Numerical and experimental investigation of increasing cooling capacity for compact variable capacity compressor

Demand for high efficiency in a compact variable displacement compressor is a necessary requirement for cooling applications as a result of increased worldwide electricity consumption and environmental awareness. The refrigerator is one of the most common household appliances in the world. With an annual average of 450 kWh of electricity consumption, it accounts for almost 14% of the total electricity consumption in the residential sector and causes an annual CO2 emissions of 450 million tons. The use of advanced technologies such as advanced insulation, efficient compressor and system optimization provides energy savings of 25%, 33% and 42%, respectively. Basically, compressors determine the performance and efficiency of the cooling system. This basic component of the refrigeration system consumes electrical energy and converts it to the energy stored in the evaporated refrigerant fluid. Despite the fact that there are many different types of compressors to achieve this goal, the hermetic reciprocating compressor in the competitive environment as a result of globalization is the most widely used type of compressor in domestic refrigerators. The overall efficiency of a compressor depends on three factors. These are electrical motor, mechanical and thermodynamic efficiencies. Current efficiency levels of reciprocating compressors which are used in household refrigerator describes the efficiency of the electric motor is 87-88%, the efficiency of mechanical systems is up to 92% and thermodynamic efficiency is much lower and is generally between 80% and 83%. Numerous researches have been made on the thermodynamic losses of most losses in compressor efficiency. According to Ribas et al. (2008), it has been found that about 47% of the thermodynamic losses are due to suction and discharge operations due to friction losses along the flow path from the suction line to the cylinder and the cylinder discharge line, respectively. A small amount of energy loss (about 4%) is due to leaks in the piston cylinder cavity. On the other hand, a significant part of the energy losses are related to the overheating of the refrigerant flowing along the suction line and within the cylinder. After overheating at the evaporator outlet in the cooling system, the refrigerant from the compressor is sent to the condenser through the suction muffler, suction plenum, cylinder, exhaust plenum and exhaust muffler, respectively. The element that transmits the refrigerant from the suction line to the compression zone is called the suction muffler. This component has many important functions for the compressor to work satisfactorily, such as guidance to the refrigerant, acoutic damping and, in some cases, thermal insulation of the gas drawn by the cylinder. Preventing of heating up of the refrigerant is important to increase the volumetric and energy efficiency of the compressor. To ensure adequate thermal isolation, existing mufflers are usually manufactured from a material with low thermal conductivity. The material of suction muffler in this study is polybutylene terephthalate material used in injection molding for engineering applications. Refrigerant temperature increases in suction line between the suction muffler inlet and outlet due to the heat transmitted to the compressor from various heat sources within the compressor. The temperature increase of the gas causes an increase in the specific volume of refrigerant, and therefore the mass of refrigerant pumped by the compressor is reduced. Since the cooling capacity of the compressor is directly proportional to the mass flow, the reduction of the mass flux results in loss of capacity. Control factors in the design of a suction muffler are two variables, "flow rate" and "pressure drop", depending on the boundary condition [6]. In this study, a prototype muffler model was developed as an alternative to the existing muffler model for reducing mechanical and thermodynamic losses. A design change was made in the flow routing channel, one of the muffler parts. Then the pressure drop between the muffler inlet and outlet for both muffler models was analyzed numerically. Computational fluid dynamics (CFD) analyzes were performed in a commercial code. First, the flow volumes of the geometries were created. When the flow volume was being created, the sharp edges joining the narrow sides of the geometry and the skewed surfaces that would not affect the flow were matched to the CFD analysis. Then, by creating a solution network, the physical component is divided into a small number of small elements. The purpose of this is to create a solution network that will provide the convergence criterion by facilitating the analysis of the solution by using the differential equations of mass, momentum and energy conservation used in the numerical method. Then, boundary conditions were created and model was analyzed. Experimental method was used to validate the numerical examination from different aspects. The effect of the new suction muffler on the mass flow rate and capacity was investigated by performing compressor calorimetry tests for the existing suction muffler and prototype muffler models produced. The tests were carried out under ASHRAE conditions of -23.3 °C (Evaporation temperature) and 54.4 °C (Condensation temperature). The