Today, refrigerators have become an essential part of every kitchen (Tatum, 2010). Refrigeration is used to store meat, vegetables among other foodstuffs at low temperatures, thus inhibiting spoilage due to microbial activity. The process of essentially, manufacturing or making a refrigerator was gradual and begain in the 18th century. It culminitated with Carl von Linden’s work in 1876. (Bellis, 2010 & Tatum, 2010)
Evidence suggests that since 500 AD, man has known to produce ice by natural processes. Egyptians and Indians made ice on cold nights by setting water on earthenware pots. Later on in the 1700’s, England servants in the 1700’s collected ice in the winter and put into icehouses, which then provided cool storage in the summer. (Bellis, 2010 & Tatum, 2010)
In 1748, William Cullen of the University of Glasgow developed an entirely new process that consequently lead to an artificial cooling medium being developed. (Tatum, 2010). His experiment produced ice. However; he was unable to explain what it meant. Around 1805, the Oliver Evans was involved in designing a refrigeration apparatus, but unfortunately, he didn’t build one until Robert Perkins improved on his creation in 1834. (Bellis, 2010). Thomas Moore coined the word “refrigerator” for these machines. However, as today Perkins and Evans’ machines are called iceboxes. In 1844, Dr. John Gorrie, a physician, was able to construct a working unit that was based on both Evan’s and Perkin’s model. constructed. It was because of a outbreak of yellow fever that led to Gorrie creating the unit, which was used to cooling the air. (Bellis, 2010 & Tatum, 2010)
Gorrie is credited as being the one who invented the refrigerator by many. (Bellis, 2010) However the situation began to change, when Carl von Linde (1842-1934), a German mechanical engineer published an essay on improved refrigeration techniques, in 1871. He proposed a continuous process of liquefying gases in large quantities. In 1873, he invented the first practical and portable compressor refrigeration machine. (Tatum, 2010) He obtained a patent for his refrigerator in 1877 from the German Imperial Patent Office. He made use of gases namely ammonia, sulphur dioxide and methyl chloride. (Bellis, 2010 & Tatum, 2010)
In the 1900’s, various refrigeration models were seen. Noteworthy refrigerator models included Servel, Frigidaire, Electroflux among others. (Bellis, 2010) These models of the 1900’s had several advancements since designs of pioneers such as Gorrie. By 1918, automatic controls were part of some models already. (Tatum, 2010) The gases used namely ammonia, sulphur and methyl chloride were replaced by Freon in the 1920’s in order to comply with safety standards. When one looks at the history, it shows that in 1918, automatic parts were already installed. This included automatic dials that aid in the operation. It was rather unfortunate that the units were not self contained as different parts were separately placed from each other. It wasn’t until 1923 that self contained refrigerators began appearing. (Bellis, 2010) Ice cube trays were also introduced. (Tatum, 2010) Although many advancements were made, the modern refrigerator was put in mass production until 1946 i.e., after the World War II. (Bellis, 2010 & Tatum, 2010)
People, in the the 1950s and 1960s were the ones that witnessed a variety of technological innovations by engineers and scientists of the day. Among them were: (i) automated defrosting and (ii) making of ice. Today, there are many features that are intertwined with the features of the olden days and includes power failure alerts, ice cabinets among others. (Bellis, 2010 & Tatum, 2010)
To present, domestic refrigerators are present in almost every home worldwide. Due to the models created by Gorrie, Cullen, Carl von Linche among others, the refrigerator has thus become one of the machines or “applicances” that is integral to us every day. (Bellis, 2010 & Tatum, 2010)
TYPES OF REFRIGERATORS
Refrigerators are classified into three types: (Suyambazhahn, 2009)
Vapour compression refrigerator
Vapour absorption refrigerator
VAPOUR COMPRESSION REFRIGERATION SYSTEM
The vapour compression refrigeration system is most commonly used in refrigerators. A refrigerant is a gas with characteristics that make is suitable for refrigeration and air conditioning. R-22 is a commonly used refrigerant. This cycle works in four phases, which are described later on because it is similar to the refrigeration cycle.
