Chapter
1 INTRODUCTION 1.1
DEFINITION OF REFRIGERATION AND AIR CONDITIONING
1.1.1 Refrigeration Literally, refrigeration stands for the production of a cool confinement with respect to surroundings. A few definitions of some authors are listed below: It may be defined as the artificial withdrawal of heat, producing in a substance or within a space a temperature lower than that which would exist under the natural influence of surroundings [1]. According to ASRE [2], it is defined as the science of providing and maintaining temperatures below that of surroundings. Elonka [3] has given several definitions of refrigeration which are equivalent to production of cooling. However, for the places where surroundings are at a temperature lower than the required condition, it has to be heated up. The refrigeration machinery which does heating is called a heat pump. The main difference between the refrigeration system and heat pump can be physically conceived of from the fact that in the former there is pumping of heat out of the system as against pumping of heat from surroundings into the system in the latter case (Fig. 1.1). Thus a refrigeration system can be used as a heat pump, just by reversing the direction of operation (as detailed in Sec. 14.8).
1.1.2
Air Conditioning
In general air conditioning is defined as the simultaneous control of temperature, humidity, cleanliness and air motion#. Depending upon the requirement, air conditioning is devided into the summer air conditioning and the winter air conditioning. The former uses a refrigeration system and a dehumidifier* against a heat pump and a humidifier** used in the latter. In addition, air conditioning is also subdivided into the comfort and industrial air conditioning. The former deals with the human comfort which as well, requires noise control while the latter is meant for the production of an environment suitable for commercial products or commodities, production shop laboratories, manufacture of materials and precision devices, printing works, photographic products, textiles, cold * A device used to remove moisture from air. ** A device used for adding moisture into air. # Also refer Sec. 14.7 for the process based air conditioning for energy conservation. Note: There may be some situations where the humidifier or dehumidifier may not be employed in the system.
3
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REFRIGERATION AND AIR CONDITIONING
storages, pharmacy, computers, dams, etc. The details of the above applications are outlined in Chapter 14. Surroundings
Surroundings
Energy input to operate heat pump Confined space
Confined space
(a)
(b)
Fig. 1.1. Schematic representation of (a) refrigeration system and (b) heat pump.
1.2 NECESSITY OF REFRIGERATION AND AIR CONDITIONING For the survival and normal functioning of human beings there should be sufficient supply of vitamins, proteins, carbohydrates, salts, etc., which can be accomplished through a balanced diet or pills containing requisite ingredients. For abnormal health of a person, the pills are provided in order to help normalise the functioning of the system appropriately. The people with normal health and their peculiar habits, prefer to take tasteful diet in order to satisfy the tongue and belly simultaneously, in addition to fulfil the requirements for normal functioning of the body organs. But, our habits or fascination for particular commodities calls for the conservation of the commodities even for those periods during which they are not naturally available. As an example, in India the mangoes are harvested during the summer season and demand for the same persists throughout the year. Therefore, an artificial environment has to be created, most suitable for the commodity, under which the minimum spoilage occurs. This necessitates the production of artificial refrigeration for industrial air conditioning. As such fruits or commodities available in the particular season have become a regular feature even in the off-season. It would not be out of place to mention that the experiments have established that the mangoes ripening at around 20°C tastes the best as compared to the ripening at other temperature [4]. Because there is percentage increase in soluble solids (sugars) and acidity resulting in better sugaracid blend. Similarly, the poultry products show better yeild if appropriate humid environment is created compared to relatively hot and dry weather. It depends considerably on temperature and sex. The males show higher fertility at around 19°C compared to 30ºC environment. On the other hand, female (hens) fertility is much lower at 30°C as compared to 8°C [5]. Another necessity of refrigeration and air conditioning lies in the development of certain scientific equipments and their operation under controlled environment in order to get the reliable results. As for example, a computer usually functions satisfactorily if the temperature is 20°C and relative humidity 50% (usually recommended values). It is worthwhile to mention that the accurate functioning of the sophisticated control devices for the space exploration is largely dependent upon the controlled temperature and humidity of the confined space. There are abundance industrial applications of refrigeration and air conditioning such as production of clothes in moistened environment causes minimum wastage of threads due to breakage*. * Details of temperature conditions for preservation of different commodities are presented in Sec. 14.9 and appendix table A-21.
