Room internal heat exchange: New design method

Abstract

The paper reviews the approach taken by the present CIBSE Guide as it handles convective and radiant exchange in an enclosure through the use of the room index temperature, ‘environmental temperature’. Certain difficulties in handling radiant exchange are identified. The paper goes on to show how the rigorous but intractable radiant exchange pattern in an enclosure can be simplified with good accuracy to a more amenable form centred on a radiant star temperature T1. It is shown that if the radiant energy from an internal heat source is taken as input at T1, it generates a (necessarily fictitious) temperature which proves to be a satisfactory approximation to the average value of the radiant temperature observable over the enclosure volume. The emissivities of the separate surfaces are included explicitly in this formulation. Further heat inputs may act at the surface nodes, T1, T1. … Convectively input heat is taken to act at the mean air temperature, T1. The temperature response of the enclosure to the various heat inputs is illustrated using this model and is compared with the results of two simpler models. In the first model (that discussed above) convective and radiant heat inputs are handled separately, as physically speaking they must. The method is conceptually simple, logical, flexible and accurate but may require the solution of simultaneous equations. In the second model convection and radiation inputs are simply lumped at an ‘air temperature node’. The method is computationally very simple but is illogical in a number of respects. It is commonly used. In the third model convective and radiant inputs are partly lumped and input at a node Tc. The model has a logical basis of a kind, but the basis is too restrictive to afford the method full credibility. Its conductances, heat flows and Tc itself are conceptually recondite through combining the two heat flow mechanisms. For preliminary and crude estimates of heat needs, the second, traditional model is suitable. For accurate assessment, the first model is needed. The third model, closely akin to the present CIBSE Guide method, provides a possible but unattractive approach.