hot water expansion pressure calculation

[BronYrAur]

I'm trying to reverse-engineer a hot water expansion situation and can't find the formulas. All I have is expansion tank sizing software, but that's not getting me a direct answer. Here is my situation:

I have a 50 gallon radiant PEX tubing system that has a 4.5 gallon diaphram tank with 2.5 gal acceptance. The system was cold filled with 50 deg water to a fill pressure at the tank of 0 psig. I don't know why it wasn't filled to the typical 12 psig, but it wasn't. The tank is at the top of the system.

The water is heated with a plate exchanger by steam. A situation occured where 220 deg water entered the PEX. I'm trying to determine based on the 50 gallon volume, the expansion tank size, and the two temps, what my elevated pressure will be. I'm thinking it's under 20 psig as I play around with the expasion tank selection software, but I have no confidence.

Any thoughts on how to calculate the pressure at 220 deg?

 

[MintJulep]

You need to know (or assume) the initial (as filled) volume in the gas side of the expansion tank.

Calculate the volumetric expansion of the water in the system.

Subtract that change in volume from the initial gas volume in the expansion tank.

P1V1 = P2V2 gets you the gas pressure in the expansion tank which will equal the water pressure.

 

[RossABQ]

I'm not clear on your description; if the pressure in the bladder tank was 0 when the system was filled, doesn't that mean that there was no expansion volume in the tank? It would have essentially zero acceptance volume with no air pressure in it. It would fill 100% with system water.

 

[Drazen]

Why the diaphragm tank is on the top of the system??!

You have to achieve pressure balance in cold/static condition to have your vessel working normally. I think you will have constant interference with regular air bleeding - when vessel is on the bottom and you specify ca. 0,5 of water gage pressure higher than actual hydrostaic height, you are protected from vacuum. But in your situation every air bleeding can lead to zeroing pressure on the top of the system?!

It looks that you will allways have air in the water side. And that is the air which can travel along the lines and cause constant problems.

 

[ChasBean1]

I think Mint's right. Probably wouldn't have to account for different T1 and T2 air temperatures as the tank's in a dead leg - pretty much room temperature under both conditions. It's a 2.4 gallon volume change from 50° to 220°. I get about a 17# rise with this estimation (using initial tank volume of 4.5 and final of 2.1 gal).

CB

 

[stevenw]

You might want to test the system after going to 220 F.
That is above the temperature limit for some PEX.