INTRODUCTION
5
Uniform stretching is a result of proper humid environment. Also the photographic materials show excellent prints when the environment is appropriately maintained. In production shop workers are capable of performing various operations in certain environment with high efficiency. If there is improper environment (specially hot), they get tired and produce much less output. Figure 1.2 exhibits the statistical study on a group of workers who put maximum efforts under the air conditioned environment. The results obtained from statistical study showed around 30% improvement in working efficiency and reduces the absenteeism by about 20%—causing significant improvement in the overall production [6]. Extensive statistical study carried out in the air conditioned class rooms shows the improvement in grades by 23%, learning and grasping by 50%, research capability by 38%, ability to concentrate 85%, effective use of learned skills 30% and effective use of study time 59% [7]. Another example of necessity of air conditioning is a film theatre. The air conditioned theatre attracts more customers or audiences than that without air conditioning, though the former charges a little higher. With air conditioning
Workability
Continuous duty
Duty with rest pause Duty with rest pause and air conditioning
0
1
2
3
4 5 Duty hours
6
7
8
Fig. 1.2. A schematic representation of the effect of environment on workability of men.
Air conditioning has become boon to mankind in curing persons suffering from high fever. In addition, the death rate of premature babies has been reduced considerably due to nursing in controlled environment. Thus, refrigeration and air conditioning find innumerable applications and scope in the present day race of the modern development of society. To sum up, the refrigeration and air conditioning which was regarded as luxurious branch of engineering in the society a few decades ago, has become the part and parcel of the present society.
1.3 HISTORY OF REFRIGERATION In the olden days around 2500 years B.C. Indians, Egyptians, etc., were producing ice by keeping water in the porous pots open to cold atmosphere during the night period (Fig. 1.3). The evaporation of water in almost cool dry air accompanied with radiative heat transfer in the clear night caused the formation of ice even when the ambient temperature was above the freezing temperature. Further references are available which supports the use of ice in China 1000 years B.C. Nero, the emperor, was using ice for cooling beverages. Further, the East Indians were able to produce refrigeration by dissolving salt in water as early as 4th century A.D., of course, on very small scale. The use of evaporative cooling is another application of refrigeration used in olden days. The cooling of water in
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REFRIGERATION AND AIR CONDITIONING Water
Fig. 1.3. Natural ancient ice harvest.
earthen pots for drinking purposes is the most common example where the evaporation for water through the pores of earthen pot is accompanied with cooling of water. The aforesaid methods of the production of cooling were not feasible for the commercial use due to very small amount of ice production. Availability of natural ice in limited regions and unavailability of good quality insulation confined the application of ice to those localities only. These all led to the development of artificial refrigeration as detailed below. Out of many pioneers’ works in the refrigeration side, a few would be presented here. Thomas Harris and John Long got the earliest British patent in 1790. Later on, in 1834 Jacob Perkins developed a hand operated refrigeration system using ether (volatile) as the working fluid (Fig. 1.4(a)). Ether vapour is sucked by the hand operated compressor and then high temperature and pressure ether vapour is condensed in the water cooled chamber (condenser). Liquid ether is finally throttled to the lower pressure, and thus evaporation of this liquid in chamber A lowers the temperature of water surrounding the vessel. Finally ice is formed. In this system, ether is used again and again in the cyclic process with negligible wastage.
Hand operated compressor
Refrigerator
Water cooled condenser
E
(a)
(b)
Fig. 1.4. (a) Ether vapour machine and (b) Sulphuric ether machine.
