Rack Systems in Low Ambient Conditions

[keepinitcool]

Thought this would be an appropriate topic since a deep freeze is moving throught the upper midwest. What is everyones take on controlling head pressure to keep freon from logging in the condenser during low ambient conditions? Whats the best way to accomplish this? How does everyone go about setting the holdback valve (A8 or ORIT)? Any particular brand of system or setup that seems to handle colder conditions better? And GO!.. . . . . .

[jpsmith1cm]

Timely topic...

Your FIRST line of defense against logging refrigerant in the condenser is properly setup condenser fan cycling controls. I've heard both pro's and con's of using them, but I'm in favor of cycling the fans off during low ambient conditions.

The ORI/ORO (A8/A9 is what I typically see) setup is to manage and maintain proper liquid pressure in the event that one of those controls fail.

Setting those valves is fairly straightforward. I like to go high on the ORI valve, then dial back to the pressure that I want and go low on an A9 and dial up to the pressure that I want. Just seems to make for a more accurate setting that way.

WHERE you set those valves is a whole topic unto itself.

A more specific question or questions is bound to get a more specific and accurate answer.

[jpsmith1cm]

Guess I forgot to include something important. Maybe the most important thing.

Make sure the rack has enough gas in it.

You are going to log SOME refrigerant in low ambients. This is just a part of life.

[IceVicious]

I think the best way is splitting the cond. some Walmart's use this along with fan cycling , im not a big fan of head masters .

Ice V

[jpsmith1cm]

I think the best way is splitting the cond. some Walmart's use this along with fan cycling , im not a big fan of head masters .

Ice V

Condenser splitting is a secondary method, used along with fan cycling.

It doesn't alleviate the need for low ambient valves.

[Phase Loss]

you're gonna need to start stacking the coil with liquid at some point if it gets cold enough.

[engineerdave]

Interesting topic. We've had cold weather-related issues here in Vegas, believe it or not. Most of our troubles manifested in Floodback in Medium Temp equipment. The problems have been tracked to two main contributing factors:

1) Most of the Medium Temp equipment are running on non-adjustableTXVs.

2) Energy Management strategies have been tweaked and re-tweaked, mandating colder & colder Dropleg Temps.

We found at various locations, Dropleg Setpoints had a parameter that would not allow Dropleg Temp to exceed 25 degf over Ambient. So we ended up with TXVs sized for 90 degf Condensing, running 50 degf Liquid.....

Can't change Operating Parameters without going through Congress. Can't install Adjustable TXVs without going through Congress...

Problem solved itself when the earth tilted the cold y'alls way.

For now.

[jpsmith1cm]

I've seen that one, too.

[Fridge Repairer]

No matter what your going to log Freon in the condenser. The hard part is settiong your A8 and A9 valve. Its best and easiest to set when outdoor ambient is about 40 Degrees F. I always set the A8 valve first. That's your holdback valve. Put your high side gauge on the A8 pressure port and turn the stem clockwise and watch the pressure rise past your desired setpoint. Be sure the manual stem is in the automatic position too. I like 180PSI for my set point for the A8 and 165 for the A9. If you want to log less gas go for a lower A8 setpoint but be carefull. If you go to low your TXV may not work properly. If your using balance port TXV's you should not have a problem.

Once your pressure at your A8 valve climbs past your desired set point you know the A8 is functioning. Now slowly start turning adjustment stem CCW and bring your A8 to your desired set point and lock it down with the lock nut.

Now put your guage on the A9 port. If your A9 if feeding the line should be hot. If its not hot its not feeding. Lets assume the line is cold. Turn the adjustment stem cw to open the A9 and bring your receiver pressure up to 165# and lock it down. Now your set.

Next check your receiver level and add freon if nessessary. Try to maintain at least 5% receiver on the coldest day of the year. The warmer it is outside the more freon you should have in the receiver. Don't ever touch the valves again.

I hope this helps you guys out. Be patient if your setting the valves on the coldest day of the year. It takes time to build up the pressure.

[markettech]

Great responses!

Ok.....now, how do you set an A8/A9 valve when the outside ambient dictates a higher SCT than your desired valve setting (summer)?

[jpsmith1cm]

Great responses!

Ok.....now, how do you set an A8/A9 valve when the outside ambient dictates a higher SCT than your desired valve setting (summer)?

The A9 valve can be set any time. Ambient isn't a factor in setting them. I've done in the middle of summer.

The A8 on the other hand.....

About the ONLY way I could see being able to set it would be to take the rack offline, isolate and pump out the piping with the A8 in it and use nitrogen to adjust it. I've never tried it, but Phase Loss has said that he's done it that way.

[markettech]

The A9 valve can be set any time. Ambient isn't a factor in setting them. I've done in the middle of summer.

The A8 on the other hand.....

About the ONLY way I could see being able to set it would be to take the rack offline, isolate and pump out the piping with the A8 in it and use nitrogen to adjust it. I've never tried it, but Phase Loss has said that he's done it that way.

Right - A9 is not a problem regardless of OAT/SCT

Let me run this scenario past you with regards to the A8 and tell me what you think:

• Gauge up at inlet to A8
• Close off the drop leg isolation ball valve (typically installed upstream of the A8)
• Close off A9 valve to receiver.
• Back locknut off to end of adjustment screw and turn A8 adjustment screw CW to stop
• Allow receiver to pump out to the the point where receiver pressure is at a value less than desired A8 pressure set point.
• Shut down pumps
• SLOWLY open drop leg isolation ball valve until gauge reading matches desired A8 set point.
• SLOWLY turn A8 adjustment screw CCW until gauge pressure drops
• Set locknut and return system to full operation

[jpsmith1cm]

It would probably get you close.


