Air Conditioning: Factors, Types and Systems | Electrical Engineering

In this article we will discuss about:- 1. Factors Influencing Air Conditioning 2. Types of Air Conditioning 3. Systems.

Factors Influencing Air Conditioning:

In this modern age everybody is trying to improve his surroundings in order to make living conditions more comfortable. Air conditioning is one of the important contribution in this respect. The technique of maintaining of air condition inside a building in such a way that produces comfort to human beings, ensures trouble free operation of delicate equipment or helps in industrial manufacturing operations is called air conditioning. It involves control of temperature, cleanliness, humidity and circulation of air.

The important factors involved in proper, efficient and complete air conditioning systems are:

1. Temperature Control:

In air conditioning, temperature control means the maintenance of a particular desired temperature within an enclosed space even though the outside temperature may be either above or below the desired inside temperature. This requires either the addition or removal of heat from the enclosed space as and when necessary. For this purpose the outside air to be supplied in the room is either cooled or heated.

In summer, air is cooled by one of the following methods:

(i) Circulation of cold water.

(ii) Evaporative system using water.

(iii) Ice activated system.

(iv) System using absorption principle.

(v) Vapour compression system employing a compressor, evaporator and a suitable refrigerant. This is commonly used method.

Three arrangements are possible under it namely:

(a) individual window type or floor mounted package type air conditioners

(b) a central plant employing chilled water in combination with unit conditioners located at various parts of the building and

(c) a central plant with a ducting system to convey cooled air to the different parts of the building.

In winter, air is warmed by one of the following methods:

(i) Electric heaters

(ii) Electromechanical heat pumps and

(iii) Heat exchangers using waste system.

2. Humidity Control:

Humidity control means the increasing or decreasing of water contents of air in order to create comfortable and healthy environments depending upon a particular season-summer or winter. Control of humidity is not only necessary for human comfort but also it increases the production and the efficiency of the workers. It has been found to improve the quality and to reduce the cost of the production of products like automobiles, clay products, drugs, chemicals, electric goods, foods, textiles etc. It is seen in factories during winter season or summer season that though there is a normal environmental temperature yet the workers feel uneasy.

That is due to lack of proper water contents or humidity of air. The humidity control is achieved by the process of humidification (addition of humidity or moisture) or dehumidification (reduction of humidity or water vapour). The humidification is accomplished either by supplying or spraying steam or hot water or cold water into the air. Dehumidification may be accomplished by using an air washer or by using absorbents. In the air washer system, the outside air is cooled below dew point temperature so that it loses moisture by condensation.

The moisture removal is also accomplished when the spray water is chilled water and its temperature is lower than the dew point of the outside air. The absorbent materials are activated alumina, silicagel and calcium chloride. These are generally used for window type small room coolers. In general, for summer air conditioning, the relative humidity should not be less than 60% while for winter air conditioning it should not exceed 40%.

3. Air-Movement and Circulation:

In order to keep constant temperature throughout the conditioned rooms, it is necessary that there should be equi-distribution of air throughout the space to be air conditioned. For this purpose the air movement and its circulation should be controlled.

4. Air-Filtration, Cleaning and Purification:

Proper filtration, cleaning and purification of air is necessary to keep it free from dust, dirt and other impurities which are very injurious to health. The dust particles are liable to clog the nose, throat and lungs of the worker seriously affecting their health. The dirt present in air acts as a carrier of germs and is responsible for disease spreading. In case of certain industries like chemicals the dirt produced as a byproduct is very harmful for health if inhaled. The presence of dirt in the air used for air cooling of electrical machines and diesel sets in power plants is liable to clog their air passage resulting in reduced efficiency and life.

Various types of air impurities other than gas or vapours called the Aerosols are also present. These are dust (earthly wet soil dust, dust of coal, cement, wheat, lint— a string like dust from cotton or wool); fumes (solid particles due to condensation and solidification of gaseous substances, zinc oxide fumes from zinc vapours); smoke (due to combustion of organic materials like tobacco, wood, oil, coal etc.); pollen (from weed, tree, flower and other vegetation—may be harmless or harmful); bacterial (living micro-organisms) etc.

There are various methods of cleaning, filtering and purifying the air from the above mentioned impurities. Mists or fogs in the air are usually removed by raising the temperature of air. This results in the formation of small suspended water particles into vapours making them invisible. The impurities like gases and vapours are removed by the process of condensation. Pollen grains responsible for hay fever, rose fever and other respiratory difficulties are removed by filters or by using a wet space. The particle of pollen grains will adhere to these wetted surfaces.

