ASHRAE Journal - February 2009 - (Page 27) On Air-Cooling Evaporators By Douglas T. Reindl, Ph.D, P and Todd B. Jekel, Ph.D, P .E., .E. Frost I n this article, we focus on the operation of air-cooling evaporators in industrial refrigeration applications at temperatures below freezing. These heat exchangers Air-cooling evaporators are refrigerantto-air heat exchangers widely used in industrial refrigeration, commercial refrigeration, and heat pump systems. Known as “forced-circulation air coolers”1 or “aircooling evaporators,” these heat exchangers use tubes to carry refrigerant with fins February 2009 are generally applied to control the environmental conditions in holding freezers, dynamic blast freezers, stationary blast cells, refrigerated docks, as well as other lower temperature conditioned spaces found in food manufacturing and distribution facilities. We review the factors that influence the formation of frost on the evaporator surfaces and discuss how frost accumulation impacts coil performance. applied to the tube exterior. Individual tubes of the heat exchanger are arranged in multiple rows of parallel circuits to achieve increased thermal performance. Refrigerant evaporates inside the tubes as it absorbs heat from air flowing over the outside surface of the finned tubes. When air-cooling evaporators operate with both coil surface temperatures below 32°F (0°C) and entering air dew-point temperatures above the coil surface temperature, moisture from the air being cooled will accumulate on the fins and tubes of the coil in the form of frost. The formation and growth of frost on the evaporator will decrease the coil’s cooling capability. What is the root cause of capacity decrease from frosted coil? a. Increased air-side pressure drop; thereby, decreasing airflow through the coil. b. Increased resistance to heat transfer between the air and the About the Authors Douglas T. Reindl, Ph.D., P .E., is a professor and director and Todd B. Jekel, Ph.D., P .E., is assistant director at the University of WisconsinMadison’s Industrial Refrigeration Consortium in Madison, Wis. ASHRAE Journal 27
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