Head pressure control method.

[mad fridgie]

I fully understand the need for head pressure control. (not that I always agree with the need in all applications)
The question is: In the USA they seem to control with a valve, do you also control fan speed or fan cycling or do you just let them run?
In NZ as general rule (has some exceptions) we control the air flow, and do not use valves.

[RANGER1]

mad fridgie , are you talking evaporative condensors or something else ?
Are you saying you use method of fan speed ie VFD ?

[mad fridgie]

mad fridgie , are you talking evaporative condensors or something else ?
Are you saying you use method of fan speed ie VFD ?

Sorry for not making clear, Air cooled condensors (dry) DX systems, Controlling the air either VSD (larger systems) Phase cutting (smaller systems) staging the fans (medium to large but on the cheap most common!)

[Sridhar1312]

The air cooled condenser fans shall be linked to the air quantity required and ambient or discharge pressure. In many cases the compressor starts after the discharge pressure is increased fan comes on that too in stages.

[Magoo]

Mad Fridgie.
Been there done that so to speak. Tried step control on pressure, total erratic control of discharge pressure.
So currently have systems on base load fans continous based minimum loading and VLT control and the balance, but have compatible motors to suit. Straight line pressure control not totally acheivable, but close. As in Christchurch area I would consider a discharge regulation PM vav to maintain discharge condition on compressor and be wide open for summer, with VLT control from there on. Add an ORIT vav to add pressure to receiver for liquid flow to system at low ambients or partial loads.

[US Iceman]

I think the use of the head pressure control valve depends on how cold it actually gets. Variable speed operation might be acceptable down to some specific temperature. However, if the ambient drops real low you might still need the head pressure control valve to block of condensing surface to maintain the discharge pressure.

In these conditions a balanced port TXV is more forgiving than an unbalanced port.

[mad fridgie]

I agree with all , thanks.
So in USA if you had for example 60Kw (cooling) stand alone air cooled refrigeration unit lets say SST -8C, would it be standard practice to install a head pressure control valve?, DO you also control the condensor fans or do you just LEAVE them running?

[mad fridgie]

Ithiunk the use of the head pressure control valve depends on how cold it actually gets. Variable speed operation might be acceptable down to some specific temperature. However, if the ambient drops real low you might still need the head pressure control valve to block of condensing surface to maintain the discharge pressure.

In these conditions a balanced port TXV is more forgiving than an unbalanced port.

I also like the idea of the double port valves (small and large port), never actually seen one in use. I believe sporlan makes these products.

[cadillackid]

most all of the systems i work on large and small have either variable speed condensor fans or have fan cyclers on them... in fact a lot of manufacturers "low ambient kit" is nothing more than a fan cycling switch.. then again ive only ever worked on domestic space conditioning and not coolers or refrigerators...

in my case there;s always been enough heat transfer at the evaporator to keep the head pressure up at the condensor with simply cycling the fans...
-Christopher

[lana]

Hi there,

As said by the others, using head pressure valve depends on ambient temperature. If it does not get so cold during winter them fan cycling/speed control is enough.
You can check it during winter: cycle the fans and then see if discharge pressure still decreases then you need valve.
Using head pressure control valve needs careful design and charging otherwise, you will have more problems than before.
Cheers

[Gary]

Fan cycling is far more common... even when I was up in Michigan where it gets very cold.

That said, I have worked on a few flooded condenser systems. They are a pain in the butt.

[isobutane]

I experienced a 12kW system with fan cycling scheme not working stable, Its hard to simulate the same condition in Lab, but in field at certain circumstance, it's easy to trigger LP switch, even when the ambient isn't too low. for lower ambient trying a accumulator with a heater working togather with a solinoid valve is Ok. Fan should be working on low speed.

[US Iceman]

I also like the idea of the double port valves (small and large port), never actually seen one in use. I believe sporlan makes these products.

Yeah, they are great. If I have to use a TXV, I always try to use a Sporlan balanced port valve. They are more forgiving than the others IMHO.

[US Iceman]

Flooded condenser with head pressure control valves can be a pain in the butt to charge. That's for sure. You have to charge them according to the lowest anticipated ambient temperature or they won't have sufficient refrigerant to flood the condenser coil. Then in the summer you have to make sure the receiver is large enough to hold the excess liquid.

I've seen some cold weather in Southern Indiana where fan cycling would not work at all.

[mad fridgie]

I have found that is the condensor coil is vertical, than fan control works well enough, but if you have a horizontal coil, then fan control is less efficient at controlling (natural convection being the problem)

[US Iceman]

I have found that is the condensor coil is vertical, than fan control works well enough, but if you have a horizontal coil, then fan control is less efficient at controlling (natural convection being the problem)

That's interesting. I never saw too many vertical coils when I pulled wrenches for a living to say one way or the other.

The flip side to what you saw though is: in that sense, a vertical coil would have more surface area of the coil exposed to the wind too, which could make control more difficult?

[mad fridgie]

That's interesting. I never saw too many vertical coils when I pulled wrenches for a living to say one way or the other.

The flip side to what you saw though is: in that sense, a vertical coil would have more surface area of the coil exposed to the wind too, which could make control more difficult?

It may be In NZ (very small population and a long way from anywhere) we tend to build condensing units in a Kit form, reduced amount of stock required (common size condensors) So if we have a larger duty we may stack a couple or more condensors on top of each other.
Wind here is not a problem as far temperature goes (If it is winding, and it gets windy then temperatures are not normally that cold) We get low ambients when the air is still