Application Research for Performance of Refrigeration Units with Flash Tank Economizer

Transcription

1 MATEC Web o Conerences 95, (07) DOI: 0.05/ mateccon/ ICMME 06 Application Research or erormance o Rerigeration Units with Flash Tank Economizer Qin Huanghui and Fan Li, Department o Marine Engineering, Nantong Shipping College, Nantong, China College o Ocean Science and Engineering, Shanghai Maritime University, Shanghai, China Abstract. This paper studies the inluence o lash economic system on perormance o rerigeration unit using Rerigerant R90. Theoretical analyses o rerigeration system with lash type economizer are carried out irstly, which provide theoretical basis or working parameters. Under dierent evaporation temperature, inluences o supply pressure on the rerigerating capacity and CO are studied and veriied through experimental. Experimental results show that, using the economizer can eectively improve the cooling capacity and CO, and achieve the eect o saving energy. With the increase o supply pressure, rerigeration capacity o rerigerating unit and shat power decreases gradually and the rerigeration coeicient rise irst and then drop, to determine the best economic position. Introduction With the improvement o people's living standard and the development o industry, rerigerating units are widely used in the ood, drinks, dairy products, petrochemical industry, coal, medicine, textile, paper making, tobacco, printing and other industries. The cooling temperature generally is between -80 C~0 C. For example, there are cooling, cold storage, rozen ood, ultra-low temperature rozen in the process o rozen ood, in detail, 0 C or the cooling room, -8 C or storage room, -5 C or rerigerating room, -0 C~-80 C or ultra-low temperature rozen. The lower the evaporation temperature o rerigerating unit, the greater the compression ratio, the sharp all in the eiciency o compression. When compression ratio is greater than 8 to 0, two-stage rerigeration system should be used []. Rerigeration unit with economizer is a simpliied twostage compression rerigeration cycle system, the rerigerating capacity and rerigeration coeicient can be improved. When the evaporation temperature is lower than -0 C~-40 C and the compression ratio is greater than 8 or 0, the eect is obvious. At present, there generally two types o economizer, they are lash type and sub-cooled type. The latter is sae and reliable, but the system is high cost and complex. And it is achieved by direct expansion. The ormer is simple in structure, easy to control and low in cost, so is mostly used in rerigeration unit []. When screw compressor works, the rerigerating capacity and the screw rerigeration coeicient can be increased by adding a new supply port at the rotor to draw in gas rom the economizer [], [4]. Yang Li, etc. rom the Shanghai Jiao Tong University studied the perormance o screw rerigeration compressor using R4a and R by numerical simulation, and compared the perormance dierence between economizer units and common rerigeration units [5]. Sun Zhen, etc. rom the Shanghai Maritime University studied perormance o screw rerigeration unit using R4a with ejector and economizer in dierent operating conditions, and optimized injection port working with economizer [6]. Sun Chao etc. rom Huazhong University o Science and Technology took numerical analysis and study on R4a applying in gas supply on rerigeration system o screw compressor [7]. This paper studies the gas supply process o the rerigeration unit system with lash type economizer. The rerigerant is propane (R90), a kind o natural rerigerants [8]. Under dierent evaporation temperature, inluences o supply pressure on the rerigerating capacity and CO are studied in this paper, and veriied through experimental. This research has an important guiding signiicance or the design and application o rerigerating units with lash type economizer. Theoretical analysis Rerigeration system with lash type economizer is shown in Fig.. The LCV throttles the liquid rerigerant, some o the liquid rerigerant change into gas ater passing this valve, and low into the lash type economizer. The opening o regulating valve-4 is controlled by adjusting the liquid level in the condenser. ressure control valve-6 maintains the supply pressure in the lash type economizer. The lashed rerigerant gas enters the economizer port o the compressor, while the saturated The Authors, published by ED Sciences. This is an open access article distributed under the terms o the Creative Commons Attribution License 4.0 (

