Purdue University Purdue e-Pubs International Refrigeration and Air Conditioning School of Mechanical Engineering Conference
1994 A Study on a Hot-Water Driven Air-Cooled Absorption Refrigerating Machine T. Ohuchi Hitachi
M. Aizawa Hitachi
A. Nishiguchi Hitachi
T. Hatada Hitachi
Y. Kunugi Hitachi
See next page for additional authors
Follow this and additional works at: http://docs.lib.purdue.edu/iracc
Ohuchi, T.; Aizawa, M.; Nishiguchi, A.; Hatada, T.; Kunugi, Y.; and Kawakami, R., "A Study on a Hot-Water Driven Air-Cooled Absorption Refrigerating Machine" (1994). International Refrigeration and Air Conditioning Conference. Paper 259. http://docs.lib.purdue.edu/iracc/259
This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact [email protected] for additional information. Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/ Herrick/Events/orderlit.html Authors T. Ohuchi, M. Aizawa, A. Nishiguchi, T. Hatada, Y. Kunugi, and R. Kawakami
This article is available at Purdue e-Pubs: http://docs.lib.purdue.edu/iracc/259 A STUDY ON A HOT-WATER DRIVEN AIR-COOLED ABSORPTION REFRIGERATING MACHINE
T. Ohuchi*, M. Aizawa**, R. Kawakami***, A. Nishiguchi*, T. Hatada*, Y. Kunugi* * Mechanical Engineering Research Laboratory, Hitachi, Ltd. ** Tsuchiura works, Hitachi, Ltd. ***Osaka Gas Co., Ltd.
ABSTRACT The water-cooled system should be replaced This paper describes the development of a hot with an air-cooled system. The key technology in water-driven air-cooled absorption refrigerating the air-cooled system lies in the air cooling of the system. The first stage of development involves absorber itself. If the current water-cooled the construction of an absorber and condenser unit absorbers were to be air cooled without changing suitable for air cooling. A multi-pass cross systems, the absorption temperature would rise in counter flow arrangement within the absorber the LiBr solution, resulting in crystallization and makes it possible to reduce the hot water rendering the cycle ineffective. To overcome this temperature. In addition, the heat transfer difficulty, heat is exchanged between the ·performance of the air coolers is improved by strip absorption tubes and cooling air by three-pass fins used to control air flow distribution. In the cross-counter exchangers. The absorption tubes second stage an evaporator for the hot-water have spiral grooves on the inside and air flow driven system is developed. Air conditioning is distribution control strip fins on the outside. attained with an indoor unit by circulating low Correct placement of the absorber and condenser temperature refrigerant from a spray-:type flash results in compactness and preserves all the evaporator. Finally, a third element is developed advantages of a water-cooled absorption chiller. for a flooded counter-flow-type high-temperature generator suitable for use in an absorption air 2. ARRANGEMENT OF THE AIR-COOLED conditioner. Combining those elements, we have CONDENSER AND ABSORBER developed an experimental model for a hot-water In an air-cooled absorber refrigeration system, driven absorption air conditioner with a water the air-cooled absorber and air-cooled condenser temperature of 160"C. The unit has a COP of 0.8 must be placed in the air heat exchanger, which for an ambient temperature of 35 "C , and a allows for the coolant air flow. The operating cycle is determined circulating chilled refrigerant temperature of 6"C. pressure of the refrigeration by the temperature and absorber concentrations by the temperature of the condenser. INTRODUCTION and of the condenser relative to Water-lithium-bromide-type absorption Therefore, the position series absorber of the configuration refrigerating machines are widely used for solar the three-pass 1 determines whether the cycle is energy and waste heat recovery. A hot water shown in Fig. The size of the absorber and driven air-cooled system should be developed for established. condenser are important, and thus it is necessary domestic use. This paper describes the methods find a configuration that minimizes the front used in this development as well as theoretical to surface area and total heat transfer surface area, and experimental results. and one that facilitates the supply of refrigerant vapor to the condenser. · 1. AIR-COOLED SYSTEM
275 that Refngeran! vapor from evaporator of the front part of the absorber as Faf and 0 1) that of the rear part of the absorber as Far· " Table 1 Simulation conditions
