Fundamentals of Refrigeration
Discuss about the Energy Use and Conservation in Domestic Dwelling.
The world energy requirement is growing day by day, with the increase in numbers of electrical appliances to be used in daily life function; the consumption of power per capital has enhanced to a great level. Last ten years has seen a huge rise of energy applications considering the domestic usage. Refrigerators in summer and heat pumps in winter are the necessity of each home in the present era. The refrigeration is the reason for the extra electricity usage in summer and is the reason of most of the load shedding in the developing countries of the world.
Heat pumps are the much better source of producing heat energy in the rooms rather than using the wood furnace or any other system. They constitute the major portion of the domestic power consumption. Three commonly used heat pumps are air to air, water source and geothermal. Water source heaters are used mostly in developed countries where water is heated in a central waste water heater, and high-pressure hot water is supplied for heating for the domestic purposes (Philip and Russell, 1950). The central heating, ventilation, and air conditioning also have a function of providing heat energy with the cooling, and it is becoming the more widely adopted solution for this purpose.
The facts show a distribution of the among fuels been utilized to supply heat energy to homes through heat engines, natural gas, and electricity has the major part as compared with other fuel types (Bellaff, 1980).There are many different methods to provide cooling during summer, but most important sources being utilized during summer are cooling the room air, with proper insulation, effective windows, and doors, shading and ventilation, that can minimize the energy usage during the hottest climate regions of the world. There are specific and principles and rules that are needed to be abided to design an efficient air conditioning system (De Cosimo, 1977)
Hot water is another accessory that is needed for various applications in domestic applications, it is reported that 18% of total home energy is utilized for heating purposes, it is necessary to select an energy efficient water heater in order to keep the energy consumption to its minimum (Lunde, 1980).
There is a big involvement of the pollution and global warming in the different sector of energy conversion, and most bitter impact is for the coal and natural gas-fired power plants that directly inject their harmful smokes and flue gasses into the atmosphere. To control global warming and air pollution, the European energy commission has aimed to reduce the production of the energy through harmful atmospheric components by 2020. Further the European countries must divert 10% of the transportation on the renewable energy (Nagy and Körmendi, 2012).
The study of refrigeration requires following definitions to be understood to clearly understand the topic
The degree of hotness and coldness of a body is measured in terms of temperature. The temperature is calculated in Kelvin.
Joules Thomson Effect
The energy of a body due to random motion of the molecules is called its internal energy.
It is force per unit area. It is measured in Pascals.
Total heat content of a body in measured in terms of enthalpy. Mathematically it is measured as
H= U + PV
U= Internal Energy
Specific heat of a substance
The amount of heat required to raise the temperature of one kg mass of substance to one degree.
The working of the refrigerator is based on Joule Thomson effect that states when the gasses undergoes through sudden expansion through throttling, they cause cooling(Garvey et al., 1983).
Ice refrigerators have been used in the early days of the history, where Ice block is placed inside a closed vessel and air inside the vessel circulate in a cyclic manner inside the vessel to cool the food products. The designing of the domestic refrigerator is much of the same kind, where the heat of the cooling cabin is circulated through the gravity different between hot and cold air. The working of the domestic refrigerator is described in detail as under.
The domestic refrigerator consists of two parts, the Ice makers, and the cooling cabin. The function of ice maker is to freeze any item stored in it, and mostly it is used to freeze the water in the form of ice, because of the lower degree of temperature required in ice maker more cooling is required in this region. The temperature in the cooling cabin is higher than that of the ice cabin; it is used to store the other materials, e.g. milk, water, and other food items. Less degree of cooling is required in this region as the temperature is higher than that of the ice cabin(Laguerre et al., 2002).
The refrigeration cycle is based on the reverse Carnot cycle, which is an ideal theoretical cycle, it consists of four processes(Borgnakke and Sonntag, 2009)
- Isentropic Compression Process
- Isothermal Heat Addition Process
- Isentropic Expansion Process
- Isothermal Heat Rejection Process
The refrigeration cycle requires modification in Carnot Cycle, the direction of all the process is reversed and further modifications are as under(Pita, 1984).
- The isentropic compression process in Carnot cycle is started from saturated vapor line, but in reverse Carnot cycle it is started from the superheated region, it is necessary as it will make the compressor work easily.
- The Isentropic expansion process is replaced with throttling process, where the refrigerant undergoes an irreversible expansion. The isentropic expansion requires larger space which is economically not justified for such a system.
- The isothermal heat addition process causes the change in phase of the refrigerant from saturated line to saturated vapor line, the heat absorption causes. To complete the heat addition at a constant degree of temperature, it is necessary to perform this process in the wet region.
- The isothermal heat extraction process, this process is performed in the condenser where the refrigerant changes its phase from superheated region to subcooled region.
