Overview
The expansion device is the metering component that separates the high-pressure and low-pressure sides of the refrigeration system. It reduces the pressure of subcooled liquid refrigerant from condenser pressure to evaporator pressure, controlling the flow rate to match the cooling load. Correct selection and adjustment of the expansion device directly affects superheat, system capacity, and energy efficiency. These topics are examined under EU Implementing Regulation 2015/2067.
Thermostatic Expansion Valve (TEV / TXV)
The TEV is the most widely used metering device in commercial refrigeration and air conditioning. It modulates refrigerant flow to maintain a set superheat at the evaporator outlet.
- A sensing bulb is clamped to the suction line and filled with a charge that responds to pipe temperature.
- Three forces act on the valve diaphragm: bulb pressure (opens), evaporator pressure (closes), and spring pressure (closes, adjustable).
- The valve opens when superheat rises (indicating the evaporator needs more refrigerant) and closes when superheat falls.
- External equalisation is required on systems with a significant pressure drop across the evaporator (e.g., distributor-fed coils).
Electronic Expansion Valve (EEV)
- Controlled by an electronic controller that reads a suction-line temperature sensor (normally a thermistor) and a pressure transducer to calculate superheat in real time.
- Offers faster, more precise response than a TEV, especially at part-load conditions.
- Common actuator types: stepper motor and pulse-width modulated (PWM) solenoid.
- Increasingly used in inverter-driven systems and heat pumps where load varies widely.
Which Metering Device Do Variable-Speed Heat Pumps Use?
Most high-efficiency variable-speed heat pumps use an electronic expansion valve (EEV). Unlike a fixed orifice or capillary tube, the EEV can continually change its opening as compressor speed, outdoor temperature and system load change. The controller uses suction-line temperature and pressure readings to maintain the target superheat.
Capillary Tubes
- A fixed-restriction device — a long, narrow-bore copper tube that reduces pressure by friction.
- No moving parts, no adjustment. Sized for a specific set of design conditions.
- Used in small, mass-produced systems such as domestic refrigerators and small split air-conditioners.
- Cannot adapt to changing loads, so performance degrades if conditions deviate from design.
TEV vs EEV vs Capillary Tube
| Metering device | How it controls flow | Best suited to | Main limitation |
|---|---|---|---|
| TEV / TXV | Sensing-bulb, evaporator and spring pressures balance across a diaphragm | Systems with a changing load but a conventional control strategy | Slower and less flexible than electronic control |
| EEV | Electronic controller drives a stepper motor or pulsed solenoid from temperature and pressure inputs | Variable-speed, heat pump and high-efficiency systems | Requires sensors, a controller and correct programming |
| Capillary tube | Fixed tube length and bore create a permanent restriction | Small, factory-charged systems with stable design conditions | Cannot adapt to load or ambient changes |
Use the superheat and subcooling calculators to practise the measurements that TEV and EEV controllers respond to.
Ancillary Components
- Filter drier — removes moisture and particulates from the refrigerant. Installed in the liquid line. Must be replaced whenever the system is opened.
- Sight glass / moisture indicator — allows visual confirmation of liquid state and moisture content. Bubbles may indicate low charge or restriction upstream.
- Liquid receiver — stores excess refrigerant charge and ensures a solid column of liquid reaches the expansion device under varying load conditions.
- Solenoid valve — electrically operated shut-off valve used to stop refrigerant flow during off cycles (pump-down control).
Exam Tip: Know the three forces acting on a TEV diaphragm and which direction each pushes. Understand when an externally equalised valve is needed versus an internally equalised one. Exam questions may describe hunting (oscillating superheat) — this is typically caused by an oversized TEV or incorrect bulb placement.
