×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

hot water expansion pressure calculation

hot water expansion pressure calculation

hot water expansion pressure calculation

(OP)
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?  

RE: hot water expansion pressure calculation

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.

RE: hot water expansion pressure calculation

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.

RE: hot water expansion pressure calculation

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.

RE: hot water expansion pressure calculation

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
 

RE: hot water expansion pressure calculation

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources