Forced Air Evaporator Coils
Air velocities across the coil should not exceed 500-600 FPM in order to avoid blowing water from the coil onto the load. Care should be taken to insure even air distribution across the coil, since uneven air flow can cause uneven loading of the refrigerant circuits. Fin spacing exceeding 6 per inch is not recommended because of the rapid build-up of frost on the fins. However, some users and manufacturers recommend spacing as low as 3 or 4 fins per inch, wile others report satisfactory experience with spacings as high as 8 per inch provided proper defrost controls are used.
Delivered air velocity should be adequate to insure good air circulation in the vehicle. Noise level is not a design limitation in a van, so velocities up to 10500 FPM or higher can be used.
Internal volume of the refrigerant tubes should be kept to a minimum to keep the refrigerant volume as low as possible. Since pressure drop at low temperatures is critical so far as capacit is concerned, multiple refrigerant circuits with fairly shor runs are preferred. Pressure drop in the evaporator should be no more than 1 to 2 psig. At the same time, it-is essential that velocities of refrigerant in the evaporator be high enough to avoid oil trapping. 5/8´´ evaporator tubes are acceptable, but 1/2´´ are preferred, and 3/8´´ tubing has been used succcessfully. Vertical headers should have a bottom outlet to allow gravity oil draining.
An evaporator face guard should be provided to protect the fins and tubing from cargo damage. Ample air inlet area should be provided, with access from both sides and the bottom if possible, to prevent blocking of alr to the evaporator by cargo stacked in the vehicle.
