EPA 608 Type 3 Study Guide: Low-Pressure Systems

EPA 608 Type 3 certification authorizes you to service low-pressure refrigeration systems, primarily centrifugal chillers used in large commercial buildings, hospitals, universities, and industrial facilities. This specialized guide covers Type 3 exam topics including low-pressure refrigerant characteristics, centrifugal compressor operation, purge unit function, vacuum recovery procedures, leak detection methods, and safety practices unique to low-pressure systems operating below atmospheric pressure.

What Are Type 3 Low-Pressure Systems?

Low-pressure refrigerants have boiling points above 50°F at atmospheric pressure (14.7 psia). Unlike high-pressure systems that operate above atmospheric pressure on both high and low sides, low-pressure chillers run below atmospheric pressure on the evaporator side during normal operation.

This creates a unique challenge: air leaks into the system rather than refrigerant leaking out. Even pinhole leaks allow atmospheric air and moisture to enter, contaminating refrigerant and requiring continuous removal via purge units.

Type 3 equipment includes:

• Centrifugal Chillers: Large water-cooled chillers using R-123 or similar low-pressure refrigerants (50-5,000+ ton capacity)
• Absorption Chillers: Heat-driven chillers using lithium bromide and water (technically exempt from EPA 608 but covered in training)
• Industrial Process Chillers: Low-pressure chillers for manufacturing processes requiring precise temperature control
• District Cooling Plants: Central chiller plants serving multiple buildings via chilled water distribution

Centrifugal Chiller Basics

Centrifugal chillers dominate the Type 3 category. Unlike reciprocating or scroll compressors that use positive displacement, centrifugal compressors accelerate refrigerant vapor using a spinning impeller (similar to a turbocharger), converting velocity into pressure.

Centrifugal advantages:

• Extremely high capacity (100-5,000+ tons) in compact footprint
• Fewer moving parts than reciprocating compressors (longer service life)
• Oil-free compression in some designs (hermetic centrifugals use oil, open-drive may be oil-free)
• Smooth, vibration-free operation (no reciprocating pistons)
• Excellent part-load efficiency with variable-speed drives

Centrifugal challenges:

• Surge potential at low loads (compressor stall causing vibration and capacity loss)
• Requires constant air purging due to below-atmospheric evaporator pressure
• High initial cost (economical only for large capacities)
• Complex controls and operating sequences
• Specialized service knowledge required

Low-Pressure Refrigerants

R-123 (HCFC-123)

R-123 is the dominant low-pressure refrigerant in modern centrifugal chillers, replacing R-11 (CFC-11) which was banned in 1996. R-123 characteristics:

• Boiling Point: 82.2°F at atmospheric pressure (14.7 psia)
• Evaporator Pressure: Typically 5-8 psia (10-20 inches Hg vacuum)
• ODP: 0.02 (low but not zero — being phased out under Montreal Protocol)
• GWP: 77 (very low compared to high-pressure HFCs)
• Toxicity: Class B (higher toxicity than most HFCs, requires special handling)
• Safety Group: B1 (toxic, non-flammable)

Because R-123 evaporator pressure is below atmospheric, any leak allows air and moisture infiltration. Systems must maintain slight positive pressure in condenser (1-5 psig) while evaporator runs in vacuum (5-8 psia absolute).

R-123 Replacements

R-123 is being phased out under HCFC elimination schedules. Replacement refrigerants include:

R-1233zd(E): HFO refrigerant with similar properties to R-123. ODP = 0, GWP = 7 (extremely low). Safety group B1. Drop-in replacement for many R-123 chillers with minor equipment modifications. Increasingly common in new chiller installations.

R-514A: Blend of R-1336mzz(Z) and trans-1,2-dichloroethylene. Very low GWP (2). Used in some newer centrifugal chillers. Non-toxic (Class A) unlike R-123.

R-1234ze(E): HFO used in some low-pressure applications. Lower pressure than R-123 (requires equipment redesign). GWP = 6, ODP = 0, Class A (non-toxic).