2 MATEC Web o Conerences 95, (07) DOI: 0.05/ mateccon/ ICMME 06 liquid through the economizer piping lows into the low pressure evaporator by the second throttling valve. In the economizer, the lashed rerigerant gas enters into the primitive volume chamber (the suction chamber) ater passing through the pressure control valve and the gas supply port in the screw compressor. The lashed rerigerant gas and the gas in the primitive volume chamber are compressed together, so the compressor power will increase. process is a polytrophic compression process. On the p-h graph, the slope o - should be greater than the adiabatic compression process (-'). I the screw compressor internal pressure ratio is less than the external the pressure ratio, that is to say exhaust pressure () is less than condensing pressure (c), the exhaust process is approx isovolumic compression process. I the compressor internal pressure ratio is greater than the external pressure ratio, that is >c, the exhaust process is constant volume expansion [0]. In short, the position o gas supply port and the oil temperature will inluence the indicating eiciency o the second stage compression, change rerigerating capacity, compressor power and CO in a certain extent [], []. LC LI. Determination o the parameters o primary compression process LC LC 0. Figure. Flow chart o rerigeration system 6 c 5' 5 m 5" 5a e Figure. -h o lash type economizer system The screw compressor with gas supply port can be considered as a two-stage system compression process, and the -h shown in Fig.. The compressor draws in the gas irst. Then, the gas suction chamber in the compressor is separated rom the suction port at the end o suction process, and the primitive volume chamber becomes an enclosed space [9]. Ater a short period o compression, the primitive volume chamber immediately connects with the gas supply port. At this position, the gas rom the lash type economizer at pressure m enters the compressor immediately. The opening o the gas supply port will increase gradually as the rotation o the screw, and more gas is drawn in, thus higher pressure is produced in the chamber. With the increase o the pressure in the chamber, the dierential pressure and the amount o supply gas decreases. At the end o gas supply process, the pressure at the primitive volume chamber is, which is less than m. In the screw compressor, the compression process is a continuous process. The gas in the gas supply chamber is at a relatively stable state, so the gas supply process is a rotating pressurization and adiabatic gas mixed combination process. When the gas supply port is separated rom the suction port the second stage compression starts. In the beginning o compression, oil is sprayed into the compression chamber. Because the temperature o oil is higher than that o gas, so gas is heated by oil. At the end o compression, the temperature o gas is higher than that o oil, thus the compression The supply port is added in the compressor rotor on the direction o the length o the screw, and the location o this port determines volume ratio. The pressure and temperature at the end o the primary compression are calculated by ormula (), () []. ( ) n () v v n n T T Among them, : compressor suction pressure, a. : ressure at the end o the primary compression, a. T : Compressor suction temperature, K. T : Temperature at the end o the primary compression, K. : Volume eiciency o the primary compression. v : Volume ratio o the primary compression. v. Mass and energy balance o economizer For the lash type economizer, the gas mass and energy balance equation in the supply process are as ollows: Among them, exhaust, kg s -. () q q q c e () qh qh qh (4) c 5 e 5' 5" q : Mass low o the compressor c q : Mass low o evaporator, kg s -. e q : Mass low o the economizer supply gas, kg s -. h : Speciic enthalpy o saturated liquid rerigerant 5' in the lash type economizer, kj kg -. h : Speciic enthalpy o saturated rerigerant gas in 5" the lash type economizer, kj kg -. h : Speciic enthalpy o sub-cooled liquid, kj kg -. 5

3 MATEC Web o Conerences 95, (07) DOI: 0.05/ mateccon/ ICMME 06. Determination o the parameters or the supply process Supply process is an approximate isentropic and adiabatic process. Gas supply equation o per rerigerant suction volume, the energy equation o gas supply process, the speciic volume equation and the speciic entropy equation are shown as ormula (5), (6), (7), (8) []: q (5) qe h h h (6) 5" v v / (7) s s s (8) 5" Among them, : Gas supply ratio o lash type economizer rerigerant system v : Speciic volume at the end o the primary compression, m kg -. v : Speciic volume at the end o the supply process, m kg -. s : Speciic entropy at the end o the supply process, kj (kg K) -. s : Speciic entropy at the end o the primary compression, kj (kg K) -. s : Speciic entropy o saturated gas at the 5" economizer pressure, kj (kg K) -..4 Exhaust temperature o compressor The compression process is a changeable compression process ater the supply process, the exhaust temperature T is calculated as ollows: T T c n n.5 Rerigerating capacity and compression power The rerigerating capacity is calculated as ollows: Q q h h L c 6 (0) For the compression power calculation: q h h h h e 5" W e d () Among them, e : Adiabatic eiciency o the compressor. d : Motor eiciency (including the transer eiciency between the motor and the compressor). In this paper, the theoretical displacement o the