Figure 1 – Vapour compression refrigerator (Suyambazhahn, 2009)
This type has various uses such as: (Suyambazhahn, 2009)
Air conditioned cinema theaters, restaurants, hospitals, residential buildings for comfort.
Advanced medicines which are manufactured and preserved only in special atmospheric conditions.
Preservation of food products.
VAPOUR ABSORPTION REFRIGERATION SYSTEM
The principle of vapour absorption was first discovered by well known scientist Michael Faraday in 1825. But this concept is applied to refrigeration during 1860s by French Scientist Ferdinand Carve. The commonly used refrigerant for vapour absorption system is ammonia, NH3. In order to change the conditions and phase of refrigerants, heat energy is utilized in vapour absorption system where as mechanical energy is utilized in vapour compression systems.
In a vapour absorption system, compressor is replaced by an absorber, a pump and a generator. The vapour at the low pressure that leaves the evaporator is then moved to the absorber. The absorber contains weak ammonia solution. The vapour leaving from the evaporator is dissolved in the weak ammonia solution to form a strong solution. Cooling water is used to cool he absorber. The strong solution from the absorber is pumped to the generator. The strong solution’s pressure is increase by the pump (10 bar) and is circulated through the system by pump.
Figure 2 – Vapour absorption refrigerator – schematic (Rajadurai, 2009)
COMMONLY USED REFRIGERANTS
Even though there are many types of refrigerants which are used in various applications, the following types are important from the subject point of view.
It is the most widely used refrigerant. It is mainly used as the refrigerant in cold storage plants and also in ice making plants. It’s boilined point at atmospheric pressure is -33 oC and it has a high latent heat and high critical temperature which are desirable properties of ammonia as a refrigerant. Also it is less expensive. But its usage becomes secondary due to the following characteristics: (Rajadurai, 2003)
It is toxic
It is flammable
It has an irritating odour
It attacks metals like copper and brass in the presence of moisture
The demerits involved in the usage of ammonia can be eliminated by using carbon dioxide. It is non toxic and odourless. It has a boiling point of -77.6 oC at atmospheric pressure. But it is not so often used because of its high operating pressure that is the operating pressure of CO2 is very high as 70 bar. (Rajadurai, 2003)
It has a boiling point of -10 oC at atmospheric pressure. IT has a very low working pressure and a large latent heat with a high critical temperature. It is non flammable and on explosive. Even though there are many positive characters mentioned, the SO2 refrigerant is very toxic and it has an irritating pungent odour. Also it is very corrosive in contact with moisture. (Rajadurai, 2003)
FREON – 12 (or DICHLOR DI FLUOROMETHANE)
It has a boiling point of -30 oC at atmospheric pressure. It is non toxic, non explosive and on flammable. It is odorless and colourless. It is non corrosive to any metal. But it is highly costlier than other types of refrigerators. But the main demerit with respect to this is type is the large amount of refrigerant that is necessary to be circulated for a given output. It is generally abbreviated as R-12 or F-12. (Rajadurai, 2003)
FREON – 22 (or DICHLOR MONO CHLORO METHANE)
It is widely used as the refrigerant for domestic refrigerants. It has all positive points like the characters posed by Freon – 12 such as non toxicity. It is colourless, odourless and non corrosive to metal. Additionally, the amount of refrigerant required is only 1.3 kg/min per tonne for refrigeration. (Rajadurai, 2003)
PRINCIPLES OF OPERATION & THERMODYNAMICAL CONSIDERATION
THE SECOND LAW
The second law of thermodynamics is described as the “most fundamental law of science” (Khemani, 2008). It is fundamental in the sense that it can be used to explain not only refrigerators and heat engines but highly advanced phenomena such as the big bang. It has been put aptly in the words of Classius as “it is impossible for a process to occur that has the sole effect of removing a quantity of heat from an object at a lower temperature and transferring this quantity of heat to an object at a higher temperature” (Mortimer, 2008). This essentially means that “heat cannot flow spontaneously from a cooler to a hotter body if nothing else happens” (Mortimer, 2008) i.e. there needs to be an external agency to effect the change.
