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Expansion tank on glycol chiller system

mena1 (Mechanical) (OP)
24 Sep 10 2:52
Hi,

I know that an expansion tank is added downstream of boilers on the hot water supply line because of thermal expansion. Why is an expansion tank added to the cold glycol supply line downstream of a chiller?

Thanks for your time.
HerrKaLeun (Mechanical)
24 Sep 10 7:59
for the same reasons. Except water being chilled contracts.
i would think they charge the tank when water is cold and then it expands when the chiller stops working and water expands.

ASHRAE has guidelines for sizing both boiler and chiller tank. due to less dT, I'd think a chiller expansion tank can be smaller.  
Ziggypump (Mechanical)
24 Sep 10 10:50
I have no idea where you came up with your expansion tank ideas, but I've always designed based on the following.

The expansion tank is added either to the air separator between the vent and separator, or between the air separator and the circulating pump(s), the point of neutralized pressure.

Now, the pumps go in the system before the chillers, and after the boilers - upstream from chillers, and downstream from boilers.

This logic would dictate that the expansion tank would be installed downstream from the boilers, and upstream from the chillers.

ASHRAE sells a book titled, "HVAC: Equations, Data, and Rules Of Thumb" by Arthur A. Bell, Jr.  It's a very handy book, and the section on Chillers has some piping schematics and they show the expansion tanks upstream of the chiller.

If I'm wrong, let me know. I would like to design properly, but I've designed based on the stated reasons.
PatPro (Mechanical)
24 Sep 10 12:59
I agree with Ziggie. I have always installed them in the "warmest" part of the system, downstream of the boiler and up stream of the chiller.  

jistre (Mechanical)
24 Sep 10 13:01
Ziggypump,

Just wanted to note that in a steam boiler system, the boiler feedwater pumps must go upstream of the boilers and deliver water at system pressure. Downstream doesn't work because the boilers are producing vapor, not hot water.

I think you were probably talking specifically about hydronic boilers, and I just wanted to clarify that steam boiler loops are a bit different.
 
Ziggypump (Mechanical)
24 Sep 10 13:04
jistre
Yeah, steam is a different beast all together, but thank you for the heads up.
HerrKaLeun (Mechanical)
24 Sep 10 13:15
why would you have the pumps downstream of the boiler? there they are warmer (shorter life) and depending on your circuits you also could get too little pressure to the boiler from primary/secondary loop if other boilers "steal" the water.

Also might be more chance of cavitation since the boiler has pressure drop.

I've seen it installed both ways, but intuitively would install the pump upstream of a large pressure drop. an in the primary loop the boiler likely is the larges pressure drop.

Correct me if I'm wrong (or confused)
Ziggypump (Mechanical)
24 Sep 10 13:30
Couple of reasons for not putting the boilers downstream of the pumps.  From talks with other Engineers I work with.

1. The boilers downstream from the pump would see spikes in pressure.  When the pumps kick on, and right downstream from the pumps is a boiler, you can get a quick spike in pressure that might trip the relief valves.  Now this is for the system pumps in a primary/secondary system.  The boiler loop pumps are designed and installed around manufacturer specs, so pay attention to suggested pumping requirements.

2. Hot water expands while cold water contracts.  Granted, while dealing with 100 degree F + water, the supply and return temps are both hot.  But wouldn't you want the water expanding before it reaches the pump, where it can be pushed into an expansion tank, instead of after the pumps, where the expansion is going to add some pressure to the supply side of the pump?

3. Air is easier to remove from hot water.  Run the water through a boiler, then an air separator, more air will be removed.  Hence as PatPro stated, place the air separator in the warmest part of the system.  Where's the warmest part of the boiler system?  Where's the warmest part of the chiller system?

These are reasons behind why I believe the pumps/expansion tank/air separator should be installed.   
mena1 (Mechanical) (OP)
24 Sep 10 14:08
Thanks, great design considerations to keep in mind.
Appreciate your feedback.

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