Principle of air conditioning

PRINCIPLE OF AIR CONDITIONING

Air Conditioning

Air Conditioning is defined as the simultaneous control of the Temperature of Air, Humidity of air, Purity of air and Motion of air for the purposes of Human Comfort, Food Processing and other Industrial Purposes.

Human Comfort

 The American Society of Heating, Refrigeration and Air conditioning Engineers (ASHRA defines the Human Comfort as:

“Human Comfort is that condition of mind which expresses satisfaction with the thermal environment."

Metabolic Rate:

 The rate at which human body produces heat is called the Metabolic Rate. The heat produced by a normal healthy person while sleeping is called the Basal Metabolic Rate. It is of the order of 60 W. The maximum value may be 10 times as much as this for a person engaged in continuous hard work.

The temperature of the body remains almost constant at about 36.9° C for tissues at the surface or the skin and about 37.2° C for the deep tissues. Body temperature in the morning after sleep is 0.5° C less than its temperature in the afternoon. A value of 40.5° C is considered serious and 43.5° C is almost fatal.

Human comfort is influenced by physiological factors determined by the rate of heat generation within the body and the rate of heat dissipation to the surrounding atmosphere by convection and by the evaporation of the sweat produced.

 

1. AIR CONDITIONING Vs REFRIGERATION

Air Cooling:

 Air Cooling is often confused with the term Air Conditioning. Air Cooling consists merely a blower with refrigerating unit. It provides only a flow of cool filtered air.

 

2. LOADS ENCOUNTERED IN AIR CONDITIONING SYSTEMS 

The air conditioning system carries the following loads:

1. The heat emitted from the bodies of the occupants in the room / conditioned space.

2. The heat transfer by conduction from the walls, floors, doors and windows of the

conditioned space.

 3. The heat emitted from the stored products, equipments, etc., in the conditioned space.

 4. The heat load due to solar radiation.

 

3. TERMINOLOGY

Psychrometry

 Psychrometry is that branch of physical science dealing with the study of the properties of air and water vapour mixture. A few psychrometric properties are defined below:

 1. Atmospheric Air

Air in the atmosphere is referred to as Atmospheric Air.

2. Dry Air

 Pure Dry Air mainly consists of Nitrogen and Oxygen and some other gases in small quantity like Carbon Dioxide, Hydrogen, etc.

3. Moisture

Atmospheric air normally contains water vapour, known as Moisture.

4. Moist Air

 Moist Air is defined as a mixture of dry air and water vapour. The maximum quantity of water vapour present in the air depends upon the air temperature.

5. Saturated Air

 Saturated Air is that air which contains maximum amount of water vapour which the air can hold at a given temperature.

6. Degree of Saturation

 Degree of Saturation is defined as the ratio of mass of water vapour in unit mass of dry air to mass of water vapour in unit mass of dry air when air is saturated at the same temperature.

7. Humidity

 Humidity is defined as the moisture content present in the atmospheric air. The atmosphere always contains some moisture in the form of water vapour.

8. Relative Humidity

 Relative Humidity is the ratio of mass of water vapour present in a given volume of dry air to the mass of water vapour required to saturate the same volume of dry air at the same temperature. It is represented in percentage. It is 0% for dry air and 100% for saturated air.

 For example, 50% relative humidity means that the air contains one-half the amount of moisture that it is capable of holding. Relative humidity changes as the air temperature changes.

9. Absolute Humidity

 Absolute Humidity is defined as the mass of water vapour contained in a given volume of air. It is expressed in gram of water vapour per cubic meter of air.

10. Temperature-Humidity Index (T.H.I.)

It is also termed as Discomfort Index. It expresses in numerical values the relationship between comfort or discomfort temperature and humidity. It is felt that T.H.I. at 20°C provides a comfortable atmosphere. 11. Dry Bulb Temperature

11. Dry Bulb Temperature(D.B.T.)

 Dry Bulb Temperature is the atmospheric air temperature recorded by a thermometer whose bulb is exposed to the atmosphere.

