Condenser Head Pressure

[BRX]

I have limited understanding of how refrigeration works. I only know the basics, my experience is more or less with PLC automation.
I was assigned the task of creating a head pressure control for a cooler condenser and one for a freezer.
Both of the condenser fans are 3 phase, so I'm controlling them with a VFD drive.
I added a pressure transducer to the condenser outlet and plan to run the fans according to psi feedback of the sensors in closed loop fashion feeding into a PID loop.
The reason for doing this is because it gets as cold as -20F in this area.
Everything works, the user basically enters a Head pressure set-point on a touchscreen and the controller is able to maintain the pressure within a PSI(or as close to the set-point as allowed by ambient temp).
My question is what is the ideal head pressure for a cooler(35-40 F)? How about the Freezer(-10-0F)?
I did a few trial runs on the cooler with different set-points.
With a set-point of 255 PSI it takes 10 minutes to bring the cabinet down to 30 with a 5 degree differential.
A set-point of 150 PSi takes 7 minutes, while a set-point of 180 takes only 5 minutes.
My question is what is the ideal head pressure for a cooler(35-40 F)? How about the Freezer(-10-0F)?
Does the ideal Head pressure set-point vary with ambient temp?
any help is appreciated.
Thanks in advance.

[crazzycajun]

What refrigerants

[pecmsg]

Why allow anyone to screw with it. There are plenty of after market controls that are non adjustable. AKA Idiot proof!

[BRX]

What refrigerants

r404A

[Olivero]

The way Head pressure is designed is dependent on ambient temperature.

We need to know the part of the world this is on, weather, ambient temperatures, inside or outside and the refrigerant being used.

The way it goes is, take your ambient temperature (Florida for example, where I work) the ambient design is normally 105*F so then you design the head pressure to be 15-20*F above that, so if you take R22 for example, 120*F is 260 PSIG, meaning when R22 is at 120*F, it's 260 PSIG, they are related.

So if you want to know what your head pressure should be, take your ambient design temperature (105*F in my example) add 15-20*F and then convert it to pressure, 260 PSIG per the R22 Pressure Temp Chart.

So that tells me, my head pressure in this system should be around 260*F.

The whole concept is to create a heat transfer between the ambient temperature and the refrigerant. If you were to design head pressure to be at your ambient, say 95*F/184.6 PSIG and your ambient was 95*F, the refrigerant as it moves through the condenser would never loose any heat and so it would never condense causing the system to not function properly and provide poor cooling.

So the refrigerant HAS to be a higher temperature than the ambient coming out of the compressor to generate that heat transfer, 15-20*F is just a rule of thumb but it works. Then as it leaves the condenser, it's cooled off and is now liquid again and can go on to do it's job properly.

Hopefully that makes sense.

[BRX]

Why allow anyone to screw with it. There are plenty of after market controls that are non adjustable. AKA Idiot proof!

I probably will lock it down after I'm comfortable with its operation.
What I meant to get across is, I have limited understanding of how the head pressure effects the total operation.
The evaporators have electronic expansion valves. If we maintain 180 PSI head pressure year round are we going to run into issues at -20F?

[BRX]

The way Head pressure is designed is dependent on ambient temperature.

We need to know the part of the world this is on, weather, ambient temperatures, inside or outside and the refrigerant being used.

The way it goes is, take your ambient temperature (Florida for example, where I work) the ambient design is normally 105*F so then you design the head pressure to be 15-20*F above that, so if you take R22 for example, 120*F is 260 PSIG, meaning when R22 is at 120*F, it's 260 PSIG, they are related.

So if you want to know what your head pressure should be, take your ambient design temperature (105*F in my example) add 15-20*F and then convert it to pressure, 260 PSIG per the R22 Pressure Temp Chart.

So that tells me, my head pressure in this system should be around 260*F.

The whole concept is to create a heat transfer between the ambient temperature and the refrigerant. If you were to design head pressure to be at your ambient, say 95*F/184.6 PSIG and your ambient was 95*F, the refrigerant as it moves through the condenser would never loose any heat and so it would never condense causing the system to not function properly and provide poor cooling.

So the refrigerant HAS to be a higher temperature than the ambient coming out of the compressor to generate that heat transfer, 15-20*F is just a rule of thumb but it works. Then as it leaves the condenser, it's cooled off and is now liquid again and can go on to do it's job properly.

Hopefully that makes sense.

Yes, that makes sense. Thanks for clearing that up.
I might be barking up the wrong tree, but please bear with me.
I have the pressure sensors reading the line coming out of the condenser after the refrigerant has cooled off and am running the fan accordingly.
I don't very well know the pressure leaving the compressor. My main goal is to keep the refrigerant from evaporating before it gets to the evaporator, and keeping a minimum load on the compressor.
Maybe I have something mixed up.

[icy78]

You said you have EEVs. With those, you can use a floating head pressure, meaning that you do not have to maintain a set point of , typically 105f condensing temperature. So with your fancy setup you can float your condensing temperature meaning that on a hotter day you'll have a higher condensing temperature on a cooler day you can have a lower condensing temperature, maybe even as low as 65-70 degrees.
Find some articles to read about it and you can leverage your expertise and your nice vfd/transducer system to save you a considerable amount of energy.