R-11 (Legacy Systems)

R-11 (CFC-11) was the original centrifugal chiller refrigerant. Production banned January 1, 1996. Thousands of R-11 chillers still operate using reclaimed refrigerant. R-11 properties similar to R-123 but with ODP = 1.0 (very high ozone depletion). Many R-11 chillers have been converted to R-123 or replaced.

Purge Units and Air Removal

Why Purging Is Critical

Because low-pressure chillers operate below atmospheric pressure in the evaporator, air continuously leaks into the system through microscopic openings, gasket imperfections, and seal surfaces. Accumulated air creates several problems:

• Reduced Capacity: Air is non-condensable, increasing condenser pressure and reducing chiller efficiency (1 psig air pressure = 3-5% capacity loss)
• Increased Energy: Higher head pressure forces compressor to work harder, consuming more electricity
• Moisture Contamination: Air brings moisture causing acid formation, freeze-ups, and corrosion
• Corrosion: Oxygen from air reacts with water forming acids that damage tubes and internal components

How Purge Units Work

Purge units continuously remove non-condensables (air, nitrogen, other gases) from the chiller refrigerant circuit. Modern purge systems use one of two methods:

High-Efficiency Purge Units: Sample refrigerant vapor from the top of the condenser (where non-condensables collect), condense refrigerant while retaining gases, vent gases to atmosphere, return liquid refrigerant to chiller. Minimize refrigerant loss during purging (< 0.1 lbs per purge cycle).

Condensing Purge Units: Older technology, less efficient. Cool sampled vapor to condense refrigerant, separate gases, vent gases. Lose more refrigerant during purge cycles (0.5-2 lbs per cycle). Being phased out in favor of high-efficiency designs.

Purge operation indicators:

• Normal purge cycles: 1-4 times per hour during normal operation
• Excessive purging (> 6 cycles/hour): Indicates system air leak requiring repair
• No purging: Purge unit malfunction or disconnected
• Continuous purging: Major leak or purge unit control failure

Type 3 Recovery Requirements

Required Recovery Level

Low-pressure systems must be recovered to 25 mm Hg absolute pressure (approximately 0 inches Hg gauge or 0.98 inches Hg absolute). This is equivalent to 29 inches Hg vacuum when measured from atmospheric pressure.

The 25 mm Hg requirement is more stringent than high-pressure recovery levels because low-pressure systems operate below atmospheric pressure normally. Deep evacuation ensures minimal refrigerant remains before opening the system.

Recovery Challenges

Type 3 recovery presents unique difficulties:

Large Refrigerant Charges: Centrifugal chillers contain 200-3,000+ pounds of refrigerant. Recovery takes hours or days depending on equipment size and recovery machine capacity. Plan accordingly — cannot complete in single service visit for large chillers.

Liquid Recovery Priority: Recover liquid refrigerant first (much faster than vapor). Most centrifugal chillers have liquid drain valves specifically for service recovery. Connect recovery machine to liquid line, drain majority of charge as liquid, then switch to vapor recovery.

Oil Contamination: Low-pressure refrigerants mix with large amounts of compressor oil. Recovered refrigerant often contains 10-30% oil requiring separation and purification before reuse. Use recovery machines with oil separators and expect extended recovery times.

System Volume: Large chiller shells have huge internal volume. After liquid recovery, vapor recovery continues for extended period to achieve 25 mm Hg. Don't rush — inadequate recovery leaves refrigerant that vents when opening system.

Recovery Procedures for Low-Pressure Systems

1. Isolate Chiller: Close all isolation valves separating chiller from building water loops. Lock out electrical power. Verify purge unit is operating (removes air accumulated during shutdown).
2. Liquid Recovery: Connect recovery machine to liquid drain valve. Open valve, recover liquid refrigerant into approved recovery cylinder. Monitor cylinder weight — don't exceed 80% fill capacity.
3. Vapor Recovery: After liquid recovery complete, connect to vapor service port (typically on condenser). Recover vapor until reaching 25 mm Hg absolute pressure.
4. Wait and Verify: After reaching 25 mm Hg, wait 30 minutes. If pressure rises, continue recovery. Pressure rise indicates refrigerant boiling from oil or trapped liquid.
5. Document Recovery: Record refrigerant type, amount recovered (by weight), recovery date, cylinder numbers. Maintain records for 3 years per EPA requirements.