12. Wet Bulb Temperature (W.B.T.)

It is defined as the temperature of air measured by a thermometer when its bulb is covered with a wet cloth and is exposed to a current of air.

Note: For saturated air, D.B.T. and W.B.T. are the same.

13. Air Purity

People do not feel comfortable while breathing contaminated air even if it is within the acceptable temperature and humidity ranges. So, proper filtration, cleaning and purification of air is necessary to keep it free from dust, dirt and other impurities.

14. Air Movement and Circulation

Even if temperature, humidity and air purity are satisfactory, a certain amount of air motion (i.e., air velocity) and circulation is necessary for human comfort.

 

4. REQUIREMENTS OF COMFORT AIR CONDITIONING or FACTORS CONTROLLING AIR CONDITIONING

The following factors are to be considered for effective air conditioning:

Human Discomfort

Moisture from human body evaporates. Thus, body heat is disposed. Hence, humidity inside a room increases, causing difficulty in disposing of body heat. Also, the room temperature rises due to heat dissipated from the body, causing human discomfort.

Human Comfort

 Thermodynamically speaking, ideal human comfort exists when the rate of heat production becomes equal to the rate of heat loss. The following parameters are required to be maintained for the human comfort in air conditioning:

1.Temperature of Air

 In air conditioning, the desired room temperature has to be maintained, though the temperature of the outside air is different. It has been found that for human comfort, we need a dry bulb temperature of 20°C.

2. Humidity of Air

The control of humidity of air means the increasing or decreasing of moisture contents of air during summer or winter respectively.

3. Purity of Air

You and I do not feel comfortable while breathing contaminated impure air, even if it is within the acceptable air temperature and humidity ranges. The conditioned air must be free from dust, bacteria, odour and toxic gases.

4. Air Circulation

A person inhales about 0.6 m3 of oxygen and exhales about 0.2 m3 of carbon-di-oxide per hour. If the carbon-di-oxide level in the room increases above 2%, it will cause human discomfort. Hence, air conditioner should supply enough quantity of fresh air. There should be continuous air circulation in the air-conditioned space.

The feeling of comfort by individuals depends upon many factors such as age, types of cloth used, duration of stay, etc. This feeling also differs from individual to individual.

 

5. CLASSIFICATION OF AIR CONDITIONING SYSTEMS

1. Classification as to Major Function

(a) Comfort Air Conditioning

Window Air Conditioner or Split Type Air Conditioner used in rooms creates atmospheric conditions conducive to human comfort. Central Air Conditioning It is used in offices, hospitals, theatres, hotels, industries, etc.

(b) Industrial Air Conditioning

 It is used in machine-part manufacturing plants, tool rooms, printing, photo-processing plants, including CAD/CAM/CIM centers, etc.

2. Classification as to Season of the Year

 (a) Summer Air Conditioning

This system controls all the four atmospheric conditions, (Temperature, Humidity, Air Purity and, Air Movement and Circulation) for summer comfort. The major problems are to cool the air and to remove excess moisture from it.

(b)Winter Air Conditioning

 Such a system maintains indoor atmospheric conditions for winter comfort. The major problems are to heat the air by electric heaters or furnaces and to bring moisture content up to an acceptable level.

(c) Year-round Air Conditioning

 This system has heating and cooling equipment with automatic controls to maintain the four parameters for human comfort round the year.

3. Classification as to Arrangement of Equipment

 (a) Unitary Air Conditioning

Unitary or Window type air conditioner is of small capacity of 1 to 2 tonnes. It is used for air conditioning of rooms, small offices, operation theaters, etc.

(b) Central Air Conditioning

The central system is used for air conditioning of entire buildings, cinemas, etc.

 

6. IMPORTANT ACTIONS INVOLVED IN THE OPERATION OF AN AIR CONDITIONER

• Air filtering, cleaning and purification

• Air movement and circulation

 • Temperature control

 • Humidity control

Thus, air conditioning is the process of treating air so as to simultaneously control its temperature, humidity, cleanliness and circulation in the conditioned space.