Dear, Dr./Prof. Maan S.M. Al-Dabbagh

The primary difference between air-operated and water-operated heat exchangers in terms of the type of pipes used lies in the medium that flows through the pipes to facilitate heat exchange:

air operated heat exchangers have air as the heat transfer fluid. For high performance of the hex material of high or reasonable thermal conductivity is preferred, e.g copper, aluminum, steel, stainless steel. An example is the domestic filament heater.

for water operated hex, water is the htf and the material similarly should be ideal for maximum heat transfer i.e., high thermal conductivity. In addition should be reasonably corrosion resistant. materials can be copper, steel, stainless steel. an example can be an economizer.

a steam boiler is a good example of a combination of both. The boiler tubes have hot gases as the htf and the shell side has water as the htf. here relatively affordable material with reasonable thermal conductivity and with an ability to withstand high pressure is required so schedule 40 steel can be used.

Air-operated heat exchangers typically use materials suitable for gas handling, while water-operated heat exchangers require materials that can withstand higher pressures, temperatures, and potential corrosion from water.