In kitchen refrigerators, the closed box inside is able to be kept cool by the removal of heat from the inside of the box and deposits it to the outside. As per the second law, the heat will not move from the cold to the hot freely so it is important for it to be made to do so, this is done by using an intermediate fluid (Littlewood, 2004) which absorbed heat on the inside. This intermediate fluid is known as a refrigerant and carries the heat outside of the box whereby it it released into the air as heat as shown in Figure 3 (Littlewood, 2004).
Figure 3 – the flow of heat within the refrigerator – a schematic (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 work effectively without violating the second law of motion. (Littlewood, 2004)
THE FIRST LAW
Refrigerator takes in energy from a region that needs to be cooled and deposits this heat energy into some other region that is outside of the refrigerator. In order to do work, there needs to be some mechanism in place, where the work done by a compressor and its electric motor is utilized. Using the First Law of Thermodynamics we can write: (Littlewood, 2004)
Figure 4 – the first law of thermodynamics (Littlewood, 2004)
QC – QH = -W
Qc – energy or heat of the cold system
QH = energy or heat of the hot system
W = work done
Since work is done on the refrigerator by the compressor, the work is done is deemed negative because of sign conventions. This is part of the first law (Littlewood, 2004).
The refrigerator is termed as a closed system and it possesses a constant composition:
U’ = U + (âˆ‚U/âˆ‚V) T dV
U’ = U + (âˆ‚U/âˆ‚T) V dT
U’ = U + (âˆ‚U/âˆ‚V) T dV + (âˆ‚U/âˆ‚T) T dT
dU = (âˆ‚U/âˆ‚V) T dV + (âˆ‚U/âˆ‚T) V dT
According to Bain (2010), there are four basic parts to any refrigerator:
The exchanging pipes are a coiled set 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 efficiently so that inside the refrigerator is kept cooled. (Bain, 2010)
A gas can be cooled by adiabatic expansion if the process is enthalphic. The gas expands through a process barrier from one constant pressure to the next and the temperature difference in observed. Insulation of the system made the process adiabatic. The result 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 µ „P
Figure 5 – schematic of a domestic refrigerator (Bain, 2010)
Figure 6 – heat transfer within a refrigerator (Popular Mechanics, 1993)
When an energy |qc| is removed from a cool source at some temperature Tc, and then deposited in a warmer sink at a temperature Th, the change in entropy is: (Atkins & dePaula, 2006)
Atkins & dePaula (2006) also indicated that the process is not spontaneous because the entropy generated in the warm sink is not enough to overcome the loss of entropy from the cold souce. And because of this more 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 performance, c:
The less the work required to achieve a given transfer, the greater the coefficient of performance 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,
We can have an expression in terms of the temperature alone, which is possible if the transfer is performed reversibly (Atkins & dePaula, 2006):
c = thermodynamic optimum coefficient of temperature
Tc = temperature of the cold sink
Th = temperature of the hot sink
For a refrigerator, it important that a very low coefficient of performance. For a refrigerator withdrawing heat from ice cold water (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, 2006)
The work to maintain 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 sample at a rate that is proportional to the temperature difference. (Atkins and de Paula, 2006).
Figure 7 – (a) “the flow of energy as heat from a cold sink to a hot sink is not spontaneous as described the first law. Notice that the entropy increases but it is larger for the hot sink as compared to the cold sink. (Atkins & dePaula, 2006). This contributes to a decrease in the ‘NET’ entropy.
(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)
The rate at which energy leaks happen is written as:
A = a constant that depends on the size of the sample and details of the simulation
Tc = temperature of the cold sink
Th = temperature of the hot sink
The minimum power, P, required to maintain the original temperature difference by pumping out that energy by heating the surroundings is:
As can be seen the power increases as the square of the temperature difference (Th – Tc).