Cooling cooling capacity Qe = 7.0 kW "" performance coefficient COP = 1.14 Strong solution condensing temp. Tc = 41.6 'C evaporating temp. Te = 6.0 'C solution concentration Fig. 1 Three-pass series absorber 61.4-58.0% cooling air temp. 35.0-43.3 'C 2.1. SIMULATION coefficients of overall Ka =0.6 kWfm2K Setting aside the specifications for the heat heat transfer Kc = 1.1 kW/m2K transfer tubes and fins, we simulated the series configuration shown in Fig. 2. McNeely's From Fig. 3, we can see that when 10% to 30 equation [2] was used to determine the thermal % of the heat transfer rate occurs in the front part properties of the aqueous LiBr solution. For the of the absorber, the total heat transfer area (Fa + front heat transfer area, F, is obtained from Fe) is small and a compact system is possible. equation (1 ). 12.-~~--~--r-~~-r~~
Front abs. Cond. Rear abs F: Area Cooling air c : Condenser a : Total absorber 9 Uf=tiF;fiiF~; a~ "E:; () r : Front absorber I
Here, F is the heat transfer area, 0 is the exchange heat, R is the water equivalent ratio, cf> is the temperature effectiveness, Ka is the overall heat 0_2 OA 0_6 0.8 transfer coefficient, Tai is the absorber inlet Oar/On solution equilibrium temperature, and Tao is the Fig. 3 Simulation results showing the heat transfer absorber outlet solution equilibrium temperature. area of the absorber and condenser Figure 3 shows the relationship between the front absorber heat transfer rate and the heat 2.2 EXPERIMENTAL STUDY transfer area, with the horizontal axis representing The specifications for the air-cooled absorber the ratio of the front absorber heat transfer rate to and condenser used in the prototype are shown in the total absorber heat transfer rate, Oaf/ Oa, and Table 2. the vertical axis representing the heat transfer area, Fig. 4 shows the relationship between the cooling F. The condenser heat transfer area is expressed air temperature and the absorption and as F c, the total absorber heat transfer area as Fa, condensation temperature. The temperature of the 276 absorber solution outlet is lower than the outlet convex strip fin was used to attain this control. temperature of the cooling air. The front absorber Fig. 6 shows examples of convex strip fins. Fig. 7 shows a rise in air temperature of about 30% of shows experimental results for heat transfer in air the total temperature increase. This shows that coolers. cross-counter heat exchange has been completed. Convex strip convention a I
Table 2 Specifications of air- cooled -~4; air flow absorber and condenser distribution air inlet area W0.92XH1.2m fins Convex strip fins 480 sheets tubes rp 15.5 Inner grooved tube 110 air cooling fan rp 600 propeller type 0.4 kW absorber 3 pass (Faf=25%) 5.36 m2 fin condenser 1 pass (2nd pass) 1.34 m2 Fig. 5 Distribution of temperature difference between fin and air flow 50 ----6 cross E --6-- condensation - . \7~ section ~ 40 ~~ - ...:::0 Solution A-A ...rd ~c~ Q.) . 0.21 q/s · A' tern10ront IZ. _ . St 1 r ~ ""' 7 6 J! 30 0.33 q/$ - Hot later 158. s-153. ot: B-B 1 2 3 4 (Pass No.) - Fig. 4 Temperature change in the air-cooled absorber and condenser Fig. 6 Convex strip fins 100 ,----.---..---.,.-----, 3. IMPROVED HEAT TRANSFER ..... improve the overall heat transfer performance in / 40 / / / air coolers consisting of fins and tubes. · The / f Pressure drop 30 ' I temperature difference( ~ T) between the fins and / Cl. / 0 I the air flow ·varies depending on their distance L / 0 Convex .strip fin I '"0 277 4. SPRAY-TYPE FLASH EVAPORATOR Air conditioning was attained with an indoor 1 . 2 J T l unit by circulating low-temperature refrigerant I ...... 1. 0 ~ from a flash evaporator. This spray-type 0 0 Ch£60 0: evaporator eliminates the need for heat transfer : ~ o. 8 ...... 11). - .._, I tubes. This increases the heat exchange efficiency I_ ,;; 0. 6 1-- .- for air in a room , ., ~ Rer. flo• rate 0. Zl q/s allowing for a more compact II L) ._, S)'llbol Hot ,_ unit. In this evaporator, liquid is sprayed 0.4 1-- water te•p . in the 0 170 t form of droplets onto a metal mesh. This II 6 160 .. 0.2 f- t 1- increases the surface area of the contact between -9 I I I I I the liquid and vapor. 