Heat exchange equipment temperature approach

In most process systems, the evaporation of the refrigerant is carried out in shell and tube heat exchange equipment, and allowance must he made for a reasonable temperature approach between the process fluid and the evaporating refrigerant. The process fluid always leaves the evaporator at a higher temperature (by 3°-15°F) than the refrigerant. 

In selecting heat exchangers an important consideration is the most economical approach or difference in temperature between the incoming large medium and the outgoing small medium. The maximum recovery of heat occurs if these two streams are at the same temperature, but in practical heat-exchange equipment a difference in temperature must exist. 

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