compressor was tested at 1300, 3000 and 4500 rpm. In the second part of the work, a special insulated valve plate design was placed between the cylinder head and the valve plate to prevent the heat transfer to the refrigerant flow in the suction plenum zone at the exit of the suction muffler. When the coolant is compressed by the piston and reaches high pressure, it is directed to the exhaust plenum and the pressure difference between the inside of the compressor housing and the exhaust plenum increases. In this case, the coolant tries to pass through the cylinder head and valve plate walls through the outer chamber. The resultant pressure loss of the refrigerant into the enclosure occurs and the refrigerant pressure to the exhaust muffler falls. In this case, the desired pressure and temperature values are not obtained in the refrigeration cycle under certain operating conditions and the compressor capacity is reduced. In order to prevent this situation, it is aimed to prevent the pressure losses in these regions by the insulated valve plate placed between the cylinder head and the valve plate and to reduce the heat transfer to the refrigerant with the relatively lower temperature taken into the cylinder volume during the suction process. It is also contemplated that the minimization of the gas leakage and the removal of the sealing element used between the cylinder head and the insulating valve table results in a cost advantage. Within this scope, it is aimed to increase the compressor volumetric efficiency and overall performance. According to the researches carried out in the literature, similar applications are found in the compressor cylinder head, suction and exhaust plenum and valve plate areas. In patent application WO2007068072, the exhaust plenum is intended to reduce heat transfer by valve plate by taking a single closed volume. Here, the valve table is formed with venting protrusions in the exhaust port periphery and bolt holes. The exhaust plenum is in the form of a chamber in a closed volume within the cylinder head. Exhaust plenum is left open to allow the fluid passing through the exhaust port to fill the exhaust plenum. Thanks to the protrusions formed on the valve plate, the hot zone of the valve plate has been reduced to a minimum. Also, as another design idea, the gap created before the exhaust port around the valve plate in the valve plate passes through the space after the gasket. The part provided as a cylinder sheath is attached to the cavity apparatus and forms an exhaust plenum as a closed volume. The cylinder head which is given as an outer jacket is passed to this assembly. In the patent application WO2012166051, an invention is presented for reducing the heat transfer on the valve plate and on the body-cylinder side. One of them had a plate with low thermal conductivity and two plates of valve plate. Thermal insulator plate and metal plate are combined by pressing, injection molding, induction heating, ultrasonic welding or gluing methods. The thermal insulation plate reduces heat transfer to the metal valve plate and the body from the exhaust plenum. In the same invention sandwich plate application is included. Here, the plate of metal valve plate is placed between two thermal insulated plates. In patent application WO2010133503, it is aimed to reduce the heat transfer between intake and exhaust plenums using a plastic cylinder head. Since the strength of the plastic cylinder head will be lower than metal, the metal support frame is passed through the cylinder head walls. The cylinder head is torqued into the body by passing bolts through the bushing portions of the metal support. As a result, both the plastic cylinder head's strength is increased and the plastic material with low thermal conductivity is considered to have less heat in the intake plenum fluid. The solid model of the insulated valve plate design was produced in the drawing program according to the determined measurements in the area to be placed in the compressor, and then the part was produced with a material determined with a low heat transfer coefficient. Then the part is assembled in the compressor and adjusted to the test setup. In the experiments performed in the compressor calorimeter test chamber, two different test equipments were prepared by using the model which is used with the existing model and the specially insulated valve table. The performance measurements were carried out under ASHRAE conditions of -23.3 °C (Evaporation temperature) and 54.4 °C (Condensing temperature). The compressor was tested at 1300, 2100, 3000 and 4500 rpm operating conditions. The effect of the newly created design on capacity and SEK (Coefficient of performance) was studied. For future work, the mass flow rate and acoustic performance of the suction muffler can be improved with some modifications in the design of the flow-guided channel and silencer chambers. On the other hand, in order to increase the positive effect of the insulated valve plate on the cooling capacity of the compressor, experimental studies can be carried out by trial production with different materials with low heat transfer coefficient used in engineering applications. In addition, a number of modifications to the solid model of the design can be included, as well as new designs in which the thermal protection in the refrigerant entering the cylinder at the suction plenum exit can be increased.