EPA 608 Type III: Low-Pressure Systems

10 min read Training Guide

Know how low-pressure chillers run under vacuum, how purge units, leak testing and recovery work, and the required levels for Type III.

Table of contents

Low-pressure appliances are mostly large centrifugal chillers in hospitals, universities and office towers. They are different from anything else in the trade, because parts of the system run below atmospheric pressure. A leak on a high-pressure system lets refrigerant out, but a leak on a low-pressure system lets air and moisture in. Almost everything about Type III follows from that.

What You Will Be Able to Do

  • State which equipment Type III covers.
  • Explain why low-pressure systems have purge units and rupture discs.
  • State the required recovery level.
  • Leak test a low-pressure system safely.
  • Recover and charge without freezing water in the tubes.

What Type III Covers

A low-pressure appliance uses a refrigerant with a saturation pressure below 45 psia at 104°F. The classic refrigerants are R-11 and R-123. Newer low-pressure chillers may use R-1233zd(E) or R-514A.

R-11 has not been produced in the U.S. since 1995. R-123 is an HCFC whose new production for servicing ends in 2030, so reclaimed supply will matter.

Why These Systems Run Under Vacuum

Low-pressure refrigerants boil at a much higher temperature than R-410A. To boil at chilled-water temperatures, the evaporator has to run below atmospheric pressure. Any leak in that part of the system draws air and water vapor in, rather than pushing refrigerant out.

Air in the system takes up condenser space and raises head pressure. Moisture leads to acid. That is why low-pressure systems have a purge unit.

The Purge Unit

Non-condensable gases collect in the top of the condenser. A purge unit draws gas from there, separates the refrigerant, returns it to the system and releases the air.

  • High-efficiency purge units release very little refrigerant with the air.
  • A purge that runs more often than usual is a sign of a leak.
  • Log purge run time, because a rising trend is an early warning.

Moisture can also enter through a leaking water tube. Signs include rising moisture levels and cloudy oil. A leaking tube is tested with the water side drained or by a hydrostatic water-side test.

The Rupture Disc

A rupture disc on the evaporator protects the machine from overpressure. It commonly bursts near 15 psig. When it bursts, the refrigerant charge can release, so the pressure in the system must stay well below that during any testing. Follow the manufacturer's rating for the machine in front of you.

Refrigerant Safety in Machine Rooms

R-123 has a low exposure limit and is a B-group refrigerant. ASHRAE 15 calls for a refrigerant sensor in machine rooms, with an alarm and mechanical ventilation. Treat an alarm as real, and leave until the area has been checked.

Required Recovery Level

A low-pressure appliance must be recovered to 25 mm Hg absolute, which is about 29 inches of mercury vacuum. The level is the same for recovery equipment made before or after November 15, 1993.

For a non-major repair that is not followed by evacuation, you may stop at 0 psig. Major repair means removing the compressor, condenser, evaporator or an auxiliary heat exchange coil, or uncovering more than 4 square inches of flow area for over 15 minutes.

If leaks prevent you from reaching the level, isolate the leak if you can, and evacuate to the lowest level you can reach, no higher than 0 psig.

Recovery Technique

  1. Recover liquid first, from the lowest point of the machine. A chiller's charge can be hundreds of pounds, and liquid moves much faster than vapor.
  2. Recover the remaining vapor until you reach the required level.
  3. Keep water moving through the tubes, or drain them. Refrigerant boiling off during recovery cools the tubes, and water that freezes inside can burst them.
  4. Watch the recovery unit's condenser and cooling water, if it uses one.
  5. Wait and check for a pressure rise.

Refrigerant dissolves in compressor oil. Study materials commonly say to heat the oil to about 130°F before you drain it, so less refrigerant leaves with the oil.

Leak Testing

Since a low-pressure system sits at or below atmospheric pressure, it often has no positive pressure to test with. You need to raise the pressure carefully.

  1. Use controlled hot water through the tubes or a heater blanket to raise pressure. Dry nitrogen is also used on many machines.
  2. Keep pressure at no more than about 10 psig, well below the rupture disc.
  3. Use a pressure regulator and a relief valve if you use nitrogen.
  4. Use a detector rated for the refrigerant, and check all joints, gaskets and seals.

Warning: Never raise pressure past the test limit. A burst rupture disc releases the charge and shuts the machine down.

Standing vacuum tests are also used. After evacuation, a rise in absolute pressure over a set time shows that air is leaking in.

Charging

Water in the tubes can freeze when refrigerant first enters a machine under deep vacuum, since the refrigerant boils at very low pressure.

  1. Run water through the tubes, or drain them.
  2. Charge vapor first, through the evaporator charging valve, until pressure rises above the point where water in the tubes could freeze.
  3. Change to liquid through the evaporator charging valve.
  4. Weigh the charge, and compare it with the data plate.

Leak Repair Reminders

Many chillers are comfort cooling, with a 10% annual leak limit at 50 lb or more for ozone-depleting refrigerants such as R-123. A system using an HFC or substitute with a GWP above 53 is under Part 84 at 15 lb or more. Many low-pressure substitutes have a GWP well below 53, so check the refrigerant before you assume either rule applies. Larger systems also need regular leak inspections and records for at least 3 years.

Exam Format

Type III has 25 questions and is closed book, with a proctor. Passing takes 70%, or 18 of 25 in practice. You also need Core.

Common Mistakes

Mistake What it costs
Thinking a low-pressure leak loses refrigerant first You miss the air and moisture coming in
Leak testing past about 10 psig A burst rupture disc
Starting recovery or charging with water still and full Frozen, burst tubes
Charging liquid first into a deep vacuum Frozen tubes
Ignoring a rising purge run time A growing leak goes unfixed

Check Your Understanding

A chiller's purge unit runs far more often than last month. What does that suggest? Air is leaking in. Find and fix the leak.

You are recovering an R-123 chiller. Which comes first, liquid or vapor? Liquid, and then vapor until you reach 25 mm Hg absolute.

You need to leak test a low-pressure machine, and its rupture disc is rated at 15 psig. How high can you go? No more than about 10 psig, using controlled hot water, a heater blanket or regulated nitrogen.

You are about to charge an empty chiller. Why not start with liquid? Liquid flashing in a deep vacuum can freeze the water in the tubes. Start with vapor.

Key Takeaways

  • Low-pressure appliances use refrigerants below 45 psia at 104°F, such as R-11 and R-123.
  • Parts of these systems run under vacuum, so leaks let air and moisture in.
  • Purge units remove non-condensables, and rising purge run time signals a leak.
  • The required recovery level is 25 mm Hg absolute.
  • Leak test at no more than about 10 psig, well below the rupture disc.
  • Recover liquid first, then vapor, and protect the tubes from freezing.