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 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 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

[Chris- Africa]

Thanks!!!!!!!!!!!!!!!!!!!!!!!

[Chris- Africa]

Currently, I am working on refrigeration and do study’s at the same time to expand my knowledge. The more I know, the better service I can give to my clients.

Could you assist me on the following questions please?

How do I know what size compressor and coil do I have to use on different sizes cold rooms or freezer rooms? For example: My client asked me he wanted a freezer room, length 10m, width 6m, height 2.4m. This size is just an example.

How do I know what size of liquid line and suction do I have to use on the above mentioned?

How do I know at what settings do I set my LP / HP switch to on the above mentioned?

What is the purpose of the solenoid on the liquid line? I assume it’s to cut off the gas, and then build up the pressure to the LP /HP switch, which is switching off the compressor. Is there any other reason for the solenoid? For example: Is the cold room switched off by the thermostat and do you still need the solenoid? If yes, why?

I would really appreciate it if somebody can help me out with the correct info.

Thanking you in advance for your time and help.

Regards,
Chris van der Walt
South Africa
E-Mail: cc.vanderwalt@gmail.com

[Electrocoolman]

Chris, you have asked a lot of questions on the back of another thread...you should really start a new thread...but here goes with help on the solenoid issue.

You would normally control the solenoid with the thermostat. The solenoid would then shut and stop the (liquid) refrigerant.
The compressor will continue to run as the refrigerant in the evaporator boils off and the pressure on the low side of the system then starts to fall, until the LP switch operates and stops the condensing unit.
This is known as a 'pump-down' system. Its main objective is to stop refrigerant from condensing and collecting in the evaporator, which would be the case if we just stopped the compressor and had no solenoid valve. Upon restarting the compressor, we would get a floodback of liquid refrigerant to the compressor which could damage the compressor and its valves.
Another advantage is that we do not need any control wiring between the thermostat / coldroom /solenoid valve and the condensing unit (which might be some distance away - say on the roof).

Hope this helps. Adrian

[FEISTY]

Just another point of caution when setting LP and HP controls. A company such as Ranco will use differential setpoints...difference between cut in and cut out. Penn Controls will set by cut in and cut out. They do not use a differential point. You need to pay attention to the small print below the scale to see how it operates. If not you can drive yourself crazy trying to figure out why it won't work properly. 18 yrs. and I still forget sometimes. Have a great day !!

[Chris- Africa]

Thanks Adrian and Feisty.All info HELPS!!!