# Thread: Low pressure switch as operating control

1. I see a lot of confusion about this subject in the field, but over the years I have learned how to do this right and it is not really that hard. This really works!

The suggested pressure settings you find for various refrigerants are merely suggested starting points, each box with its particular compressor, TXV, evaporator (or evaporators) and associated tubing is slightly different. Change any of those components on that machine and the set points for that machine may need to be a little different.

I'll throw in another variable: the numbers on the faces of Ranco, Danfoss and Penn pressure switches might as well just be left off and an arrow pointing up and down painted in their place. This is in contrast to the temperature controls by the same makers; 70 on a Penn A-19 ABC24 commercial cold control is almost always exactly 70, 30 is 30 and 0 is 0.

Cut out setting: 3 things determine the cut out. Box temperature, an acheivable low pressure and reasonable run/off time. If everything is OK these occur together, but some junk just doesn't agree/work. What I mean by this is that the cut out set point pressure must be acheivable without excessive run time or frosting of the evap. Using R-22 as an example lets say that the unit cruises along during the normal cooling part of the cycle at 48 psi. 48 psi converts to 24 boiling point for R-22. Some temperature drop across the metal to the air and we are getting 30-34 discharge air off of our evap; just right during the need for cooling. Once the box temp falls a little more the pressure will fall closer to 45 psi. Hopefully the TXV is sized correctly and doesn't make a lot of weird swings and dips as the box cools down. As the box cools to 35-38 the pressure should be somewhere around 40 to 46psi. If the particular parts don't pull to 45psi before the box runs for 90-100 minutes, the evap ices and/or the box temp falls to 33-32 .... then 45 psi is too low for that particular box. The cut out must be acheiveable; otherwise the compressor runs forever and ices up the evaporator {first the temp will be low, then high}. If the pressure dips to 43psi momentarily during the first 3-6 minutes and bounces up to 50psi then 42 psi is too high for that box's cut out (as long as 40-41 psi cut out is OK in the long haul)*<<note at bottom>>.

Cut in setting: 3 things determine the cut in. Box temperature, run time and FULL DEFROSTING OF THE EVAP. Too many techs try to use box temp and run time as the only cut in setting factors for a pressure switch controlled cooler. Factor #1 must be complete defrosting of the evap. You can manually watch the ice melt on a commercial cooler evap to determine the cut in pressure. If all the ice isn't melted when the system cuts back on, you are going to get a call back. If the cooler is still below 38 when all the ice is melted - the cut in can be set higher than the pressure observed when the last ice melts. PT conversion for 32 degrees for R-22 is about 57.5 psi, so for R-22 your pressure switch controlled cooler cut in is never going to be 58 psi or lower. Ice will still be present on the evap with 58 psi showing on your gage. A more likely cut in pressure for R-22 will be 68 psi. The off time for a pressure switch controlled cooler is the only defrost time (unless you have some super fancy unit with defrost heaters & time clock ... very rare).

With all the components properly sized, working, arranged and clean - setting the pressure switch like this will give you approximately 68% on (run) time and 32% off (defrost) time. The warmer the box is the longer BOTH times will be and the cooler the box is the shorter the times will be (at first, later the off time will increase). 40 minutes on and 20 off is acceptible and 8 on and 4 off is equally acceptible. If the box is in a relatively cool room, has good insulation, large product filling and the doors stay shut - the times may ease to 50-50 or better (nice long defrost period) after many hours of stable operation .... but how often are WE there to see those conditions?

When you replace a component like a TXV or compressor - the cut out setting may need to be changed. In fact, changing the superheat setting on the TXV may require a change in the cut out. Changing (replacing) the evaporator may require a change in the cut in pressure setting, too.

Going back to the pressure switches themselves. Again the numbers on the face mean very little. Test the operation of the new pressure switch with your gages. If you are replacing a pressure switch that has stopped working you are starting from the begining with that unit. If it switched ok, but had a leaky diaphragm, you can use the original set points (determined with your gages) as a guide.

Adjusting the temperature: Observing the operation with your gages before changing anything is extremely important. If the box is too cold, first try setting the cut out a little higher. Second try setting the cut in a very little higher. Observing the operation with your gages before changing anything is extremely important. If the box is too warm try setting the cut out a little lower. With each decrease in the cut out set point remember to prove (by normal cycling of the box) that the pressure is acheivable and that you haven't changed the cut in setting.

Remember that on most switches you are setting a set point on one screw and differential on the other. Changing the set point changes both points, changing the differential only changes one.

* note: too much "hunting" is a problem with the TXV - problem with bulb location, no insulation on the bulb or the TXV is internally worn out or the TXV is too large. A little "hunting" is OK as long as the unit can be made to function.

[Edited by boat racer on 10-23-2004 at 09:31 AM]

2. Regular Guest
Join Date
Aug 2004
Posts
138
Post Likes

## boat racer

Thanks. I'm not in low temp,but that was a good read.

What can you tell me about the CFM's for that evaporator.
Arent they much higher than A.C.?Like about 1500 CFM per ton?------thanks.

3. Originally posted by boat racer
I see a lot of confusion about this subject in the field, but over the years I have learned how to do this right and it is not really that hard. This really works!

agree .....and you know what??
some thermostat, on some job acts as a dummy....

now let me read the rest of this long post

4. Professional Member
Join Date
Aug 2002
Posts
6,047
Post Likes
Boat .. that was good.

but in the future ... try and keep em as short and sweet as you can. D4 & I are trying to win a content contest and posts like yours are screwing us up.

Afterall, D4 & I have an "image" to uphold.

5. Regular Guest
Join Date
Aug 2004
Posts
138
Post Likes

## edit

Oops ,I meant medium temp.

What are typical air flow volumes and how critical are they to setting pressure controls,ect...
How do you measure?

6. Professional Member
Join Date
Feb 2003
Posts
145
Post Likes
Very informative. Thanks for taking the time to share.

7. That's a good post alright. However......the time it takes to set one up and check operation properly (usually requiring an additional trip) is the main reason why I still prefer to use a thermostat and timeclock wherever possible.

8. highlimit we don't have any choice in the fans, you just use what the evap coil comes with .... you have to go far out of your way to buy an evap and fan separately for medium temp.

I 100% agree that the prefered control is a thermostat, but there are a lot of pressure switch controlled boxes out there that will need to be maintained for years to come.

icemeister is right .... an initial start up of a field built up system might take 4 hours (including charging) to do if the idiot doing the design work specified (can you say "skimped on"?) a pressure switch as the operating control; compared to 60 to 90 minutes to charge and test a tstat & solenoid system..... you don't normally need to even watch them fully come down to temperature.

R-12, my girlfriend once aced an essay exam in college that had the question "What is the essence of communication?"

Her answer in full: "The essence of communication is brevity."

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•

## Related Forums

The place where Electrical professionals meet.