RSES Expansion Devices 2 — Questions and Answers
Question 1: What happens to refrigerant pressure as it passes through a thermostatic expansion valve (TXV)?
- Pressure increases significantly
- Pressure drops from high-side to low-side pressure (Correct answer)
- Pressure remains constant
- Pressure fluctuates based on compressor speed
Correct answer: Pressure drops from high-side to low-side pressure
The TXV creates a pressure drop that allows the refrigerant to flash from liquid to a low-pressure mixture entering the evaporator.
Question 2: A TXV with an external equalizer senses evaporator pressure at which location?
- At the TXV inlet
- At the compressor suction port
- At the evaporator outlet (Correct answer)
- At the condenser outlet
Correct answer: At the evaporator outlet
The external equalizer line connects to the evaporator outlet, allowing the valve to sense actual evaporator pressure rather than inlet pressure.
Question 3: Which condition would cause a TXV to be in the fully closed position?
- High superheat at the bulb location
- Low superheat at the bulb location (Correct answer)
- High evaporator load
- High ambient temperature
Correct answer: Low superheat at the bulb location
Low superheat indicates excessive liquid refrigerant at the evaporator outlet, causing the TXV bulb pressure to drop and close the valve.
Question 4: What is the typical superheat setting range for a TXV used in air conditioning applications?
- 1-3°F
- 8-12°F (Correct answer)
- 20-25°F
- 30-40°F
Correct answer: 8-12°F
Most air conditioning TXVs are factory set for 8-12°F superheat to ensure full evaporator use while protecting the compressor.
Question 5: An automatic expansion valve (AEV) maintains constant:
- Suction superheat
- Evaporator pressure (Correct answer)
- Condensing temperature
- Refrigerant flow rate
Correct answer: Evaporator pressure
The AEV modulates to maintain a constant evaporator (low-side) pressure regardless of load changes.
Question 6: Which type of expansion device is most commonly used in small hermetic refrigeration systems like household refrigerators?
- Thermostatic expansion valve
- Float valve
- Capillary tube (Correct answer)
- Electronic expansion valve
Correct answer: Capillary tube
Capillary tubes are inexpensive, have no moving parts, and are well-suited to fixed-load applications like household refrigerators.
Question 7: What effect does increasing the length of a capillary tube have on system performance?
- Increases refrigerant flow rate
- Decreases refrigerant flow rate (Correct answer)
- Has no effect on flow rate
- Increases evaporator pressure
Correct answer: Decreases refrigerant flow rate
A longer capillary tube increases friction resistance, reducing refrigerant flow rate and lowering evaporator pressure.
RSES Expansion Devices 2
What happens to refrigerant pressure as it passes through a thermostatic expansion valve (TXV)?