Static pressure, pump pressure, and flow rate. > GreenBuildingTalk - Green Building Forums on Insulating Concrete Forms (ICF), Structural Insulated Panels (SIP), Radiant Heating, Geothermal Heat Pumps, Solar Power, Green Construction Projects

Forums

Members

Forums > Green Building Technologies > Geothermal Heat Pumps > Subject: Static pressure, pump pressure, and flow rate.

You are not authorized to post a reply.

Author

Messages

Glen W

Registered Users

New Member

Posts:11

11/04/2009 11:39 PM
If you are using a closed system with a static pressure of 20 PSI and have a pressure running the pump of 25 PSI and assume .49 PSI pre foot of head or 10.2 feet of head can you use the pump performance curve to estimate the flow through the system? Example running the pump with no back pressure should not increase the pressure within the closed system. This is of course impossible due to the friction of the water moving through the system. Therefore the increase in pressure from static is a result of this friction or obstructions to the flow through the system. The pump manufacturer publishes a pump performance curve indicating the GPM of the pump according to the force resisting the pump (Feet of Head). This should give at least a good indication of the flow through the system, similar to the pressure drop across the heat exchanger charts the heat pump manufacturers publish. I am resorting to this because the pressure drop information is unavailable for my unit. Any thoughts?

cnygeo

Registered Users

Basic Member

Posts:121

11/05/2009 12:08 AM
Yup, you're on the right track - just measure the pressure drop across the pump (ignore the static pressure). Then assuming you have a pump curve it should be about as accurate as doing the same thing with a heat exchanger pressure drop chart. Maybe even a bit better, since the pump will be less affected by viscosity than most heat exchangers. It will certainly get you in the ballpark. Your conversion might be off a bit, though - I believe the standard is 2.31ft of water per psi, so going the other way it would be .43, not .49.

engineer

Registered Users

Veteran Member

Posts:1157

11/06/2009 7:13 AM
If closed loop has antifreeze density will be different from that of pure water so 2.31 may not apply

Without data, you only have an opinion.

craigb93

Registered Users

New Member

Posts:31

11/06/2009 7:53 AM
Here's a LINK to sort out the gravity influence. http://www.engineeringtoolbox.com/propylene-glycol-d_363.html This says if I read it right, 26 F water mixed w/ 10% 60F glycol weighs SG 1.008.This would make the multiplier change not much, to 0.436.

cnygeo

Registered Users

Basic Member

Posts:121

11/06/2009 9:50 AM
Yes, but it doesn't matter what he has in his system, just what was in the system that the manufacturer used to calibrate the pump curve, pressure drop chart, etc. which is probably water near room temperature. He's just changing units from psi to ft of head. There will be a correction factor for different fluids at different temperatures but it will depend on viscosity more than density in most cases.

Glen W

Registered Users

New Member

Posts:11

11/10/2009 1:26 AM
In a closed system gravity has no influence on back pressure only viscosity and friction and obstruction of the flow. If you pump the fluid uphill it must return down hill in the same amount the effort to pump is uphill will be recovered on the downhill similar to, but different then a siphon. Water seeks it own level.

Alex_in_FL User is Offline

Registered Users

New Member

Posts:80

11/13/2009 10:18 PM
Me thinks the viscosity change would be of more concern than density. But the answer the poster is looking for is yes, using actual data is better than reading from the chart. Why? Because all pumps are not exactly the same and performance varies even with the same model. Thus your pump might be slighly better or slightly worse than the chart shows.

You are not authorized to post a reply.

Forums > Green Building Technologies > Geothermal Heat Pumps > Static pressure, pump pressure, and flow rate.