Overview
The evaporator is where useful cooling takes place. Low-pressure liquid refrigerant absorbs heat from the product, space, or process being cooled, changing state from liquid to vapour. Correct evaporator operation is critical to system efficiency and product safety. F-Gas technicians assessed under EU Implementing Regulation 2015/2067 must understand evaporator types, how to measure and interpret superheat, and how defrost cycles work.
Evaporator Types
- Direct expansion (DX) — refrigerant evaporates directly inside the coil. The most common type in commercial and domestic systems. Fed by a thermostatic or electronic expansion valve.
- Flooded evaporators — the coil is kept full of liquid refrigerant, providing excellent heat transfer. Used in larger industrial systems. Requires a surge drum or separator to prevent liquid carry-over to the compressor.
- Plate evaporators — compact plate heat exchangers used in chillers and process cooling. High efficiency in a small footprint.
- Forced-air (unit coolers) — finned-tube coils with fans that circulate air across the evaporator. Used in cold rooms, display cabinets, and air-conditioning systems.
- Natural convection (static) — no fans; air circulates by natural convection. Used where low air movement is required (e.g., some wine stores, sensitive product storage).
Superheat
Superheat is the temperature of the refrigerant vapour above its saturation temperature at the evaporator outlet. It confirms that all liquid has evaporated before reaching the compressor, preventing liquid slugging.
- Measured by subtracting the saturation temperature (from suction pressure via a PT chart) from the actual suction line temperature (from a pipe clamp thermometer).
- Typical superheat: 5–8 K for most DX systems.
- Low superheat risks liquid flood-back; high superheat indicates the evaporator is starved of refrigerant, reducing capacity.
Defrost Methods
In sub-zero applications, frost and ice accumulate on evaporator surfaces, insulating the coil and blocking airflow. Common defrost methods include:
- Off-cycle (air) defrost — fans run with the compressor off; ambient air melts frost. Only effective above 2–3 °C room temperature.
- Electric defrost — heater elements mounted on the coil are energised at timed intervals.
- Hot-gas defrost — high-pressure discharge gas is directed through the evaporator, melting ice from the inside out. Fast and efficient.
- Reverse-cycle defrost — used in heat pump systems; the cycle reverses so the outdoor coil becomes the condenser temporarily.
Defrost scheduling must balance energy use against frost build-up. Over-frequent defrosts waste energy and raise product temperatures.
Exam Tip: Be able to calculate superheat from given pressure and temperature data. Understand that excessive frost on an evaporator reduces both capacity and efficiency. Exam scenarios may describe symptoms such as poor cooling, high suction superheat, or iced-up coils — connect these to the underlying cause.
