Head Pressure and Flow Issues - Liebert Glycol Unit

[quonix]

I have a 20-ton Liebert CRAC unit, glycol condenser cooled.

The unit has a 2" glycol supply and return feeding it. Its a building loop that I have no control over. The 2" pipe runs horizontal for about 70 feet, that joins into a 3" main tap.

Glycol entering temp is 88 degrees, exit temp is 112 degrees. Yeah, I know, its high, thats the problem.

When the unit runs on 1 compressor, exit temp is fine, but when that 2nd compressor comes online, it gets hot. The buildings mechanical contractor built the 2" loop for me and they were supposed to qualify it and verify the capacity.

I really need to figure out if there is anything on my end that could be causing the issue, or could help the issue. Its like moving mountains to get the building to lower the loop temp. The mechanical contractor that does the loop is reputable, and when they installed these new 3" main taps, the return tap was a homerun all the way back to the tower on the roof (4 story building), the supply tap is on the 1st floor - where the Liebert is. Because of that return tap on the roof, they claim flow should not be an issue.

Here are some ideas:

1. Head Pressure Regulating valve. We know its wide open, but could it still be restricting flow even in its open state. Would cutting out the regulating valve potentially help flow? This unit runs 24x7 (dataroom), so the valve is always open anyway.

2. Econocoil. The unit has an econocoil, but its never used. As as result, the glycol has to first go through the econocoil valve, could that valve be restricting flow? Again, would cutting out the econocoil valve help flow? I've heard some people make comments that econocoil systems tend to have issues because of that valve.

3. Dirty Condenser Tubes. Unit is only 4 years old, on a 50% glycol/water mix. Is it really that likely that the tubes are dirty causing poor heat transfer?

Thanks
John

[m8j4c]

Is there pressure taps on both sides of the condensor loop to check pressure drop? I cant remember with crac units... If there is theres usually some reference chart to see if your flow through the condensor is in the right range. Also, since it is a data room unit dont mess with the head pres valve. The only thing that would make that ok is if you had a speed drive on the fan motor in your cooling tower that is controlled by the water temp.

[uhPrintUs]

You'll need to determine the glycol flow through the condenser. Also, the specific gravity & specific heat of the glycol/water mixture is needed.

Water flow is typically designed at 3 gpm per ton, but glycol has a lower heat transfer rate, so additional flow (compared to water) is required.

Various methods are available to determine fluid flow through the condenser. If there is no circuit setter installed, reading the pressure drop of the glycol mixture across the condenser can be compared to Liebert's data.

There are electronic flow meters available that have a couple of probes to attach to the piping, so flow is measured without tapping into the glycol lines.

Also, with a couple of wet-bulb temp readings of supply & return air, you can calculate the gpm. Use the wet bulb temps to get enthalpy of the air by using a table or a psychrometric chart.

Heat Transfer Formula for Standard Air:
Btuh = 4.5 x cfm x (H1 - H2)
H is enthalpy

If you're concerned that your air is not close enough to "Standard Air" conditions, then substitute (60 x density of actual air) in place of the 4.5

This will tell you the true Btuh capacity of your operating system.

Insert that Btuh reading into the next formula with the glycol/water readings.

Btuh = 500 x gpm x delta T x Specific Heat x Specific Gravity

OR: Gpm = Btuh divided by 500 x delta T x SH x SG

If it used water, the SH & SG would both be 1.

Have fun!

[ispip]

112 degree exiting? that is not really that abnormal. Like the guys said above you should get your flows threw the condenser. But to be honest if your unit is not tripping on head pressure and from what is sounds like you wasted money getting econo-cool units. don't worry about it, these units can handle 95 degree entering water temp.

If you could get a list of info together would help: head pressure, suction pressure(with both compressors running.) 2 way or 3 way valves, super heat and sub-cooling.

because these units are controling the water flow threw the condenser based on compressor head pressure. so just using water temp delta t across the condenser is kinda pointless. but still you need to check the basics with y strainers and circuit setters and making sure your condenser is not fouled.

why are they running a 50/50 mix?? are you hooking in to a condenser loop?

[uhPrintUs]

If there is a 24 degree rise in the glycol/water temp through the condenser with 20 tons of cooling, you're not getting much more than 1 gpm per ton (about 22 gpm total). According to the Liebert DS System Design Manual, with 85 degree entering water, you need approximately 67 gpm.