You could also set it at the lowest point local ambient will permit, then reduce the setting per the X# per turn (I've seen it written somewhere) to approximate target desired.

[markettech]

You could also set it at the lowest point local ambient will permit, then reduce the setting per the X# per turn (I've seen it written somewhere) to approximate target desired.

I am aware there is literature out there which details x psig per turn, but I'm still not following you.....can you expand on this a little?

[jpsmith1cm]

I am aware there is literature out there which details x psig per turn, but I'm still not following you.....can you expand on this a little?

Ok, let's say you're trying to set an A8 for a 65 SCT, but it's 85 out.

So, dial it in as low as you can, say 90-95 SCT, then figure out how much lower the pressure needs to go, adjust the stem blind per the pounds per turn formula to reach an approximate setting.

[Fridge Repairer]

Best to set it when you do start up. if its summer out and system is running it will be tough to set the A8. Bypass all your fans on and turn off a bunch of system circuits. See if you can creat a condition your head pressure goes below 180#. If not you won't be able to set it till cooler weather arrrives. During summer operation the A8 valve is not a factor anyway. A8 does nothing.

[markettech]

Best to set it when you do start up. if its summer out and system is running it will be tough to set the A8. Bypass all your fans on and turn off a bunch of system circuits. See if you can creat a condition your head pressure goes below 180#. If not you won't be able to set it till cooler weather arrrives. During summer operation the A8 valve is not a factor anyway. A8 does nothing.

This thought process befuddles me.

Since the A8 valve is out of play during the summer it is okay to do a half-assed job of servicing the customer's equipment, apparently.

I can't count the number of follow ups I've seen over the years by techs who are either too lazy to do their job, too dimwitted to think outside the box, or both.

Bet those customers love seeing a service invoice come through in December resulting from work that was actually done the previous June....

[Dowadudda]

You can dial a rack with low ambient valving arrangement any time of the year in my opinion, unless it's so hot it's not possible. It's certainly easier to do on a cold day. Less time and work.

One regulates pressure inlet to it. One regulates the pressure difference of the inlet to the outlet. Manipulate the rack to give you those pressures and dial it in. if no one messes with it, it ought to perform on that low ambient day no problem.

This is why seeing 50%-60% receiver in summer may be normal. We need that liquid in winter to stack the condenser.

This is why splitting is nice to have. If we split we reduce that overall charge. In split, and still require the stacking to keep minimum receiver up, I only have to stack that active condenser. much less liquid needed.

In Michigan and Wisconsin, where I came from in the trade. You always had to fan cycle, obviously. You almost always had equipped Split either automated or manual means, You always had low ambient valving. And on them cold days, really cold days, you may have had to add gas to stack that condenser so much so, that if you never did anything else to that rack, in June you would see a receiver at 60% or more. Not abnormal in that situation. It did what it was suppose to do when it was suppose to.

So now I think about applying the knowledge gained on floating head. All the low ambient stuff above stays the same. Cause in the strategies of floating I have been taught, we kinda basically stop anything at 65 to 70 SCT depending. And I have to ask myself, what happens if it's so cold out, like I have years of time on, and even with any fans off, were still dropping. \

Well. Easy. Still split. Still Stack. If we go below our limit block of floating, where stop doing anything, then we have other lines of defense.

I would love to go back up north and put this all in together. You have no idea how many stores up there miss the savings between 70 SCT and 95 SCT. And it's because the mind set is about them Cold Winters. So not much is concentrated on the sweet spot. It's literally amazing. How much money is sitting up there ready to be made.

[Fridge Repairer]

This thought process befuddles me.

Since the A8 valve is out of play during the summer it is okay to do a half-assed job of servicing the customer's equipment, apparently.

I can't count the number of follow ups I've seen over the years by techs who are either too lazy to do their job, too dimwitted to think outside the box, or both.

Bet those customers love seeing a service invoice come through in December resulting from work that was actually done the previous June....

To be honest, I can tell I'm not nearly as smart as you BUT, your the one that is to dimwitted that your the one un-able to think outside the box just by the nature of your response. Let's be honest here if you the start -up guy did your job correctly you would have set the A8 and A9 valve at start up while you were charging the system.

I don't understand how you can call my response lazy and ignorant. My response was accurate, knowledgeable and understandable. Where do you get off be-rating me from?

[jpsmith1cm]

To be honest, I can tell I'm not nearly as smart as you BUT, your the one that is to dimwitted that your the one un-able to think outside the box just by the nature of your response. Let's be honest here if you the start -up guy did your job correctly you would have set the A8 and A9 valve at start up while you were charging the system.

I don't understand how you can call my response lazy and ignorant. My response was accurate, knowledgeable and understandable. Where do you get off be-rating me from?

Markettech is one of the finest techs I've had the privilege of speaking to.

Of course this should be done during start up, and I'm sure that he does it when he starts equipment up.

That said, things change, A8 valves require repair, adjustment and other service throughout the life of a piece of equipment and that service cannot always be scheduled when it is most convenient for the technician.

Flippantly dismissing the idea of taking time to set the A8 valve during the summer because "the A8 valve is not a factor anyway. A8 does nothing." does strike me as a bit on the lazy side. This is the attitude that keeps supermarket guys working long hours when they should be at home drinking a little beer or spending time with their kids. Summed up, that attitude reads:

It's not important today, so I don't care. I'll fix it tomorrow.

That attitude has kept my bills paid and my belly full for years and will continue to do so for as long as guys have it.