- (a) Sketch a thermostatically controlled expansion valve for a vapour compression refrigeration system, labelling all components.
(b) Describe the operation of the valve sketched in part (a).
(a) Thermostatically Controlled Expansion Valve (TEV) – Sketch and Components (5 marks)
Your sketch should show the following labelled parts:
- Inlet (from liquid line/condenser)
- Valve seat and needle
- Diaphragm
- Spring (adjustable for superheat setting)
- Capillary tube
- Sensing bulb (attached to evaporator outlet)
- Outlet to evaporator
(Diagram would show liquid refrigerant entering from the condenser, passing the needle/seat, controlled by diaphragm deflection, and then going into the evaporator.)
(b) Operation of a Thermostatic Expansion Valve (5 marks)
- The sensing bulb is clamped to the evaporator outlet and filled with a charge (same or similar to refrigerant).
- As refrigerant vapour leaves the evaporator, its temperature (superheat) is detected by the bulb.
- The fluid in the bulb expands or contracts with temperature, exerting pressure via the capillary tube on the top side of the diaphragm.
- Opposing forces:
- Bulb pressure (opens valve) proportional to evaporator outlet superheat.
- Spring pressure (closes valve), adjustable to set required superheat.
- Evaporator pressure (closes valve) acts on underside of diaphragm.
- Balance of forces regulates the valve needle position:
- If outlet superheat rises → bulb pressure increases → valve opens wider → more liquid refrigerant admitted.
- If outlet superheat falls → bulb pressure reduces → spring and evaporator pressure close the valve → less refrigerant admitted.
Purpose: Maintains correct refrigerant flow, prevents liquid carryover (floodback), and ensures efficient evaporator operation.