Hermetik kompresörlerde kullanılan elektrik motorlarının kompresörler içi ısı geçişi açısından deneysel ve analitik olarak incelenmesi

Abstract

Hermetik pistonlu kompresörler, buhar sıkıştırmalı soğutma sisteminin en temel elemanlarından birisidir. Küçük boyutları nedeniyle uzun yıllardır ev tipi buzdolaplarında yoğun olarak kullanılmaktadır. Yılların getirdiği birikimle olgunlaşan teknoloji ve dünya çapındaki rekabet koşulları sayesinde gelişimini hızla sürdürmektedir. Gelişen teknolojiye bağlı olarak kompresörlerdeki mekanik, elektriksel ve termodinamik kayıplar azalmaya ve yüksek performanslı kompresör üretimi hız kazanmaya başlamıştır. Kompresörün toplam kayıpları içerisinde elektrik motorundan kaynaklanan kayıplar önemli yer tutması nedeniyle bu tez çalışmasında hermetik kompresörlerde kullanılan elektrik motoru, kompresör içi ısı geçişi açısından deneysel ve analitik olarak incelenmiştir. Tez çalışmasının ilk bölümünde kompresör ve elektrik motorunda ısı geçişine yönelik çalışmalar paylaşılmıştır. İki konu üzerinde literatürde fazlasıyla makale bulunmasına rağmen elektrik motorunun kompresör üzerindeki etkisini özel olarak inceleyen çalışmalara birkaç makale dışında pek rastlanılmamıştır. Tez çalışmasının ikinci bölümünde mevcut kompresörde sıkıştırmaya ayrılan iş, kayıplar ve performansı ayrıştırmak adına yapılan PV ölçüm çalışmaları, kompresör geneli ve motor özelinde yapılan detay sıcaklık çalışmaları ve motor performansının görülebilmesi için yapılan motor testleri paylaşılmıştır. Tez çalışmasının son kısmında öncelikle elektrik motoru analitik olarak modellenerek mevcut durum ve soğutma yöntemleri uygulandıktan sonraki sıcaklıkları parametrik olarak elde edilmiş, sonrasında kompresör genelinde oluşturulan analitik modelle motorun kompresör genelinde sıcaklık değişimine etkisi incelenmiştir.

