1、4705 Axial-Flow Air4apor Compression Refrigerating System for Air Conditioning Cooled by Circulating Water Shaobo Hou Hefei Zhang ABSTRACT This paper presents an axial-flow air-vapor compression refrigerating system for air conditioning cooled by circulating water and proves its feasibility through
2、performance simulu- tion. Using this new system may simplifi air-conditioning systems and reduce the amount of initial investment. This new system has a smaller section of supply duct, two fwer heat exchangers, and one fer water-circulating system. Satu- rated air with a temperature of about 0C is o
3、btained by this system; therefore, the adoption of this system will make air- conditioned rooms more comfortable and cut down on the size of the supply duct section. Furthermore, there are no CFCs or HCFCs in this entirely new air-conditioning system concept, and the working medium is wet air from o
4、utdoors at no cost to the user Simulations show that the new system S COP depends mainly on the turbine inlet temperature of the wet air and the eficiencies of the axial compressor and the turbine, and the COP varies with the wet-bulb temperature of the atmosphere. This system is very suitable for u
5、se in muggy areas, und it will still he eficient ifcooled by circulating water AIR-VAPOR AS WORKING FLUID Air compression refrigeration has been studied for years, but several disadvantages prevented air from being used as a working fluid in refrigeration cycles. They include a low volu- metric refr
6、igerating effect, which may result in a large compressor, and low COP due to low efficiencies of compres- sor and expander. With the development of the aeronautical industry, highly efficient axial compressors and turbines have become a reality. At present, the stagnation isentropic efficiency of a
7、single- stage axial compressor can reach 0.88-0.91 and that of a turbine to about 0.88-0.91 (NPU 198 I). The characteristics of turbo-machinery are large volume flow rate and high e%- ciency compared to other types of compressors and expanders. Consequently, it is possible for us to use axial air-va
8、por compression refrigeration for air conditioning. This refriger- ating system intakes wet air from outdoors and some vapor is condensed during air cooling. For this reason, air-vapor is the working fluid for this system. Air-vapor has the following advantages when it is used as the working fluid i
9、n refrigeration air conditioning: 1. Thermally stable and chemically inert at its working temperature range. 2. Neither toxic nor flammable. 3. Possible to simpliQ air-conditioning system and to reduce initial investment. Wet air is free and available everywhere, and its character- istics are well k
10、nown. 4. SYSTEM Flow and H-s diagrams of an axial-flow air-vapor compression refrigerating system for air conditioning cooled by circulating water are shown in Figure 1. The outdoor air is drawn into the atomizing chamber, cooled into saturated air with some fine water droplets at 3, and then compre
11、ssed by an axial compressor. A flow of compressed air with higher temperature is obtained at 4. Then the compressed air is cooled by circulating water in a surface heat exchanger between the axial compressor discharge and the turbine inlet. Some vapor is condensed and the saturated air with high pre
12、ssure is gener- ated at 7. Then the saturated air is expanded and cooled in the Shaobo Hou is a lecturer in the Department of Energy and Power, Zhanjiang Ocean University, Zhanjiang, Guangdong, China, and is also a doctoral student in the College of Aeroengine and Thermal Power Engineering, Northwes
13、tern Polytechnical University, Xian, Shaanxi, China. Hefei Zhang is a professor in the College of Aeroengine and Thermal Power Engineering, Northwestern Polytechnical University, Xian, Shaanxi, China. 02004 ASHRAE. 125 H - S 7 ri-, AC - 8 cooling tower I i I I AC Atomizing chamber C Compressor SHE S
14、urface heat exchanger T Turbine Figure 1 Circuit diagram of the axial-$ow air-vapor compression refrigerating system for air-conditioning cooled by circulating water. turbine. The cool air leaving the turbine at 8 is then ducted to the air-conditioned rooms. The water injection before the axial comp
15、ressor aims at decreasing both the temperature of the working fluid and the polytropic exponent of the compression process. Thus, we can save some work in the compression process. The problem of impingement of the droplets on the rotor blades will not occur because the mount of very fine water dropl
16、ets is very small, about 20-30 g/kg(d.a.). This method has been used in jet engines when fighter planes speed up. However, the difference is that what is injected in a jet engine could be water, alcohol, etc. (NPU 1981). The water vapor in the compressed air can easily be extracted by a surface heat
17、 exchanger. The surface heat exchanger can be the current used in an ordinary air-condi- tioning system. With the same temperature, the humidity ratio of the saturated wet air at high pressure P, is only about P,/ P, of that at P,. The method of using compressed air to dry air has been used in some
