313A Metering Devices — Questions and Answers
Question 1: What is the primary function of a metering device in a refrigeration system?
- To measure the amount of refrigerant in the system
- To reduce the pressure and temperature of the liquid refrigerant before it enters the evaporator, and to control refrigerant flow rate (Correct answer)
- To measure the superheat at the compressor
- To regulate the speed of the compressor
Correct 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.
Question 2: What is a thermostatic expansion valve (TXV/TEV) and how does it operate?
- A valve that responds to room temperature
- A valve that modulates refrigerant flow based on the superheat at the evaporator outlet, using a sensing bulb, equalizer, and spring (Correct answer)
- A valve that opens and closes based on time
- A valve that controls water flow through the condenser
Correct 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).
Question 3: What is a capillary tube and in what applications is it commonly used?
- A large-diameter pipe for refrigerant storage
- A small-diameter fixed-restriction tube that meters refrigerant by friction; commonly used in residential refrigerators, window air conditioners, and small heat pumps (Correct answer)
- A tube that measures capillary pressure in the system
- A tube used to connect pressure gauges
Correct 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.
Question 4: Where should the sensing bulb of a TXV be mounted?
- On the liquid line leaving the condenser
- On the suction line at the evaporator outlet, at the 10 o'clock or 2 o'clock position on a horizontal line (Correct answer)
- On the discharge line of the compressor
- Inside the evaporator coil
Correct 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.
Question 5: What is a fixed-orifice metering device (piston) and how does it differ from a TXV?
- A piston that compresses refrigerant
- A precisely sized orifice that provides a fixed restriction regardless of load; unlike a TXV, it cannot modulate flow to match changing conditions (Correct answer)
- A device that measures oil flow
- A variable-size orifice controlled by a thermostat
Correct 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.
Question 6: What happens to system performance if a TXV sensing bulb loses its charge?
- The system operates normally
- The valve closes because the opening force is lost, starving the evaporator of refrigerant and causing low suction pressure and poor cooling (Correct answer)
- The valve opens fully, flooding the compressor
- Only the defrost cycle is affected
Correct 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.
313A Metering Devices
What is the primary function of a metering device in a refrigeration system?