Thermal Radiation and Operative Temperature

In buildings away from outside perimeter walls, air and surface temperatures are usually approximately equal. The heat losses from a person by radiation (q ) and convection (q ) are then flowing to the same temperature level. In such uniform spaces, the radiant and convective losses are about equal and together account for about 80-90% of the total heat loss of a sedentary comfortable individual. In the presence of hot or cold surfaces, as may occur in perimeter or other locations in a building, the average surface temperature of the surroundings (called mean radiant temperature) as seen by the person s body may be substantially different from air temperature. If the mean radiant temperature (MRT) is greater or less than air temperature (T,) the person will feel warmer or colder than in a thermally uniform space where MRT =.  

To simplify the effects of radiation and convection on dry heat transfer, the concept of operative temperature is often used. By definition operative temperature is the temperature of a uniform environment (= MRT) that has the same total dry heat loss (convection + radiation) as the actual environment where MRT. 

Dry heat losses q y) from the person s surface at temperature T,. can be expressed as 

A is less than the skin area A y because some of the skin of fingers, arms, legs, and feet radiates to other skin and is not as effective for radiant heat loss. Equation (5.15) can be rearranged to 

Ar higher air speeds h convective heat loss becomes greater than radiation and approaches Tj. For such conditions Eq. (5.20) is recommended 

---