Discharge Ductworks Arrangement
for Air Cooled Condenser
with Fan Cycling Control
By TK NG
Air cooled condensers of air-conditioning/refrigeration systems installed in the vicinity of inhabited areas are usually provided with acoustic treatment to minimize the noise nuisance generated. For air cooled condensers equipped with fan cycling control Note, care should be taken in the design of fan discharge ductworks connecting to acoustic silencers to prevent air short-circuiting during fan cycling.
Using a condenser with three pairs of cooling fans (see Fig. 1) as an example. The first pair (1 & 2) will be in constant operation, while the second (3 & 4) and third (5 & 6) pairs will be cycled on and off in stages in response to the ambient temperature variation.
When individual silencer is connected to each cooling fan or each pair of fans, everything should be fine if the fan static pressure can cater to the additional pressure loss through the silencer (see Fig. 2).
In case the cooling air is ducted before being discharged through the silencers, at least separate ducting for each pair of fans should be provided. Otherwise, part of the air discharged from those fans in operation may be circulated through those cycled off and their associated condenser sections. The two situations are illustrated in Fig. 3. The short-circuited hot air may cause the condenser to shut down due to high temperature cutout as heat is not fully dissipated but accumulated.
When designing ductworks for connecting condenser fans and acoustic silencers, the first thing to check is whether on-off cycling control is adopted for the condenser fans to prevent the condensing temperature and pressure from dropping too low at time of low ambient temperature. The ductworks must be so designed to prevent discharge air short-circuiting.
Air cooled condensers are usually subjected to varying ambient temperature at different time of day, month and year. Both the condensing temperature and pressure will drop with the ambient temperature for the refrigeration system to reach a new equilibrium state. The resulting lower pressure differential across the expansion valve leads to capacity loss albeit lower ambient temperature enhances heat dissipation from the condenser. Fan cycling is a way of control to prevent the refrigerant condensing temperature and pressure from dropping too low by reducing the number of fans in operation and hence the amount of cooling air circulating through the condenser coils.