THE REFRIGERATION CYCLE
The gas is pumped continuously at a steady pressure, the heat exchanger (which brings the required temperature) and then through a porous plug inside container that is thermally insulated. A phase change heat pump uses a liquid, as described earlier, that has a very low boiling point, which is used to move heat from an area where it is cooler to one where it is warmer. The refrigerant requires energy so that it can evaporate, which essentially allows it remove the heat from the surroundings by absorbing it. When the vapor condenses, the energy absorbed in the process is released which is also in the form of heat as might be expected. 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 latent 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 vapour form and vice versa. This process occurs repeatedly 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 widely used refrigerant is hydro fluorocarbon (HFC) known as R-134a 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 good refrigerant. 1930 (MCMXXX) was a common year starting on Wednesday (link is to a full 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 cooling effect. The compressor, in itself does not create a “cooling effect” directly, as might be expected. The “cooling effect” is fashioned when the refrigerant absorbs the heat so that it is removed and the area becomes cooler. This is accomplished with a heat exchanger. (Bambooweb, 2009)For other uses, see CFC (disambiguation). …A heat exchanger is a device 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 parts:
The liquefaction stage
The evaporation stage
The 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 contributes 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 it’s liquid state, hence the name for the condenser. (Mortimer, 2003 ; Brain, 1994 ; Bellis, 2010)
Using the Joule-Thompson coefficient: For a perfect gas µ = 0
Cp + Cv = (âˆ‚H/âˆ‚T)p – (âˆ‚U/âˆ‚T)p
Introducing: H = U + pV = nRT into the first term:
Cp – Cv = (âˆ‚U/âˆ‚T)p + nR – (âˆ‚U/âˆ‚T)p = nR
As the refrigerant leaves the condenser, the next part of the cycle begins. This is accomplished 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 relatively 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. … (Mortimer, 2003 ; Brain, 1994 ; Bellis, 2010)
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. (Mortimer, 2003 ; Brain, 1994 ; Bellis, 2010)
The measure of a refrigerator performance is defined as the efficiency expressed in terms of the coefficient of performance (). Since the purpose is to extract the most heat () per unit work input (), the coefficient of performance for a refrigerator, , is expressed as their ratio: (Mortimer, 2003 ; Brain, 1994 ; Bellis, 2010)
Where, the conservation relationship 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. (Mortimer, 2003 ; Brain, 1994 ; Bellis, 2010)
Figure 8 – A diagram of the vapor – compression refrigeration cycle that is used in heat pumps. The cycle shows the following: (i) condenser, (ii) expansion valve, (iii) evaporator, (iv) compressor. (Karin, 2003)
It is commonly believed that by opening a refrigerator, it’ll cool the kitchen. However this is entirely opposite, opening a refrigerator or freezer heats up the kitchen because the refrigeration cycle does not accept the air from the outside (Karlin 2003). The heat is referred to as the heat dissipated from the compressor’s work and also includes that heat that s removed from within the refrigerator as well. (Karlin, 2003)
The COP (in a heating or cooling application), provided that it undergoes steady – state operation, is given by the following equation:
Î”Qcool is the heat extracted from a cold reservoir,
Î”Qhot is the heat delivered to a hot reservoir.
Î”A is the dissipated work by the compressor.
THE CARNOT ENGINE
The Carnot refrigerator is the maximum limit to the COP (efficiency) of a refrigerator system. Although we cannot make the carnot refrigerator, it tells us the maxium or best performance that can be garnered from a real refrigerator. The carnot refrigerator is sort of ideal in its design. As described earlier by Atkins & dePaula (2006) with the Carnot engine, the COPc of a Carnot refrigerator depends (i) the temperature of the region that needs to be kept cool which has a characteristic temperature, TC and the temperature of the region where the heat needs to be transferred to, having a characteristic temperature, TH. It is equal to: (Littlewood, 2004)
The efficiency of a refrigerator is described by a special “coefficient” known as a coefficient of performance and is defined in terms of the following parameters:
SUMMARY OF THERMODYNAMICS OF A REFRIGERATOR AFTER ONE CYCLE
Change in internal energy = 0
Change in heat is > 0
Total work > 0
Total volume change = 0
Change in Gibb’s free energy = 0
Entropy change of the system = 0
Entropy change of the universe > 0
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