pressure switches

[wilks]

hello guys ,

still making the transition to refrigeration and was wondering how to read hp / lp pressure switches.
i know lp is when we have low pressure so system cuts out and the same for hp , but how do i read them , i get confused with diff:confused: any help is much appriciated

thanks

[Brian_UK]

It depends on the switch make up, I always have to read the instructions or sit a have a think before doing things.

For e.g. a switch may have a 'cut-in' and a 'diff' range. So if we want the switch to changeover at 3 bar and back again at 2 bar we would set the cut-in at 3 bar with a diff of 1 bar.

You could perhaps think of the diff as being a 'range' setting.

[taz24]

hello guys ,

still making the transition to refrigeration and was wondering how to read hp / lp pressure switches.
i know lp is when we have low pressure so system cuts out and the same for hp , but how do i read them , i get confused with diff:confused: any help is much appriciated

thanks

I see lots of people who struggle with this.
It does not help that the LP and the HP work opporsite to each other.
HP cuts out on rise.
LP cuts out on fall.

I remember it as LP cut out is cut in, less differential.
You set the cut in and adjust the diff for the cut out.

HP I think is easier.
You just set the pressure to what ever you want to cut out at. Adjust the diff to allow a safe period before it cuts in again.

R134a is about 1bar @ -10degC.
At 0degC it is about 2bar.
So for a fridge if you set the lp to cut in at 2bar with a 1bar diff you will have it opperating at cut out -10degC cut in 0degC. (pumpdown would be cut out at 0.2bar and cut in at 2b)

R134a is nearly 15 bar at 55degC.
So if you set the HP to 15 bar. 32degC is about 7 bar so adjust the diff to 8bar diff.
The HP will cut the comp if the discharge gets to 55degC and then will cooldown to 32degC before it cuts in (auto resset).


Cheers taz.

[paul_h]

On LP, you'll have "range" which could be called just cut in, and you'll have differential, which is how much lower than the cut in you set , will be it's cut out pressure.
eg: setting range (cut in) to 500 KPa and differential to 300 means the unit will cut in @ 500 KPa and cut out @ 200KPa. If you set the diff to 400 then the cut out will be 100KPa, if ou set both to 500 then the cut out will be 0 KPa

edit:
For HP it's the much the same, but they often don't have a diff setting as they are a fixed diff or manual reset. But if they do have two setting, then you're going to be setting the range as the cut out and the diff as the cut in.

[wilks]

thanks guys for all your help