Metering Devices Flashcards

What is the primary function of a metering device in a refrigeration system?

Answer: To reduce the pressure and temperature of the liquid refrigerant before it enters the evaporator, and to control refrigerant flow rate

The metering device (expansion device) serves two critical functions: it reduces the high-pressure liquid refrigerant to a low-pressure mixture of liquid and vapour, and it controls the flow rate of refrigerant into the evaporator to match the system's cooling demand.

What is a thermostatic expansion valve (TXV/TEV) and how does it operate?

Answer: A valve that modulates refrigerant flow based on the superheat at the evaporator outlet, using a sensing bulb, equalizer, and spring

A TXV/TEV modulates refrigerant flow to maintain a constant superheat at the evaporator outlet. It uses three forces: the sensing bulb pressure (opening force, responds to suction line temperature), the equalizer pressure (closing force, responds to evaporator pressure), and the spring pressure (closing force, sets superheat).

What is a capillary tube and in what applications is it commonly used?

Answer: A small-diameter fixed-restriction tube that meters refrigerant by friction; commonly used in residential refrigerators, window air conditioners, and small heat pumps

A capillary tube is a long, small-diameter (0.5 mm to 2 mm) copper tube that meters refrigerant through friction-based pressure drop. It is a fixed restriction with no moving parts, making it inexpensive and reliable. It is used in small, fixed-capacity systems where the load is relatively constant.

Where should the sensing bulb of a TXV be mounted?

Answer: On the suction line at the evaporator outlet, at the 10 o'clock or 2 o'clock position on a horizontal line

The TXV sensing bulb must be clamped securely to the suction line at the evaporator outlet. It should be mounted at the 10 o'clock or 2 o'clock position (not on top or bottom) on horizontal lines to avoid oil influence. It must have good thermal contact, be insulated from ambient air, and be located before any external equalizer connection.

What is a fixed-orifice metering device (piston) and how does it differ from a TXV?

Answer: A precisely sized orifice that provides a fixed restriction regardless of load; unlike a TXV, it cannot modulate flow to match changing conditions

A fixed-orifice (piston) metering device has a precisely sized hole that creates a fixed pressure drop. Unlike a TXV, it cannot adjust its opening to match varying loads. Systems using fixed orifices must have a critically accurate refrigerant charge and are less efficient at part-load conditions compared to TXV systems.

What happens to system performance if a TXV sensing bulb loses its charge?

Answer: The valve closes because the opening force is lost, starving the evaporator of refrigerant and causing low suction pressure and poor cooling

If the TXV sensing bulb loses its refrigerant charge (from a leak or physical damage), the opening force is lost. The spring and equalizer forces push the valve closed, severely restricting or stopping refrigerant flow to the evaporator. This results in a starved evaporator, very low suction pressure, high superheat, and little to no cooling.