Compressors are mechanical devices that increase the pressure of the gases. There are many types of compressors that have different working principles like screw compressors, scroll compressors, rotary compressors, linear compressors, reciprocating compressors etc. They have many application fields and they are widely used in industry. In this thesis hermetic reciprocating compressors and experimental-theoretical investigation of induction motor's effects are studied. Hermetic reciprocating compressors are the main units of the vapor compression refrigeration systems. These compressors are commonly used due to its smallness and compactness. Especially, the majority of household refrigerators have these type of compressors. In recent years, environmental concerns and energy consumption become more important. Therefore, compressor manufacturers try to design high-efficiency reciprocating compressors. Owing to developing technology, mechanical, electrical and thermodynamic losses of hermetic compressors are reduced. Nevertheless, the effects of electrical motors are significant phenomena for hermetic compressors. The thermal behavior of an electrical motor is very important for motor performance, but also has a key role for compressor components heat transfer characteristics. In the first part of this thesis the previous investigations about heat transfer of hermetic compressors and induction motors are presented, important articles and patent applications are summarized. Although there are many articles about hermetic compressors, and also induction motors, there are very few combined studies which are used both these two topics together. Most of the studies about hermetic compressors are focused on heat transfer characteristics among the compressor components and motor is thought just a lumped parameter. In these articles, there are many correlations to calculate heat transfer coefficients and predict temperature distribution inside the compressor. Beside these studies, patent search is given in the first part of this thesis. It can be easily seen from European Patent Office web site that, there are thousands of invention patent applications about hermetic compressors and electrical motors. However, similarly the articles, very few of them are about the motors of hermetic compressors. The patent applications that are given in the first part of this thesis are about cooling the motor inside the compressor housing. For this aim, mostly coolant fluid and oil are used to reduce motor temperatures. In the second part, the heat transfer between the components of the compressor and the electrical motor of the hermetic reciprocating compressor and also its effects are studied experimentally. The compressor performance, indicator diagram, motor tests and the detailed temperature measurements of electrical motor and compressor components are given. Experimental part of the study begins with calorimeter device introduction. To measure compressor performance, calorimeter devices are used. With these devices, cooling cycle is controlled and performance parameters like cooling capacity, input power and coefficient of performance (COP) can be specified. In this thesis, compressor measurements were done for ASHRAE working conditions. Evaporation pressure and temperature were fixed at 0.624 bar and -23.3 °C, condensation pressure and temperature were fixed at 7.61 bar and 54.4 °C. To get solo performance of compressor motor, motor measurements were done. With these measurements motor torque, power and efficiency according to the speed are determined for cold and hot conditions. Hot and cold tests were done between 2895 rpm to 2970 rpm. The increase of efficiency was seen until 2930 rpm and 2940 rpm in cold and hot condition tests respectively. The most efficient intervals were determined from 2920 rpm to 2925 rpm (87.4 %) and from 2930 rpm to 2940 rpm (86.2 %) in cold and hot conditions respectively. In cold condition tests, the torque of motor was changed 0.304-0.252 Nm, while the efficiency of motor were maximum (on the 2920-2925 rpm interval). Similarly with torque, according to increasing the speed, the power of motor was decreased. The power of motor were changed from 100.8 W to 95.3 W and from 100.5 W to 89.7 W in cold condition tests (2920-2925 rpm interval) and hot condition tests (2930-2940 rpm interval) respectively. Indicator diagram of compressor is used to calculate the energy which transferred to the refrigerant. It was measured by using optical encoder and pressure transducers. A compressor model geometry was modified and pressure transducers were placed inside valve plate (for cylinder pressure measurements), cylinder head (for discharge plenum measurements), discharge muffler and suction plenum. An optical encoder was placed top of the shaft and linked with a coupling. While the modificated compressor was running on calorimeter, pressure and angle data were collected. According to the test results, compression work of the compressor was found 81% of the input power. The temperature distribution in hermetical reciprocating compressor directly effects compressor COP and also motor efficiency. To measure average temperatures of compressor components and electrical motor, measurement points were determined and several thermocouples were placed on these points. Temperature data were collected by data acquisition system while compressor was running on calorimeter device. In order to get temperature distribution of electrical motor, 40 thermocouples are located on the lamination and windings. The test results show that motor temperatures almost in the same range. There are no significant differences between points. Moreover, lamination and winding temperatures are very close. After motor temperature measurements, compressor was modified again and new thermocouples were used to specify temperature of hermetic compressor components. According to these measurements, as expected, the hottest components were found discharge plenum and cylinder head, the coldest component was found suction muffler. The other components and fluid temperatures were measured and temperature distribution of compressor was determined. At the end of the experimental studies section, a motor cooling method is presented. In order to increase motor performance heat pipes were used. Heat pipes are passive heat transfer devices that generally used in electronic industry for cooling. A heat pipe is one of the most effective device for transport thermal energy between two points. They have simple structure and have no moving parts. Inside the heat pipes generally water or acetone are used. The fluid inside the pipe picks up heat and evaporates and then the vapor moves to the other end of the pipe. At this end of the heat pipe, the vapor releases the heat and condenses. After condensation process the fluid turn back to the evaporation end thanks to capillary effect or gravity. All advantages of the heat pipe make it useable inside the compressor housing. Two heat pipes were located on the motor lamination to pick up heat from surface and release it to the oil. In addition to these two heat pipes, another heat pipe was used for cooling oil which was heated by the other two heat pipes. Calorimeter results show that heat pipes were reduced input power of the compressor nearly 1 W, COP of the compressor was increased 0.017 W/W. In the last part of the this thesis, analytical models of induction motor and compressor components are presented. This part describes analytical models to predict the temperatures of motor and the other parts of hermetic reciprocating compressor. By using Motor-CAD and Matlab softwares, the behaviors of compressor components with different motor temperatures are investigated. For motor analytical model, the fluid around the motor was modeled as R600a (isobutane) refrigerant. Motor speed was entered 2937 rpm. This speed value was taken when compressor run in calorimeter for ASHRAE conditions. After modeling studies, model was run by Motor-Cad software and temperature distribution was determined. According to the results, similarly experimental measurements, temperature differences between points are almost negligible. In addition to this baseline simulation, the motor cooling methods which available on Motor-Cad software were applied. In the first, spray cooling method was used. This model shows a design that end winding of stator is cooled by oil passing dawn the shaft channel and firing it to the end windings. According to the spray cooling method simulation results, motor temperatures were reduced about 6-7 °C. After spray cooling model, spiral water jacket was modeled and parametric study was done for different flow rates and channel sizes. Motor water jacket is a motor cooling method that spiral shape channels surround the motor laminations and water flows inside these channels. The simulation results show us, by using water jacket on electric motor laminations, motor temperatures can be reduced effectively. Different flow rates from 10-7 m3/s to 8.10-7 m3/s were reduced temperatures from 77 °C to 73 °C and 63 °C respectively. It can be easily seen from simulation results that channel sizes of spray cooling design did not affect motor temperatures as much as flow rates. Besides the spiral water jacket model, R600a was used as fluid instead of water. Although motor temperatures can be reduced by using R600a, the flow rates are limited by compressor model and performance. The last motor cooling model, axial water jacket is presented after spiral water jacket method. The procedure which applied for spiral water jacket was repeated, same flow rates and same channel sizes were applied for this model. According to the results for the same flow rates, motor temperatures were found lower than spiral water jacket model. For example, for 8.10-7 m3/s flow rate, motor temperatures were reduced to approximately 54 °C. In the end of the third part of this thesis, analytical model which was build to predict compressor component temperatures is presented. Compressor was divided into different control volumes and conservation of energy was used for each of them. At first, model was simulated and temperatures were determined for original situation for compressor model. After that motor temperatures were reduced and temperature differences of compressor components were investigated.