Subject:- Refrigeration and Air Conditioning

Topic:- Design of air conditioning systems

Difficulty:- Low

The degree of warmth or cold felt by a human body depends mainly on following three factors: (a) Dry bulb temperature (b) Relative humidity (c) Air velocity.

Effective temperature is used to evaluate the combined effect of all these factors. It is defined as that index which correlates the combined effects of air temperature, air velocity and relative humidity on human body.

The numerical value of effective temperature is made equal to temperature of still saturated air (i.e. 5 to 8 m/min air velocity) which produces same sensation of warmth or coolness as produced under the given conditions.

Factors affecting ‘Effective Temperature’ are:

a) Climatic and seasonal differences: There is a relationship between optimum indoor effective temperature and outdoor temperature which changes with seasons.

b) Clothing: Person with light clothing needs less optimum temperature than person with heavy clothing.

c) Age and sex: Women of all ages require higher effective temperature than men. Children also need higher ET than adults.

d) Duration of stay: If the stay in a room is shorter then higher ET is required than that needed for long stay.

e) Kind of activity: When activity of person is heavy such as people working in factory then low ET is needed than for people sitting in cinema hall or auditorium.

f) Density of occupants: The effect of body radiant heat particularly in a densely occupied space like auditorium is large enough which requires little lower ET.