It is a system for controlling the humidity, ventilation, and temperature in a building or vehicle, typically to maintain a cool atmosphere in warm conditions.
Human comfort
The human comfort depends upon physiological and psychological condition.
The most acceptable definition, from the subject point of view, is given by the American Society of Heating, Refrigeration and air Conditioning Engineers (ASHRAE) which states human comfort is that conditions of mind, which expressed satisfaction with the thermal environment.
Factors Affecting Human Comfort
1. Effective temperature
2. Heat production and regulation in human body
3. Heat and moisture losses from the human body
4. Moisture content of air
5. Quality and quantity of air
6. Air motion
7. Hot and cold surfaces
8. Air stratification
Effective Temperature:
The degree of warmth or cold felt by a human body depends mainly on the following three factors:
1. Dry bulb temperature,
2. Relative humidity and
3. Air velocity.
In order to evaluate the combined effect of these factors, the effective temperature is employed. It is defined as that index which collates the com binned effects of air temperature, relative humidity and air velocity on the human body. The numerical value of effective temperate is made equal to the temperature of stills (i.e 5 to 8 m/min air velocity) saturated air, which produces the same sensation of warmth or clones as produced under the given conditions.
The practical application of the concept of effective temperature is presented by the comfort chart.
This chart is the result of research made on different kinds of people subjected to wide range of environmental temperature, relative humidity and air movement by the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE). In the comfort chart, the dry bulb temperature is taken as abscissa and the wet bulb temperature of ordinates.
Heat Production and Regulation in Human Body:
The rate of heat production depends upon the individual’s health, his physical activity and his environment. The rate at which the body produces heat is metabolic rate. The heat production from a normal healthy person when a sleep (called based metabolic rate) is about 60 wtts and it is about ten times more for a person carrying out sustained very hard work.
Heat and Moisture Losses from the Human Body:
The heat is given off from the human body as either sensible or latent heat or both. In order to design any air-conditioning system for spaces which human bodies are to occupy, it is necessary to know the rates at which these two forms of heat are given off under different conditions of air temperature and bodily activity.
Moisture Content of Air:
The moisture content of outside air during winter is generally low and it is above the average during simmer, because the capacity of the air to carry moisture is dependent upon its dry bulb temperature. This means that in winter, if the cold outside air having a low mist rue content leaks into the conditioned space, it will cause a low relative humidity unless minster is assed to the air by the processes of humidification. In summer, the reveres will take place unless moisture is removed from the inside air by the dehumidification process.
Quality and Quantity of Air:
The air in an occupied space should, at all times, be free room toxic, unhealthful or disagreeable fumes such as carbon dioxide. It should also be free from dust and odour.
Air Motion:
The air motion which included the distribution of air is very important to maintain uniform temperate in the conditioned space. The air velocity in the occupied zone should not exceed 8 to 12m/min.
Cold and Hot Surfaces:
The cold or hot objects in a conditioned space may cause discomfort to the occupants.
Air Stratification:
The movement of the air to produce the temperature gradient from floor to ceiling is termed as air stratification. In order to achieve comfortable conditions in the occupied space, the air conditioning system must be designed to reduce the air stratification to a minimum.