Bacteria present in the air are removed either by filter chemicals or by using a sterilising light of ultraviolet rays. For proper air sterilisation special ultraviolet ray lamps of radiation 2,600 Angstrom units are used. These are generally placed near the outlet grille on its inside end. They are shielded properly to avoid exposure of skin or eyes. Ultraviolet lamps are largely used in meat packing houses and storage rooms to avoid the growth of bacteria. Fine powder of propylene or triethylene glycol is also sprayed in the air to kill bacteria.

To remove dust particles from the air four methods using equipments like centrifugal devices, water filters or air washers, adhesive impregnated filters, electrostatic precipitators are used.

Types of Air Conditioning:

1. Comfort Air Conditioning:

This is the type of air conditioning whose objective is to produce comfortable and healthy conditions for occupants. In other words, it is the process of treating air so as to control simultaneously the prevailing conditions of temperature, humidity, cleanliness and distribution in the conditioned space to meet the comfort and health requirements of the occupants. It includes the air conditioning of spaces, places and surroundings which are occupied by human beings.

Some of the important places intended for human occupancy and requiring comfort air conditioning are the residences, departmental stores, public buildings including theatres, cinema halls, dancing halls, skating rings, restaurants and night clubs, hospitals etc. In order to understand the comfort air conditioning the human requirements should be known.

Human Requirements:

Our body is thermostat with inner temperature of 98.4°F (37°C) and outer temperature of about 26.5°C. Under normal conditions our body emits 2,500 k cals per 24 hours; 45% of which is dissipated by radiation, 30% by convection and remainder 25% by evaporation. Heat dissipated by radiation, convection and evaporation is governed by surrounding temperature, air temperature and humidity of air.

It is necessary for our body to dissipate the above proportion of heat in order that we may feel convenient and comfortable. For producing comfort it is necessary to maintain surrounding atmosphere at a temperature between 22°C to 25°C during summer and 17°C to 20°C during winter and humidity between 40 to 60 per cent. It is not desirable however, to maintain more than a 8 to 11°C difference between indoor and outdoor temperatures since a higher temperature difference will cause discomfort when a person goes out of a room and comes back.

In Indian Railway air-conditioned coaches the temperature is maintained at 24°C during summer and 20°C during winter, as these are most accepted temperature by passengers.

2. Industrial Air Conditioning:

A large number of manufacturing, inspection measuring processes require a close control of temperature and relative humidity. Industrial air conditioning is generally defined as the process of simultaneously controlling the temperature, relative humidity, ventilation, cleaning and movement of air within an enclosure intended for the manufacture, inspection and measurement of a product which is sensitive to atmospheric conditions.

It covers a large number of industries requiring controlled air conditions for their efficient and accurate manufacture as well as healthy and comfortable environments for the workers. Air conditioning is very essential for the manufacture of rayon and various plastics, paper, colour printing, in tobacco industry, in the precision manufacturing and measurement works for year-round production, textiles, felt goods, leather, electrical equipment, photographic material manufacturing, candy etc.

Effective Temperature:

Comfort requirements for human beings excluding radiation, air odours and cleanliness generally depend upon the dry- and wet-bulb temperatures of the air and its movement rate. An arbitrary index which combines in a single value the degree of sensation warmth or coolness felt by the human body due to the effect of air, temperature, humidity and air movement is called the effective temperature.

Its numerical value cannot be measured directly but is taken as that temperature of the saturated still air (at a velocity of 4.5 to 7.5 metres per minute) which induces the same sensation of warmth or coolness as those created by the air surrounding a person. An atmospheric condition will have an effective temperature of 20°C if it induces the same sensation of warmth or coolness as created by saturated still air at 20°C.

From the investigations and results of various studies it has now been proved that the effect of humidity shown by effective temperature scale are not great if the air temperature and the human activities are such that very little heat is lost by evaporation of the perspiration. For an individual doing an activity in sitting position with air temperature between 23°C and 25°C a relative humidity in the range of 30 to 65% will have no effect on his comfort.

At the same temperature, a higher rate of activity may result in a sensation of warmth but a relative humidity of 40% would be more comfortable than 60% relative humidity under similar conditions. In case of a large number of persons doing different activities like sitting, playing, dancing etc. in the same space, a 23.5°C temperature with 40% or less relative humidity generally provides most comfortable condition for all.

3. Summer Air Conditioning System:

It is the most important type of air conditioning, in which air is cooled and generally dehumidified. Figure 9.10 shows a schematic arrangement of a typical summer air conditioning system.

The outside air flows through the damper, and mixes up with re-circulated air (which is had from the conditioned space). The mixed air passes through a filter for removing dirt, dust and other impurities. The air now passes through a cooling coil, which has a temperature much below the required dry bulb temperature of air in the conditioned space.

The cooled air passes through a perforated membrane and loses its moisture in the condensed form which is collected in a sump. After that, air is made to pass through a heating coil to heat up the air slightly. This is accomplished for bringing the air to the designed dry bulb temperature and relative humidity.

