expansion tank pressure

[hinesguy]

Our expansion tank (glycol system for comfort cooling) runs at a positive pressure when the system is off and goes into a vaccuum when running, sometimes as much as 20 Hg. Is this normal.Seems like a pretty high vaccuum.
System: Three 540 ton chillers, two 260 ton chillers. supplying 6 main cooling fans and multiple small chilled water units in the ceiling.

 

[Yorkman]

Where is the compression tank? Is it "the compression tank" on the suction side of the pump. Is there only one compression tank in the system? I assume this vacuum is at the suction of your pump too? Are there multiple pumps? Are they pumping into the chillers or out of them? Could you be starving the pump(s) when you're running at light loads and several chillers are isolated?
I have seen the suction side of a pump drop into a "deep" vacuum on a closed loop, but it was when the pump was pumping against the compression tank, "the point of no pressure change". This was on a primary/secondary system and the primary pump was discharging into the secondary loop near the compression tank.
Is this a new system or an existing problem? Could you be pumping against a large air trap?
Lots of possibilties.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[KenRad]

One more question for the list -- is this tank a sealed chamber with air in contact with the water, or a bladder or diaphragm-type tank?

---KenRad

 

[hinesguy]

There is positive pressure at the pump inlets of around 180 psi. The only vaccuum reading we get is at the expansion tank at the top of the loop. This is a primary only system. Do not believe the tank has a diaphragm. This is the main expansion tank on the 35th floor, all the pumps and chillers are on the 3rd floor.

 

[hinesguy]

On second thought the tank is probably a diaphragm type, like an Amtrol.

 

[Yorkman]

Hinesguy,
Is this 180 psig hydrostaic head, with the pumps off? If it is, it looks like you may not have the system up to full fill pressure. A rough calculation at 180 psig accounting for the placement of your pumps on the third floor gives you a rough calculated elevation of 415 Ft. divide that by 32 floors puts you about 13 Ft. per floor average does that sound right? This would mean you might be at 0 psig with out the pumps running, and when you turn them on that could be what's putting you into a vacuum on the return side, depending on the connection point of the compression tank. I would prefer to see at least 5 - 10 psig at the top of the system minimum, when the system is running.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[hinesguy]

Just checked the supply and return lines for the upper house fan coils. They where at 0 or below, no compound gauge to see. But we are getting decent cooling out of them. This is getting weird. It would seem we are starving the coils, but they are cooling.

 

[Yorkman]

It does sound like the system may be short, although you obviously have water in the system. But remember when ever you go into a vacuum any trapped air in the system will expand, in a coil that can reduce coil capacity. The other issue would be any leaking packing glands or auto vents could be drawing air into the system, causing a possible air lock problem. I would try increasing system presure so that you consistantly see a positve system presure at the top. It doesn't need to be a lot 5-10 psig maybe? Just my own opinoin.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

 

[hinesguy]

We are going to add glycol to the tank after the system has been off and warmed up for a while. Going for 10 psi. System has to be short, and I think in the past they added to it while running which created the blow off problem . This way that can't happen as it will only contract as it chills down.
Thanx.

 

[Yorkman]

A blow off problem??
Are you saying that when you fill the system up to a normal 10 psig of pressure while it's cold, if the system is shut down and warms up, the pressure rises above the safety relief valve setting and opens? Where is the relief valve located and what is it's setting?
That sounds like it might be a symptom of the bladder in your compression tank has failed and filled with water, or you may not have a large enough compression tank for the system volume. Assuming a chilled water temp of 40-45 degrees, and when the system goes down an ambeint temp. of 85-95 max. I would think that the compression tank should be able to handle the volumetric expansion of 40-50 degrees for water without risk of over pressurizing the system.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI

http://home.comcast.net/~offsetaj

 

[hinesguy]

I think at one point someone overfilled the system, probably by filling it when the system was running. I just added some to the tank which was warm and at 0 psi. It's at 6 psi now and I will check it once I start up.