expansion tank pressures

[mfi]

Our expansion tank is on the 35th floor, the chiller plant is on the 3rd. The upper house chilled water (glycol) coils are about 12 feet above the tank on 35. The question: The guage on the tank reads 20 Hg of vaccuum when the sytem is on. This seems like a very deep vaccuum. We are not sure anymore what it should read and are wondering if the system is short of glycol. Filling it too much will cause it to flood out when the system is off and heats up. Any ideas.

[referteacher]

You need to have 10 to 15 psi positive pressure at the highest point in the system. Reason is, there should be air vents at the highest point. Need to have positive pressure to push the air out. Air vents are float actuated, one-way, check type of vents. They do not hold against a vacuum and air will draw in if you don't have positive pressure.

[mfi]

Just checked the supply and return lines feeding the upper house fan coils and they where at 0 or below, no compound gauge to see. It would seem we are starving the coils but they are providing decent cooling. 45 chilled water, 55 degree air supply temp. This is weird. The lower house coils are at 140 psi.
Possible they might be at a few psi above 0, gauges aren't too accurate, pretty old.

[nun]

mfi what is your supply and return pressure at the highest coil? Sounds to me your system is low. When you say system "floods out" do you mean the relief pops? If so you have other issues. Maybe undersized or not setup properly. How many tanks are there?

[onesidedcoin]

Originally posted by mfi
Just checked the supply and return lines feeding the upper house fan coils and they where at 0 or below, no compound gauge to see. It would seem we are starving the coils but they are providing decent cooling. 45 chilled water, 55 degree air supply temp. This is weird. The lower house coils are at 140 psi.
Possible they might be at a few psi above 0, gauges aren't too accurate, pretty old.

The 140 one the lower floors is normal due to the weight of the water say you have a building that is 231 feet to the highest piping with the pump off you would read 100 psig differance between the high point and low point.
Sounds like you may have a two pipe change over system in that case the expasion tank would have to be sized for your system volume and temp rise of 45 to 180.

[duke of earl]

Ever been to B&G website? They can help with any hydronics question you have.(Bell and Gossett)

[mfi]

We are going to add more glycol to the system after it has been off and warmed up. Figure on getting about 10 psi reading on the tank gauge. System is definitely short. Thanx all.

[referteacher]

My calculations ( ass-u-me ing 12 ft per story) times the 31 storys between the chiller plant on third floor and expansin tank on 35th floor(if you don't have the 13th floor) would be 375 feet. This times .433 psi per foot and add 15 psi extra to pressurize the top coil would add up to a total of at least 175 psi at the expansion tank.

[absrbrtek]

Im missing something here. You would want 175 PSI on an expansion tank on the roof/top floor? Wouldn't that give you over 300 PSI at the chiller plant? Remember, the ex tank is on the 35th floor not in the chiller plant on the 3rd floor.

Personaly I would set it up at 10 to 15 PSI. Just enough to ensure the top AHUs have a positive pressure on them and can bleed air properly.


Originally posted by referteacher
My calculations ( ass-u-me ing 12 ft per story) times the 31 storys between the chiller plant on third floor and expansin tank on 35th floor(if you don't have the 13th floor) would be 375 feet. This times .433 psi per foot and add 15 psi extra to pressurize the top coil would add up to a total of at least 175 psi at the expansion tank.

[esdd]

Is the tank on the top floor a baldder type tank? I recently found a syatem with a ruptured bladder, the tank fed from the top, no air in the system. Then gauge on the tank would go from 20" vac to 140 psi, then back to 20" when the pumps cycled and the water in the loop expanded.

[Carnak]

I think if you got the water up on the top floor with a little bit of positive pressure you would be fine

[core]

Is there a bladder in this expansion tank or is it a non-captive air tank? If its non-captive loose the air vents because you will render the tank useless.

You pointed out that your gauges where old and crusty. Start by replacing them first.
If your readings aren’t accurate your pissing in the wind.

Core

[referteacher]

OOPS!!!!!!!!!!!Brain fart this holiday weekend.......175 psi at the central plant on the third floor

[mfi]

We replaced about every gauge in the plant, except for the ones on the supply/return lines. They are 20 feet in the air and I'm not going up there, yet. Added some glycol today. The tank was at 0 psi and fully warmed up. It's now at 6 psi. Going to start up around 9pm tonight so I'll have to wait and see.

[dutchcool]

Is the vessel in the return or in the supply?

It should be in the return.


http://www.hvac-talk.com/vbb/showthread.php?threadid=99361

[mfi]

Well, looks like I got more pressure since adding some more glycol. Tank is now at 15 Hg running. Sup/ret lines are at 5 psi. An improvement, but we need more glycol. I'm on the right track.

[Carnak]

Originally posted by dutchcool
Is the vessel in the return or in the supply?

It should be in the return.


http://www.hvac-talk.com/vbb/showthread.php?threadid=99361

As in the low pressure point of the system?