Now the conditioned air is supplied to the conditioned space by a fan. From the conditioned space, a part of the used air is exhausted to atmosphere by the exhaust fans or ventilators. The remaining part of the used air, known as recirculated air, is again conditioned as illustrated in Fig. 9.10. The outside air is sucked and made to mix with the recirculated air in order to make up for the loss of conditioned (or used) air through exhaust fans or ventilators from the conditioned space.

4. Winter Air Conditioning System:

In winter air conditioning, the air is heated, which is usually accompanied by humidification. The schematic arrangement of winter air conditioning system is shown in Fig. 9.11.

In this system, the outside air flows through a damper and mixes up with the re-circulated air coming from the conditioned space. The mixed air is passed through a filter for removing dirt, dust and other impurities. The air is now passed through a preheater so as to avoid the possible freezing of water and to control the evaporation of water in the humidifier. After that, the air is made to pass through a reheat coil to bring the air to the designed dry bulb temperature. Now, the conditioned air is supplied to the conditioned space by fan.

From the conditioned space, a part of the used air is exhausted to atmosphere by the exhaust fans or ventilators. The remaining used air is again conditioned as shown in Fig. 9.11. The outside air is sucked and made to mix with re-circulated air, in order to make up for the loss of conditioned (or used) air through exhaust fans or ventilators from the conditioned space.

5. Year-Round Air Conditioning System:

The year-round air conditioning system has equipment for both the summer and winter air conditioning. The schematic arrangement of a modern year-round air conditioning system is shown in Fig. 9.12.

The outside air flows through the damper and mixes up with the re-circulated air coming from the conditioned space. The mixed air is passed through a filter so as to remove dirt, dust and other impurities. In summer, the cooling coil is operated to cool the air to the desired value. The dehumidification is obtained by operating the cooling coil at a temperature below than the dew point temperature (apparatus dew point). In winter, the cooling coil is made inoperative and the heating coil is operated to heat the air. The spray type humidifier is also used in the dry season to humidify the air.

Systems of Air Conditioning:

1. Unitary Air Conditioning System:

In this system, factory assembled air conditioners are installed in or adjacent to the space to be conditioned.

The unitary air conditioning systems are of the following two types:

(i) Window Units:

These are self-contained units of small capacity of 1 tonne to 3 tonnes, and are mounted in a window or through the wall. They are employed to condition the air of one room only. If the room is of bigger size, two or more units are installed. Now smaller units of 1 tonne, even 3/4 tonne are also available.

(ii) Vertical Packed Units:

These are also self-contained units of larger capacity of 5 to 20 tonnes and are installed adjacent to the space to be conditioned. This is very useful for conditioning the air of a restaurant, bank or small office.

The unitary air conditioning system may be adopted for winter, summer or year-round air conditioning.

2. Central Air Conditioning System:

This is most important type of air conditioning system, which is adopted, when the cooling capacity required is 25 tonnes or more. This system is also adopted when the air flow exceeds 300 m³ per minute or different zones in a building are to be air conditioned.

In this type of air conditioning various essential components or units of air conditioning are situated at some central place usually in basement from where conditioned air is led through sheet metal ducts to the various rooms or space of building. There are return ducts for carrying air from these rooms back to the central place where it is dehumidified, cooled and recharged with fresh air.

The circulation of air is achieved either naturally or by a fan. Advantages of this type of air conditioning system are high efficiency of operation and robustness of the equipment. The main disadvantages are absence of any adjustment of individual room temperature, necessity of costly ducting system and mixing of odours, smoke and bacteria present in the return air from infected rooms and redistributing them to healthy rooms. This type of air conditioning is best suited for big installations, factories and industrial concerns.

3. Unitary Central Air Conditioning System:

The main disadvantage of the central system is that it needs large sized ducts which are costly and occupy large space. The ducts have to carry full volume of air (outdoor plus return) to be supplied to all the rooms. The size of ducts is, therefore, made large enough to carry the whole supply of air. Similarly for return ducts, the same remarks apply. To overcome this problem, unitary central system is used.

In this type of air conditioning the features of both the central type and unitary type air conditioning systems have been combined in order to use it for all purposes and to make greater life expectancy. In this type of air conditioning heating and cooling equipments are installed in the central room. The various unit air conditioners in different rooms are supplied with heat energy (in the form of steam or hot water) or chilled water through the pipes. The air conditioning unit in this case comprises of heating and cooling coils, filters, blowers, dampers, valves, control and other associated equipment.

All the disadvantages of central type are done away with. There will be no return air duct, no mixing of odours and bacterial of one room and redistribution of the same among other rooms. Temperature of each space unit can be controlled by room unit. Fresh air duct required to supply only 15% fresh air, rest of 85% air being re-circulated, will be of very much small size. Fresh air introduced will displace corresponding quantity of air which will escape through openings etc.