In push-pull recovery the machine draws vapor off the recovery cylinder and pushes it into the appliance while liquid flows out of the appliance's liquid line into that cylinder. What condition has to be met before this setup will actually transfer refrigerant?
- AThe recovery cylinder must be warmed above the appliance's saturation temperature so the higher cylinder pressure drives the liquid through the transfer hose
Push-pull works by lowering cylinder pressure, not by raising it. Heating a cylinder to build pressure reverses the direction of the transfer and is a serious overpressure hazard on its own.
- BThe appliance's own compressor must run for the whole transfer, since it is what keeps the liquid column moving to the cylinder
The recovery machine supplies the entire pressure difference. The appliance is normally off and locked out during a push-pull transfer, and a dead compressor does not stop the method from working.
- CThe hookup must be made through the suction service valve, since liquid will not flow out through a liquid line service port
Liquid has to leave through a liquid line or receiver port, which is the whole point of the hookup. Pulling from the suction valve gives vapor, and there is no liquid column left to push.
- The appliance must hold enough liquid refrigerant for the machine to build and hold a pressure difference between the appliance and the cylinder
Why D is correct
The method works only while a pressure difference exists across a continuous column of liquid. A large liquid charge lets the machine establish and hold that difference long enough to move most of the refrigerant quickly, which is precisely why push-pull is a large-system technique.
What this question is testing
This tests understanding of the physical mechanism behind push-pull rather than a memorized hookup diagram, plus the judgment to know when the method is worth setting up. It also probes whether you can recognize dangerous or backwards descriptions of the technique.
On the job
Push-pull earns its keep on chillers and supermarket racks where hundreds of pounds sit in a receiver. Techs watch the sight glass in the transfer hose and the scale under the cylinder: bubbles in the glass and a stalled scale mean the liquid is gone. At that point you break down the push-pull hookup, go to standard vapor recovery, and finish. Overfilling is the real danger here, since liquid moves fast and the cylinder can pass the safe fill level in minutes if nobody is watching the scale.
Memory technique
Push vapor in, pull liquid out, and only bother when there is a lake of liquid to move.
Exam tip
Push-pull questions hinge on two things: it moves liquid, and it needs a large charge. Never heat a recovery cylinder to speed anything up.
Where to look it up
Push-pull is a liquid recovery technique for large charges; finish with vapor recovery and keep the cylinder within its safe fill level.