You stated the water regulating valve is 100% open.

Have you tried closing the water reg valve on the non-operating compressor to confirm the temp rise through only one condenser?

With no change in Gpm being circulated through the unit, if you have one half the tonnage, (only one compressor operating) then you would have 1/2 the Temp rise through the condenser.

[quonix]

Thanks for all the replies... Here is an update.

We are doing a pressure test tomorrow for the condenser. The building (who maintains the loop) also took a glycol sample to check the specific gravity to make sure the ratio isn't out of spec.

Here are my specific units numbers (20-ton Liebert Deluxe 3, with econocoil that is never used):

Return Air Temp: 86
Supply Air Temp: 66

Glycol Enter Temp: 89
Glycol Exit Temp: 115

Comp #1: Discharge 255#, Suction 80#, LL Temp 105, SL Temp 63, Subcooling 13.2, Superheat 15.6

Comp #2: Discharge 260#, Suction 80#, LL Temp 108, SL Temp 65, Subcooling 12, Superheat 17

Also, if I shut off one compressor, the glycol exit temp comes down to around 99 degrees. I dont have suction and discharge numbers since its not easy to take the unit down to one compressor right now, but the last we did this, the head pressure numbers do come down (so more like 235# and 70#). My other Lieberts (which are mostly 5-ton Challengers) have better loop taps, and only have a 8-10 degree delta on the glycol, they also have have much better numbers, 225# and 65# - and those unit have the same return air and supply air characteristics.

[uhPrintUs]

I HAVE to ask this! Did you obtain subcooling by comparing saturation temp to the discharge pressure? I assume you did since you didn't include a "Liquid Line" pressure. Most condensers have enough pressure drop to cause the "so-called" subcooling measurement to be inaccurate. It's a mistake made quite often by many people, so don't feel all alone if you did this. I just want to point it out so it may help some who have been taught incorrectly. I've seen start-up reports by techs who figured subcooling the wrong way and it resulted in impossible data, such as the amount of subcooling would indicate the liquid refrigerant was cooler than the medium that cooled it.

[r22eric]

Hello all I have a McQuay water source heat pump model: CCH036CMFS in the ceiling of my facility i can only access the high side fitting.
the unit kicks out on high head according to the diagnostics on my mark IV control board
it runs for approx 30 to 45 seconds and i watched the highside guage climb to 345 psig and it kicks off on high head pressure???
it has a tube within a tube condensor strainer is clean as a whistle.........

any thoughts??

[uhPrintUs]

Hello all I have a McQuay water source heat pump model: CCH036CMFS in the ceiling of my facility i can only access the high side fitting.
the unit kicks out on high head according to the diagnostics on my mark IV control board
it runs for approx 30 to 45 seconds and i watched the highside guage climb to 345 psig and it kicks off on high head pressure???
it has a tube within a tube condensor strainer is clean as a whistle.........

any thoughts??

r22eric,

Since this is your first post, I suppose you don't understand how the forums work, because you should start a new thread instead of beginning a new topic in the middle of another thread.

One way is to go to the bottom of the thread and click on "Forum Jump" to select a forum, such as "Residential" or "Commercial". Once you've been taken to your selected forum, look near the top of the page on the left side and click on "New Thread".

Then give as much info as possible so technicians will be able to help you, otherwise you may be ignored if you don't provide enough info for someone to help you. It's better to give too much information than to provide too little. Nobody knows why a water cooled unit is going off on high head pressure if you don't give a few clues. What kind of refrigerant? Is this the only unit with a high head pressure problem? What's the age of the system. Has there been a problem with scaling in any part of the system. What size is the piping at the unit? What temp is the water entering the condenser? Is the water valve open? Is there some type of volume control for the water to the condenser? If so, what method: circuit setter, secondary pump, water regulating valve???

Most of those questions are based on a water source heat pump in the cooling mode, but you didn't actually state if the unit is in cooling or if it's in heating. We need to know different information if the problem is in the heating mode.

However, based on the info you provided, I can take a stab at the reason for your problem. I think you have a system with R-22 and the problem is in the cooling mode, or possibly the heating mode, and the actual reason the diagnostics board indicates the unit is going off on high head pressure is because your high pressure switch is working correctly and tripping when the head pressure gets too high. :) Maybe a dirty air filter!

[r22eric]

WOW!! Right on, thank you for the heads up.
i was grasping at straws on this one
thanks for the help.
:cheers: