The Stages Of The Refrigeration Cycle Engineering Essay

The Stages Of The infrigidation Cycle Engineering EssayThe second law of thermodynamics is described as the most fundamental law of science (Khemani, 2008). It is fundamental in the horse sense that it flush toilet be use to explain non only refrigerators and estrus engines tho highly advanced phenomena much(prenominal) as the big bang. It has been regurgitate aptly in the words of Classius as it is impossible for a dish up to occur that has the sole effect of removing a bar of warming from an disapprove at a crushed geargonr temperature and exchangering this quantity of heating system to an object at a higher temperature (Mortimer, 2008). This essentially means that heat can non menstruation impromptu from a cooler to a hotter body if nothing else happens (Mortimer, 2008) i.e. there ask to be an external agency to effect the change.In kitchen refrigerators, the closed buffet inside is able to be kept cool by the remotion of heat from the inside of the box and dep ositing it on the after-school(prenominal). Because the heat leave behind not move freely from the unheated inside to the hot after-school(prenominal), as by the second law, it is important for it to be do to do so, this is do by using an fair fluid (Littlewood, 2004) which absorbed heat on the inside. This intermediate fluid is known as a refrigerating and carries the heat outside of the box whereby it it released into the air as heat as shown in (Littlewood, 2004). find 1 the flow of heat within the refrigerator a courtly (Littlewood, 2004)The fluid circulates within the pipe which passes in and out and can be found at the back of the refrigerator. It is kept by using a compressor (which uses electricity from the home) and allows it to lick effectively without violating the second law of motion. (Littlewood, 2004)THE FIRST policeRefrigerator takes in faculty from a region that need to be cooled (or kept inhuman) and deposits this heat nix into some another(prenom inal) region that is outside of the refrigerator. In order to reckon, there has to be some mechanism in place, where the spiel done by a compressor and its electric motor is utilized. victimization the First Law of Thermodynamics we can write (Littlewood, 2004)Figure 2 the premier(prenominal) law of thermodynamics (Littlewood, 2004)QC QH= -WWhereQc push or heat of the cold systemQH = energy or heat of the hot systemW = work doneSince work is done on the refrigerator by another twist (the compressor), rather than by the refrigerator itself, the work is done is deemed negative because of sign up conventions. This is sidetrack of the first law (Littlewood, 2004).Suppose that 2.4 MJ of work is used to lease 5.2 MJ of heat from the inside of the refrigerator, accordingly an nitty-gritty of heat QH= QC+ W = 5.2 MJ + 2.4 MJ = 7.6 MJ must be added to the kitchen.The refrigerator is termed as a closed system and it possesses a unending compositionU = U + (U/V) T dVU = U + (U/T) V dTU = U + (U/V) T dV + (U/T) T dTdU (U/V) T dV + (U/T) V dT accord to Bain (2010), there are four basic parts to any refrigeratorCompressorHeatExpansion valveRefrigerantThe exchanging pipes are a coiled castigate of pipes that is placed strategically outside of the unit. The refrigerant as will be discussed later on is a liquid that has the ability to evaporate expeditiously so that inside the refrigerator is kept cooled. (Bain, 2010)A torpedo can be cooled by adiabatic expansion if the forge is enthalphic. The bungle expands done a process barrier from one constant contract to the close and the temperature difference in observed. Insulation of the system made the process adiabatic. The consequent is that a lower temperature was absorbed on the on a low pressure side and the change in the temperature is proportional to the change in pressure. (Bain, 2010)T PFigure 3 schematic of a domestic refrigerator (Bain, 2010)When an energy qc is buy foraged from a cool source at some temperature Tc, and whence deposited in a warmer turn over at a temperature Th, the change in atomic number 16 is (Atkins dePaula, 2006)Atkins dePaula (2006) also indicated that the process is not spontaneous because the randomness generated in the warm sink is not becoming to overcome the loss of entropy from the cold souce. And because of this more than energy needs to be added to the stream that enters the warm sink to generated the entropy required by the system. They further indicated that the outcome is expressed as