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[Olivero]

Yes, that makes sense. Thanks for clearing that up.
I might be barking up the wrong tree, but please bear with me.
I have the pressure sensors reading the line coming out of the condenser after the refrigerant has cooled off and am running the fan accordingly.
I don't very well know the pressure leaving the compressor. My main goal is to keep the refrigerant from evaporating before it gets to the evaporator, and keeping a minimum load on the compressor.
Maybe I have something mixed up.

Yeah, that makes it a little tricky I think, considering you need to balance the head pressure to keep the condensing temp above the ambient, you'll need a way to adjust that, by having it on the outlet, you can only adjust it based on what comes out of the condenser but without knowing what it is on the inlet, it'll make it tricky.

I guess you could do it in a way where if the temp of the refrigerant goes below the ambient temp or close to it, then the fans kick off, allowing the head pressure to go back up, raising your refrigerant temp and increasing the heat transfer from refrigerant to ambient.

I think that would work.

The compressor will pump regardless, the only reason your compressor won't have a load is if the box is nearing temp and the TXV or in your case EEV is closing down and so beginning to starve the comp until it eventually shuts off. But that's how it works, no way around that and if you really want to keep it fancy, make it a pump down system with a liquid solenoid valve to prevent liquid migration, that's probably one of the most important things you can do.

I think you're well on your way, we can help you get everything sorted, most of these guys forgot more than I'll know but I'll try to help you.

[icy78]

I have limited understanding of how refrigeration works. I only know the basics, my experience is more or less with PLC automation.
I was assigned the task of creating a head pressure control for a cooler condenser and one for a freezer.
Both of the condenser fans are 3 phase, so I'm controlling them with a VFD drive.
I added a pressure transducer to the condenser outlet and plan to run the fans according to psi feedback of the sensors in closed loop fashion feeding into a PID loop.
The reason for doing this is because it gets as cold as -20F in this area.
Everything works, the user basically enters a Head pressure set-point on a touchscreen and the controller is able to maintain the pressure within a PSI(or as close to the set-point as allowed by ambient temp).
My question is what is the ideal head pressure for a cooler(35-40 F)? How about the Freezer(-10-0F)?
I did a few trial runs on the cooler with different set-points.
With a set-point of 255 PSI it takes 10 minutes to bring the cabinet down to 30 with a 5 degree differential.
A set-point of 150 PSi takes 7 minutes, while a set-point of 180 takes only 5 minutes.
My question is what is the ideal head pressure for a cooler(35-40 F)? How about the Freezer(-10-0F)?
Does the ideal Head pressure set-point vary with ambient temp?
any help is appreciated.
Thanks in advance.

For some reason I can't download the link, but Google RSES, then.... EEV floating head pressure control.
Nice little article.

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[BRX]

Thanks guys.I'll do some reading.
I appreciate the help.

[Spitz]

-20 on a windy day stopping or modulating your condenser fan motors will not be enough. A Headmaster should be installed.

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[BALloyd]

-20 on a windy day stopping or modulating your condenser fan motors will not be enough. A Headmaster should be installed.

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X2

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[Cool breeze 38]

being your low ambient I'd also install a wind baffle along with the controls.

if I were doing your task I would either modulate the fans at a minimum of 85 deg line temp or 185 psi If its warmer then they will be full speed as you would want. You can then adjust the minimum as you see fit. A stable pressure as you are trying to do is best for any modulating valve (txv, eev or anything adjustable) so it will be the most stable and not hunt.
You could do it with a bms type controller and write a program, or a Honeywell controller or the like. Or use a https://www.icmcontrols.com/productdetails/ICM334

Warning. you are using a freq drive on motors that may be sleeve bearings and not grounded shafts. See if you can write your program to speed the fan up 100% for 3 seconds then slow it down to help protect the sleeves. The ICM head controller does this also.. Good luck

[Cool breeze 38]

-20 on a windy day stopping or modulating your condenser fan motors will not be enough. A Headmaster should be installed.

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he is making a head master set up.

[Spitz]

How do you figure?

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[Cool breeze 38]

How do you figure?

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what is a head master and what does it do?

[Olivero]

From what I understand, it's basically a valve that short circuits the refrigeration system and maintains a certain head pressure.

It basically allows the discharge line to bypass the condenser to keep the head pressure up.

https://www.hvactrainingsolutions.ne...on-headmaster/

[Cool breeze 38]

I wasn't thinking that was the name of it. Yes that would be helpful along with fan control. Good advice spitz

From what I understand, it's basically a valve that short circuits the refrigeration system and maintains a certain head pressure.

It basically allows the discharge line to bypass the condenser to keep the head pressure up.

https://www.hvactrainingsolutions.ne...on-headmaster/

[Spitz]

You may have been thinking of a Motormaster. Close name but very different. Liebert runs there fans at full speed year round. No fan speed controls.

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