Chiller Heat Rejection

A cooling tower is not selected on the chiller's tonnage. It is selected on the heat it has to throw away, which is the building load plus the work the compressor put in to move that load — and the heat rejection factor is the single number that carries the difference. For a modern electric centrifugal it is about 1.25, so a 250 ton chiller hands its tower 312.5 tons, roughly 3.75 million BTU/h. Read it backwards and it is a useful audit: an absorption machine whose tower is sized for 180 tons of rejection against 100 tons of cooling is running at HRF 1.8, which is exactly why absorption towers dwarf the electric equivalent.

Ordering tower on the wrong side of this factor is one of the classic expensive mistakes in the trade. Buy a 250 ton tower for a 250 ton chiller and you are 25% short of duty on the hottest afternoon of the year — the approach walks out, condensing pressure climbs, and the chiller quietly loses capacity and burns extra compressor power for the rest of its service life, a bill far larger than the tower ever saved. The factor itself is a shorthand for 1 + 1/COP and it moves with the machine: about 1.25 at 0.6 kW/ton, closer to 1.3 on an older or air-cooled-condenser-retrofit reciprocating plant, 1.7–1.8 for single- and double-effect absorption, and effectively 1.0 for a free-cooling or waterside-economiser mode where no compressor is running at all. Take it from the manufacturer's selection where you can; the 1.25 is a placeholder for a number the chiller submittal already knows.