9.3 Type III Recovery and Service Procedures

9.3 Type III Recovery and Service Procedures

Recovery and service procedures for low-pressure systems differ significantly from high-pressure systems due to vacuum operation, large refrigerant charges, and the unique physical characteristics of low-pressure refrigerants.

Recovery Requirements for Low-Pressure Systems

25 mm Hg absolute is a very deep vacuum — nearly complete evacuation of the system. This is measured with a micron gauge or absolute pressure gauge, NOT a standard compound gauge.

Understanding the Pressure Measurements

Recovery Procedures for Low-Pressure Chillers

Step 1: Prepare for recovery

• Shut down the chiller
• Allow pressures to equalize
• Connect recovery equipment to the chiller service connections
• Ensure recovery cylinders have adequate capacity (low-pressure chillers often contain hundreds of pounds)

Step 2: Liquid recovery

• Transfer liquid refrigerant from the evaporator and condenser shells
• Low-pressure refrigerants are relatively easy to handle as liquid because they boil at high temperatures
• Can often gravity-drain liquid into recovery containers in cooled environments

Step 3: Vapor recovery

• After liquid is removed, use a recovery machine to pull remaining vapor
• Continue until required evacuation level is reached (25 mm Hg absolute)
• May need to apply gentle heat to the chiller shells to vaporize remaining liquid

Step 4: Verify evacuation

• Use an absolute pressure gauge or micron gauge to verify the evacuation level
• Allow the system to stabilize before taking the final reading
• If the pressure rises above the required level, continue recovery

Before Opening a Low-Pressure System

Before opening a low-pressure appliance for service, the system must be pressurized to 0 psig (atmospheric pressure) to prevent:

• Air rushing into the system when it is opened
• Sudden pressure changes that could be dangerous

Important: The system is first recovered to the required level (25 mm Hg), then pressurized to 0 psig using recovered refrigerant or nitrogen before opening.

Charging Low-Pressure Systems

Charging a low-pressure chiller differs from high-pressure systems:

• Refrigerant is added as liquid to the evaporator (not the suction line)
• The liquid is introduced below the water tubes in the evaporator shell
• Charging through the suction line could cause liquid slugging
• Because the system is in vacuum, liquid will flash to vapor quickly
• Large charges may take several hours to add

Rupture Disc Safety

Low-pressure chillers have rupture discs as a critical safety device:

• Set to burst at 15 psig — to relieve pressure before it reaches dangerous levels
• Since these systems normally operate in vacuum, 15 psig represents a significant over-pressure
• Rupture discs are one-time use devices — they must be replaced after bursting
• Never valve off or bypass the rupture disc during system operation
• During repair, a pressure relief valve may temporarily substitute while the rupture disc is being replaced

Preventing Over-Pressurization

Low-pressure systems can become dangerously over-pressurized if:

• The system is shut down and ambient temperature exceeds the refrigerant boiling point
• Water side isolation causes the refrigerant to be heated by warm condenser water
• NEVER isolate a chiller with a full charge and then shut off the water — the stored heat can cause pressure buildup
• Always leave water flowing or recover the charge before extended shutdowns

For the Exam: Recovery to 25 mm Hg absolute for newer equipment. Pressurize to 0 psig before opening. Rupture disc bursts at 15 psig. Charge liquid into the evaporator. Never isolate a full charge from water flow during shutdown.