RSES Expansion Devices 4 — Questions and Answers
Question 1: What three forces act on the TXV needle/pin to control refrigerant flow?
- Bulb pressure, spring pressure, and atmospheric pressure
- Bulb pressure, evaporator pressure, and spring pressure (Correct answer)
- High-side pressure, low-side pressure, and gravity
- Ambient temperature, suction temperature, and spring tension
Correct answer: Bulb pressure, evaporator pressure, and spring pressure
The TXV balances bulb pressure (opening force) against evaporator pressure plus spring pressure (closing forces) to modulate flow.
Question 2: When installing a TXV sensing bulb on a suction line larger than 7/8 inch OD, the bulb should be mounted at:
- The 12 o'clock (top) position
- The 4 or 8 o'clock position (Correct answer)
- The 6 o'clock (bottom) position
- Any position since size doesn't matter
Correct answer: The 4 or 8 o'clock position
On larger suction lines, mounting the bulb at the 4 or 8 o'clock position ensures it contacts refrigerant vapor rather than oil that pools at the bottom.
Question 3: Which expansion device is best suited for a system with widely varying load conditions?
- Capillary tube
- Automatic expansion valve
- Thermostatic expansion valve (Correct answer)
- Short-tube orifice
Correct answer: Thermostatic expansion valve
The TXV modulates based on superheat sensing, making it ideal for systems with variable loads unlike fixed-restriction devices.
Question 4: A clogged or restricted TXV will cause the suction pressure to:
- Increase above normal
- Remain at normal operating pressure
- Drop below normal (Correct answer)
- Fluctuate erratically between high and low
Correct answer: Drop below normal
A restricted TXV starves the evaporator of refrigerant, reducing suction pressure and causing abnormally low evaporator temperatures.
Question 5: What is meant by the term 'static superheat' in a TXV?
- Superheat measured during system shutdown
- The superheat setting due to spring tension alone when bulb and evaporator pressures are equal (Correct answer)
- Superheat measured at maximum load
- The superheat caused by ambient temperature effects
Correct answer: The superheat setting due to spring tension alone when bulb and evaporator pressures are equal
Static superheat is established by the valve spring's initial tension and represents the minimum superheat before the valve begins to open.
Question 6: In a low-temperature refrigeration application, the TXV sensing bulb charge should be:
- The same refrigerant as the system charge
- A cross-charge designed to prevent valve closure at low ambient temperatures (Correct answer)
- Air or nitrogen for stability
- A higher-pressure refrigerant than the system uses
Correct answer: A cross-charge designed to prevent valve closure at low ambient temperatures
Cross-charged bulbs use a different fluid blend to maintain valve responsiveness and prevent the bulb from condensing at low evaporating temperatures.
Question 7: What is the purpose of the MOP (Maximum Operating Pressure) feature in some TXVs?
- To limit compressor discharge pressure
- To prevent the evaporator pressure from exceeding a set maximum during pulldown (Correct answer)
- To control condenser fan cycling
- To regulate refrigerant charge quantity
Correct answer: To prevent the evaporator pressure from exceeding a set maximum during pulldown
MOP prevents the TXV from opening fully during high-load pulldown conditions, protecting the compressor motor from overloading.
RSES Expansion Devices 4
What three forces act on the TXV needle/pin to control refrigerant flow?