By Eamonn Ryan
At a SAIRAC Cape Town Centre TechTalk, Marius la Grange – general manager of Thermocoil Cape Town and a past SAIRAC president – took delegates through the fine art of choosing the right evaporator. This is part one of a three-part series.
Far from a matter of simply picking a unit from a Thermocoil catalogue, he stressed, it’s a disciplined balance of science, standards and experience. Correct selection determines not only how well a system cools, but how efficiently, reliably and comfortably it performs over its lifetime.
The starting point, La Grange explained, is understanding nominal duty – the amount of heat the blower coil must remove to maintain the desired room temperature (high and low). That begins with four fundamentals: the room set-point, the suction temperature, the refrigerant and airflow characteristics.
Consider a customer wanting to keep a space at 16°C. With a ΔT (difference between room and suction) of 10K, the suction temperature must be 6°C. Using the manufacturer’s conversion tables, this yields a correction factor of 1.6. Thus, a fin-and-tube evaporator rated at 2.4kW under standard conditions would easily handle the duty required for those conditions.
Every manufacturer’s catalogue – be it Thermocoil, Heat Exchange Centre or Colcoil – follows the same Eurovent-certified method, which allows direct comparison between brands under identical laboratory conditions. Eurovent defines standard test points: SC2 for medium-temperature rooms (air in at 0°C, suction -8°C) and SC3 for low-temperature storage (air -18°C, suction -25°C). These nominal values are the baseline, to be corrected for the refrigerant used and the coil’s material.
Two key correction factors apply: F1, which adjusts for the refrigerant’s latent heat of vapourisation, and F2, for fin or tube materials. Multiply the nominal duty by both to find true coil capacity. “Each refrigerant absorbs heat differently. A kilogram of R290, for example, removes almost twice the heat of R22 when it boils,” said La Grange.
Coil design and application
Evaporator coils are fin-and-tube heat exchangers on the low-pressure side of a direct-expansion system. As liquid refrigerant evaporates inside the coil, it absorbs heat from the room air and exits as superheated vapour ready for compression and heat rejection at the condenser.
Medium-temperature rooms – such as dairies or produce stores – typically use 4mm fin spacing to maximise surface area and heat transfer. Freezers, operating at much lower temperatures and higher humidity, require 7mm spacing to reduce frost build-up and allow longer intervals between defrosts. “The closer the fins, the greater the heat transfer – but also the quicker the icing,” La Grange warned.
Misunderstanding the temperature regime can be costly. A butchery cold room around 0°C is medium-temperature, but a freezer holding meat or ice cream must maintain at least -18°C. One client, he recalled, stored frozen pineapple juice at -10°C; the enzymes remained active and burst the drums within weeks.