Vacuum Decay on a New Frozen-Food Walk-In
A crew installs a new 12 ft x 16 ft x 10 ft frozen-food walk-in with two unit coolers, a 3 HP R-448A condensing unit, and 80 ft of line set with a 25 ft vertical rise to a remote receiver. They nitrogen-purge during brazing and pressure-test at 300 psig for 24 hours with no drop. For evacuation they use a manifold with 1/4 in. hoses and a core-depressor valve. After 3 hours the micron gauge reads 480 microns. They valve off the pump; the gauge rises to 1,900 microns in 10 minutes and stays there. Outdoor ambient is 85°F. The unit nameplate charge is missing. The installer plans to charge until the sight glass is clear and the suction line sweats, then 'set superheat by feel.' The store opens in 12 hours.
1. Interpret the vacuum readings. Is the system ready to charge? Explain the standing vacuum test and the likely cause of the rise.
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480 microns is a good ultimate vacuum, but the standing test fails: after isolating the pump the gauge rises from 480 to 1,900 microns in 10 minutes and holds. A common pass criterion is a rise of less than about 500 microns in 10 minutes; this system is far above that. Because the 300 psig nitrogen pressure test held for 24 hours, a large leak is less likely, but it is not ruled out. The more likely problem is moisture or a small leak releasing gas from a void. Do not charge. Continue evacuation, use larger hoses and a micron gauge at the system, replace the liquid-line drier, and perform a triple evacuation with dry nitrogen breaks. Repeat the standing test.
2. Describe the correct charging and commissioning sequence for this system, including what to do because the nameplate charge is missing.
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Charge by weight, not by sight glass. Use the condensing unit and unit cooler data to establish the base charge, then add the calculated line-set and receiver allowance; if the nameplate is missing, get the manufacturer's charging table or use the remote-charge formula from the installation manual. R-448A is a blend, so charge liquid into the liquid line and use dew-point temperature for superheat and bubble-point for subcooling. Set superheat at design conditions, verify subcooling, check compressor amps, oil level, defrost termination, and drain. Record the final weight and readings so the next technician has a baseline.
3. What line-set and receiver issues must be checked given the 25 ft vertical rise?
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With a 25 ft vertical rise, check oil return: the suction riser must be sized for minimum load and trapped at the bottom, with additional traps if the manufacturer requires. Verify the receiver is located and valued so liquid can drain from the condenser and the head-pressure control works at low ambient. Check the liquid-line sizing and static pressure drop for the lift, and make sure the suction line does not create an oil trap at the evaporator. Confirm the defrost drain and heater wiring are complete before charging.