Low Pressure Cut-off switch on Liquid line

[Vishnukurup]

Few days back, I saw a York Unitary Package, the DCE series 4.0 TR unit. This is a cooling only unit. I saw two mini pressure switches in the liquid line. Initially, I assumed one is high pressure switch and the other is fan cycling switch for low ambient operation. When checked the wiring, these are wired in series in the control circuit for compressor. Then it can not be for fan cycling. So, checked the settings. HP switch cut-off setting is 435psig. For the other one the settings are cut-off = 7 psig and cut-in = 22 psig. So, it is an LP switch. Normally I saw LP switches in suction line only. Why York is using LP switch on liquid line? Is there any advantage by doing so? There was no LP switch on suction line in this unit. The other compressor protection device used is antifreeze sensor on suction line. In the catalogue, they mentioned that "Loss of gas protection is provided" in the unit.

Somebody Please explain why is this done.

Bye

[Rtic]

Can I know what temperature you are trying to get?
What Gas are you on?

BTW: - The HP Pressure switch seems to be set VERY high.

[old gas bottle]

sounds odd,maybe wrong part or is it situated after a magnetic valve that shuts when it reaches its target temperature and pumps down, not seen this before.

[Pooh]

Not the best method of protection but in the event of refrigerant loss the switch will obviously shut the comp off, a similar system is used on certain car AC systems, the obvious disadvantage is that the suction pressure will drop into a vac if the leak is bad enough it can allow enough air to be drawn in to keep the discharge pressure above the setting of the protection letting the comp run until it dies.

Ian

[Brian_UK]

I suppose it could be used as a first start safety switch. If the system has had a major leak then the compressor will not start.

[gham]

this method of protection is used to prevent nusence LP trips upon startup as you can often get when the LP is in the suction line. It will also pick up an LP situation if you have a blocked filter dryer. The compressor will normally trip its internal O/L when running for peroids with low suction cooling and thus generate a compressor or evaporator alarm due lack of temperature change

[Vishnukurup]

Can I know what temperature you are trying to get?
What Gas are you on?

BTW: - The HP Pressure switch seems to be set VERY high.

The application is residential air conditioning. Temperature required is around 25°C.
The refrigerant used is R-22.

[Vishnukurup]

sounds odd,maybe wrong part or is it situated after a magnetic valve that shuts when it reaches its target temperature and pumps down, not seen this before.

No, there is no solenoid valve for pump down control in this unit.
I also saw this first time, LP switch on high pressure side for compressor protection!
Interestingly, there is no short cycling timer for compressor in this York Unit!
I saw Hitachi packages ACs where in LP switch is not used. They are using discharge gas thermostat to put the compressor off in the event of gas charge.

[absrbrtek]

Carrier used to use it and they still might. Its been a long while since Ive done any RTU work. It was called a "Loss of Charge" switch. I would have to think York is trying the same thing.

[bobkellyb]

It is a "Loss of charge" switch!

______________________
XXX Gold mmmmmmmmmmmmmmmmm

[bobkellyb]

It does not have a LLSV so the loss of charge switch
will not allow a start if the system pressure is below
the the LOC setpoint. (somthing else to go wrong)

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xxxx gold mmmmmmmmmm

[Chris Burton]

lot of heat pumps have lp switch on high pressure side to prevent trips when switching from heating to coooling, probaly fitted as standard to cooling units as well.

[will smith]

It is a loss of charge "safety". Carrier has been using them for years-same thing, located in the liquid line. I remember 10 years ago or so(probably longer) Carrier used a Tecumseh compressor that had an internal accumulator of sorts to prevent slugging; after a night setback, you could expect suction pressures from 5-15 lbs for up to 15 minutes-no problem if you know that's normal, but at first, we were all looking for restrictions or shoddy valves. Carrier offered no help with the troubleshooting at the time, so they ended up with mountains of "failed" compressors. It was then I first noticed loss of charge safeties on their rtus:you couldn't use a low pressure switch on these units unless you also had a time delay to bypass the potential low pressure startups. I still see loss of charge safeties on new equipment all the time-personally, I wonder if the emperor has no clothes. You can have a compressor running so low that it gets encased in ice with a liquid side too high to trip the loc safety. Maybe somebody ordered a LOT of pressure safeties with the wrong range (too low for hpc, too high for lpc), but they needed to do something with 'em, and just invented the "loss of charge" protection. Or maybe I just watch too many X-Files reruns-

[Lincoln]

The lp switch is being used as a loss of charge switch.it prevents nuisance trpping on start up,but if the expantion valve fails you will probably lose the compressor. lincoln

[paul_h]

Wow! Zombie thread

edit: you seem to know your stuff, so keep posting on RE.
But your answer for this situation has already been said two years ago