Total External Pressure

[UAAAlumni]

When calculating total external pressure (static and velocity) of a system, how do you account for a specified pressure differential (say, 0.05" W.C. pressure differential relative to outside barometric pressure) that will be accomplished by a modulating damper who's controls are linked to a pressure diffential transmitter?

Would this be considered the total static pressure for that damper (go with 0.05" W.C. for the damper rather than what the vendor specifies which was lower), or would this be in addition to the static pressure of the damper?

Thanks for the help!

 

[imok2]

Normally determoned be face velocity as in this graph go here

http://www.engineersedge.com/hvac-systems/damper-pressure-drop.htm

 

[cry22]

You mean that your system does not have enough fat to take care of 0.05" W.C?
Normally, you would add 0.5" just for system effect alone if this is an AHU. On top of a 10% minimum overall safety factor.
0.05" W.C. is so insignificant that you should not waste time with it at all.

 

[ChasBean1]

External pressure of a system has nothing to do with velocity. You don’t account for barometric pressure in HVAC control. Barometric pressure changes are on the order of magnitude of fourteen times (terms of inches Hg) than typical HVAC space pressures (terms of inches water column). Elevations in buildings resulting in pressure magnitude changes are accounted for by relative pressures. Stack effect is another story. Buildings are negative in lower floors and positive in higher floors when cold out; reverse when warm out. True building pressure should be measured when the OA temperature is about 70-72°F.

You may have to clarify your question to get a good answer.