The evaporator is where the actual cooling happens. Low-pressure refrigerant enters as a cold mix of liquid and vapour and boils as it picks up heat from the cabinet air. In fan-forced cabinets the evaporator is a finned coil with a fan blowing across it; in static units it may be a plate, a wall of tubing or a shelf-mounted coil.
Because the evaporator is colder than the cabinet, moisture from the air condenses or freezes on it. In a chiller it simply melts off and drains away during the off-cycle; in a freezer it builds frost that must be removed by a defrost cycle. A coil choked with ice cannot pass air or absorb heat, so the cabinet warms up even though the compressor keeps running.
Evaporator size is chosen against the load. A larger coil can work with a smaller difference between coil and air temperature, which dries food less. That is why produce and sushi display cases favour generous coils. Keeping product and cartons clear of the evaporator fan and air outlets is essential for even temperature on every shelf.