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Low-pressure appliances must be evacuated to 25 mm Hg absolute, a deep vacuum roughly equal to 29 inches of mercury vacuum. This level is the same whether the recovery equipment was made before or after November 15, 1993. The inches-of-mercury figures apply to high-pressure systems, not low-pressure chillers.

A low-pressure appliance must never be pressurized above 10 psig. The shell is protected by a rupture disk that typically relieves at 15 psig, so staying under 10 psig keeps a safe margin below that disk. Higher pressures risk bursting the rupture disk and venting the charge.

The rupture disk on a low-pressure chiller is generally set to relieve at 15 psig, protecting the low-pressure shell from over-pressurization. This is exactly why leak-test pressure must stay at or below 10 psig. The disk must never be bypassed or blocked.

A purge unit removes non-condensable gases, mainly air and moisture, that leak into a chiller running under vacuum and collect at the top of the condenser. Removing them lowers head pressure and restores efficiency. A high-efficiency purge unit also recovers refrigerant vapor before venting the air.

HCFC-123 is a classic low-pressure chiller refrigerant, along with CFC-11 and the newer HFO R-1233zd. R-410A, R-22, and R-404A are all high-pressure refrigerants that operate above atmospheric pressure and fall under Type II. Low-pressure refrigerants boil above about 50°F at atmospheric pressure.

Since the low side runs below atmospheric pressure, a leak pulls air and moisture inward rather than pushing refrigerant out. That incoming air becomes non-condensable gas the purge unit must remove, and the moisture can cause acid and corrosion. This inward leakage shapes nearly every Type III service procedure.

Water that leaks into the refrigerant side reacts to form acids, corrodes metal parts, can freeze and damage tubes, and contaminates the refrigerant. Because the chiller runs under vacuum, a tube leak lets cooling water be pulled inward. Any sign of water intrusion calls for investigation, not just resetting controls.

Recovery moves fastest when the refrigerant is warm and the recovery cylinder is cooler, so vapor naturally migrates toward the cold cylinder. Warm the chiller by circulating warm water or using built-in heaters, never a torch. Overpressurizing with nitrogen or applying open flame is unsafe.

A rising purge rate means air is leaking into the vacuum side of the chiller, so the purge unit runs more often to remove it. The correct response is to find and repair the leak, not simply purge more. Modern high-efficiency purge units also track how much they run as a diagnostic.

Low-pressure refrigerants boil above roughly 50°F at atmospheric pressure, so the appliance operates at or below atmospheric pressure. Refrigerants boiling between minus 50°F and 10°F are high-pressure, and those below minus 50°F are very-high-pressure. This boiling-point distinction sets which certification type applies.

Dry nitrogen is the correct pressurizing gas, sometimes with a small amount of refrigerant so an electronic detector can sense the leak. Oxygen or compressed air can combine with refrigerant oil and explode, and acetylene is a fuel gas. Even with nitrogen, never exceed 10 psig in a low-pressure appliance.

Unlike the high-pressure evacuation table, the low-pressure requirement of 25 mm Hg absolute is the same for recovery equipment made before or after November 15, 1993. There is no split based on equipment age or chiller size for low-pressure machines. This single deep-vacuum figure is a heavily tested Type III fact.

25 mm Hg absolute is a deep vacuum equal to roughly 29 inches of mercury vacuum on a compound gauge, since atmospheric pressure is about 760 mm Hg or 30 inches Hg. This is far deeper than the 4 to 15 inches used for high-pressure systems. It reflects how completely a low-pressure chiller must be evacuated.

Non-condensable gases collect at the high point of the condenser and raise head pressure, which forces the compressor to work harder and cuts cooling efficiency. That is why the purge unit exists. Rising non-condensables also point to an air leak on the vacuum side that should be repaired.

A high-efficiency purge unit recovers refrigerant vapor before releasing the non-condensable air, so it loses far less refrigerant per pound of air purged than older units. It reduces both refrigerant emissions and operating cost. It does not replace the need to find and repair the air leak causing frequent purging.

Circulating warm water through the chiller tubes gently raises refrigerant temperature and speeds recovery without risk. Using the machine's built-in heaters is also acceptable. An open flame such as a torch, or introducing oxygen, is dangerous and prohibited.

Low-pressure chillers typically use centrifugal compressors, which move large volumes of low-pressure refrigerant vapor to cool commercial buildings. Reciprocating, rotary, and scroll compressors are more common in higher-pressure and smaller systems. Recognizing the centrifugal chiller helps identify Type III equipment.

A tripped limit control is a warning that should be investigated, since it may signal non-condensable buildup, water intrusion, or another fault. Simply resetting or bypassing a safety control hides the real problem and is unsafe. Never alter or bypass the rupture disk or limit devices.

Recovering liquid first moves the bulk of the charge quickly, then vapor recovery pulls the remaining refrigerant down to the required 25 mm Hg absolute. Recovering vapor alone is far slower on a large charge. The purge unit removes air but is not the recovery method.

The rupture disk is a critical safety device protecting the low-pressure shell and must be kept in good condition and never bypassed, plugged, or adjusted. Test pressure must stay below its typical 15 psig setting, which is why 10 psig is the leak-test limit. Tampering with it endangers the technician and the equipment.

