Human Comfort/Human Comfort Conditions

Human comfort can be defined as that condition of mind which express satisfaction with the thermal environment.

Thermal Exchange of human body:

•  The human body like any other machines works best at a   certain temperature, it can not tolerate a wide range of   temperature. To maintain that temperature human body uses three different modes of heat transformation:

1.  Evaporation
2.  Convection
3.  Radiation

•  A human body feels comfortable when the heat produced     by metabolism of human body is equal to the sum of the   heat dissipated to the surrounding and the heat stored in   human body by raising the temperature of body tissues. It   can be written as:

QM - W = QE ± QR ± QC ± QS

• Here, Qm is metabolic heat produced by body,W is rate of working, Qm-W is heat dissipated to atmosphere,Qe is heat loss by evaporation, Qr is heat gain or loss by radiation, Qc is heat loss or gain by convection, Qs is heat stored in body.

• The metabolic heat produced depends upon the rate of food energy consumption in the body. A fasting, weaker or sick man will have less metabolic heat production.

• The heat loss by evaporation is always positive. It depends upon the vapour pressure difference between skin surface and surrounding air.

• Qe becomes zero, when the surrounding air temperature is equal to the skin temperature. It cannot be negative because if the surrounding temperature is less than the skin temperature then skin do not absorb moisture from surrounding atmosphere.

• The heat loss or gain by radiation (Qr) depends upon the mean radiant temperature. When mean radiant temperature is lower than the dry bulb temperature of air in the room, Qr is positive i.e. the body will undergo a radiant heat loss and vice-versa.

• Mean radiant temperature is the average surface temperature of surrounding objects, when properly weighted and varies from place to place inside the room.

• The heat loss by convection (Qc) is positive when the temperature of surrounding is lower then the temperature of body i.e. the heat will be lost by the body and vice-versa.

• When Qe,Qr,Qc are high and positive and their sum is greater than (Qm - W), then heat stored in body is negative i.e. the body temperature falls down and the person feels cold.

• A men gets fever when high internal body activities increase Qm  to such extent so that Qs become positive for the given Qe,Qr and Qc.

• The ambient temperature for human body is 37℃ (at Qs=0). If the temperature of human body falls below 36.5℃ or rise above 40.5℃ it is dangerous.

• To maintain this ambient temperature human body uses two mechanisms:

1. Vasomotor Control : This control regulates the blood supply to the skin. When the human body temperature rises, then the vasomotor control increases blood circulation which increases conductivity of tissue. When the body temperature is falling down the vasomotor control decreases the circulation of blood which decreases the conductivity of cell.
2. Sudomotor Control : This control regulates sweat production, whenever heat loss by Qr and Qc is negative due to high atmospheric temperature then only way to control body temperature or heat transfer is evaporation.

Physiological hazards from Heat: 

• In summer the surrounding temperature is always higher than human body temperature because of this the human body gain heat from surrounding by means of radiation and convection. Now, human body can release heat only by means of evaporation of sweat. When this system is unable to cope with the heat gain, several physiological hazards can occur.

1. Heat Exhaustion: It is due to failure of normal blood circulation. The symptoms of heat exhaustion are headache, fatigue, vomiting, dizziness and abnormal mental reactions such as irritability. It does not cause any permanent damage and body rapidly recovers when person is placed in cool place.
2. Heat Cramp: It results because loss of salt due to an excessive rate of perspiration. It causes pain in calf and thin muscle. It can be avoid by taking salt tablets.
3. Heat Stroke: When the body temperature is rise above 40.5℃, it causes heat stroke and can leave permanent damage to brain or the person may enter in coma with imminent death. To avoid such condition, drink water at frequent intervals and rest at a cool place.
   

 Source: 1) A textbook of Refrigeration and Air conditioning by R.S.Khurmi & J.K.Gupta
              2) Refrigeration and Air conditioning by R.K.Rajput.