Air cooling evaporators for display cases, coldrooms, blast freezers and air — conditioning have finned pipe coils (see Figure 7.1). In all but very small coolers such as domestic and small retail units there will be fans to blow the air over the coil.

Construction materials are the same as for air-cooled condensers. Aluminium fins on copper tube are the most common for the halocarbons, with stainless steel or aluminium tube for ammonia. Frost or condensed water will form on the fin surface and must be drained away. To permit this, fins are vertical and the air flow horizontal, with a drain tray provided under. 91



Figure 7.1 Air cooling evaporators. (a) Floor mounted. (b) Ceiling mounted (Searle)

The size of the tube will be such that the velocity of the boiling fluid within it will cause turbulence to promote heat transfer. Tube diameters will vary from

9 mm to 32 mm, according to the size of coil.

Fin spacing will be a compromise between compactness (and cost) and the tendency for the interfin spaces to block with condensed moisture or frost.

Spacings will vary from 2 mm on a compact air-conditioner to 12 mm on a low — temperature coldroom coil.

Air cooling evaporators are invariably of the direct expansion type.

Posted in Refrigeration and Air Conditioning