The flow chart for subjective perception of comfort

  The flow chart for subjective perception of comfort

1.6) Human heat balance:

 Heat is a form of energy that flows from higher to lower direction. Human burn food for energy & must discard the excess heat. Excess heat is discarded by conduction, convection, radiation& evaporation process. For health reason heat loss or heat gain must not occur too quickly. Thermal equilibrium that is comfortable condition is maintained when heat loss is exactly equal to heat generation . Thermal heat balance can be expressed mathematically by the following equation: Heat production = heat loss.7 M-W = CV + CK + R + ESK + ERes + CRes 

Where, 

M = Metabolic rate (thermal heat production) 

W = External work 

CV = Heat loss by convection. 

R = Heat loss by radiation 

 ESK = Heat loss by evaporation through skin 

ERes = Evaporative heat loss by respiration. 

CRes = Sensible heat loss by respiration. 

CK = Heat loss by conduction. ( All units are W/m2 )

 Human body if alive always produces heat, hence metabolic rate ( M ) is always positive, but varies with degree of activities. 

 Normally combined heat loss is less than heat generation. 

 Excess heat generated by human body makes the body warmer. 

 The body reacts to correct the situation by increasing blood flow to the skin surface and increasing perspiration. 

 As a result, the body heat loss is increased, thereby maintains the desired body temperature, which is expressed by heat balance equation. 

 When heat loss is greater than the body heat generation, the reverse situation occurs and the body starts shivering to increase metabolic rate to maintain heat balance. 

 The means of heat loss by human body is briefly discussed below : 

( a ) Radiation : 

  Heat loss may occur from warmer surface and heat gain may occur in cooler surface. 

 All bodies and surfaces emit thermal radiation and loss of heat by radiation occurs in the form of infrared wave. 

 Radiation is the net exchange of radiant energy between two bodies across an open space in the direct line of sight. 

 An example is, in the direct sunlight human body feels warm due to heat gain through radiation process but if under the shadow of a tree, does not feel warm as because radiation from sunlight can not occur. 

 Heat loss or gain by radiation process from human body depends on surrounding temperature. If the surrounding temperature is less than the human body temperature, then heat loss by radiation will occur feeling cool 

 During hot season , the reverse situation may occur if surrounding temperature is more than body temperature.8 

 A nude person staying in a room at normal temperature may loss about 60 % of the total heat by radiation.