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EPA 608 Technician Certification: Low-Pressure Chillers, Purge Units, and Safety Concepts

Importance ★★★☆☆ · 25% (estimate)

Overview

This chapter covers how low-pressure chillers are built and the safety rules for the rooms they sit in. You will learn why they run in a vacuum, what the purge unit and rupture disc do, which refrigerants they use, and what to watch for during a large leak.

Low-pressure refrigerants such as R-123 boil at low temperatures only under vacuum, so the evaporator of a running chiller is below atmospheric pressure. R-123, an HCFC, replaced R-11, a CFC, in many of these chillers. R-123 boils at about 82°F at atmospheric pressure, so a shut-down chiller in a 75°F room shows a slight vacuum.

Air and moisture that leak in collect at the top of the condenser. The purge unit draws from there, removes the noncondensables, and returns refrigerant to the system. The rupture disc on the evaporator protects the chiller from overpressure and is usually set at 15 psig.

Refrigerant vapor is heavier than air and can displace oxygen in a closed machinery room. Under ASHRAE Standard 15, a machinery room with R-123 needs a refrigerant sensor with an alarm. Use a self-contained breathing apparatus for a large leak.

Key concepts

  • Purge unit

    A device on a low-pressure chiller that removes air and moisture from the top of the condenser while returning refrigerant to the system.

    Memory tipPurge = gets air out.

    Source: EPA Section 608 Test Topics · as of 2025

  • Rupture disc

    A safety disc on the chiller's evaporator that bursts to relieve pressure, usually set at 15 psig.

    Memory tipDisc 15 psig; leak test 10 psig.

  • R-123

    An HCFC low-pressure refrigerant that replaced R-11 in many centrifugal chillers.

    Memory tipR-11 out, R-123 in.

  • Running in a vacuum

    While running, the evaporator of a low-pressure chiller is below atmospheric pressure, so leaks pull air in.

    Memory tipLow side below 0 psig.

  • ASHRAE Standard 15

    The safety standard for refrigeration machinery rooms. EPA's test topics note it calls for an oxygen-deprivation sensor for all refrigerants and a refrigerant sensor for R-123.

    Memory tipR-123 room: refrigerant sensor.

    Source: EPA Section 608 Test Topics · as of 2025

Often tested

  • Noncondensables collect at the top of the condenser.
  • R-134a is medium-pressure, not low-pressure.
  • A large leak in a closed room can cause oxygen deprivation.

3 sample questions

Verified practice questions from this unit.

Practice questionReviewedType III (Low-Pressure) › Low-Pressure Chillers, Purge Units, and Safety★★★★☆

Question 1. What does the purge unit on a low-pressure chiller do?

  1. ① Adds refrigerant automatically
  2. ② Cools the compressor oil
  3. ③ Relieves pressure if the chiller gets too hot
  4. ④ Removes air and moisture (noncondensables) from the system
▼ Show answer and explanation▲ Hide answer and explanation

Answer: ④ Removes air and moisture (noncondensables) from the system

Key point: Purge unit removes air and moisture

Air and moisture leak into low-pressure chillers. The purge unit removes these noncondensables while returning as much refrigerant as possible to the system. High-efficiency purge units release very little refrigerant.

Wrong choices

  • Adds refrigerant automatically: The purge unit does not charge refrigerant.
  • Cools the compressor oil: It does not cool the oil.
  • Relieves pressure if the chiller gets too hot: Pressure relief is the job of the rupture disc or relief valve.
Practice questionReviewedType III (Low-Pressure) › Low-Pressure Chillers, Purge Units, and Safety★★★☆☆

Question 2. R-123 replaced which older CFC refrigerant in many low-pressure chillers?

  1. ① R-12
  2. ② R-502
  3. ③ R-22
  4. ④ R-11
▼ Show answer and explanation▲ Hide answer and explanation

Answer: ④ R-11

Key point: R-123 (HCFC) replaced R-11 (CFC)

R-123, an HCFC, became the common replacement for R-11, a CFC, in low-pressure centrifugal chillers. Both are low-pressure refrigerants.

Wrong choices

  • R-12: R-12 is a medium-pressure CFC used in other equipment.
  • R-502: R-502 is a high-pressure blend.
  • R-22: R-22 is a high-pressure HCFC.
Practice questionReviewedType III (Low-Pressure) › Low-Pressure Chillers, Purge Units, and Safety★★★☆☆

Question 3. While a low-pressure chiller is running, what is the pressure in its evaporator?

  1. ① Below atmospheric pressure (a vacuum)
  2. ② About 300 psig
  3. ③ Exactly 0 psig at all times
  4. ④ About 100 psig
▼ Show answer and explanation▲ Hide answer and explanation

Answer: ① Below atmospheric pressure (a vacuum)

Key point: Low-pressure evaporator runs in a vacuum

Low-pressure refrigerants such as R-123 boil at low temperatures only under vacuum, so the evaporator runs below atmospheric pressure. This is why leaks let air in.

Wrong choices

  • About 300 psig: 300 psig is typical of high-pressure systems, not low-pressure chillers.
  • Exactly 0 psig at all times: The pressure is normally below 0 psig while running.
  • About 100 psig: 100 psig is far too high for a low-pressure evaporator.
Practice 5 questions from this unit →

AI-written · Verified · Written by AI. Only units that passed a separate AI check (re-reading facts and numbers) are published. Tell us if you find a mistake.