Friday, April 24, 2015

Keeping Your Cool!

Hot weather is coming, and that can mean slow refrigerant recovery. It is common to have high recovery tank temperatures and pressures shut you down as the temperature rises. The condensing temperature of the refrigerant in the recovery unit has to be higher than the outside temperature, and so the refrigerant leaving the recovery unit can be hot, especially on a hot day. This raises the recovery cylinder temperature and pressure, which in turn makes the pressure leaving the recovery unit climb even higher since it must be higher than the cylinder pressure. You often need to cool the recovery cylinder to keep going. There is the old stand-by: ice and a bucket. It works great, but involves the extra bother of carrying ice and a large bucket to the job site. You can use the recovery machine to cool off the cylinder.

Connect the vapor valve of the cylinder to the inlet of the recovery unit and the liquid valve of the recovery cylinder to the outlet of the recovery cylinder. Purge both hoses of air. Start the recovery unit with both cylinder valves open. After the unit gets started, turn the liquid valve on the recovery cylinder clockwise to create a restriction. Be careful NOT to completely close it off. You have just made a refrigeration system with the refrigerant recovery cylinder acting like a flooded evaporator.

The recovery unit is provides the compressor and condenser, and the partially closed liquid valve on the cylinder acts like a metering device. Liquid in the cylinder boils as vapor leaves, cooling off the remaining liquid. The returning liquid is at a lower pressure and temperature, thanks to heat being removed by the recovery unit and the pressure drop at the liquid valve. It takes a bit of practice to learn where to set the liquid valve, but you can monitor the two pressures on the gauges of the recovery unit. You should see both pressures drop as the unit operates.

I have made cylinders frost on the bottom using this technique. One caveat – if you wait until the high pressure switch on the recovery unit opens, you can’t use this method to cool the cylinder down since the recovery unit won't run at that point. You need to make sure and cool the cylinder before that happens. One manufacturer used to make a recovery unit with this feature built in. You connected two lines from the recovery unit to the cylinder, and anytime you needed to cool the cylinder down you would just set the recovery unit valves to subcool. This kept you from having to move your hose connections after the cylinder was cooled down. However, I don’t think it is available any more.

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