Why it is needed
If the surface temperature is below the dew point of the air and below zero, the moisture in the air turns to frost on the coil. The frost first behaves like insulation and lowers the heat transfer, then closes the gaps between the fins and cuts the air flow. Together the two drop the capacity, and the system runs longer and makes more ice.
The light and heavy arrangements
Planer evaporators have two standard arrangements. In the light defrost system the electric heaters are only on the heat exchanger, and it is for rooms running in warmer conditions. In the heavy defrost system the heaters are in the drain tray as well as on the exchanger; it is for low temperature conditions.
The difference is in the tray. At frozen-room temperature the melted water freezes again in the tray and the drain line; a tray without a heater turns into a unit whose defrost works but which cannot get the water out of the room. That is also why the drain line heater is on the options list.
What SC1–SC4 mean
The defrost recommendations in the catalogues are given against condition steps running from SC1 to SC4. Those steps are EN 328's standard measurement conditions and descend from a warm storage room towards deep freezing. The exact temperature and humidity values for each step are written in the catalogues; we do not repeat them here, because a condition written down wrong shifts the whole calculation.
The routes other than electric
Hot gas defrost takes the heat from the system's own cycle and clears the coil faster while leaving less heat in the room than an electric heater; in return it needs a circuit design and an extra valve. Water defrost is effective against thick ice build-up but calls for drainage and freezing to be managed.
All three are optional and the choice is made against the room. Send us your room temperature, the product type and the daily door traffic; we will settle which arrangement is needed together.