Experiment 2: Test on Domestic Refrigerator for evaluation of EER. Specifications of Domestic Refrigerator: Model: Videocon refrigerator (single door) Capacity: 150 lit. Compressor: Hermetically Sealed Compressor (1/8 th T.R. capacity) Condenser: Air Cooled Condenser Expansion Device: Capillary Tube COMPONENTS OF REFRIGERATOR SYSTEM There are four major components of vapour-compression refrigeration system. These components are compressor, condenser, capillary tube and evaporator. 3.1 Compressor In a refrigeration cycle, the compressor has two main functions. The function of the compressor is to pump the refrigerant vapour from the evaporator. The second function is to increase the pressure of the refrigerant vapour through the process of compression. Due to this automatically increase in the temperature of refrigerant vapour takes place at discharge end. Discharge pressure is equal to condenser pressure. The most common compressor used in domestic refrigeration is a hermetically sealed reciprocating type. Refer Fig. 1 1. Motor Rotor 2. Motor Stator 3. Cylinder 4. Piston 5. Connecting Rod 6. Shaft 7. Crank 8. Casing 1
Fig. 1 Cut-way of Reciprocating Compressor [4] Fig. 2 Actual photograph of Hermetically Sealed Compressor 3.2 Condenser This is important component of the refrigeration system which needs more consideration in design and construction. The condenser removes heat from refrigerant carried from evaporator and added by compressor. Condensor converts the vapour refrigerant into liquid refrigerant. It is a heat exchanger in which heat transfer takes place from high temperature vapour to low temperature air or water as per the cooling medium. There are different types of condensers and selection of condenser depends upon the capacity of the system, refrigerant used and medium of cooling available. Types of Cooling Medium Available The cooling mediums available by nature are air and water. Both can be used as a cooling medium either separately or combined as per the availability and the requirement. The condensing temperatures adopted with air as cooling medium is 55 0 C to 60 0 C where with water it is 31 0 C to 37 0 C. The temperature of water cooled condenser also depends upon the purity of water. Few more factors considered with air or water cooled condenser are listed below: 2
Quantity of cooling medium. Area of cooling medium. Velocity of cooling medium. Type of refrigerant. Purity of refrigerant. Classification of condenser Based on the external fluid, condensers can be classified as: a) Air cooled condensers b) Water cooled condensers c) Evaporative condensers a) Air-cooled condensers As the name implies, in air-cooled condensers air is the external fluid, i.e., the refrigerant rejects heat to air flowing over the condenser. Air-cooled condensers can be further classified into natural convection type or forced convection type. Natural convection type Fig. 3 Schematic of a wire-and-tube type condenser used in Domestic Refrigerator [4] 3
In natural convection type, heat transfer from the condenser is by buoyancy induced natural convection and radiation. Since the flow rate of air is small and the radiation heat transfer is also not very high, the combined heat transfer coefficient in these condensers is small. As a result, a relatively large condensing surface is required to reject a given amount of heat. Hence these condensers are used for small capacity refrigeration systems like household refrigerators and freezers. Flat black or Plate Surface Type Condensor The natural convection type condensers are either plate surface type or finned tube type. In plate surface type condensers used in small refrigerators and freezers, the refrigerant carrying tubes are attached to the outer walls of the refrigerator. The whole body of the refrigerator (except the door) acts like a fin. Insulation is provided between the outer cover that acts like fin and the inner plastic cover of the refrigerator. It is for this reason that outer body of the refrigerator is always warm. Since the surface is warm, the problem of moisture condensation on the walls of the refrigerator does not arise in these systems. These condensers are sometimes called as flat back condensers. The finned type condensers are mounted either below the refrigerator at an angle or on the backside of the refrigerator. In case, it is mounted below, then the warm air rises up and to assist it an air envelope is formed by providing a jacket on backside of the refrigerator. The fin spacing is kept large to minimize the effect of fouling by dust and to allow air to flow freely with little resistance. In the older designs, the condenser tube was attached to a plate and the plate was mounted on the backside of the refrigerator. The plate acted like a fin and warm air rose up along it. In another common design, thin wires are welded to the serpentine tube coil. The wires act like fins for increased heat transfer area. Figure 3.3 shows the schematic of a wire-and-tube type condenser commonly used in domestic refrigerators. Regardless of the type, refrigerators employing natural convection condenser should be located in such a way that air can flow freely over the condenser surface. 4
TEST PROCEDURE To carry out test on domestic refrigerator we have followed certain steps. The sequential test procedure is explained as below: 1. Fill the vessel with the known quantity of water. 2. Switch on the main supply, temperature indicator. 3. Measure the suction and discharge pressure of refrigerant with the help of pressure gauges. 4. Initially take energy meter reading for compressor work. Also measure the initial temperature of water in the evaporator cabin. 6. Measure the evaporator (L.P.) and condenser (HP) pressures of the refrigerant. Energy Efficiency Ratio (EER) We have defined COP of a refrigerator system as the ratio of refrigerating effect to the work input. It is a dimensionless quantity. However, in U.S.A the performance of refrigerator and air conditioning system are expressed in terms of EER. The energy efficiency ratio is defined as, the amount of heat removed from space in B. Th. U. (British Thermal Unit) in a HVAC (Heating, Ventilation & Air-conditioning) system per Watt hour of electric energy consumed. Mathematically, EER =Heat removed in B. Th. U. / Energy consumed in Watts- hour =Heat removed in B. Th. U. per Hour / Energy consumed in Watt 1 Wh = 3.412 B. Th. U. Or 1 kwh = 3412 B. Th. U. Therefore a refrigerator which removes heat 1Wh of energy will have COP as 1. Therefore, EER = 3.412 * (COP)R 5
Energy Efficiency Ratio (EER): Performance of smaller chillers and rooftop units is frequently measured in EER rather than kw/ton. The higher the EER, the more efficient the unit. in kj/h at full-load conditions. kw/ton rating: Lower kw/ton indicates higher efficiency. 1 Btu/h = 1.055056 kj/h; 1 kj/h = 0.947817 Btu/h OBSERVATIONS For ------ ml of water: Initial Temperature of Water: Time: Final Temperature of Water: Natural Convection Condensor Observation Table: Calculate Theoretical COP Calculate Actual COP: Represent the cycle on P-h chart. 6