Low-Pressure Chiller with Air Ingress and Frequent Purge Cycling
A 250-ton R-123 chiller serves a data center (comfort cooling). The charge is 400 lb. The purge unit, which normally cycles every 6 hours, is now cycling every 15 minutes. The chiller's evaporator pressure is 12 inches Hg vacuum. The technician performs a standing vacuum test: with the chiller off, the pressure rises from 12 inches Hg vacuum to 2 inches Hg vacuum over 2 hours. There is no obvious refrigerant odor, and an electronic refrigerant leak detector outside the chiller reads zero. The owner reports that the chiller has been losing capacity and the condenser pressure is high. The technician suspects a leak but cannot find it with a standard refrigerant detector. The chiller must be repaired without venting refrigerant.
1. Explain why the standard refrigerant leak detector reads zero and why the purge unit cycles frequently.
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A low-pressure chiller operates below atmospheric pressure, so any leak draws air and moisture into the system rather than allowing refrigerant to escape. An electronic refrigerant leak detector placed outside the chiller reads zero because no refrigerant is leaking out; air is leaking in. The purge unit removes non-condensable gases such as air from the system. When a leak admits air, the purge unit must operate more often to remove it, so cycling every 15 minutes instead of every 6 hours is a strong indicator of air ingress. The high condenser pressure and loss of capacity are also consistent with air in the system.
2. What leak detection methods should the technician use on this system?
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Because the system is under vacuum, the technician should first use the standing vacuum test already performed to confirm a leak. To pinpoint the leak, pressurize the system with dry nitrogen to a positive pressure of about 5 to 10 psig, never exceeding 15 psig, and use an electronic leak detector or soap bubbles at suspected joints: gaskets, flanges, valve packing, and the rupture disc. An ultrasonic leak detector can also be used while the system is under vacuum because it detects the sound of air entering. The technician should also inspect the purge unit and its connections, since frequent purge cycling can sometimes be caused by a fault in the purge unit itself.
3. After finding a leaking gasket, what evacuation level must be achieved before recharging, and what precautions apply?
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Before recharging, the low-pressure appliance must be evacuated to 25 mm Hg absolute, the Section 608 requirement for low-pressure appliances. Because the chiller barrel contains water, the technician must drain or isolate the water side before pulling a deep vacuum to prevent freezing and tube damage. Use a low-pressure recovery machine and replace the filter-drier after the repair. After evacuation, charge the system with R-123 and perform a leak check and a standing vacuum test to confirm the repair.