An experimental investigation on the cylinder pressure and valve displacement of reciprocating compressors

Pistonlu kompresörler soğutma, iklimlendirme ve proses endüstrileri başta olmak üzere bir çok farklı alanda havanın veya soğutucu akışkanın basıncını arttırmak amacıyla kullanılmaktadır. Bu kompresörlerde emme ve egzos valf yapraklarının zamana bağlı hareketi nedeniyle oluşan basınç salınımları, hem kompresörün enerji verimliliğini veya soğutma etkinlik katsayısını (SEK), hem de kompresör ses gücü düzeyini önemli ölçüde etkilemektedir. Bu etkileri incelemek amacıyla yapılan çalışmanın burada sunulan ilk kısmında, soğutucu akışkan kompresörlerinde silindir içerisindeki basıncın ve valf yaprağı deplasmanının zamana bağlı değişiminin deneysel olarak incelenmesinden elde edilen bulgular tartışılmıştır. Bu deneysel çalışmalar şu başlıklar altında sıralanabilir: i-) “Kompresör Kalorimetresi” adı verilen bir test sisteminden yararlanılarak, belirli şartlar altında kompresör soğutma kapasitesinin ve giriş gücünün ölçülmesi; ii-)Optik kodlayıcı ve yüksek frekanslı basınç sensörlerinden yararlanılarak, silindir içerisindeki basıncın kompresör krank açısına bağlı olarak ölçülmesi. 50 Hz nominal çalışma frekansında sıkıştırma yapan bir kompresörde, genişleme, emme, sıkıştırma ve egzos safhalarından oluşan bir tam çevrim yaklaşık 20 ms sürmekte ve bu sürenin yarısından daha az bir zamanda, emme valf yaprağı birden fazla kez açılma kapanma hareketi yaparak silindire gaz geçişine izin vermektedir. Bu nedenle, emme valf yaprağı hareketinin kompresör krank açısına bağlı olarak ölçülebilmesi için de bir yöntem geliştirilmiş ve örnek bir model kompresörde uygulanmıştır.   Anahtar Kelimeler: Pistonlu kompresör, silindir basıncı, valf yaprağı titreşimi, pulsatif akış.The energy efficiency and the sound power level of hermetic reciprocating compressors used in refrigeration and air conditioning applications are effected by the pressure pulsations caused by the vibrations of valve leaves during the suction and discharge phases. In the first part of this study a compressor calorimeter set-up is used to measure the refrigeration capacity and the input power of a specific compressor model at standard rating conditions. High frequency pressure transducers and an optical encoder are used to measure the cylinder pressure as a function of compressor crank angle. A method to measure the suction valve displacement as a function of crank angle by standard strain gauges is also developed and applied to the compressor model used in the study. The compressor calorimeter set-up used to measure the mass flow rate and input power of compressors at specified operating conditions is indeed a basic refrigeration circuit equipped with different pressure, temperature and power measurement devices. The suction and discharge pressures of the compressor which correspond to the evaporation and condensation temperatures in the refrigeration circuit are measured by pressure transducers. The subcooling after the condenser and the superheating at the end of the evaporator are measured by thermocouples where these temperatures are 32.2°C at the standard ASHRAE test conditions. The temperature at the inlet of the compressor is also 32.2°C and by specifying these values the vapor compression cycle is fully determined. The refrigeration capacity of the compressor is determined via two different methods and the relative difference between the results should be lower than 3 % for a stable test. The first method is to measure the electric power utilized by the heater which is placed in a well insulated shell with the evaporator where the second method relies on the measurement of the mass flow rate of the compressor. The input power of the compressor is directly measured and the coefficient of performance (COP) for the compressor can be calculated from the refrigeration capacity and input power. The refrigeration capacity and input power of the specific compressor model under consideration are found to be 189.01 W and 127.06 W with standard deviations of 0.38 and 0.26 % respectively. In addition to the calorimeter set-up a specific test system is used to measure the instantaneous cylinder pressure as a function of compressor crank angle. An optical encoder which gives 360 signals per revolution is placed at the top of the crank shaft of the compressor and a specific pressure transducer is placed in the valve plate where it can detect the cylinder pressure. Another pressure transducer is also placed in the suction plenum of the compressor in order to see the pulsation effects caused by the suction valve movement. The nominal running frequency of the compressor under consideration is 50 Hz which corresponds to a 20 ms cycle time where the complete cycle consists of four processes: expansion of the gas in the dead volume, suction of fresh to the cylinder from the plenum, compression of the gas in the cylinder and finally the discharge to the discharge plenum. Since the complete cycle takes only 20 ms, the suction phase where the suction valve opens and closes several times per one cycle takes 8 to 9 ms. Therefore a strain-gage was used to investigate the suction valve leaf vibration behavior as a function of the compressor crank angle. Both the cylinder pressure and valve displacement measurements were carried out at approximately 18 kHz. The results of the tests conducted for the cylinder pressure can be used to identify the losses caused by the suction and discharge port and valve leaves and it can be concluded that the experimental indicator diagram of reciprocating compressors is an important tool for research activities to develop high efficiency compressors. The results of this study showed that the pressure drop losses associated with the suction and discharge port and valve leaves consist 5.1 % of the net piston work done on the refrigerant gas for the compressor model under consideration. Though the test carried out both with the cylinder pressure transducer and the strain-gage shows that the 60 mm thick strain-gage has an effect on the pressure pulsations, and hence the force exerted on the valve leaf, the general vibration characteristics of the valve leaf can be observed with such a method. Keywords: Reciprocating compressor, cylinder pressure, valve leaf vibration, pulsating flow