Pressure Vessels & Heat Exchangers
An air cooled heat exchanger is the only exchanger whose cold side you do not control. The air arrives at the temperature the site gives you, it changes through the day and through the year, and the whole design turns on a single decision: which ambient temperature you call the design temperature.
Techso designs air cooled heat exchangers to API 661 / ISO 13706 for the manufacturers who build them: thermal rating, mechanical design of headers and bundle to ASME VIII and PED, and the supporting structure. Our main client on this equipment is Astra Refrigeranti, an Italian manufacturer of heat transfer equipment.
Picking the design dry bulb temperature is not a meteorological question, it is an economic one. Design for the annual maximum and the unit is oversized for every hour of the year except a handful. Design for the mean and it will not meet duty exactly when the plant needs it most, in the middle of a summer afternoon. The usual practice is a percentile of the site record, stated explicitly on the datasheet, together with what happens above it.
This is also where a revamp usually starts. Plants designed twenty years ago on a design air temperature that the site now exceeds regularly become the bottleneck of the whole unit on the hottest days, and the question arrives as 'why are we cutting production in August'.
The approach is the gap between the process outlet temperature and the design air temperature. It sets how much surface the duty will cost, and it does so very non-linearly: the surface climbs steeply as the approach closes. Below roughly 10 K an air cooler stops being the economical answer, and the usual arrangement is to let it take the bulk of the duty and finish the last few degrees with a water trim cooler.
Forced draft puts the fans under the bundle, in cold air: lower power for the same duty, easier maintenance access, cheaper drives. Induced draft puts them above, in the hot air leaving the bundle: more power and a hotter environment for the drive, but a far more uniform air distribution across the bundle and much less exposure to hot air recirculation, which is what quietly ruins the performance of a badly laid out bank.
API 661 is a purchase specification more than a calculation method: it fixes what the buyer is entitled to expect, and a large part of it is about the things that fail in service rather than the things that size the unit — fan coverage and tip clearance, plenum geometry, header design and access for tube plugging, vibration switches, guards and walkways, and the noise the machine is allowed to make.
Contact us for a tailored consultation on Air Cooled Heat Exchangers — API 661.
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