Comfort Air conditioning basic principle theory.pptx

Comfort air conditioning theory

1. In order to obtain desired results from winter or summer air conditioning systems one has to be quite familiar with a number of factors which physically or physiologically affect human comfort. The important factors include : Effective temperature Heat production and regulation in man Heat and moisture losses from the human body Moisture content of air Air purity Air motion Cold and hot surfaces Air stratification Comfort Air Conditioning

2. Factors Affecting Comfort The comfort of occupants is affected by several environmental variables, including the following: Temperature Humidity Airflow (Air velocity at about 8m/s) Filtration Ventilation Radiation

3. ( i) Effective Temperature : Excluding such factors as radiation, air odours and cleanliness, the main factors on which comfort conditions for individual depend are: dry bulb temperature wet bulb temperature rate of motion of air. The arbitrary index which combines in a single value the degree of warmth or cold felt by the human body in response to the air temperature, moisture content and its motion is known as effective temperature. Effective temperature cannot be measured directly but is fixed as the temperature of saturated still air (velocity 3 to 8 m/min) which induces the same sensations of warmth or coolness as those produced by the air surrounding a person.

4. Figure shown below is an ASHVE comfort chart for still air conditions. The diagonal temperature lines represent constant effective temperature. The chart reveals that there are several combinations of wet bulb and dry bulb temperature with different relative humidities which will produce the same effective temperature. This however may not be true for either extremely high or low relative humidities which may produce conditions of discomfort regardless of the existent effective temperature. The mucous membrane and skin become too dry when R.H. is below 30% and there is sticky feeling when the R.H. is above 70 %. Curves at the top and bottom of in the above shows the percentage of people, participating in tests who found various effective temperature satisfactory for comfort. The test covered both winter and summer conditions.

5. Wet bulb temperature D ry bulb temperature

6. For winter conditions the chart indicates that a maximum of 97.7% people felt comfortable for an effective temperature of 20 C For summer conditions the chart shows that affective temperature of 21.6 C was desired by 98% of the people. The values of recommended effective temperature obtained from various tests conducted at Defence Institute of physiology and Allied Sciences Madras of Indian conditions are given below The values were obtained on the basis of maximum percentage (90%) of people showing similar reactions to varying inside conditions. The values of equivalent room state given in the test are of 50% R.H. while any value of R.H. could be taken and corresponding D.B.T. obtained from the E.T. chart.

7. Climate Effective temperature °C. Equivalent Room State Hot and dry 21.1 to 26.67 23.9°C, 50% R.H. Hot Humid 22 to 25.6 24.2°C 50% R.H. Monsoon 20.5 to 24 23°C 50% R.H. Winter 19.7 to 22.2 22°C 50% R.H.

8. (ii) HEAT PRODUCTION AND REGULATION IN MAN The human body may be considered as a heat engine which derives its energy from the food. The food eaten by a human being is converted into energy by a process of oxidation effected by the air breathed into the lungs (called as Metabolism) Since thermal efficiency of the body is only 20%, so the remaining 80% of the heat has to be rejected to the surroundings. The rate and the manner of rejection of this heat is controlled by a bodily system of automatic regulation . The rate at which the body produces heat is termed as the metabolic rate, the value of which depends upon a number of factors such as individual's health, his physical activity and his environments . In order to balance the loss of heat the body supplies more blood to the capillaries in the skin from where heat transfer may take place by radiation, convection and by evaporation

9. In the event either the process of radiation or convection or both fails to produce necessary loss of heat, the sweat glands become more active. As a result, more moisture is deposited on the skin and the same evaporates thus carrying away the requisite amount of heat. As long as the surroundings are below blood temperature heat removal may take place by the methods of radiation and convection . When the temperature of the surroundings is above the blood temperature removal of heat will take place only by evaporation . If the body fails to transfer the required amount of heat, accumulation of heat results which will cause discomfort . When the body is exposed to cold it maintains its temperature by withdrawing blood from the outer portion of the flesh by decreased blood circulation and by the increased rate of metabolism.

10. HEAT AND MOISTURE LOSSES FROM THE HUMAN BODY The human body loses heat in the form of either sensible heat or latent heat (by evaporation) or both. While designing a comfort air conditioning system it is necessary to know the rates at which the human body gives off these two forms of heat under different conditions of air temperature and bodily activity. Figure show heat losses in the form of sensible heat and latent heat with the increase in dry bulb temperature (DBT), by taking rate of activity as parameter (i) Curve A when man working at a rate of 91 kJ/hr. (ii) Curve 8 when man working at a rate of 45.5 kJ/hr. (iii) Curve C when man working at a rate of 22.80 kJ/hr. (iv) Curve D when man seated at rest.

11. (i) Curve A when man working at a rate of 91 kJ/hr. (ii) Curve 8 when man working at a rate of 45.5 kJ/hr. (iii) Curve C when man working at a rate of 22.80 kJ/hr. (iv) Curve D when man seated at rest.