INTRODUCTION
7
In 1851, Dr. John Gorrie of Florida, a physician, obtained the first American patent of a cold air machine to produce ice in order to cure people suffering from the high fever. Instead of air or ether, sulphuric ether was used by Dr. James Harrison of Australlia in 1860, the world’s first installation of refrigeration machine for brewery [8, 9] (Fig. 1.4(b)). The steam engine works as a power source which drives the compressor for the pressurization of sulphuric ether vapour, which is, in turn, condensed and is allowed to expand and evaporate in order to produce refrigeration. Dr. Alexander Kirk of England constructed a cold air machine in 1861 similar to that of Dr. Gorrie. The air was compressed by a reciprocating compressor driven by a steam engine running on coal. His actual machine consumed about 1 kg of coal to produce 4 kg of ice (approximately). In the 19th century, there was tremendous development of refrigeration systems to replace natural ice by artificial ice producing machines. Unfortunately steam engine, a very low speed power developing source, was used to drive the compressor, rendering very poor performance of the refrigeration system. In the beginning of 20th century, large sized refrigeration machines were under progress. By 1904 about 450 ton* cooling system for air conditioning the New York Stock Exchange was installed. In Germany people used air conditioning in theatre for comfort purposes. In around 1911 the compressor speed was raised between 100 to 300 rpm. The first two-stage modern compressor was brought under use in 1915. During the civil war there was an acute shortage of the supply of natural ice from the north. Hence, Ferdinand Carré of USA developed a vapour-absorption refrigeration system using ammonia and water (Fig. 1.5). The system consists of an evaporator, an absorber, a pump, a generator, a condenser and an expansion device. The evaporated vapour is absorbed by the weak ammonia-water mixture in the absorber yielding strong aqua ammonia. The pump delivers this strong solution into the generator NH3 ¾® 100°C
Condenser
Exp valve
Evaporator
Generator
30°C Ice tray 15°C
Weak
Refrigerator temp. 45°C
Pump
Burner Absorber
Fig. 1.5. Vapour-absorption machine of Ferdinand Carré. * See page 11 for definition.
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REFRIGERATION AND AIR CONDITIONING
where heat transfer from a burner separates ammonia vapour and the weak ammonia water returns to the absorber. On the other hand the ammonia vapour condenses in the condenser before being throttled. The throttled ammonia liquid enters the evaporator resulting in completion of the cyclic process. In the beginning of two decades of the twentieth century, the development in refrigeration system was confined to refinement in cold air machines and vapour-compression systems. In third decade and onwards of 20th century, there has been considerable diversification in the growth of refrigeration systems leading to new developments such as solar powered vapour-absorption system, use of mixed (non-zeotropic) refrigerants in vapour-compression machines, vortex tube, pulse tubes, steam-jet refrigeration, thermoelectric devices, cryogenics, etc. Moreover, the world energy crisis has led to utilization of waste heat, solar energy, bio-energy, wind energy, etc. for the functioning of some of the refrigeration systems. There is concerted effort by various governments and private agencies to develop commercial units these days to cope up with growing requirements of refrigeration and decreasing the dependence on conventional energy sources. Use of primary energy has led to developments of refrigeration systems of capacities beyond 1000 tons in one unit.
1.4 METHODS OF REFRIGERATION 1.4.1 Dissolution of Certain Salts in Water When certain salts such as sodium chloride, calcium chloride, salt-petre, etc., are dissolved in water, they absorb heat. This property has been used to produce refrigeration. By this method the temperature of water can be lowered much below 0°C, the freezing temperature of water. Calcium chloride lowers the water temperature upto around –50°C while sodium chloride upto –20°C. The salt used for refrigeration has to be regained by evaporating the solution. On one hand the refrigeration produced is quite small and on the other hand, the regaining process of salt is so cumbersome that this is not feasible for commercial exploitation.
1.4.2 Change of Phase If a substance such as ice is available, it is possible to get refrigeration due to phase change, i.e., conversion of soilid into liquid. The required cooling is: .
.
Qc = m hsf
(1.1a)
where m and hsf are the rate of fusion of ice and enthalpy of fusion respectively. The value of hsf for ice at standard atmospheric pressure is about 335 kJ/kg (80 kcal/kg). Refrigeration can be produced by change of phase from solid to vapour known as sublimation which occurs if the solid is maintained below triple point. Then, .
.
(1.1b) Qc = mhsv where hsv is the enthalpy of sublimation. As an example, solid carbon dioxide (also called dry ice*) at one atmosphere pressure produces 573 kJ/kg of refrigeration maintaining a temperature of – 78.5°C. Refrigeration can be had due to phase transformation from liquid into vapour, i.e., (Fig. 1.6): .