0 In this case, thermocouples 2 4 6 8 are fitted on the metal mesh and exposed to the Qa (kW) vapor. Fig. 8 Performance of the flash evaporator Table 3 Specifications for the spray-type 5. FLOODED COUNTER-FLOW-TYPE HIGH flash evaporator TEMPERATURE GENERATOR Number of nozzles 6 The flooded counter-flow-type high- Wire mesh size ¢0.2-0.635 X 0.635 temperature generator can use hot water from Wire mesh size 165WX985LX lOOH solar energy and waste heat. Fig. 9 shows a cross section Case size 165WX985LX IOOH of the high-temperature generator. Table 4 Specifications for the high-temperature To evaluate the performance of the flash generator evaporator, the following equation for temperature Size effectiveness was used. 152WX 1,105LX200H Number of U-tubes 14 U-tube size (outer/inner) evaporators in terms of temperature effectiveness is shown in Fig. 8. The effectiveness of the spray-type flash evaporator is 95%. In contrast the effectiveness of the conventional water-cooling system is 70%. Hot water Flash evaporators can produce refrigerant at 6 "C Fig. 9 Cross-section of the high-temperature with an evaporation rate of about 500 kWjm3 generator The perfonnance of the high-temperature generator is evaluated usmg temperature effectiveness. 278 6 Specifications for the indoor unit c.pHG = twHGi-tWHGo ...... ( 3) Table fwHGi -THG Heating and cooling 3.5 kW capacity Here, 6. EXPERIMENTAL MODEL FOR HOT WATER DRIVEN AIR -COOLED ABSORPTION REFRIGERATING MACHINE Combining the above technologies, we developed an experimental model for a hot-water driven air-cooled absorption refrigerating machine, as shown in Fig. 11. Specifications for the experimental model for this machine are shown in Table 5, and those for the indoor-unit are shown in Table 6. An experimental apparatus for this machine and Fig. 11 Experimental model for the hot-water two indoor units are shown in Fig. 12. driven air-cooled absorption refrigerating machine Table 5 Specifications for the prototype Cooling capacity Q e = 7.0 kW Absorption Evaporator spray-flash type (table 3) refrigerating machine Air cooler convex strip fin (table 2) Low-temp. generator flooded type 0.41 m2 Hi-temp. generator flooded type (table 4) Solution heat exchanger plate-type 0.8 m2 Heating heat exchanger spray type 0.027 m2 Size 1.240W X 570D X 2,000H Fig. 12 Experimental apparatus for the hot-water Weight (include base) 700 kg driven air-cooled absorption refrigerating machine 279 Figure 13 shows the performance of the performance of the air coolers was improved by prototype for the hot-water-driven air-cooled strip fins used to control the air flow distribution. absorption refrigerating machine. In the second stage an evaporator for the hot From this prototype, an experimental model of water-driven system was made. Air conditioning a hot-water-driven absorption air conditioner was attained with an indoor unit by circulating operated at a water temperature of 160 "C was low-temperature refrigerant from a spray-type made. The unit has a COP of 0.8 at an ambient flash evaporator. Finally, a third element was temperature of 35 "C , and a circulating chilled developed for a flooded counter-flow-type high refrigerant temperature of 6 "C . This machine is temperature generator to be used in an absorption shown to be useful for domestic applications. air conditioner. Combining those elements we developed, an experimental model for a hot-water driven absorption air conditioner with a water I I I 25 f- p - - temperature of 160 'L. The unit has a COP of 0.8 .,_0 ea ...... 20 f- :& ~ ~- for an ambient temperature of 35 'C , and a 15 8 circulating chilled refrigerant temperature of 6'C. 8 f- 00 - 5: ..:.:: 6 f- - KEYWORDS - 6. 4 f- Ill Waste heat recovery, Absorption refrigeration, 0"' /1).- 0 Lithium bromide, Air cooling, Double-effect 2 - Hoi water flow rale 0.33kg/s - Rofrigrerant flow rate 0.2lkg/s absorption cycle, Strip fins 0 _l_ Symbol - Hot water temp. p 15 0 170 "'C - REFERENCES /:,. 10 r-- 160 "'C f:{:y._ 0 1. McNeely, L. A. "Thermodynamic ~ 0 properties 5 f--0 c(;O lk - of aqueous Lithium Bromide water solution 0 I _I I "ASHRAE Transactions, Part. 1,85(1979) 1.0 r-- - 2. T. Ohuchi, Y. Kunugi, M. Aizawa, A. 0.8 f--0 oo - Yoshida, and R. Kawakami " Study on direct a_ 6. ~ 0 0.6 r-- refrigerant circulating absorption air conditioning u - system, 3rd report, Air cooled absorber and air 0.4 f-- &_ 0 cooled condenser " 25th. JAR & JAC Joint 0.2 - - Meeting Report, (1991.4), pp. 177-180 0 I J I 27 30 35 40 45 Out door temperature , "C Fig. 13 Performance of the prototype hot-water driven air-cooled absorption refrigerating machine 7. CONCLUSION The first stage of development involved constructing an absorber and condenser unit suitable for air cooling. A multi-pass cross counter flow arrangement within the absorber made it possible to reduce the hot water temperature. In addition, the heat transfer 280