18、workshops in southern China, The system above is a new concept and differs from conventional air-cycle systems. There are many differences between this and older systems. First, an axial compressor and a turbine are used in the above system. The characteristics of turbo-machinery are large mass flow
19、 rate and high efficiency. Other types of compressor and expander have none of the advantages above. Second, this refrigeration system intakes the precooled wet air with fine water droplets and some of the vapor is condensed during air-cooling. The amount of the water extracted from high-pressure we
20、t air can reach 18-30 g/kg(d.a.). The amount of heat discharged from water vapor condensed is about 45-75 kW/kg(d.a.). This exceeds that from air, which is 30-50 kW/kg(d.a.). Therefore, refrigeration with the new system depends on a combination of air and vapor. In order to bring the machine to mark
21、et, the axial compressor and turbine should be designed by turbomachin- ery engineers. The requirements are for high efficiency and working well at 85-1 15% of mass flow rate ofthe design point. The current available efficiency of a three-stage axial compressor at the design point is 0.87-0.9, and t
22、he current available efficiency of a one-stage turbine at the design point is 0.89-0.92 (NPU 1981). We can choose different mass flow rates of compressor and turbine at series production, such as i O, 15,20,25,30,50 kg/s, and the corresponding refrigerating capacities are about 600, 900, 1200, 1500,
23、 1800, 3000 kW, respectively. This new machine could be driven by an electro- motor with transmission case. For this system, the two key parts are the axial compressor and turbine. The developing budget of this system in China is estimated at no more than US$l,OOO,OOO for the first refriger- ating s
24、ystem, while the cost of batch production will be much lower in China. The surface heat exchanger in this system should be fixed in firm rectangular case because the diameters of the axial compressor and turbine are small, and the case size of the surface heat exchanger is large. There should be a t
25、ransitional duct between the axial compressor and surface heat exchanger and between the turbine and surface heat exchanger. Thus, we can get a flow of air with low flow speed, high pressure, and high temperature in the surface heat exchanger. This causes a rise of the convective heat transfer coeff
26、icient of the air side and finally leads the overall heat transfer coefficient of the surface heat exchanger to increase. PERFORMANCE SIMULATION Wet Air The humidity ratio of wet air, d, is obtained from Pvap B - Pvap d = 621.98 126 ASHRAE Transactions: Research The enthalpy of wet air, h, is calcul
27、ated from h = 1.006t+0.001d(2501 + 1.805t). (2) The adopted relation for water vapor between saturation pressure and saturation temperature, P, = f( t,) , is selected from ASHRAE (2001). Axial Compressor During the compression process of wet air, the fine water droplets in the air may evaporate. Bec
28、ause the evaporation of water intakes heat, we can regard the ideal compression process of wet air in the compressor as a polytropic process. Hence, we can obtain the ideal work of the compressor per kilogram dry air, W, from: r L J where n is the polytropic exponent for the compression process. The
29、 practical work consumed by the axial compressor is WJq, in which qc is the thermal efficiency of the compressor. Turbine The expansion of the saturated air in the turbine cannot be regarded as an adiabatic expansion of an ideal gas. With the decrease of the wet air pressure in the turbine, the temp
30、erature of the wet air decreases, and some heat is discharged during the condensation of some water vapor. The heat discharged may cause an increase in both the temperature of the turbine outlet and the work done in the expansion. For this problem, we can imagine that no phase change exists and that
31、 there is some heat added to the wet air during the expansion process when we calculate the work done in the expansion process. According to the above assumption, this problem can be simplified to a problem of the polytropic expansion of an ideal gas. Therefore, we can obtain the ideal work done by
32、the expansion, Wt, through iteration and then obtain the practical work generated by the turbine and the temperature of the turbine outlet. PERFORMANCE The refrigerating capacity per kilogram of dry air, q2, can be determined by the enthalpy difference between the inlet of the compressor and the out
33、let of the turbine by using the following formula: The work consumed by the refrigerating system is calcu- lated by (5) The COP of this refrigerating systemis calculated by the following formula. The work consumed by the circulating cooling-water system is not included in w,. q2 COP = - The losses o
