There are more parameters to be considered in the thermal design of ACHEs than for shell and tube exchangers. ACHEs are subject to wide variety of constantly changing climatic conditions which pose problems of control not encountered with
shell and tube exchangers. Designers must achieve an economic balance between the cost of electrical power for the fans and the initial capital expenditure
for the equipment. A decision must be made as to what ambient air temperature should be used for design. Air flow rate and exhaust temperature is initially unknown and can be varied in the design stage by varying the number of tube rows and thus varying the face area.
Because the number of tube rows, the face area, the air face velocity, and the geometry of the surface can all be varied, it is possible to generate many solutions to a given thermal problem. However, there is obviously an optimum solution in terms of capital and operating costs