Secondly, in ‘free cooling’ mode a system designed for refrigeration is fairly inefficient. As a result, ‘free cooling days’ are not those with temperatures up to 18-20C, the inlet temperature to the air cooler unit would in theory need to be below 14C.
In practice, however, external chiller units are generally installed in ‘chiller-farms’ either on the roof or on the ground. There can be significant leakage of hot exhaust-air back into the chiller inlet. This means that the inlet is almost never below 14C. So, in some installations, despite the theory, you’ll practically get zero ‘free-cooling’ days.
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