compressor is 650m h -. The rerigerating capacity, CO value o rerigeration unit, and compressor power are (9) showed in Fig. 4. The curve is the theoretical calculation results at dierent economizer pressure when the suction pressure is 5 kpa and exhaust pressure is 70 kpa. Experimental Rerigerant unit experimental is completed in environmental simulation laboratory. The experimental devices include brine system and rerigerant system. Technical parameters and precision o experiment instruments are shown in Table. The serial number Table. Experimental instruments and equipment. Name o equipment Technical parameters recision Current transormer 0600A 0. level Digital power meter Electromagnetic lowmeter V/ 0.040A 0.5 level 000m h % 4 T temperature sensor -60~60 C A level 5 ressure sensor ka 0.0% The lash type economizer is applied in the rerigerating unit, using R90 as rerigerant, with the open type twin-screw compressor, the water-cooled shell and tube condenser, and hal bundle evaporator. The rotor diameter o the screw is 6.mm. The length diameter ratio is.65. The theoretical displacement o compressor is 640.7m h -. The throttling valve adopts Fisher LCV to control the rerigerant level o condenser. The pressure control valve controls the pressure o the economizer. Using ID, it can achieve the stability o the economizer pressure and liquid level at dierent load and pressure. 4 Experimental data analysis At dierent supply pressure, the theoretical and experimental results o the unit capacity, power and CO are shown in Fig.. The experimental conditions are: evaporate at -5± C, condensate at 40± C, superheat C at the 00% load. The Fig shows that the unit capacity decreases with the increase o the economizer supply pressure, the same is true or compressor power. The Cal. capacity is about 96.7% to that o the Exp. capacity, the Cal. power is about 0.%, and the Cal. CO is about 95.5%. So or the rerigeration unit with lash type economizer, the theoretical calculation results are basically consistent with the experimental data. Both to theoretical and experimental data, the rerigerant system CO is parabola distributed with the increase o gas supply pressure. CO increases irst, then decreases, because o the reducing rate o rerigerating capacity is less than the rate o power when the supply pressure is closer to the suction pressure. And the opposite occurs

4 MATEC Web o Conerences 95, (07) DOI: 0.05/ mateccon/ ICMME 06 when the supply pressure is closer to the exhaust pressure. As can be seen rom Fig., rerigerating capacity curve is approximate linear curve, and the power curve is about quadratic curve. So there must be an optimal supply pressure or lash type economizer system. Theoretical calculated CO reaches a peak at the pressure o 40.9 ka, and the experimental test is about 4 ka. This shows that when the gas supply pressure is.98 ~.05 times o the compressor suction pressure, CO is the highest, and the gas supply port position is reasonable. Capacity(kw) Cal. Capacity Cal. ower Cal. CO Exp. Capacity Exp. ower Exp. CO CO Cas Compensating ressure(ka) Figure. T&E curve in dierent supply pressure Capacity(kw) 600 W/E Capacity W/E ower W/E CO Wo/E Capacity Wo/E ower Wo/E CO Figure 4. The thermal perormance o dierent evaporation temperature Flash type economizer has great impact on the perormance o rerigeration unit. In two dierent working conditions (lash type economizer put into use and all out o use), when the condensing temperature remains at 40 C, the thermal perormance (such as rerigerating capacity, shat power and CO) at dierent evaporation temperature is shown in Fig. 4. Regardless o whether the economizer is put into use, with the increase o the evaporation temperature, the cooling capacity and the compressor shat power will increase, and both are basic linear change. When evaporate at -5 C, the rerigerating capacity o the unit with lash type economizer increased by 0% than without the economizer, shat power increased by 0%, and the cop increased by 7.%. And when evaporate at - 50 C, the rerigerating capacity o the unit with lash type economizer increased by 7%, shat power increased by 8%, and the CO increased by 45.5%. Experimental data show that the capacity and shat power are greatly ower(kw) improved with the use o economizer. With the decrease o the evaporation temperature, the change rate o rerigerating capacity is greater than that o power. Because the CO o the unit with economizer is higher than without economizer, rerigeration perormance o lash type economizer unit is obviously improved. In lower evaporation temperature, that is to say, compression ratio is higher, the greater the CO is. Thereore, in low-temperature rerigeration occasions, rerigerating units using lash type economizer can greatly improve the rerigerating capacity and cooling eiciency o the unit. The change trend o rerigerating capacity and shat power can be explained by theoretical analysis o rerigeration unit. With the change o air supply pressure, air