Leak Detection in Low-Pressure Systems

Leak Detection Challenges

Finding leaks in low-pressure systems is fundamentally different from high-pressure work. Because the evaporator operates below atmospheric pressure, air leaks in rather than refrigerant leaking out. You cannot use traditional leak detection methods that rely on detecting escaped refrigerant.

Pressurization Testing

The most effective leak detection method for low-pressure systems:

1. Recover Refrigerant: Remove refrigerant to 25 mm Hg or lower
2. Pressurize with Nitrogen: Introduce dry nitrogen to 10 psig (do NOT exceed low-pressure system test pressure — typically 10-15 psig maximum)
3. Add Tracer: Inject R-22 or other detectable tracer gas (1-2% concentration) OR add electronic leak detector tracer
4. Electronic Detection: Use electronic leak detector to find escaping tracer gas
5. Bubble Testing: Apply soap solution to suspected leak areas (flanges, gaskets, tube joints)

Important: NEVER pressurize low-pressure chillers above nameplate test pressure. Evaporator and condenser shells are not designed for high pressure — over-pressurization causes rupture and injury. If test pressure not specified, use 10 psig maximum.

Ultrasonic Leak Detection

Ultrasonic detectors locate air leaks by detecting high-frequency sound of gas movement. Effective for finding where air enters evacuated systems. Point detector at suspected leak areas with system under vacuum, listen for characteristic leak sound. Works well for large leaks but may miss very small leaks.

Common Leak Locations

Low-pressure chiller leaks typically occur at:

• Tube-to-Tubesheet Joints: Evaporator and condenser tubes expand into tubesheets. Corrosion or improper expansion causes leaks between tubes and tubesheet.
• Gasket Surfaces: Shell flanges, handhole covers, sight glasses use gaskets. Aging gaskets leak air. Replace gaskets during major service.
• Purge Unit Connections: Purge unit piping connections prone to leaks. Check thoroughly during leak investigation.
• Pressure Relief Valves: Relief valve seats develop leaks over time. Test relief valves annually.
• Mechanical Seals: Open-drive centrifugal compressors use mechanical shaft seals. Seal wear allows air infiltration. Monitor seal condition closely.

Evacuation and System Preparation

Deep Vacuum Requirements

Low-pressure chillers require deep evacuation to remove moisture and non-condensables before charging. Target vacuum: 500 microns or lower, same as high-pressure systems, but achieving this in large chiller shells takes longer.

Evacuation procedure:

1. Equipment Setup: Connect high-capacity vacuum pump (12+ CFM for chillers) to chiller service ports. Use short, large-diameter hoses (minimum 1/2" ID) to minimize flow restriction.
2. Initial Pulldown: Evacuate chiller to 1,000 microns. Large chillers may take 2-4 hours for initial pulldown due to shell volume.
3. Moisture Removal: If vacuum stalls above 1,000 microns, moisture is present. Use heat lamps to warm shell (accelerates moisture evaporation) or perform triple evacuation.
4. Final Evacuation: Continue evacuation to 500 microns or lower. Hold vacuum for 30 minutes minimum. Vacuum should remain stable — rising pressure indicates moisture or leak.
5. Verify and Charge: After successful vacuum hold, break vacuum with refrigerant vapor, complete charging per manufacturer specifications.

Triple Evacuation Method

For chillers with moisture contamination or after major repairs involving air exposure:

1. Evacuate to 1,000 microns, break vacuum with dry nitrogen to 5 psig
2. Evacuate to 500 microns, break vacuum with dry nitrogen to 5 psig
3. Evacuate to 500 microns, hold 30 minutes, proceed with charging

Triple evacuation is most effective moisture removal method, using nitrogen to sweep water vapor from the system between evacuation cycles.

Charging Low-Pressure Chillers

Charging Methods

By Weight: Most accurate. Manufacturer specifies operating charge on nameplate. Use electronic scale, charge refrigerant to specified weight. For R-123, typical charges range from 200-3,000 lbs depending on chiller size.