A float valve meters refrigerant flow to maintain the proper level in a low-pressure chiller. The rupture disk is a pressure-relief safety device, the purge unit removes non-condensables, and an accumulator protects the compressor from liquid. Knowing the float's role helps in diagnosing chiller performance.

HCFC-123 has a relatively low allowable exposure limit, and like most refrigerants it is heavier than air and can accumulate near the floor in a machine room, displacing oxygen. Good ventilation and a refrigerant monitor protect the technician. It is not flammable, but exposure must still be controlled.

The refrigerant must be recovered to 25 mm Hg absolute before the machine is opened for major service such as a tube-bundle replacement. Venting is illegal, and pressurizing above 10 psig risks the rupture disk. Only after proper recovery may the low-pressure shell be opened.

R-1233zd is a low-global-warming-potential HFO refrigerant used as a replacement for HCFC-123 in new low-pressure chillers. Like other low-pressure refrigerants it still runs in a vacuum and must be recovered to 25 mm Hg absolute. It has zero ozone depletion potential and a very low GWP.

Leak-test pressure must stay at or below 10 psig to remain safely under the roughly 15 psig rupture disk. Approaching the disk setting or using compressed air is dangerous. Only dry nitrogen, sometimes with a trace of refrigerant, should be used.

Recovered refrigerant, including from low-pressure chillers, must be stored in a refillable DOT-approved recovery cylinder, never a disposable one-trip cylinder. The cylinder must be within its five-year test date and filled to no more than 80 percent. Using improper containers is illegal and dangerous.

Because the chiller normally runs in a vacuum, a leak pulls air in rather than out, so a detector cannot sense escaping refrigerant. Raising pressure slightly above atmospheric, staying under 10 psig, makes refrigerant flow outward at the leak so it can be found. The pressure must never approach the rupture-disk setting.

Low-loss fittings seal automatically to release as little refrigerant as possible each time hoses are connected or disconnected. This reduces emissions and conserves refrigerant on every job. They are good practice on all recovery work, not only low-pressure.

CFC-11 (R-11) was the classic low-pressure chiller refrigerant, but U.S. CFC production ended January 1, 1996, so it is now available only from recovered and reclaimed stock. R-134a, R-410A, and R-22 are not low-pressure CFC chiller refrigerants. This scarcity makes careful recovery of R-11 especially important.

Refrigerant vapor is heavier than air and can pool near the floor of a machine room, displacing oxygen and exposing workers. A refrigerant monitor and mechanical ventilation warn of a leak and clear the space. This is a standard safety requirement for chiller machine rooms.

An open flame can break refrigerant down into toxic gases such as phosgene and hydrogen chloride, and localized heating can raise pressure dangerously and cause burns. Warm water or built-in heaters are the safe way to add heat. This is why torches are prohibited for warming the shell.

The evaporator of a low-pressure chiller runs below atmospheric pressure, that is, in a vacuum, because the refrigerant boils at a low temperature under low pressure. This vacuum is why leaks admit air and moisture. Understanding this is central to all Type III procedures.

If the vacuum will not hold and pressure rises after the machine stops, refrigerant is still off-gassing from the oil, or air is leaking into the appliance or hoses. You must continue recovery or find the leak before considering the job done. A stable 25 mm Hg absolute is the target.

HFC-404A is a high-pressure blend used in commercial refrigeration, not a low-pressure chiller refrigerant. CFC-11, HCFC-123, and R-1233zd are all low-pressure refrigerants that run in a vacuum. Sorting refrigerants by pressure class is a common exam task.

Because low-pressure refrigerants boil at low temperatures under vacuum, they can chill intruding water below its freezing point, and the expanding ice can rupture the chiller tubes. Water intrusion also causes acid and corrosion. This is one more reason to keep the machine tight and free of leaks.

Even low-pressure refrigerant cylinders must never be filled above 80 percent by weight, leaving room for liquid expansion, and should be stored cool and upright below 125°F. Overfilling can lead to hydrostatic rupture as temperature rises. Always weigh the cylinder rather than guessing.

Tracking the purge rate turns the purge unit into a leak-detection tool; a climbing rate signals that more air is entering the vacuum side, so the leak should be located and repaired. Ignoring it wastes refrigerant and energy. Modern purge controls log and alarm on excessive purging.

Low-pressure refrigerant boils near room temperature, so recovering it fully requires pulling the appliance down to a deep vacuum of 25 mm Hg absolute to draw out the remaining vapor. Shallow vacuums would leave significant refrigerant behind. The deep level ensures thorough recovery from these vacuum-operating machines.

To make a leak show outward, pressure must be raised above atmospheric, using nitrogen with a trace of refrigerant while staying at or below 10 psig. A deeper vacuum keeps leaks pulling air inward where a detector cannot sense them, venting is illegal, and oxygen is dangerous.

The 10 psig limit exists to keep test pressure safely below the rupture disk, which typically bursts near 15 psig to protect the low-pressure shell. Exceeding 10 psig risks rupturing the disk and releasing the charge. This is why nitrogen leak testing on these machines is done at low pressure.