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Refrigeration air coolers (evaporators) are widely used to cool and circulate air in cold storage warehouses and food processing facilities. Manufacturers of air coolers publish cooling capacities based on differing assumptions and rating methods. It is important for refrigeration design professionals to understand these different rating methods and to apply them appropriately. In extreme cases, air coolers can be grossly undersized even though nominal catalog ratings appear to satisfy the calculated refrigeration load.
DT1 ratings are based on the initial temperature difference, or "TD", defined as Entering Air Temperature minus Evaporating Temperature. DTM ratings, on the other hand, are based on an artificial average temperature difference defined as Average (Mean) Room Temperature minus Evaporating Temperature. DTM ratings will always appear to be significantly higher than DT1 ratings for the same air cooler operating at the same "temperature difference". In the case of DTM ratings, the refrigeration system designer must be careful to clearly understand the manufacturer’s definition of "temperature difference"! Air cooler ratings are also affected significantly by room relative humidity and resulting imposed latent load on the evaporator coil. Ratings which include the latent load will always appear to be higher than ratings which are based on sensible cooling only (i.e. Sensible Heat Ratio (SHR) = 1.0).
American air cooler manufacturers have traditionally published capacity ratings based on SHR = 1.0 (all sensible) and DT1. European manufacturers typically include latent cooling in their air cooler ratings, indicated by an air on relative humidity typically between 85% and 95%. European manufacturers also publish ratings based on either DT1 or DTM, or both. Colmac President, Bruce Nelson, has written a new Technical Bulletin on this important topic which illustrates the differences in these rating methods and highlights the importance of selecting air coolers using ratings suited to the operating conditions. In the Bulletin, Mr. Nelson observes that "Misapplication of DTM and/or total cooling ratings can result in severely undersized air coolers and the consequent failure of the refrigeration system to perform to energy efficiency and cooling capacity expectations."
For more information and requests for quotation for accurately sized refrigeration air coolers, please contact Jeremy Olberding, Sales Manager at 1-800-845-6778 or contact us online.
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