- Components of Refrigerators
The refrigerator consists of following main components; all these components are joined to produced cooling through a thermodynamic cycle known as vapor compression cycle
1) Compressors2) Condensers3) Expansion Valves4) Condenser5) Liquid Line6) Storage Tank7) The refrigerant8) Suction Line.
The domestic refrigerator uses a reciprocating compressor, where the tro and fro motion of the piston of the compressor is utilized for the increasing the pressure of the compressor. The basic purpose of the compressor is to perform isentropic compression process that increases temperature and the pressure of the refrigerant. The domestic compressor is known as the hermetic motor compressor (Elhaj et al., 2008).The compressor is the main source of the input to the refrigeration cycle, as no other component is provided an input power; therefore the proper selection of the compressor is necessary for the efficient operation of the refrigerator.
The most important part of the refrigerator is the refrigerant whose chief function is to take the heat from the evaporator or the cooling cabin of the refrigerator and throw it into the condenser. The most important properties that a refrigerant should comprise are that it should have low boiling point,
- Be safe,
- Be chemically inactive,
- Should not dissolve water,
- Should not impact on ozone and
- Should not cause global warming.
The most commonly used refrigerants are R12, R22, R134a (J Steven Brown PhD, 2009). A number of recent development on the refrigerants is in progress, as the refrigerant containing high percentage of chlorofluorocarbons has been controlled by Montreal Protocol.
The function of the condenser in the refrigeration cycle is to desuperheat, condense, and subcool the refrigerant present in the refrigerant. There are multiple purpose condensers available in the market, but most general are air cooled, water cooled and evaporative condensers, among these air cooled condensers are more suited for the domestic refrigerators (Colburn and Hougen, 1934). The refrigerators are needed to be placed at the windy place to perform the refrigerators operation efficiently.
The purpose of the expansion valve is to keep the low-pressure compartment of the refrigerator separated from high-pressure compartment. Different expansion valves being used are float valve, thermostatic expansion valve, thermoelectric expansion valve, capillary tube and hand operated valves. Capillary tube is most commonly used in the domestic refrigerators (Broersen and Van der Jagt, 1980). The control of the temperature of the refrigerator is the control of the pressure at the exit of the expansion valve, as a change in pressure will always change the temperature at the refrigerant’s boiling point, and the same will be the temperature of the refrigerator.
The function of the evaporators is the change of phase of refrigerant, the expansion valve and evaporator functions simultaneously, once the expansion is complete the refrigerant is into the evaporator where it will absorb heat from the products stored in the refrigerator and change to saturated vapor or superheated vapor. The different kinds of evaporators being used are the bare tube, plate surface, and finned tube evaporators; the finned tube evaporators utilize extra fins that are required to the increase the heat transfer area and heat is transferred in less time (Ayub, 2003). The cooling cabin and ice making cabins have the zigzag piping of the evaporator that the mostly of a conductor material.
The liquid line takes the refrigerant from the condenser to the expansion valve. It also performs the function of the subcooling the refrigerant while reaching towards the expansion valve.
The refrigerant once condensed in stored in the receiver tank, where cooling caused subcooling of the refrigerant (Nakamura, 1992). The tank provides desired space for subcooling the refrigerant if the COP is needed to increase by external means.
The purpose of the suction line is to take the refrigerant from evaporator to the compressor, the extra superheating is done in the suction line, that is necessary for the proper operation of the compressor. The suction line can be covered with insulation to keep the refrigerant from superheating.
The performance of the domestic refrigerator is measured by its coefficient of performance (COP) or Energy Efficiency Ratio (EER) value. The COP is defined as the ratio of refrigeration effect to work of compression, where refrigeration effect is the amount of heat absorbed by one kg of refrigerant from the cooling space and work of compression is the heat equivalent of the compressor.(Blanchard, 1980) The EER value is defined as the ratio between cooling capacity and power input.
The Refrigeration Cycle
Numbers of different developments are already made and others are in progress to increase the efficiency of the vapor compression cycle, some of these are mentioned here.
The coefficient of performance improvement of the vapor compression cycle is due to the increase in the refrigeration effect of the cycle, the two possible ways to perform the subcooling are dedicated mechanical subcooling and integrated mechanical subcooling that employs two reciprocating compressors in different locations in the cycle and that causes an increase in coefficient of performance of the refrigeration cycle(Yu et al., 2007).
The superheating of the refrigerant in the evaporator causes an increase in efficiency of the refrigerator, the extra heat required to superheat the refrigerant is taken from the evaporator and the refrigerant becomes superheated, the increase in coefficient of performance of the cycle is due to the increase in the refrigeration effect of the vapor compression cycle(Selba? et al., 2006).
The liquid suction heat exchanger combines subcooling and superheating, through a heat exchanger, the refrigerant from the receiver tank is subcooled, and the refrigerant from the evaporator is superheated, while passing the heat exchanger, it has different effect on different refrigerants, e.g. coefficient of performance decreases for R22, R32, R717 and the COP increases for R507a, R134a, R12, R404a, R290, R407c, R600(Domanski, 1995).
