.5 m3 h1. The number of passes in chilled water side is 4, and the inlet and outlet of chilled water were on the same side, as shown in Figs. 7 and 8. K-type thermocouples are used to measure the inlet and outlet temperatures of cooling and chilled water, whose accuracy is 0.1 K. The pressure is measured by a transducer with an accuracy of 1% of the span 0e20 bar. The R134a
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1
3
2
6
7 8 9 4
5
Fig. 8 e Structure of the FFE: 1. top inlet; 2. top outlet; 3. intlet of chilled water; 4. outlet of chilled water; 5. bottom receiver; 6. primary distributor and second distributing tray; 7e9. sight window.
volumetric flow rates Vsuc and Vmot are measured by using electromagnetic flow meters with an accuracy of 0.5% of the span 0.2e6 m3 h1; and the volumetric flow rate Vcond is measured by using an electromagnetic flow meter with an accuracy of 1% of the span 2.5e50 m3 h1. The volumetric flows of cooling and chilled water are measured by using electromagnetic flow meters with a repeatability of within 1.0% of the span 25e400 m3 h1.
4.
Results and discussion
4.1.
The validation of RRS
While adjusting the valve opening of RV at the motive port of the EJ, the refrigerant flow rate induced by the high-pressure motive fluid varies gradually, hence the total refrigerant spraying flow rate in the inlets of FFE changes. Fig. 9 depicts the intertube refrigerant distribution of the bundle observed by the sight window 7. Fig. 9a is obtained while the regulative valve is closed. From the Fig. 9a, we can see that the flow pattern between tubes is a discrete droplet. Similarly, Fig. 9b is inspected at the half valve opening of the regulative valve. Fig. 9b indicates that the flow pattern between tubes is then a column liquid while adjusting the RV at half valve opening. In addition, while the flow pattern between tubes is a discrete droplet, the dryout may potentially emerge on the external
surface of tubes. In other words, if the RRS is closed, the external surface of bottom tubes of the bundle would easily suffer from dryout. Consequently, the RRS is necessary.
4.2.
Experimental results
Fig. 10 shows the variations of the evaporating and condensing temperatures with the motive flow rate of the ejector. From the Fig. 10, it can be seen that the influence of the motive flow rate on the evaporating and condensing temperature is small; and the evaporating temperature slightly increases first and then drops as the motive flow rate rises. On the contrary, the condensing temperature slightly descends at first and ascends at last as the motive flow rate increases. The influence of the motive flow rate on the suction flow rate and entrainment ratio is illustrated in Fig. 11, both the suction flow rate and the entrainment ratio are monotonically increasing with the motive flow rate. The suction flow rate of the ejector enhances sharply with the increase of motive flow rate. Somewhat differently, the impact from the motive flow rate on the entrainment ratio is rather limited, and the average value of the entrainment ratio is about 2.03. The experimental entrainment ratio of this liquideliquid ejector is less than the average value of 2.41 obtained by Dopazo and Seara (2011); they utilized a commercial nozzle with a nozzle outlet diameter of 1.2e1.6 mm. In the present work, the nozzle
Fig. 9 e Intertube refrigerant distribution observed by the sight window 7: a) at zero valve opening; and b) at half valve opening.
315
°C
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 4 0 ( 2 0 1 4 ) 3 0 9 e3 1 6
Fig. 10 e Variations of the evaporating and condensing temperatures with the motive flow rate.
Fig. 12 e Effects of the motive flow rate on the recirculation spraying flow rate and re-circulation ratio.
outlet diameter of the ejector is designed as 2 mm. Generally speaking, the smaller the nozzle outlet diameter, the bigger the entrainment ratio of the ejector; however, the smaller the nozzle outlet diameter, the higher the manufacturing costs. Fig. 12 shows the experimental re-circulation spraying flow rate of the FFE and the re-circulation ratio calculated as a function of the motive flow rate of the ejector. Increasing trends are observed in both cases when the motive flow rate rises. The recirculation spraying flow rate rises 18.4%, from 6.32 kg s1 with motive flow rate of zero, to 7.64 kg s1 with motive flow rate of 0.56 kg s1, while the increment observed in the re-circulated ratio is up 18%, from 1.0 to 1.18. In Fig. 13 the experimental results of the evaporating capacity of the FFWC and the COP are represented as a function of the motive flow rate. Similar increasing-and-decreasing trends in both cases can be appreciated. It is worth pointing out the special behavior observed when the measurement value of motive flow rate is 0.43 kg s1. At this point, the obtained peak value of the evaporating capacity is 985.5 kW as well as the maximum COP 5.133. The extremum obtained at this point can be linked with the intertube flow pattern
observed by the sight window (depicted in Fig. 9). As previously indicated, by adjusting the valve opening of the RV, the RRS can control the inter-tube flow pattern of tube arrays to involve all three flow modes of droplet, column and sheet. When the valve opening is bigger, the larger re-circulation spraying flow rate of the FFE will lead to intertube continuous sheet, the effect of the accumulated film outside of tubes on heat transfer will decreases the evaporating capacity of the FFWC. On the other hand, while the valve opening is smaller, the less spraying flow rate of the FFE will result in intertube discrete droplet, the dryout emerged outside of tubes will deteriorate the heat transfer to reduce the evaporating capacity of the FFWC. Fig. 14 shows the variation of the performance coefficient with the refrigerant re-circulation ratio. It can be claimed that, the performance coefficient reaches a maximum and then drops down with the increase of the re-circulation ratio, and the performance coefficient rises 2.3%, from 5.017 with the recirculation ratio of one, to 5.133 with the re-circulation ratio of 1.135. Fig. 14 points out that the optimal re-circulation ratio of the FFWC is about 1.135, and the corresponding cooling COP is about 5.133.
Fig. 11 e Influences of the motive flow rate on the suction flow rate and entrainment ratio.
Fig. 13 e Influences of the motive flow rate on the evaporating load and performance coefficient.
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i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 4 0 ( 2 0 1 4 ) 3 0 9 e3 1 6
Fig. 14 e Variation of the performance coefficient with the refrigerant re-circulation ratio.
5.
Conclusions
A falling-film water chiller is presented, the experimental analysis about the effect of the refrigerant re-circulation system on the performance of the FFWC has been proposed, and the following conclusions can be drawn: By adjusting the valve opening of the RV, the RRS can control the inter-tube flow pattern of tube arrays to involve all three flow modes of droplet, column and sheet, and the RRS is feasible and necessary. The influence of the motive flow rate on the entrainment ratio of the ejector is small, and the average value of the entrainment ratio is about 2.03 at average evaporating and condensing pressure of 3.436 bar (4.5 C) and 9.656 bar (38.1 C). Increasing gradually the valve opening of the regulating valve, the evaporating capacity of FFWC rises 4.8%, from 940.2 kW with the motive flow rate of zero, to 985.5 kW with the motive flow rate at 0.43 kg s1. The performance coefficient reaches a maximum and then drops down with the increase of the re-circulation ratio.
Acknowledgments The authors gratefully acknowledge supports through the Financial Grant from 12th Five-year National Science Project of China (2011BAJ03B05) and the General Financial Grant from the China Postdoctoral Science Foundation (2012M520970).
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