By Refrigerant Level: Many chillers have sight glasses showing liquid refrigerant level. Charge until refrigerant reaches operating level mark. Less accurate than weight but useful when nameplate charge unknown.

By Operating Conditions: Charge while monitoring approach temperature (difference between leaving water temperature and refrigerant saturation temperature). Target approach: 1-3°F for efficient operation. Add or remove refrigerant to achieve proper approach.

Startup After Charging

Low-pressure chiller startup requires careful procedure:

1. Verify Water Flow: Confirm chilled water and condenser water pumps operating before starting compressor. No-flow conditions damage tubes.
2. Check Oil Level: Verify compressor oil level at operating mark. Low oil causes bearing damage.
3. Purge Air: Operate purge unit to remove non-condensables accumulated during service.
4. Start Compressor: Follow manufacturer startup sequence. Monitor discharge pressure, bearing temperatures, oil pressure.
5. Monitor Operation: Watch for unusual vibration, noise, or pressure fluctuations indicating problems.

Safety Considerations

R-123 Toxicity

R-123 is Class B (toxic) requiring special handling. Exposure limits:

• TLV-TWA: 50 ppm (8-hour time-weighted average)
• AEL: 1,000 ppm (acute exposure limit — 1 hour maximum)
• Symptoms: Dizziness, drowsiness, irregular heartbeat at high concentrations

R-123 safety practices:

• Work in well-ventilated areas — chiller mechanical rooms should have mechanical ventilation
• Use refrigerant monitors with alarms set at 25 ppm (half TLV-TWA)
• Wear respiratory protection when working on open systems or during major refrigerant releases
• Never enter confined spaces with R-123 refrigerant without proper confined space procedures
• Evacuate area immediately if refrigerant monitor alarms — high concentrations displace oxygen

Low-Pressure System Hazards

Implosion Risk: Low-pressure vessels under vacuum can implode if structural integrity compromised. Never strike or weld on evacuated shells. Relieve vacuum before performing any shell work.

Water-Side Pressure: While refrigerant side operates at low pressure, water side operates at 30-150 psi. Use caution when opening water-side connections — depressurize water loops first.

Large Equipment Hazards: Centrifugal chillers are massive — 5,000+ lbs for smaller models, 20,000+ lbs for large chillers. Use proper rigging when moving components. Rotating compressor components present crushing hazards.

Type 3 Exam Study Tips

Focus on Low-Pressure Specifics

Type 3 exam emphasizes differences from high-pressure systems. Know that evaporators operate below atmospheric pressure causing air infiltration (not refrigerant leakage), purge units continuously remove non-condensables, and recovery requires 25 mm Hg absolute vacuum.

Understand Centrifugal Operation

Questions cover centrifugal compressor basics: impeller accelerates vapor, diffuser converts velocity to pressure, surge occurs at low loads, oil-lubricated vs. oil-free designs, capacity control via variable inlet guide vanes or variable-speed drives.

Memorize R-123 Properties

Know R-123 safety classification (B1 — toxic, non-flammable), ODP (0.02), evaporator pressure (below atmospheric), and why purge units are required (air leaks into vacuum-side evaporator).

Study Leak Detection Methods

Understand why traditional leak detection doesn't work on low-pressure systems (refrigerant doesn't leak out during normal operation). Know pressurization testing procedure: recover refrigerant, pressurize with nitrogen + tracer to 10 psig maximum, detect leaks electronically or with bubbles.

Completing Universal Certification

Type 3 is the final piece for Universal certification. If you already hold Core + Type 1 + Type 2, adding Type 3 completes your Universal credential authorizing work on all equipment types.

Universal certification benefits:

• Maximum employment flexibility — work on any refrigeration equipment
• Higher earning potential — employers prefer Universal-certified technicians
• Career advancement — supervisory and specialized roles often require Universal
• No equipment restrictions — service calls don't require checking certification limits

Next steps:

• Universal Strategy: Review Universal Guide for efficient multi-section exam approach
• Core Review: Strengthen fundamentals with Core Guide
• Type 2 Comparison: Understand high-pressure vs low-pressure differences using Type 2 Guide