Walk-In Cooler: High Superheat With a Clear Sight Glass
A medium-temperature walk-in cooler (35°F setpoint) uses a remote air-cooled condensing unit and a TXV with an external equalizer; the refrigerant is R-448A. The box is holding 46°F. At steady state you measure: suction pressure 40 psig, which the PT chart lists as 22°F saturation; suction line temperature at the sensing bulb 54°F; liquid line temperature 105°F with a 113°F condensing saturation temperature; the sight glass is clear; compressor amp draw is slightly below RLA; the evaporator is cold right after the distributor and warm at the outlet. Condenser ambient is 88°F. The owner says another company "topped off the charge twice this summer" and the problem keeps coming back.
1. Calculate superheat and subcooling from these readings. What does each number tell you about the low side and the high side of this system?
Read the worked answer
Superheat = suction line temp − suction saturation temp = 54°F − 22°F = 32°F. On a medium-temperature walk-in with a TXV, the target superheat at the bulb is roughly 8–12°F, so 32°F means the evaporator is badly starved - the refrigerant that enters is boiled off long before the outlet, and the last circuits are doing almost no work. Capacity is down; a warm box follows.
Subcooling = condensing saturation temp − liquid line temp = 113°F − 105°F = 8°F. That is in the normal range for an air-cooled system and tells you the liquid line is full of liquid with no flash gas - so there is no restriction upstream of the TXV, no undercharge, and no non-condensable problem big enough to matter.
Read together: liquid is available and stacked normally behind the metering device, but it is not getting through. The fault is at or downstream of the TXV, not in the charge or the condenser.
2. The technician proposes adding 2 lb of R-448A to "bring the suction pressure up." Explain why that is the wrong move, what the added refrigerant would actually do to your readings, and what non-invasive test you would run first.
Read the worked answer
Adding refrigerant attacks the wrong end of the system. The subcooling of 8°F and the clear sight glass already prove the high side is properly charged; feeding more liquid into a system whose meter will not pass it simply stacks liquid in the condenser.
What you would actually see:
- Subcooling climbs (to 15–25°F), head pressure and compressor amps rise, capacity does not.
- Superheat barely moves, so the box stays warm and the tech concludes "it needs more."
- If the restriction later clears or the valve starts hunting, the extra liquid slugs the compressor.
Before touching the charge, run a temperature-drop test across the TXV body and its inlet screen (a drop of more than a couple of degrees means a restriction), verify the bulb is clean, tight, insulated, and at the correct clock position on the suction line, and check each distributor circuit for an even frost pattern/temperature. Compare the reading to the fact that superheat should respond to the adjustment stem - a valve that will not respond is plugged or failed.
3. You find a 14°F temperature drop across the TXV inlet screen and one distributor circuit noticeably colder than the others. Give your repair plan, including refrigerant handling and EPA 608 obligations.
Read the worked answer
Findings point to a restricted TXV inlet screen plus a partially plugged distributor tube - debris that the old liquid line filter-drier should have caught.
Plan:
- Recover the charge into a recovery cylinder with a recovery machine. Under EPA Section 608 you may not vent R-448A; record the recovered weight.
- Pump down/isolate, then replace the liquid line filter-drier (the source of the debris) and clean or replace the TXV inlet screen; replace the valve if the screen body is damaged.
- Purge with nitrogen while brazing, flow nitrogen during the repair, and install a new filter-drier last.
- Pressure-test with dry nitrogen (never with refrigerant), then evacuate to 500 microns and perform a standing vacuum test to confirm no leak and no moisture.
- Weigh in the nameplate charge; do not charge by feel.
- Restart and verify: superheat 8–12°F at the bulb, subcooling per manufacturer's spec (roughly 8–12°F), even frost on all circuits, box pulldown, and amp draw at or below RLA.
Document the recovered/charged weights on the record - that is a 608 requirement for appliances over 5 lb, and it is also your defense if the owner calls back.