Rad-air temperature: The global temperature in an enclosure

Abstract

It is first established that heat losses from an enclosure must be based on a global index temperature, rather than on any observable value. Environmental temperature is such an index temperature but it and its associated model are ill-conceived and unsuitable as a general method of handling radiative and convective exchange in merged form in a room. Rather, one should use the binary star model if one wishes to keep these processes separate, or the rad-air model if they are to be merged. The rad-air temperature is the true global temperature in a room and is exactly equivalent for indoor use to the well established sol-air temperature (which relates to an outer surface.) Holmes has recently presented a comparison of computational methods in which he has demonstrated that the binary star method has no advantages in estimating surface temperatures over the environmental temperature method as set out in the 1970 IHVE Guide. With Holmes's choice of surface emissivities and convection coefficients, this result is to be expected; his results support the use of the rad-air model in connection with the rooms he analyses. The exact equivalence of the rad-air and sol-air concepts is established. It is demonstrated that the Guide method of arriving at environmental temperature amounts to a delta-to-star transformation of the real enclosure conductances; and this proves to be invalid. The scaling factor which is needed in handling radiation is treated through an equivalence theorem. The 'surface factor' technique in the Guide is shown to underestimate certain enclosure temperatures.