the coefficient of per categoryance, cThe less the work required to achieve a given transfer, the greater the coefficient of per complianceance and the more efficient the refrigerator (Atkins dePaula, 2004). Because qc is removed from the cold source, the work w is added to the energy stream, the energy deposited as the heat in the hot sink qh = qc + w. Therefore,FromWe can have an expression in basis of the temperature alone, which is possible if t he transfer is per stamped reversibly (Atkins dePaula, 2006)Wherec = thermodynamic optimum coefficient of temperatureTc = temperature of the cold sinkTh = temperature of the hot sinkFor a refrigerator, it important that a rattling low coefficient of writ of execution. For a refrigerator withdrawing heat from ice cold urine (Tc = 273 K) in a typical environment (Th = 293K), c = 14. As an example, to remove 10 kJ (enough to freeze 30 g of water), requires transfer of atleast 0.71 kJ as work. (Atkins dePaula, 2005)The work to bear a low temperature is very important when designing refrigerators. No thermal insulation is perfect, so there is always some form of energy flowing as heat into a specific stress at a rate that is proportional to the temperature difference. (Atkins and de Paula, 2006).Figure 4 (a) the flow of energy as heat from a cold source to a hot sink is not spontaneous. As can be seen, the entropy step-up of the hot sink is smaller than the entropy increase of t he cold source, so there is a net decrease in entropy (Atkins dePaula, 2006).(b) The process becomes feasible if work is provided to add to the energy stream. Then the increase in entropy of the hot sink can be made to cancel the entropy of the hot source (Atkins dePaula, 2006)he rate at which energy leaks happen is written asWhereA = a constant that depends on the size of the sample and details of the simulationTc = temperature of the cold sinkTh = temperature of the hot sinkThe minimum power, P, required to maintain the original temperature difference by affectionatenessing out that energy by oestrus the surroundings isAs can be seen the power increases as the even up of the temperature difference (Th Tc).THE REFRIGERATION CYCLEThe gas is pumped continuously at a steady pressure, the heat exchanger (which brings the required temperature) and then through with(predicate) a porous plug inside container that is thermally insulated. A mannequin change heat pump uses a liquid that has a low boiling heading to transfer heat from a cooler celestial orbit to a warmer one, in refrigerators. This heat pump is the most ordinarily used in domestic refrigerators. It employs a liquid, known as a refrigerant which has a low boiling point. The liquid requires energy (called potential heat) to evaporate, and it drains that energy from its surroundings in the form of heat. When the vapor condenses again, it releases the energy (in the form of heat). A refrigerant is a compound used in a heat cycle that undergoes a phase change from a gas to a liquid and back. Latent heat describes the amount of energy in the form of heat that is required for a material to undergo a change of phase (also known as change of state). Two possible heats are typically described. (Bambooweb, 2009)For other uses, see CFC (disambiguation). The pump operates a cycle in which the refrigerant changes state from its liquid form to the desiccation form and vice versa. This process occurs r epeatedly and I known as the refrigeration cycle. In this cycle, the refrigerant condenses and heat is released in one point of the cycle. It is the boiled (or evaporated) so that it absorbs heat in another point of the cycle. The wide used refrigerant is hydro fluorocarbon (HFC) known as R-134a (1, 1, 1, 2 tetrafluoroethane) and CCl2F2 (dichlorodifluoromethane). Other substances such as liquid ammonia, propane or butane, are be used but because of their highly flammable nature, they are disregarded as a straightforward refrigerant. 