supply low is dierent. When the air supply port is more close to the suction chamber, supply pressure decreases, the gas supply low increases, and the rerigeration capacity increases greatly. And the second stage compression section o the compressor will extend, the increasing amplitude o shat power becomes large. When the air supply port is more close to the discharged port, supply pressure increases, the gas supply low decreases, and the rerigeration capacity decrease greatly. And it will shorten the second stage compression section o the compressor, causing the increasing amplitude o shat power becomes small. So the change trend o rerigerating capacity with supply pressure is the same as shat power. With the increase o the supply pressure, the actual growth rate o polytrophic compression power will become large. Because o the injection o higher pressure gas into the compression chamber, more power will be consumed than the equivalent low pressure gas, thus cause the compressor shat power growth rate becomes larger. However, with the gas low becomes smaller the growth rate o theoretical adiabatic power will become lower. So there must exist an optimal supply pressure makes the most eiciency o the compressor. That is, the greatest ratio o theoretical adiabatic power to the actual polytrophic compression power. The above experimental and theoretical analyses show that the low temperature cooling capacity and eiciency o rerigerating unit are improved obviously with the use o lash type economizer. With the use o lash type economizer system, the scope o low temperature rerigeration has been eectively expanded, and the operation reliability o rerigeration unit greatly improved. 5 Conclusions This paper makes an experimental in thermo perormance o rerigeration unit with lash type economizer using Rerigerant R90. The data show that, using the economizer can eectively improve the unit's cooling capacity and CO, so as to achieve the eect o saving energy. And when the compression ratio increases, the energy saving eect will be more signiicant. The experimental data o the eect o supply pressure on the compressor rerigeration eiciency is consistent with theoretical analysis. For the rerigerating unit, there 4

5 MATEC Web o Conerences 95, (07) DOI: 0.05/ mateccon/ ICMME 06 is an optimal point o economizer supply pressure. The actual supply pressure is slightly higher than this point. For the application o environmental protection rerigerant R90 in rerigerating units, the technology is still in research stage, especially in the lash type economizer. In this paper, theoretical analysis have an important guiding signiicance or the design o rerigerating units with lash type economizer, and provide theoretical basis and practical support or improving the rerigerating unit s perormance at low evaporation temperature in long-term reliable operation. Acknowledgment We grateully acknowledge the support o this eort rom the sponsors o the Rerigeration Technologies and Applications Laboratory o Nantong Shipping College. Reerences. X. Xu, Y. Hwang, R. Radermacher, Transient and Steady-State Experimental Investigation o Flash Tank Vapor Injection Heat ump Cycle Control Strategy, Int. J. Rerig. 4,9-9 (0). J.H. Qiu, Design Analysis o Flash Type Economizer System or Ammonia as a Rerigerant, China Journal o Rerigeration, (), 9- (0). Z.W. Xing, Screw Compressor - Theory, Design and Application, China Mechanical Industry ress, (008) 4. J.J. Tian, Z. Zhang, Experimental Study on Air- Cooled Heat ump on the Frosting Conditions, Rerigeration and Air Conditioning ( China Sichuan), (): (008) 5. L. Yang, W. Wang, Y.F. Bai, Inluence o Economizer on Compression Rerigeration Cycle, Journal o Rerigeration (China), 4, (00) 6. Z. Sun, W. Chen, R. Yang, et al. Research o Rerigeration System with Ejector and Economizer, China Journal o Rerigeration,, 5-0 (00) 7. C. Sun, Numerical Analysis and Study on R4a Applying in Gas Supply on Rerigeration System o Screw Compressor, Compressor Technology (China),, -4 (0) 8. Analysis o Vapour Compression Rerigeration System with R404A, R407C and R40A, International Journal o Mechanical Engineering and Robitics Research, (), H.G. Wu, X.Y. eng, Z.W. Xing,.C. Su, Experimental Study On the Eects o Supereed ressure o Ecomizer on Twin-Screw Rerigeration Compressor erormance, Journal o Rerigeration (China), 4, 0- (00) 0. L. Fu, G.L. Ding, C.L. Zhang, Z.J. Su, Development and Application o Screw-type Water Chiller Steadystate Simulation Model, HVAC, 5(5), 7-75 (005). H.H. Qin, Experimental Study on erormance o Air Cooled Heat ump Device with Flashing System Economics, Journal o rerigeration (China), 4(5), (0. Y.C. Li, Z.H. Cai, Y.F. He, et al. Study on erormance Characteristic o Heat ump with Economizer, Journal o Rerigeration(China), (6), 7-9,, (0). Q.H. Chai, G.Y. Ma, Y. Jiang, J.J. Xia, et al. Thermodynamic Analysis o Scroll Compressor Rerigeration Cycle with Economizer, Journal o Tsinghua University (Natural Science Edition), 4(0), (00) 5