10.3 Type III Recovery and Procedures

The Recovery Standard for Low-Pressure Systems

Type III recovery is governed by one number you must commit to memory: the required evacuation level. Before a major repair or disposal of a low-pressure appliance, the technician must reduce the system to a deep vacuum, and the exact target depends on when the recovery equipment was built:

25 mm Hg absolute is a near-complete evacuation — only 25 mm of mercury above a perfect vacuum. Read it on a micron (absolute) gauge, never a compound gauge. The notorious exam trap is confusing 25 mm Hg with 25 in. Hg: 25 mm Hg is roughly 0.98 in. Hg, so the two differ by about 25×. The same 25 mm Hg absolute level applies before disposing of a low-pressure appliance.

Pressure References You Must Keep Straight

Type III is the only section that mixes vacuum scales, so anchor these reference points:

The Freezing Hazard — Keep Water Flowing

Here is the procedure point the exam loves. As you pull a vacuum, the remaining liquid refrigerant boils violently at the low pressure and absorbs heat from the water tubes. If the cooling and chilled-water pumps are off, that heat is drawn from the now-stagnant water, which can freeze and rupture the evaporator or condenser tubes. The rule: keep water circulating through the tubes during recovery so the moving water resists freezing and feeds heat to vaporize the refrigerant. Stop the water and you risk thousands of dollars in burst-tube damage.

Recovery Procedure and Pressurization Limits

A typical low-pressure recovery sequence:

1. Prepare — shut down the chiller, let pressures equalize, connect recovery equipment, and stage enough recovery cylinders (charges are often hundreds of pounds).
2. Keep water flowing — circulate condenser and chilled water to prevent tube freezing.
3. Liquid recovery — transfer liquid from the shells first; low-pressure refrigerant is easy to handle as a liquid because it boils so high.
4. Vapor recovery — pull the remaining vapor with the recovery machine; apply gentle heat to the shells to vaporize the last liquid.
5. Reach the target — continue until 25 mm Hg absolute (or 25 in. Hg vacuum for older gear) is achieved.
6. Verify — isolate, let the system stabilize, and read the final level on a micron/absolute gauge. If the pressure rises back above the target, keep recovering.

The Hard Pressurization Limit

Never pressurize a low-pressure appliance above about 10 psig. This rule has two reasons. First, the rupture disc bursts at ~15 psig — exceeding 10 psig leaves almost no safety margin and risks blowing the disc (and your charge). Second, refrigerants like R-123 are not flammable in normal vacuum service, but raising pressure with air or oxygen-bearing gas can create unsafe conditions and can drive moisture into the machine. For any leak test you pressurize with dry nitrogen capped at 10 psig (covered in 9.4), and before opening the machine you bring it gently to 0 psig (atmospheric), not above it.

Example: A technician recovering a 700-lb R-123 charge wants to "speed things up" by adding nitrogen to push refrigerant out, and lets the gauge climb toward 14 psig. The rupture disc — rated at 15 psig — lets go, venting nitrogen and refrigerant and forcing a costly disc replacement. The correct method is to keep any pressurization at or below 10 psig, recover with the machine, and pressurize only to 0 psig before opening.

For the Exam: Recover to 25 mm Hg absolute (post-Nov 15, 1993 equipment) or 25 in. Hg vacuum (older). Keep water circulating to prevent tube freezing. Never pressurize above ~10 psig — the rupture disc relieves near 15 psig. Charge liquid into the evaporator, and verify the vacuum on a micron gauge after the system stabilizes.