Brown (1990) discusses run-around and indirect evaporative pre-cooling.
Two basic concepts are applicable—run-around heat recovery and indirect evaporative pre-cooling. These concepts have been limited to schemes that rely on piping the cooling fluid to a finned-tube heat exchanger within the makeup air unit. While there are other variable heat recovery or indirect evaporative cooling concepts, they are predicated on a close-coupled arrangement of makeup air and exhaust air. This results in a physical arrangement not considered practical for most cleanroom system designs. For the run-around heat recovery arrangement, the recovery coil in the exhaust duct can be placed upstream or downstream of the scrubber. When placed downstream, the entering coil temperature is less (due to adiabatic cooling in the scrubber), thereby resulting in greater cooling potential. However, while enhancing cooling opportunities, preheating opportunities are reduced. With the coil placed upstream, cooling heat recovery is decreased, while preheating opportunities are enhanced. To select the best location, it is necessary to evaluate how the exhaust stream is partitioned. Generally, only the acid exhaust is scrubbed and could amount to 50% or less of the total exhaust. Further, as the semiconductor industry moves toward more dry processing vs. wet, the opportunities to benefit from better cooling potential associated with scrubbed exhaust diminish.