The improvement in the COP of the refrigerant due to the expansion of the refrigerant through the expender is causes an increases in COP, where the rotor attached to expender rotates with the expansion of the refrigerant while passing through it and this rotation is forwarded to compressor in order to compress the refrigerant that causes decreased voltage requirement for the compressor and overall COP of the vapor compressor cycle increases(Kornhauser, 1990).
It offers increased coefficient of performance, decreased compressor displacement, and decreased compression ratio for the same operating conditions. These benefits are obtained by the addition of equipment that is intrinsically durable and low in cost. The jet ejector is low in cost and able to handle a wide range of multi-phase flows .without damage. It is proposed that an ejector should be used as a refrigerant expander(Hassanain et al., 2015).
The utilization of the solar energy for power production is quite useful and in progress, but the use of solar energy for the cooling and air conditioning is even more effective, it has an advantage of the direct conversion of solar heat into cooling without the conversion of solar energy into electric energy as number of conversions increases the efficiency of the solar energy increases (Kreider and Kreith, 1975).
It is the total cooling load in terms of KW or tone refrigeration that is needed to be absorbed from the evaporator and to throw it outside in the atmosphere. Numbers of different factors are involved in increasing the heat content of the evaporator and are mention below.
- The wall gain load
- The air circulation load
- The Product load
An excel file attached give the detailed calculation of the cooling load for the domestic refrigerator.
The wall gain load
The wall gain load is the amount of heat added into the evaporator through temperature difference between the cooling cabin and the surrounding it is calculated by Fourier law of heat conduction (Xirouchaki et al., 2008)
Where A= Area of the contact between outside air and the outer surface
U= Overall heat transfer coefficient
TD= Temperature Different
The U value depends on the type of the material being utilized for the construction of the walls of the cooling cabin. Different books in the literature have the detailed values of heat conduction coefficient to calculate the value of U-factor. The domestic refrigerator consists of layers of number of materials, e.g. the outer sheet metal of steel or aluminum, with paint on it, in between glass fiber insulating material and inner plastic or metal sheet. The U-factor for a wall consisting of a number of different materials is calculated by following relation (Agrawal and Menon, 1993).
U= 1/(1/f1 + x1/k1+ x2/k2 + x3/k3+ x4/k4….xn/kn+ 1/f0)
Where f1 shows coefficient of convective heat transfer at the inside wall and f2 is the coefficient of convective heat transfer at the outside wall
X= thickness of the conductive materials
K= Conductive heat transfer coefficient.
The wall gain load (Calculate)= 0.684 KW
The Air Circulation
It is due to the bulk movement of the air molecules from surrounding to the cooling cabin especially through the leakages present at the mating surfaces. The air change load is calculated by
Q= m(h0-hi) OR
Q= (Infiltration Rate L/s)(Enthalpy Change kJ/L)
Q= Air change load
H0= Enthalpy of the outside air
Hi= Enthalpy of the inside air
m= mass of the air entering the space
Infiltration rate the total leakage of the air inside the cooling cabin.
The air change load (Calculated) = 0.329 KW
The Product Load
It is the load due to the thermal or heat energy of the products placed inside the cooling region; it is calculated by
Q= mc ?T/ Desired Cooling Time in seconds
Q= The product load
m= Mass of the product stored in the refrigerator
C= Specific heat above freezing in KJ/kgK.
?T= Change in the product temperature
The product load (Calculated) = 4.88 KW.
Factor of Safety
The load is calculated by adding all the loads that can cause a rise in temperature, and the factor of safety equal to 10% of the total cooling load is added to the calculated value, to make up for all kinds of mistakes that could occur in the calculation of the load.
Factor of Safety (Calculated) = 0.589 KW
Total Cooling Load
The total value of cooling load is calculated by considering the running time of the equipment. The equipment is considered to be running for about 18 hours in a day and rest of the time the system is in idle condition. The total refrigeration load is calculated to be 2.5 tones of refrigeration. The tone of refrigeration and kilo watt are related as the following equation
I ton refrigeration = 0.284 KW
The total load can be calculated by the following relation
Q= 24h/RT (Qt)
Total Load (Calculated)= 2.5 Tone of refrigeration.
The paper discusses the importance of the refrigeration system for the daily life usage and method of calculation of the cooling load for a single family home. The precise calculation of the refrigeration system keeping in view the flexible loads that could often incur, are quite important in the energy usage. The world is heading towards energy crisis where the fossil fuels are depleting, and renewable energy resources are focused. The study concluded that the proper usage of the existing sources: decreases the wastes and the losses by precisely using the calculation for the selection of the cooling systems could easily fulfill the world’s energy demand.
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