1930 (MCMXXX) was a common year starting on Wednesday (link is to a liberal 1930 calendar). (Bambooweb, 2009)For other uses, see CFC (disambiguation). In the refrigerator the fluid used (e.g. CCl2F2 ) fluid is liquefied by compression then vaporized by sudden expansion which gives a engine cool system effect. The compressor, in itself does not bring in a cooling effect directly, as might be expected. The cooling effect is crapd when the refrigera nt absorbs the heat from the cooled space. This is accomplished with a heat exchanger. (Bambooweb, 2009)For other uses, see CFC (disambiguation). A heat exchanger is a subterfuge built for efficient heat transfer from one fluid to another, whether the fluids are separated by a solid wall so that they never mix, or the fluids are directly contacted. The refrigeration cycle can be divided in two partsThe liquefaction constituteThe evaporation stageLIQUEFACTION STAGEThe refrigerant vapour undergoes recycling by itself into the liquid form by the extraction of heat from a vapour at a higher temperature. The refrigerant is compressed by the compressor where a low pressure and low temperature condition is created. This is accomplished by an evaporating coil. During the compression process, the vapour of the refrigerant undergoes a temperature change (as an effect of the compression process). Additionally, the work of compression to create the high temperature and pressure vapour also co ntributes to the temperature change experienced by the vapour. The condenser that is located where the temperature is higher (i.e. the higher temperature heat sink) collects the vapour. Heat is then removed from the refrigerant and in lieu of this it condenses to its liquid state, hence the piss for the condenser.Using the Joule-Thompson coefficient For a perfect gas = 0Cp + Cv = (H/T)p (U/T)pIntroducing H = U + pV = nRT into the first termCp Cv = (U/T)p + nR (U/T)p = nREVAPORATION STAGEAs the refrigerant leaves the condenser, the next part of the cycle begins. This is accomplishe when a high temperature and high pressure liquid passes through a metering device that is found within the refrigeration. The valve allows a specific quantity of liquid coolant to pass into the evaporation chamber. Evaporation chambers are comparatively low pressure and this encourages coolant evaporation. Newly evaporated coolant is drawn though the cooling coils (typically a fan is used to blow air over the coils). Thus, the evaporative process produces the cooling effect. The refrigerant then is pulled to the compressor in the suction line where it will be compressed into a high temperature, high pressure gas and sent to the external heat sinking coils. Capillary action or capillarity is the ability of a narrow tube to draw a liquid upwards against the force of gravity. A refrigerator pumps heat up a temperature gradient. The cooling efficiency of this operation depends on the amount of heat extracted from the cold temperature reservoir (the freezer compartment), , and the work needed to do so. Since a practical refrigerator operates in a cycle to provide a continuous removal of heat, for the cycle. Then, by the conservation of energy (or first law), , where is the heat ejected to the high temperature reservoir or the outside.The measure of a refrigerator exercise is defined as the efficiency expressed in terms of the coefficient of performance (). Since the purpose is to ex tract the most heat () per unit work arousal (), the coefficient of performance for a refrigerator, , is expressed as their ratioWhere, the conservation kin given above is used to express the work in terms of heat.For normal refrigerator operation, the work input is less than the heat removed, so the is greater than 1. Refrigerators are commonly referred to as heat pumps of more specifically a it is a reversible heat pump because they basically pump heat.Figure 5 A simple stylized diagram of a heat pumps vapor-compression refrigeration cycle 1)condenser, 2)expansion valve, 3)evaporator, 4)compressor.Opening a food refrigerator or freezer heats up the kitchen rather than cooling it because its refrigeration cycle rejects heat to the indoor air. This heat includes the compressors scatter work as well as the heat removed from the inside of the appliance.The COP for a heat pump in a heating or cooling application, with steady-state operation, isWhereQcool is the amount of heat extra cted from a cold reservoir at temperature Tcool,Qhot is the amount of heat delivered to a hot reservoir at temperature Thot,A is the compressors dissipated work.EFFICIENCYThe efficiency of a refrigerator (known as the coefficient of performance, COP) is defined asFor example, if 20 MJ are removed from the inside of the refrigerator by doing 7.5 MJ of work, then the coefficient of performance is equal to 20/7.5 = 2.67.SUMMARY OF THERMODYNAMIC OF A icebox AFTER ONE CYCLEChange in internal energy = 0Change in heat is 0Total work 0Total volume change = 0Change in Gibbs free energy = 0Entropy change of the system = 0Entropy change of the universe 0