.
d
i
.
Q = m hg − h f = m h fg
* For detail refer Sec. 14.4.
(1.2)
INTRODUCTION
9
Liquid + Vapour phase
f
T
Triple point temperature
g
s
g Solid + Vapour phase s
Fig. 1.6. Refrigeration due to phase change.
1.4.3 Throttling Process If a fluid at high pressure is expanded through a valve or constriction, either of the three effects are expected depending upon initial and final conditions: (i) Te > Ti , (ii) Te = Ti and (iii) Te < Ti as shown in Fig. (1.7), where Ti is the inlet temperature and TB , TA and TC are the corresponding values of the exit temperature Te for the above respective cases. The rise or fall in the temperature at the end of throttling is dependent upon the state after throttling on the constant enthalpy curve (Fig. 1.7). As the pressure after throttling decreases, the temperature rises until it becomes a maximum. This maximum value occurs at a location of the curve where Joule-Thomson coefficient defined as: (1.3) μ = (∂T/∂p)h is zero. This point is also called inversion point.* The positive value of μ is taken for cooling processes.
T
Inversion curve h Constant
TB
i
TA TC PC PA PB (a) Throttling device
p
Pi
(b) T-p diagram
Fig. 1.7. Throttling process.
1.4.4 Expansion of Gas through a Turbine or Behind a Piston If a gas at pressure p1 and temperature T1 expands behind a piston to pressure p2 (p2 < p1), the temperature, T2, after expansion is lowered: * For detail refer Sec. 2.6.
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REFRIGERATION AND AIR CONDITIONING
T2 = T1 (p2/p1)(n – 1)/n (1.4) where n is the index of the expansion process. Using the initial temperature 313K and pressure ratio p1/p2 = 6.5, the temperature T2 is found to be 183.5 K (= 313/6.50.4/1.4) or – 89.5°C.
1.4.5 The Ranque Effect When a high pressure gas is allowed to expand through a nozzle fitted tangentially to a pipe there is simultaneous discharge of the cool air core and hot air periphery*.
1.4.6 Material Properties Magnetic materials possess a property due to which cooling is achieved. If a substance is magnetized, temperature increases which is, in turn, cooled by the evaporation of helium giving about 4 K of temperature. When the magnet is allowed to demagnetize by removing the magnetic field suddenly without helium being used for cooling, the temperature is lowered further. In this process the temperature of the order of 0.001 K has been achieved [10] (Fig. 1.8). Using some special techniques, the temperature is lowered to 10–6 K [11, 12]. But this ultra-low temperature is of interest to scientists only. Thermocouples are another example of production of cooling. If two dissimilar metals are joined together and direct current is passed through them, the temperature at one junction gets increased whereas at the other junction it decreases depending upon the material combinations (Fig. 1.9). (For details refer Chapter 8.) Nylon thread
Vacuum for insulation
Liquid helium Liquid hydrogen
Magnet N
Helium gas
S
Paramagnetic substance
Fig. 1.8. Schematic diagram of supercooling.
* For detail refer Chapter 6.
INTRODUCTION
11 I
Current
Hot end
Cool end + – D.C. source
Fig. 1.9. Thermoelectric cooling.
1.5
UNIT OF REFRIGERATION AND COP
In the refrigeration industry the unit is ton. It is equivalent to the rate of heat transfer needed to produce 1 ton (2000 lbs) of ice at 32°F from water at 32°F in one day, e.g., in 24 hours. Now the enthalpy of solidification of water from and at 32°F is 144 Btu/lb in British Thermal Unit*. 1 ton of refrigeration = (2000 lb/day) (144 Btu/lb)/[(24 hr/day) (60 min/hr)] = 200 Btu/min or 12,000 Btu/hr. If a refrigeration system is capable of cooling at a rate of 300 Btu/min, it is a 1.5 ton machine. A machine of 20 ton rating is capable of cooling at a rate of 20 × 200 = 4000 Btu/min. In case of MKS (Metre, Kilogramme and Second) system one can proceed as given above to obtain the ton unit which yields 5.56 kcal/min(= (1000) (80)/(24 × 60)). However, in order to keep the specifications of refrigeration system identical with Btu value, the accepted practice is to use 50 kcal/min equal to one ton in MKS. If Btu ton unit is expressed into SI** system, it is found to be 210 kJ/min or 3.5 kJ/s or 12,600 kJ/h. In refrigeration, an important term called refrigeration effect is defined as the amount of cooling produced by a system. This cooling is obtained at an expense of some form of energy. Hence, it is customary to define a term known as Coefficient of Performance (abbreviated as COP): COP = Refrigeration effect/Energy input (1.5) Here the refrigeration effect and energy input should be expressed in the same units. Sometimes it is desirable to use relation between ton of refrigeration and the power of the machine to get COP, i.e., if Tn is the capacity of the plant in tons requiring PkW of energy then for the MKS system: (1.6a) COP = (3000 Tn kcal/hr) (4.187 kJ/kcal)/(3600P kJ/hr) = 3.49/(P/Tn) and COP = (3000 Tn kcal/hr) (4.187 kJ/kcal)/(0.736P × 3600 kJ/hr) (1.6b) = 4.74/(P′/Tn) where P′ is the Metric Horse Power (MHP) in MKS system being equal to 75 kgf-m/s = 736 W or 0.736 kJ/s. The above relation in the SI system is seen to be: (1.6c) COP = (210 Tn kJ/min)/(60P kJ/min) = 3.5/(P/Tn) with P in kW. Example 1.1. A refrigeration system produces 30 kg/hr of ice at 0°C from water available at 25°C. Find the refrigeration effect per hour and tonnage of the unit. If it takes 1 kW, find COP. Take solidification of water at 0°C as 335 kJ/kg and specific heat of water 4.19 kJ/kg-°C. * Btu is the unit of heat in the British System. ** SI System International.
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REFRIGERATION AND AIR CONDITIONING
Solution: 1 kg of ice at 0°C requires: qc = Enthalpy of solidification + Enthalpy due to cooling from 25 to 0°C = 335 + 4.19 (25 – 0) = 440 kJ/kg Refrigeration effect per hour: .
.
Qc = m qc = (30)(440) = 13,200 kJ/hr. .
Tonnage = Qc /12,600 = 13,200/12,600 = 1.048 The COP is obtained as: .
COP = Qc /Power = 13,200/[(1)(3600)] = 3.667 Example 1.2. An ice plant produces 1000 kg of ice per hour at –10ºC from water available at 30°C. Taking enthalpy of solidification (hfs) of ice and specific heat of ice below 0°C as 335 kJ/kg and 2.09 kJ/kg °C, respectively, obtain refrigeration effect, tonnage and COP for the power consumption* of 40 kW. Solution: Here ice is to be formed at –10°C, the heat transfer for cooling requires to cool water from 30°C to 0°C and then to solidify the water. Finally, ice is cooled from 0°C to –10°C. Therefore, refrigeration effect per kg of ice is: qc = 4.187 (30 – 0) + 335 + 2.09 (0 – (– 10)) = 481.5 kJ/kg Refrigeration effect per hour is: .
.
Q c = m q c = (1000) (481.5) = 4,81,500 kJ/hr.
The corresponding tonnage is: .
= Q c /12,600 = 4,81,500/12,600 = 38.21 tons .
Then, COP = Q c /40 kW = 4,81,500/[(40) (3600)] = 3.344 COP can also be determined directly by Eqn. (1.6c) as: COP = 3.5 × 38.21/40 = 3.344 As explained earlier energy is needed for a refrigeration system either in the form of mechanical or electrical or thermal. A schematic diagram (Fig. 1.10) represents a refrigeration system operated by a device which takes energy in the thermal form. A part of this energy is rejected to surroundings and the rest part of the energy is used to execute the device which absorbs heat from the confined Energy rejected
.
Surroundings
Cooling produced
Q Energy supply
.
QC
Heat engine
Fig. 1.10. A generalized representation of refrigeration system. * Energy is never consumed but is transformed from one form into another. However, we are so much conservant with such misuse (improper term) that it is used here.
