In addition to the Five fingers rule, referring to the five parameters required for the correct sizing of a heat exchanger, there is also a Four fingers rule, which is employed when designing a thermal machine.

A thermal machine that can be a dry cooler, or a refrigerating group, or a cooling tower. In this case, there are indeed four parameters required for the proper sizing of the machinery: the kind of fluid to be cooled and its flow rate; the inlet temperature, which is the temperature of the fluid as it comes from the process; the outlet temperature, which is the temperature of the fluid required when it goes back to the process; the design ambient air temperature.

 

 

Ambient air temperature that in case of a chiller is related to the condensation process, in case of a dry cooler is the temperature of the air that will cool the fluid. And finally, in case of an evaporative tower is the wet-bulb temperature, a key value for the evaporation process.

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