Comfort Conditions | Human Comfort | Factors Affecting Human Comfort

Comfort Conditions | Human Comfort | Factors Affecting Human Comfort– There are lot of definitions for Human Comfort. But the most appropriate from thermal point of view is: Human Comfort is that condition of mind, which express satisfaction with the thermal environment.

Factors Affecting Human Comfort

  • Effective Temperature
  • Heat production and regulation in human body
  • Heat and moisture losses from the human body
  • Moisture content of air
  • Quality and quantity of air
  • Air motion
  • Hot and cold surfaces
  • Air Stratification

Comfort Conditions | Human Comfort | Factors Affecting Human Comfort

Comfort Conditions | Human Comfort | Factors Affecting Human Comfort

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

3.Air Velocity

    In order to evaluate the combined effect of these factors, the term effective temperature is employed. It is defined as the index which correlates the combined effect of air temperature, relative humidity and air velocity on human body.

Heat production and regulation in human body- The body acts like a heat engine which gets its energy from the combustion of food within the body. The process of combustion produces heat and energy due to the oxidation of products in the body by oxygen obtained from the inhaled air. 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 termed as metabolic rate. Since the body has a thermal efficiency of 20%, therefore the remaining 80% of the heat must be rejected to the surrounding environment, otherwise accumulation of heat results which causes discomfort.

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 rate at which these two forms of heat are given off under different conditions of air temperature and bodily activity.

Moisture content of air- As we know that the dry bulb temperature, relative humidity and air motion are interrelated. The moisture content of outside air during winter is generally low and it is above the average during summer, 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 moisture content leaks into the conditioned space, it will cause a low relative humidity unless moisture is added to the air by the process of humidification. In summer the reverse will take place unless moisture is removed from the inside air by de-humidification. Thus, while designing an air conditioning system, the proper dry bulb temperature for either summer or winter must be selected in accordance with the practical consideration of relative humidities which are feasible.

Quality and quantity of air- The air in an occupied space should, at all the times, be free from toxic, unhealthful or disagreeable fumes such as carbon dioxide. It should also be free from dust or odour. In order to obtain these conditions, enough clean outside air must always be supplied to an occupied space to adequately dilute the sources of contamination.

The concentration of odour in the room depends upon many factors such as hygienic habits of occupants, type and amount of outside air supplied, room volume per occupant, and type of odour sources.

Air motion- The air motion which includes the distribution of air is very important to maintain uniform temperature in the conditioned space. No air conditioning system is satisfactory unless the air handled is properly circulated and distributed. Ordinarily, the air velocity in the occupied zone should not exceed 8 to 12m/min. the flow of air should be preferably towards the faces of individual rather than form the rear in the occupied zone. Also for the proper and perfect distribution of air in the air-conditioned spaces, down flow should be preferred instead of up flow.

Hot and cold surfaces- The cold or hot objects in a conditioned space may cause discomfort to the occupants. A single glass of large area when exposed to the outdoor air during winter will produce discomfort to the occupants of  room by absorbing heat from them by radiation. On the other hand, a ceiling that is warmer than the room air during summer causes discomfort.

Air Stratification-When the air is heated, its density decreases and thus it rises to the upper part of the confined space. This result in a considerable variation in the temperature between the floor and ceiling level. The movement of the air to produce the temperature gradient from floor to ceiling is termed as air stratification.In order to have comfort conditions, the air-conditioning should be designed to reduce the air stratification to a minimum.

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