A critique of the environmental temperature model

Abstract For many years, the index temperature to describe the warmth of a room was termed ‘air temperature’, t ai . This was not logical since the index was taken to drive a longwave radiant flux to a room surface, in addition to driving the convective flux that is properly associated with air temperature. Environmental temperature, t ei was introduced in the 1960 s; it was composed of air and surface temperature components and was intended to replace air temperature as the room index. Certain compound conductances were associated with t ei and the package of ideas was incorporated into the 1970 IHVE Guide to carry through calculations which involve heat transfer with the surfaces of a room. The procedure—the environmental temperature model—has not proved easy to understand. The model is based on three erroneous ideas: it reduced the 21 conductance network that represents radiant exchange in a six surface enclosure to just a single conductance, and so failed to distinguish between emissivity and geometrical aspects of radiant exchange; the argument appeared to suggest—wrongly—that the temperature t ei so arrived at was a meaningful enclosure parameter; it assumed—also wrongly—that heat could be input at t ei . This t ei is a nonsensical quantity and the model leads to some absurd conclusions. The value of t ei as it is evaluated is a different quantity from t ei as it is defined and is numerically larger when heat is input radiantly to the enclosure. The operational value of t ei has a valid theoretical basis for an elementary enclosure but in general it oversimplifies enclosure heat exchange and may lead to somewhat crude numerical estimates. The environmental temperature model comprises conductances, heatflows and the temperature t ei itself which are based on a mixture of convective and radiant processes. The model is unable to express radiant temperature in explicit circuit form. ‘Mean radiant temperature’ (mrt) should denote either the space-averaged observable temperature, or the star temperature of the star pattern used to express radiant exchange; both are global parameters and both are needed. In fact the CIBSE Guide sees mrt as a local parameter, which is inappropriate, and actually identifies it with mean surface temperature, which is grossly in error. Finally the expression for comfort or dry resultant temperature is conceptually inappropriate.