The laboratory session for refrigeration system was performed in order to investigate the main concepts of the vapour-compression refrigeration cycle. Simple refrigeration system studied in the laboratory is a prototype for a variety of engineering applications utilized in industrial, domestic and scientific purposes. The detailed goals include the study of how the refrigeration process is affected by the water flow rate through the condenser and evaporator. The optimal flow rate is determined for condenser and evaporator. Finally, the efficiency of the refrigeration system is analyzed. Theory
Both refrigerators and heat pumps are used to alter the temperature in the specific area. However, the refrigeration systems perform the heat transfer from source with low temperature to sink with higher temperature, while heat pump provides the region with heat. Owing to similarity in general scheme, devices as space cooling are utilized as refrigerator and a heat pump. The efficiency of refrigerators is evaluated in terms of the Coefficient of Performance (COPR).
COPR= Desired outputRequired input= Cooling effectWorking input= QLWnet,in where QL is a heat removed from the low-temperature region and Wnet,in is a required net work input to the refrigerator. The vapor-compression cycle is a popular type of refrigeration cycles. It consists of evaporator, compressor, condenser and thermal expansion valve (Figure 1).
Figure1. Schematic representation and T-s diagram for ideal vapor compression cycle (Cengel, 2009). The behavior of the ideal vapor compression cycle is described by the following processes: (1-2) Isentropic compression.
(2-3) Isobaric heat rejection.
(3-4) Adiabatic expansion.
(4-1) Isobaric heat absorption (Cengel, Y. 2009).
RA1 Refrigeration Unit was utilized for laboratory purposes. RA1 is a vapor compression refrigeration system with sensors on essential parameters of the system. The sensors are connected to specific USB interface devise for recording and saving the data. Also, the key variables could be viewed, extracted and analyzed. Moreover, the operating parameters could be adjusted by the use of the computer programme. Thus, the flow rate for water though condenser and evaporator could be altered. At the same time, readings for the refrigerant flow rate had to be taken on the unit itself and typed into the mimic diagram of the programme.
In order to estimate the effect of the water flow in the condenser on the refrigeration process, in Section A the flow rate of the water was altered, while keeping other parameters constant. The settings were as follows: speed of condenser water pump at 50%, speed of evaporator water pump at70%, compressor motor speed at 80%. Some time was required for system stabilization, after the run. Relatively stable results were recorded by the program. General procedure consisted of the data collection for changes of the velocity in the condenser water pump by 5% until the temperature at the condenser inlet (T4) was 60˚C for other parameters kept stable. In Section B, the cycle was evaluated for changes in the water flow in the evaporation part of the system. All parameters were fixed, except water flow rate through evaporator. This time, the speed of the evaporator water pump was lowered by 5% until the pre-evaporator temperature (T7) was -1˚C. Finally, in Section C, data collected in Sections A and B was analyzed in order to determine the relation between water flow rate and refrigeration system for specific parts by means of appropriate equations.