Expansion tank Volume Calculations for High Rise Building by Considering P1 & P2 Pressure Values

[Buildtech2]

I am currently doing calculations for the expansion tank of high rise tower. The chiller plant and associated expansion tank is located on ground floor. The expansion tank is located to the suction side of secondary chilled water pumps at ground floor. The highest level of piping in the system is at 120 m higher than the location of expansion tank. This is the lowest pressure point in the system which is at 120 m from expansion tank. Therefore, the static pressure rise from lowest pressure point to the point of connection of expansion tank is simply the elevation difference between the two points which is 120 m = 170 psig.
This elevation difference plus the minimum pressure required at lowest pressure point to maintain the positive gauge pressure and prevent air leaking into the system is the minimum or initial pressure for the calculation of expansion tank volume is P2.

The maximum pressure P2 is the lowest pressure setting of components / equipment of the chilled water system is generally 125 PSI.

As you can see above that P1 is higher than P2 which should not be the case as we cannot get proper volume calculations for the expansion tank with these pressure conditions.

Appreciate, if anybody guide me to resolve this issue.

 

[LittleInch]

Either
a) lower the building (probably not practical) or b)
b) increase the pressure of your components and equipment to handle the required pressure ( probably about 200PSig, but might be higher on the discharge side of the pump.
c) If the 125 psi stuff is on the chiller plant and expansion tank, put the plant and the expansion tank on the roof like many other people do.
d) If it's only the expansion tank you're worried about, put it on the roof.



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[Buildtech2]

@LittleInch thanks for your valuable response. In case if expansion tank cannot be relocated to upper floors, The maximum pressure (P2) will be much higher than required. In my case,
P1 = static height + fill pressure = 185 psia
P2 = P1 + pump head = 185 + 78 = 263 psia

Above 263 psia pressure may not be acceptable to the chilled water system as the lowest pressure rating for the chilled water components is 125 psi, even if we consider working pressure as 1.5 x 125 = 187.5 psia which is less pressure than P2.

So with above P2 pressure, the chilled water components may not be suitable with 263 psia. Please advise.

 

[danschwind]

Then you will need to change your chilled water components to those of higher pressure rating...I don't see any other way.

Daniel
Rio de Janeiro - Brazil

 

[LittleInch]

Why can't you / haven't you just increased the pressure of the chilled water?

Only other way is to split into two vertical sections and put the chillers on the middle or top floor for the top section.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Buildtech2]

@Danschwind...thanks for your input.

@LittleInch..The chilled water pressure has already increased to 263 psia mainly due to maximum static height of 120 m and fill pressure but standard HVAC components are generally not compatible with such max. pressure rating. So either we need to increase the pressure rating of the HVAC components which may be an expensive exercise or we can relocate the expansion tank to higher level to minimize the static height.

Appreciate, if somebody can also evaluate the way I have calculated P1 and P2.

 

[LittleInch]

Looks right to me.

The other option is to beef up the distribution system but install pressure regulation for the lower floors down to below 125 psig. You need good quality valves and also have a relief system but is used in talk buildings to reduce pressure in the lower floors.

You can install prvs that feed 3 to 5 floors at a time downstream.

You're the designer, design away with your particular restrictions and pesky architect who doesn't think of these things.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Compositepro]

If you divide the elevations into zones, you will need energy to move fluid between zones going up, and it is likely not practical to recover the energy when the fluid comes back down. In essence The pump would be working as though it were pumping to a tank on the roof and the the energy in the fluid would "go down the drain".

 

[BronYrAur]

No matter where you put the expansion tank, the static pressure alone greatly exceeds 125 psig. The chillers themselves may not be able to handle the pressure, along with other piping components. Many chillers are put on the roof/penthouse for this reason.

Another option would be to install a heat exchanger bank somewhere in the middle of the building so that the static pressure in the basement is less. This of course adds a whole pumping system for everything above the heat exchangers.

I hope you are still in the design stages of this building and not trying to engineer as construction is progressing.

 

[TBP]

If you can put the equipment in a penthouse (normally where it would be) the pressure it sees will be very low. You only need enough pressure to push the air out of the top of the system, plus a few PSI.