Effect of glycol on heat transfer in chilled water loop

JJ_4

I have a chiller that serves 3 air handlers. One air handler serves an area that, until we had the roof replaced and more insulation, was difficult to cool above 85 degrees OAT.

This past fall we had a leak in a coil and after the repair checked the system for glycol protection and found it was in the 35-40 degree range (I can't remember exactly)...glycol was added to get this to -4.

Now in my first cooling season with this concentration of glycol...I am having trouble cooling the previously difficult area at temps above 85. Prior two summers with lower concentration I didn't start "loosing" the building until above 95 degrees OAT.

My question, can the high glycol concentration reduce the heat transfer to the point where the situation I describe can occur? What concentration (degree protection) should I specify? I have a software based Mixed Air Low Limit loop that closes the OSA damper at 38 degrees MAT and hardwired freeze stats.

Thanks

don_201

Its possible

Its possible that it may have reduce your flow.

You should get some numbers at the airhandler.

You would want to check the water temps into the coil and out of the coil.You would also want to check the temps into the airhandler and out of the airhandler.Drybulb temps
along with wetbulb temps.

That way you can check the capacity of the evaporator and see if meets rated capacity.

Are you controlling the water flow to the coil with bi-direction acturator?

Bob Gagnon plumbing and heating

Radiant Cooling WIth 56 Degree Well Water

I didn't notice any difference in the performance of my system when I added antifreeze.

Thanks, Bob Gagnon

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JJ_4

Thanks..

We tried reducing flow to better match the coil. No effect. Decided to clean coils...though they didn't look dirty...no effect. Going to dilute glycol (its at 40% now for a -4 protection) to see what happens. This is sort of our last resort. If this works, may have to dilute for summer and refill in winter...not the best of situations, but the system is at the edge of its capabilities.

hot_rod

the colder the fluid

the thicker it becomes and more pump power is required. Look at the Engineering Manual from the glycol manufacturer, they have the charts and correction factors for various %'s at various temperatures.

Dow has an excellent manual, for free, I think it is now downloadable at Dow Heat Transfer Solutions site.

Are the pipes exposed that you need a -4 freeze protection?

hr

JJ_4

Coils...

The coils are in built-up AHU's in a mechanical penthouse. Pipes leading to/from the coils are insulated. I've read that protection should be 5 degrees less than the lowest expected winter temp...design temp here (Denver)is -10 on the charts I found, but I think that really is for the mountains not the front range. We rarely get below zero. I can't seem to get our mechanical contractor to make a recommendation for the concentration...probably hedging against liability.

My boss really freaks out about making sure we have glycol where it is...even though we have 3 levels of protection (Normally Open heating valves that are open when the unit is off or when freeze stat trips, fan shutdown on freeze stats, and mixed air low limit programming for the economizer).

bob_46

Double Whammy

What H.R. said plus the greater the concentration the lower the specific heat.
So to move the same amount of heat as water you have to pump more GPM.

don_207

you may

You may be putting all your thought into a area that is really not the issue.

From what i'm hearing is you have a system that cools,dehumidified and reheats for better comfort control.
Where is the heating coil in the system? before the evaporator coil or after.

That why it so important to check the airside number as mention above.Now you become the weather man.I'm sure your mechanical contractor should be able to find these numbers for you.

You have to start thinking like a weather man now.You know like today dewpoints are going to be high with all the snow on the mountains.

JJ_4

Problem resolved....hopefully....looks good so far today

We reduced the glycol concentration from 40% to 20%(which means we will have to re-fill in winter) and seem to have resolved the problem. It is 80 today (10AM)...and we are holding a DAT of 57 and maintaining the duct static setpoint of 1.8...space temp average is 71. Yesterday, before the dump, at 80 we could only get 63 DAT, duct static (many VAV's open) dipped to 1.2, with VFD at 100% and average floor temp was 76.

Thanks for everyone's help and suggestions. It really helped out in decision making...and I learned some new things to look for too!

UPDATE 6-26-09: It's 88 outside now at 12:15. Maintaining the following: 1.8 inches duct static (setpoint Max) with VFD at 86%, 57.6 DAT, average space temp is 71.7. I think I can definately say that the glycol concentration and resultant diminished heat transfer was our probem.

paul_79

glycol

if you have the capability on your building controls you can have the chilled water pump come on if the freeze stat trips and also open any valve to the air handler chilled water loop, we have it on our buildings in utah and it sounds as if you have about the same low temps.

don_207

I often

I often think about you guys up there above sea level and how you all adjust your equipment for specific volume and enthalpy difference at non sea level elevations.

Gald to hear all is well.

JJ_4

Good idea...

Thanks Paul. One of our techs from the mechanical company suggested something similar...starting the CW pump at an adjustable outside air temp. Freeze stat application you mention may be better as the HW valve will be open on trip.

JJ_4

Office Building Environment

I have mainly worked in an office building environment. First, for a temperature controls company (design and project management) and now as a facilities manager. We don't really deal with specific volume (not really familiar with this term...that is how often it came up) and enthalpy...even in our computer rooms, which are now mainly PC based servers that tolerate by spec wide temperature and humidity ranges. I suppose in a process manufacuring situation these issues may come into play, but we just consider return air temp versus outside air temp when enabling the economizer. The control loops are designed to keep mechanical cooling, free cooling, and heating with minimal overlap (0-33%, 33-66%, 66-100%). RH in Denver averages 30%...we have highs of 60% or so, but only occasionally.

van_8

What do you have for an air elimination on the system air in a glycol is very hard to remove and it has a big effect on system prefomance a microbubble separator may take care of the problem without having to adjust glycol concentrations.