In this article we will discuss about:- 1. Definition of Air Conditioning 2. Requirements of Air Conditioning 3. Factors 4. Equipments 5. Types 6. Vapor Absorption System.
Definition of Air Conditioning:
Air conditioning can be defined as the treatment of indoor air in order to control certain conditions required for human comfort. The desirable conditions may be temperature, humidity, dust particle level, odor level, and air motion.
It is known that the physical properties of air can be controlled by cooling, heating, humidification, and dehumidification. These processes may be employed to maintain specific conditions desirable for comfort. Thus, simultaneous control of temperature, humidity, air motion, and cleanliness is known as air conditioning.
Requirements of Air Conditioning:
Human body releases about 100 W to 450 W/person depending on the activity of the person due to metabolism. The body temperature is maintained to be 97°C. But the body surface temperature changes according to the surrounding temperature and relative humidity. The body heat must be dissipated from body surface to the surrounding.
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If the surrounding temperature is less than the body temperature, the flow of heat from body becomes quite easy and normal flow. If the surrounding temperature is low as in winter, the rate of flow of heat from the body is rapid and the person will feel cold. If the surrounding temperature is too hot, there would be no flow of heat.
In such situation, sweat glands become activated. The moisture of body gets evaporated which brings the temperature normal. If the outside temperature is hot and humid, little evaporation of moisture will occur from the body skin and so the person will feel hot and uncomfortable. The movement of air by fan helps to keep body comfortable.
When the room temperature becomes high due to heat gain, it causes human discomfort. When the room moisture becomes high, the increased humidity causes difficulties in disposing the body heat. For human comfort, the indoor temperature of 20°C and relative humidity 60% is quite good. Any air conditioning unit will be able to achieve the above requirement and maintain the conditions for comfort.
Factors Affecting Air Conditioning:
The process of air conditioning in auditorium, office building, houses, and classrooms is meant for maintaining comfort conditions for occupants.
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The four important factors for comfort conditioning are to be observed and maintained:
(a) Temperature,
(b) Humidity,
(c) Purity/cleanliness, and
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(d) Air motion.
(a) Temperature:
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The control of temperature is necessary in air conditioning. Even though the outdoor temperature is varying, the indoor temperature is maintained to be constant which is the desired condition. The heat may be either removed or added to the conditioned space depending upon the surrounding conditions. The person may feel comfortable when the temperature is 20°C and relative humidity is 60%.
(b) Humidity:
Humidity control means an increase or a decrease in moisture content inside the space to be air-conditioned. It is necessary not only for human comfort but also to increase the working capability. In summer, the relative humidity should be 60% and in winter it should it should be 40%.
(c) Purity/Cleanliness:
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It is one of the most important factors which affect the air conditioning. In addition to the control of temperature and humidity for human comfort, it is necessary to clean air, i.e., to make the indoor air free from dust, dirt, and odor. It is necessary that proper filtration and purification of air should be done and the supply of air free from dust and dirt should be made in air-conditioned space.
(d) Air Motion:
Air motion or proper circulation of air is also a factor affecting the human comfort. In order to maintain constant temperature throughout the conditioned space, it is necessary that there should be equal distribution of conditioned air in the space. The air movement is maintained at the desirable velocity of about 8 m/min using appropriate distribution system, grills, etc.
Equipments Used for Air Conditioning:
Following are the important equipments used for air conditioning:
(a) Air circulation fan.
(b) Air-conditioned unit-This will have cooling and dehumidification system or heating and humidification system.
(c) Supply duct
(d) Supply outlet (grill)
(e) Return outlet duct
(f) Filter
Types of Air Conditioners:
There are mainly two types of air conditioners:
(a) Window air conditioner
(b) Central air conditioner
(a) Window Air Conditioner:
Window air conditioner is a simple air conditioning unit fitted with the room wall or window. In this unit air is not supplied to the room through duct system. This consists of a complete vapor compression system having compressor, condenser, evaporator, expansion device with motor, blower, fan, air filter, grills, fresh air damper, and control panels as shown in Fig. 6.16.
The unit draws air continuously from the space to be cooled and it is cooled by cooling the coil of the unit and delivered back into the same space to be cooled. The process of drawing, cooling, and recirculation cools the space at a lower temperature required for the comfort. Regarding the installation of this unit, the evaporator unit should be mounted inside the room and the condenser will be mounted on the outer side of room wall.
The air supply grills have adjustable louvers or deflectors for changing the direction of air flow. It is generally operated with a 220-V single phase ac supply. The cooling capacities for such unit are available in market between 0.5 TR and 3 TR.
(b) Central Air Conditioning:
Centralized air conditioning system is a large-capacity plant which has a cooling capacity of 30 TR or more. This is also adopted when the air flow requirement is more than 5 m3/s. The systems employed for air conditioning of theater, restaurant, auditorium, and public buildings [Figs. 6.17(a) and 6.17(b)].
There is a separate machine room and the conditioned air is distributed to different places to be cooled by means of a ducting system. The unit will have the provision of cooling and dehumidification, heating and humidification, and proper ventilation to the room. The system will have the provision of return air ducting system also. The system includes a complete refrigeration system, blower, air ducts, and a plenum where the outdoor air is mixed with indoor air.
Vapor Absorption System of Refrigeration:
The vapor absorption system is the cycle used for the production of low temperature. It is an established fact that some substances have more affinity towards other substances at particular conditions and less affinity at some other conditions. This peculiar behavior generated the working of a vapor absorption system.
The vapor absorption system can be classified into two categories:
1. Intermittent absorption system
2. Continuous absorption system
1. Intermittent Absorption System:
In the beginning, the use of absorption system culminated from the experimental observation. Michael Faraday, in 1824, was performing experiments to liquefy certain gases, knowing this fact that AgCl (silver chloride), white powder, has the property of absorbing large amount of ammonia gas at NTP.
He performed experiments on two chambers A and B connected by a pipe as shown in Fig. 6.20.The white power was kept in chamber A and NH3 gas was supplied through pipe C. AgCl started absorbing ammonia until it became fully saturated and at the same time chamber B was cooled by the supply of water. Then, pipe C was sealed, chamber A was heated, and chamber B was cooled.
After liquid ammonia was obtained at the cooler end, heating was stopped. Ammonia gas again started coming from the cooler end and got absorbed again in AgCl. Traces of ammonia vapor were seen in the liquid. It was quite surprising to find that chamber B was very cold. The process was repeated and cooling was obtained rendering the concept of intermittent absorption system.
2. Continuous Absorption System:
i. Aqua Ammonia Absorption System:
The aqua ammonia absorption system was invented by Ferdinand Carre in around 1860 and later on the system was patented by the USA.
In this system, ammonia was used as a refrigerant and water was used to absorb ammonia. If the compressor is replaced by a generator and an absorber assembly, the unit will act as the basic absorption system. But the unit will require a liquid pump for its operation.
Figure 6.21 represents the schematic diagram of a basic vapor absorption system which uses ammonia as a refrigerant and water as an absorber. The aqua ammonia from the absorber at temperature Ta is supplied by the pump to the generator at temperature Tg.
After raising the pressure, heat is supplied to the solution at a temperature higher than the environment temperature to separate ammonia from solution. The heat is transferred to the generator which leads to the separation of ammonia vapor from aqua ammonia leaving behind weak aqua solution. The weak solution returns back to the absorber through an expansion valve V.
On the other hand, NH3 vapors condense in a condenser at temperature Ta. The liquid ammonia is throttled to the evaporator pressure where the refrigerant gets evaporated by the transfer of heat. The vapor formed in the evaporator is absorbed and the cycle is thus completed.
It is clear that the compressor used in the case of vapor compression cycle is replaced by an absorber, a generator, a pump, and a valve (V). The rest of the unit, i.e., condenser, throttle valve, and evaporator, is identical in both the systems. However, the system becomes more complex and a pump becomes necessary which requires mechanical energy for its operation which is a drawback of this unit.
A rectification unit is also necessary which requires a complex designing of the system. The system is able to produce low temperature and requires an efficient rectification unit after the generator to prevent any water vapor being carried to the condenser. Otherwise, if present in refrigerant line, it may form ice at a temperature less than 0°C, thereby blocking the passage or the expansion valve.
Energy balance yields, qa + qc = qz + qg + wp
Heat coefficient of performance, HCOP = qz/ (qg + wp)
Where, qg is the heat supplied to generator, wp is the work required for pump, qz is the amount of heat removed by evaporator, and wz is the work required for pump.
ii. Improved Absorption System:
The basic cycle works to produce refrigeration but the performance is very low which can be improved by reducing the water content. The refrigerating effect is reduced which requires greater flow and quantity of heat transfer.
Another improvement can be made by introducing heat exchanger between the generator and the absorber as shown in Fig. 6.22. An improved version of aqua ammonia vapor absorption cycle will have the rectification unit and the heat exchanger unit incorporated in the basic cycle for improvement in the performance.
A rectification unit is necessary which requires a complex designing which is installed after the generator to prevent any water vapor being carried to the condenser; otherwise the presence of water will choke the refrigerant line due to the formation of ice at low temperature.
The ammonia vapor which is almost pure enters the condenser at (1). The liquid ammonia leaving the condenser at (2) is throttled by the throttle valve. The saturated ammonia vapor leaving the evaporator at (4) enters the absorber which contains a weak aqua ammonia solution. Ammonia vapors are absorbed by the solution.
The pressure difference caused by the absorption maintains the flow of vapor from evaporator to absorber. The strong solution is pumped through the heat exchanger. The temperature is raised in the generator. The ammonia vapor goes up at (1) and the weak solution comes out at (8). A schematic diagram of the improved absorption system is shown in Fig. 6.22.
In this system, two devices have been added, which modify the basic cycle, in order to reduce the water concentration of the vapor entering the condenser. These devices are rectifier and dephlegmator. The analyzer is a direct contact heat exchanger consisting of a series of baffles mounted above the generator.
The strong solution from the absorber flows downward to cool the vapor rising from generator. The heat exchanger not only reduces the heat transfer but it also condenses some of the water vapors rising from generator. The final reduction in water content occurs in a dephlegmator, a water-cooled heat exchanger which condenses water vapors.
The generator, separator consists of four sections the liquid pump to which heat is added to raise ammonia vapor. The rectifier’s r1 and r2 improve the quality of vapors. In the dephlegmator, water is circulated to remove heat to improve the purity of the vapor to the desired value.
Thus, the absorber pump-generator combination receives ammonia vapors at lower pressure from the evaporator and supplies ammonia vapor to the condenser at higher pressure. The higher pressure is determined by T0 condenser temperature and the lower pressure is determined by evaporator temperature, Tz.
Per kg of vapor at (1),
Amount of heat qc (heat) leaving the condenser.
Amount of heat qz (heat) entering the evaporator.
Amount of heat qa (heat) leaving the absorber.
Amount of heat qg (heat) entering the generator.
Amount of heat qd (heat) leaving the dephlegmator.
wp is the pump work supplied.