313A Metering Devices

Question 1

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

Accepted Answer

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

Answer

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.

Question 2

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

Accepted Answer

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

Answer

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).

Question 3

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

Accepted Answer

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

Answer

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.

Question 4

Where should the sensing bulb of a TXV be mounted?

Accepted Answer

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

Answer

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.

Question 5

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

Accepted Answer

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

Answer

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.

Red Seal HVAC 313A Practice Test

Red Seal HVAC 313A Practice Test

313A Metering Devices