Physics Concepts - how Refrigerators Work - Report Example

PHYSICS CONCEPTS-HOW REFRIGERATORS WORK A refrigerator is any form of enclosure (e.g. a room or box) whose internal temperature is kept considerably lower than the surrounding environmental temperature. It basically operates on the physics principle of latent heat as well as other physics applications. Several fundamental refrigeration techniques are used: the ice box, cold air systems, vapor compression, vapor absorption and the thermoelectric A typical standard home refrigerator uses the technique of vapor compression. It consists of four main parts: the expansion valve, compressor, condenser, and evaporator. All refrigerators contain a refrigerant i.e. working fluid (Whitman et al p 36). FIG.1 Home refrigerator Vapor compressor This component applies the concept of thermodynamics and gas laws. Electric energy that goes into the refrigerator is converted to work energy used to run the compressor. The compressor adiabatically compresses cold vapor (composed of fluorocarbon gas) from the evaporator. This vapor condenses into liquid, reaching its boiling point and flows into the heat exchanging tubes at the back of the refrigerator. The vapor is cycled to the condenser while the heat goes to the surrounding since it’s not required in the refrigerator. During this process, work is done on the gas due to the pressure applied by the compressor. Temperature of the vapor increases as pressure is applied since Pressure is directly proportional to temperature (pressure law). (Pressure law) FIG.2 Vapor compressor The refrigerant also goes through a change in volume due to increase in temperature in the compressor that forces it into the condenser. This can be explained using Charles law (volume is directly proportional to absolute temperature at constant pressure), (Charles law) The condenser This comprises a set of heat-exchanging tubes with outer fins located behind the refrigerator. Its main purpose is to remove heat emitted as the vaporized refrigerant is condensed into liquid. Some heat is produced as temperature drops to condensation temperature, then additional heat (specifically the latent heat of condensation) is let out as the refrigerant liquefies. Hot vapor from the compressor undergoes isothermal and isobaric condensation, releasing latent heat of vaporization. The condenser tubes are designed and arranged to maximize their surface area to absorb heat from anything stored in the refrigerator. The compressed vapor temperature is higher than room temperature as the vapor goes into the condenser coils, thus heat will flow out of the refrigerant and be taken away by the surrounding air. However, the vapor pressure remains constant as it gets converted into liquid. This stage applies the concept of latent heat of condensation. The condensation process releases heat which contributes to cooling of the system. The vapor refrigerant cools, condenses and turns into liquid, a process which repeats again when the refrigerant is cycled to the expansion valve and evaporator. The electric energy supplied to the refrigerator is also used in running the fan in the condenser. This energy is converted into mechanical energy which rotates the fan that transfers heat produced into the air. Cooling is achieved and helps to condense the hot vaporized refrigerant back into a cooler liquid hence the refrigeration cycle continues. Expansion valve Also known as thermostatic expansion valve, this component of the refrigerator performs following functions: a) It reduces the pressure of the refrigerant from the high condenser pressure to the evaporator pressure. It is modified to form a constriction whish causes the pressure of the refrigerant passing through it to drop suddenly to the level of the evaporator pressure. It applies the pressure law (pressure is directly proportional to temperature), thus temperature of the refrigerant also drops, producing a cooling effect on the evaporator. b) It keeps the evaporator active by regulating flow of the refrigerant according to the cooling load inside it. The flow of the refrigerant is increased at higher load and reduced at lower loads. This allows the evaporator to operate according to specification requirements minimizing wastage of the evaporator capacity. c) By allowing flow of the refrigerant as per the requirements, it prevents flooding of the liquid refrigerant to the compressor and efficient working of the whole refrigerator. The expansion valve applies both the concepts of electricity and pressure to its operation. It is connected though capillary tubing to a sensing bulb which transmits bulb pressure to the top of its diaphragm (Wang, p26). These forms a thermostatic element .The motion of the diaphragm is transmitted to the pin and its carrier by means of pushrods, which allow in and out movement of the pin in the valve port. A spring under the pin carrier and a spring guide puts the spring in position. The spring pressure can be adjusted. The sensing bulb pressure (PBS) acts on the top of the valve diaphragm opening the valve whereas equalizer (PE) and spring pressures (SP) act together below the diaphragm causing it to close. When operating the valve normally, the sensing bulb pressure has to be equal the equalizer pressure plus the spring pressure, i.e. PBS=PE+SP FIG.3 expansion valve The expansion valve functions by controlling the difference between bulb and equalizer pressures by the amount of the spring pressure. The sensing bulb detects the temperature of the refrigerant vapor as it leaves the evaporator. In ideal situation, the bulb temperature and the refrigerant vapor temperature will be equal. Increase in bulb temperature causes bulb pressure to increase thus the valve pin shifts away from the valve port, and more refrigerant flows into the evaporator. The valve continues to open until equalizer pressure increases so the sum of equalizer and spring pressures balance with the bulb pressure and vice versa. Then expansion valve allows cool liquid to expand and evaporate as a gas. The evaporation results in quick drop in temperature and the refrigerator becomes cold. This very cold gaseous refrigerant goes to the heat-exchanging tubes inside the refrigerator. Evaporator This is the component of the refrigeration system that does the actual cooling. It is placed in the area to be cooled and absorbs heat into the refrigeration system the evaporator. The refrigerant flows in, regulated by a flow control device, then released to the compressor (Rathakrishnan, p112). The evaporator works based on the concepts of thermal conductivity of materials and latent heats. It consists of finned tubes that absorb heat from the air blown through a coil by a fan. To maximize on heat transfer by conduction, the material of the fins and tubes metals with high thermal conductivity (Alefeld & Reinhard, p116) .The heat absorbed by the operator is used to vaporize the refrigerant. The evaporating refrigerant absorbs latent heat of vaporization from the system causing a cooling effect. Energy conversions in the operation of a refrigerator Electrical energy is converted into mechanical energy via an electric motor whereas the refrigerant is converted from vapor to liquid and back again by compression. Conversion of liquid to gas is endothermic hence balanced by an exothermic process. The lower temperature source of energy is pumped into a higher temperature heat sink, thus Insulation reduces the work and energy required to achieve low temperatures (Massoud, p49). Overall thermodynamics of a refrigerator after one cycle 1. Change in internal energy =0 2. Total work >0 3. Change in heat >0 4. Total volume change =0 5. Entropy change of the universe > 0 6. Entropy change of the system = 0 7. Change in Gibb’s free energy = 0 Works cited Massoud, Mahmoud. Engineering Thermofluids: Thermodynamics, Fluid Mechanics, and Heat Transfer. Berlin [u.a.: Springer, 2005. Print. Whitman, William C, William M. Johnson, and John A. Tomczyk. Refrigeration & Air Conditioning Technology. Australia: Thomson Delmar Learning, 2005. Print. Rathakrishnan, E. Fundamentals of Engineering Thermodynamics. New Delhi: Prentice Hall, 2005. Print. Alefeld, Georg, and Reinhard Radermacher. Heat Conversion Systems. Boca Raton: CRC Press, 1993. Print. Wang, Shan K. Handbook of Air Conditioning and Refrigeration. New York [u.a.: McGraw-Hill, 2001. Print. Read More