34、f the electromotor and transmission are not included in COP calculations. RESULTS .There are many factors that may affect the COP of an axial-flow air-vapor compression refrigerating system for air conditioning cooled by circulating water. These include the pressure ratio of the axial compressor, P,
35、IP, the efficiencies of the axial compressor and turbine, the wet-bulb temperature of the atmosphere T, and the wet air temperature of turbine inlet, T7. During simulation, the pressure ratio of the axial compressor varied from 1.6 to 2.5 and the wet-bulb tempera- ture of the atmosphere from 20C to
36、27C; the wet air temper- ature of the turbine inlet is 7 - 11C higher than the wet-bulb temperature of the atmosphere. There is a 300 Pa pressure loss before the axial compressor and 300 Pa between the turbo compressor and the turbine and after the turbine. Some encouraging results are illustrated i
37、n Tables 1 and 2. Table 1 gives simulation results of an axial-flow air-vapor compression refrigerating system for air conditioning cooled by circulating water when qc= 0.90, q, = 0.90, T3 = 300 K, and T7 = 298 - 3 13 K. Table 2 gives its simulation results when qc = 0.90, qt= 0.90, and T7 -T,= 10 K
38、 (T3 = T,) . The results show that the temperature of the turbine inlet, T, affects COP heavily and that a lower temperature results in a higher COP. This indicates that a fall in T7 is one method of raising COP. T7 is determined by the temperature of the cooling water, which is about 4 K higher tha
39、n the wet-bulb temperature of the atmosphere. The difference between T7 and T, is about 70 -1 1 K if cooled by circulating water. T7 will drop lower if cooled by other cooling water with lower temperature, such as the refilled groundwater, the water having been cooled by the air leaving the air-cond
40、itioned room, and other easily available cool water (Hou and Li 1992). Consequently, the assumption of the temperature of the turbine inlet, T7, in this paper is reasonable or conservative. T7 has room to drop with other measures, and the COP of this refrigerating may rise further. The COP lines of
41、efficiency of the axial compressor and turbine at 75%, 80%, 85%, 88%, and 90% are illustrated in Figure 2, which shows the efficiencies of the axial compressor and the turbine heavily influence the COP. Still, this system is feasible. First, this new turbo-machinery works near the design point, and
42、efficiencies of the axial compressor and turbine are very high at the design point, about 0.89-0.91. Second, the intake air is clean and without dust; therefore, the efficiencies of the axial compressor and turbine will not drop ASHRAE Transactions: Research i 27 Table 1. Simulation Results of an Ax
43、ial-Flow Air-Vapor Compression Refrigerating System for Air Conditioning Cooled by Circulating Water When oqc=0.90, qt=0.90, and T3=300 K T7, K 298.0 299.0 300.0 T, K q1, kJWW 9% kJ/kg(d.a) w, kJ/kg(d.a) COP 261.1 120.85 93.67 27.18 3.45 262.7 118.39 91.55 26.85 3.41 264.3 115.86 89.35 26.51 3.37 I
44、301.0 I 265.9 I 113.24 I 87.07 I 26.17 I 3.33 I 302.0 303.0 1- I I I l I 1 267.6 110.53 84.70 25.83 3.28 269.2 107.73 82.25 25.48 3.23 - 7 I 304.0 I 270.8 I 104.84 I 79.71 I 25.13 I 3.17 I 305.0 306.0 - 272.4 101.84 77.07 24.77 3.11 274.0 98.73 74.32 24.41 3.04 I 307.0 I 275.6 I 95.51 I 71.47 I 24.0
45、4 I 2.97 1 - - 308.0 309.0 - 277.2 92.18 68.51 23.67 2.89 278.7 88.72 65.42 23.30 2.81 310.0 311.0 I 312.0 I 283.4 I 77.55 I 55.39 I 22.16 I 2.50 1 280.3 85.13 62.21 22.92 2.71 281.8 81.41 58.87 22.54 2.61 I 313.0 284.9 73.54 5 1.76 21.77 2.38 T3, K 293 294 295 296 297 298 299 128 T, K T7, K Ts, K q
46、i, kJ/kg(d.a) q2, kJ/kg(d.a) w, kJ/kg(d.a) COP 342.0 303 269.2 76.98 54.47 22.51 2.42 343.1 304 270.8 77.97 55.4 1 22.56 2.46 344.3 305 272.4 79.01 56.40 22.61 2.50 345.4 306 274.0 80. I I 57.44 22.66 2.53 346.6 307 275.6 81.26 58.55 22.71 2.58 347.8 308 277.2 82.48 59.71 22.77 2.62 348.9 3 09 278.7
47、 83.77 60.93 22.84 2.67 ASHRAE Transactions: Research I 300 2.71 350.1 310 280.3 85.13 62.2 1 22.92 2.8 _I 2.0 i io 292 293 294 295 296 297 298 299 300 301 T,.K (T,=T,+IO) Figure2 The COP of an axial-flow vapor compression refrigerating system for air conditioning cooled by circulating water at dife
48、rent eciencies of the axial compressor and turbine. greatly while working. Third, there is no very complex combustion chamber and high-temperature turbine in the turbo-machinery. Consequently, it is much easier to accom- plish than many people thought. Last, the efficiencies of the axial compressor
49、and turbine have room for improvement with additional design measures. CONCLUSIONS This study shows the feasibility of an axial-flow vapor compression refrigerating system for air conditioning cooled by circulating water. The results show: The system given in this paper is a new cycle, and its work- ing fluid is a mixture of air and vapor. Its refrigeration depends on both air and vapor and differs fium a conven- tional air-cycle system. The use of turbo machinery makes this possible. Humid air is a good working fluid for refrigeration in central air con
