Air cooling evaporators working below 0°C will accumulate frost which must be removed periodically, since it will obstruct heat transfer.
Where the surrounding air is always at +4°C or higher, it will be sufficient to stop the refrigerant for a period and allow the frost to melt off (as in the auto-defrost domestic refrigerator). This method can be used for coldrooms, packaged air-conditioners etc., where the service period can be interrupted.
For lower temperatures, heat must be applied to melt the frost within a reasonable time and ensure that it drains away. Methods used are as follows:
1. Electric resistance heaters. Elements are within the coil or directly under it.
2. Hot gas. A branch pipe from the compressor discharge feeds superheated gas to the coil. The compressor must still be working on another evaporator to make hot gas available. Heat storage capsules can be built into the circuit to provide a limited reserve of heat for a small installation.
3. Reverse cycle. The direction of flow of the refrigerant is reversed to make the evaporator act as a condenser. Heat storage or another evaporator are needed as a heat source.
In each of these cases, arrangements must be made to remove cold refriger-ant from the coil while defrosting is in progress. Drip trays and drain pipes may require supplementary heating.
Posted in Refrigeration and Air Conditioning