For several years, refrigerant retrofitting has been a common procedure for a lot of the kitchen and restaurant equipment I work on, so I'll go over some of the things I've learned that might help you save some time and money on your own retrofit projects.
Our TXV system retrofits have been fairly straightforward; it's the capillary tube systems that have been kind of challenging.
For both types of systems, you have to be sure that you open the lines at good spots so you can blow all the old mineral oil out with nitrogen; and you also need to be sure you get all the old mineral oil out of the compressor.
With hermetic compressors you'll probably have to remove them and turn them upside down so the old oil will drain out; and for semi-hermetics, you'll need to pump the oil out with a hand pump.
In either case; carefully measure the oil that you remove, and replace it with the same amount of new oil.
The refrigerant manufacturers and compressor manufacturers have good guidelines about refrigerant retrofitting and the operating characteristics to expect with the new refrigerants; and the best thing to do is to follow those guidelines.
We have several walk in freezers that were originally R-502.
These were retrofitted to R-404A before I came on board, and they've been running fine.
Some are still using the original R-502 expansion valves, and there haven't been any capacity or control problems.
We also have a number of 400 lb. capacity R-502 and HP-81 ice machines.
Over the years, these have also been refrigerant retrofitting projects, and they've been working fine with R-404A.
Retrofitting the old R-12 capillary tube reach in units to use R-134A has been more difficult.
R-12 is too expensive to be cost effective these days, and even 409A is considerably more expensive than 134A, so we've been changing these old R-12 units over to 134A.
We tried charging a couple of old R-12 units with 134A without changing the cap tubes, just to see if it would work.
It doesn't work; so if you're changing over an old R-12 unit to 134A, plan to change the capillary tube too.
The selection of the capillary tube size and length is where we've had problems.
We've found that for our refrigerant retrofitting projects, the chart from the capillary tube manufacturer is way off.
We had a series of change-overs where we used the recommended cap tube size, and ended up with starving evaporators, high superheat, high subcooling, and poor cooling capacity.
For a while, no matter what size cap tube the chart recommended, we just used #4 cap tube, first cut to 75% of the length recommended by the chart.
We eventually found that although this worked with older equipment; with newer generations of equipment it would take some experimentation with shortening the cap tubes to get the units to run correctly.
Now we simply replace the cap tube with a 1/4 flare type brass refrigeration ball valve.
We adjust it 1/4 open at start up, and it takes a minimun amount of adjusting, if any at all, to get pressures and temperatures balanced out.
When replacing a capillary tube or other type of metering device during a refrigerant retrofitting project, it's a good idea to remove the evaporator, or at least remove the mounting screws so you can move it enough to have good access to the inlet line where the cap tube is brazed in.
Sweat the old cap tube out carefully, then use nitrogen to blow the old mineral oil out of the evaporator, condenser, and lines.
After that, braze everything back together normally with the new metering device installed, and be sure to install a new drier.
When its time to charge and run the unit, we charge to 5° subcooling if possible, keeping a close eye on suction pressure and the compressor amp draw.
If the unit is a 1 door we place a one gallon container of cold water in it. If it's a 2 door, we put two gallon containers of cold water in it.
If our saturated suction temperature is within 10° of what we want to see, which would be 25° for a medium temp unit, or -10° for a freezer; and if superheat at the compressor is no more than 70°, (it's always high at this point), we let the unit run until the box is within 5° of setpoint.
Once we're at the point of a refrigerant retrofitting project where the box has pulled down to within 5° of setpoint, we look for the saturated suction temperature to be in the range of 20° to 30° for a medium temp unit, or -15° to -5° for a freezer; with 20° to 30° of superheat at the compressor (since we don't solder the cap tube or liquid line to the suction line), 5° to 15° of subcooling, and a normal compressor amp draw.
If necessary, we'll add or remove refrigerant; and if we can't bring our pressures and temperatures to within 5° of these conditions all the way around, we used to cut or lengthen the cap tube to get the correct readings, but now that we use the ball valve, a slight adjustment of the valve is all it takes.
Something to keep in mind is that although these readings would be perfect, I rarely see them all in the perfect range.
If the readings are within 5° of what would be perfect, and the box is cooling well with a good compressor amp draw, I consider the job done.
I don't have an explanation about why we had problems with the cap tube charts; but this method has worked well for us, and the units run fine when we're done.
We've worked on 5 to 20 year old Taulsen, Delfield, True, Hobart, and other brands of units; and if we can get a few more years of relatively trouble-free use out of them after these refrigerant retrofitting projects, it's worth a little extra effort.
We have also replaced cap tubes with small expansion valves.
Check the manufacturer's specifications for the refrigeration capacity of the condensing unit and compressor of the unit, and install an expansion valve of that capacity.
At start up, try to charge to 1° of subcooling.
An expansion valve will open fully at start up, and until the box cools down suction pressure will be high, superheat will be high, head pressure will be a little high, and compressor amps will be high.
When the box starts cooling down you should see compressor amps drop, head pressure drop, suction pressure drop, superheat should start to drop, and subcooling may increase.
When the box is within 5° of setpoint, look for pressures and temperatures to be in the range described previously.
I hope this page has helped, and please, feel free to contact us with any specific HVAC questions you might have, including questions about refrigeration on Guam, or air conditioning on Guam.
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