5 in Hg Is Not 15: Recovery Level on a 275 lb Chiller
A 2004 centrifugal chiller with a 275 lb R-134a charge needs its compressor replaced; the machine still runs but is noisy. Outdoor ambient is 40°F and the machine room is unheated. The technician hooks a recovery unit to the service ports with 1/4 in hoses, pulls liquid and then vapor, and after 45 minutes the manifold gauge reads 5 in Hg vacuum, where it holds. He tells the customer that "the rest is only a couple of pounds" and that pulling deeper will take hours and waste a tank. Four DOT recovery cylinders with 100 lb water capacity are on the truck, one already holding 60 lb of recovered R-134a from another job.
1. What evacuation level must be reached before the system is opened, and is 5 in Hg acceptable? What instrument should confirm it?
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Required level: 15 inches Hg vacuum.
The table is applied in two steps:
- Manufactured after November 15, 1993 (this unit is a 2004) → the stricter tier applies.
- Charge over 200 lb → 15 in Hg.
(If it were post-1993 and 200 lb or less, the figure would be 10 in Hg; pre-1993 equipment stops at 4 in Hg regardless of size.)
5 in Hg is not acceptable. Stopping there leaves a substantial residual charge in the shell, the oil, and the low points, and it is a reportable failure to recover - the "only a couple of pounds" argument is an admission of venting when the shell is opened.
Measurement: the level is read at the system service port with a vacuum gauge; a manifold gauge in inches Hg is the regulatory reference, but a micron gauge gives far better resolution for confirming the level is actually reached and holding. Confirm the reading at the system, not at the recovery machine's discharge, and confirm it holds after the machine is valved off.
2. The technician says reaching 15 in Hg at 40°F will take too long. Give the practical steps that make it achievable without cutting corners.
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Cold ambient is the reason it is slow: at 40°F the refrigerant's vapor pressure is low, so the machine is fighting a low-density vapor stream.
Do this:
- Liquid first, vapor second. Use the liquid port (or push-pull) to move the bulk as liquid, then switch to vapor recovery. Vapor-only recovery on 275 lb is the slow path.
- Remove restrictions: pull the Schrader cores, use the largest hoses and shortest runs available, and check for a restricted filter or an undersized access port.
- Use equipment rated for R-134a and for the vacuum level, and let it run to the machine's rated cutout rather than short-cycling it.
- Warm the shell and oil sump with heat blankets or by warming the room - never with an open flame. Chiller oil holds a meaningful amount of refrigerant that only boils out when warmed.
- Do not use the chiller's own compressor to evacuate or to pull itself into a vacuum.
Judgment: the target is fixed by rule; the time is a planning variable. Bid the job with the recovery time included rather than negotiating the vacuum level with the customer.
3. One of the four cylinders already contains 60 lb of R-134a from another job. How should the technician manage fill levels, and what could go wrong if he does not?
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Rule: never fill a recovery cylinder past 80% of its water capacity by weight, and determine the fill by weight on a scale, not by pressure, sound, or a float alone.
Application here:
- Add only to cylinders whose tare weight is stamped and legible; compute allowed net contents from the stamped water capacity and the refrigerant's liquid density, and stop below the 80% mark.
- The 60 lb already in the truck cylinder counts against that limit. Topping it off with a different batch is also a record-keeping problem: the cylinder label must reflect what is actually inside.
- Never mix refrigerants in the same cylinder. A cylinder that is cross-contaminated is a reclaimer's problem, not a top-off.
- Cylinders must be DOT-approved for the refrigerant and in current requalification (commonly a 5-year test interval for recovery cylinders), with valid markings and a functioning relief device.
What goes wrong otherwise: a liquid-full cylinder has essentially no vapor space. A modest temperature rise - a truck bed in the sun, a warm room - produces hydraulic pressure that can rupture the cylinder or blow the relief, injuring someone and releasing the entire charge. It is the single most common serious injury in recovery work.