A technician must pull a 400 lb charge out of a supermarket rack before replacing the receiver, and the store has only a few hours of downtime. Where should the recovery machine inlet be connected to move the most refrigerant in the least time?
- AThe suction service valve, because vapor passes through a recovery machine faster than liquid does out of a system
This has the physics backwards. Vapor is far less dense, so a machine pulling vapor moves only a small fraction of the mass per minute that it moves when it is pulling liquid.
- BThe compressor discharge port, because standing head pressure pushes the charge out on its own at first
System pressure alone pushes out some refrigerant at the start but equalizes quickly and cannot bring the rack down to the required level. A self-contained machine has to finish the job.
- The liquid line at the receiver, because pulling liquid moves far more refrigerant per minute than pulling vapor
- DThe suction line in push-pull mode, because push-pull transfers only work off the vapor side of the rack
Push-pull is a liquid transfer method. The machine pulls vapor off the recovery cylinder and pushes it into the appliance so liquid flows out of the liquid line into the cylinder.
Why C is correct
Density is the whole story. A pound of liquid occupies a tiny fraction of the volume of a pound of vapor, so a recovery machine handling liquid clears the charge many times faster. Starting on the liquid line and finishing on the vapor side is the standard sequence on any large high-pressure system.
What this question is testing
This item tests practical recovery technique on large high-pressure systems and whether you understand why liquid recovery dominates the time budget on a big charge. It also checks that you can recognize push-pull as a liquid transfer method rather than a vapor method, and that system pressure alone cannot finish a recovery.
On the job
On big rack and chiller work, recovery time is the job. Techs plan the hookup before touching a valve: liquid first from the receiver king valve, a large-diameter hose to cut restriction, a scale under the cylinder, and a second cylinder staged and ready. Undersized hoses and vapor-only hookups are the two most common reasons a night shutdown runs into morning. Watching the sight glass and the cylinder weight tells you when the liquid is gone and it is time to switch over and finish on vapor.
Memory technique
A gallon of liquid beats a roomful of vapor: pull liquid first, mop up vapor last.
Exam tip
Any question asking how to recover a large charge quickly is pointing at liquid recovery or push-pull. Vapor recovery is the finishing step, never the fast one.
Where to look it up
Recovery practice under 40 CFR 82.156: remove liquid first on large charges, finish on vapor to reach the required level.