Calculating gpm from a Pressure Diff reading?
Calculating gpm from a Pressure Diff reading?
(OP)
Hello all,
First time poster here.
I have a CW AHU (55Tons Nominal) and I am trying to figure out how many actual tons of cooling I am currently getting out of it. For this, I need to know the gpm's going through the coil. Since I don't have a flow meter, I believe there is a way to calculate gpm, if I can read a pressure drop (diff) from a specific location on the unit's CWS and CWR lines.
Can anyone offer any assiatance on this? I am basically trying to figure out if the unit is starving for air or starving for water. Here are the current conditions on the unit:
- CWS Temp: 42'F
- CWR Temp: 56'F
- Supply Air Temp: 55.5'F
- Return Air Temp: 70.5'F
- CW Valve Open: 100%
- Supply Air Setpoint: 55'F (this is min value allowed)
Thanks in advance!
jordi29.
First time poster here.
I have a CW AHU (55Tons Nominal) and I am trying to figure out how many actual tons of cooling I am currently getting out of it. For this, I need to know the gpm's going through the coil. Since I don't have a flow meter, I believe there is a way to calculate gpm, if I can read a pressure drop (diff) from a specific location on the unit's CWS and CWR lines.
Can anyone offer any assiatance on this? I am basically trying to figure out if the unit is starving for air or starving for water. Here are the current conditions on the unit:
- CWS Temp: 42'F
- CWR Temp: 56'F
- Supply Air Temp: 55.5'F
- Return Air Temp: 70.5'F
- CW Valve Open: 100%
- Supply Air Setpoint: 55'F (this is min value allowed)
Thanks in advance!
jordi29.





RE: Calculating gpm from a Pressure Diff reading?
you can compute for the actual cooling load capacity (not 55ton) using your air side information (SA temp, RA temp or mixing temp, air volume at main duct) then get the GPM using your water side info and computed cooling capacity from air side information.
RE: Calculating gpm from a Pressure Diff reading?
More air isn't going to improve things.
Not enough water.
The coil manufacturer should be able to get you a flow vs. pressure curve for the coil.
RE: Calculating gpm from a Pressure Diff reading?
no need to call the coil manufacturer to get coil water flow rate.
if he/she is engineer, he can compute for coil water flow rate.
.002
RE: Calculating gpm from a Pressure Diff reading?
You're correct that you can do a heat balance.
However, the water side of the coils may be fouled, or there might be a chunk of debris obstructing flow.
Heat balance won't tell you that. Manufacturer's curves will.
RE: Calculating gpm from a Pressure Diff reading?
RE: Calculating gpm from a Pressure Diff reading?
RE: Calculating gpm from a Pressure Diff reading?
RE: Calculating gpm from a Pressure Diff reading?
My ultimate goal is getting the actual tonnage being delivered by this unit. However, I believe there is a formula where, if I have the gpm, I can get the tonnage. Can't remember the formula, it was something like:
BTU = (gpm)(500)(dT), where dT is the Delta of the water.
Not sure if this formula is correct.
Eaasy with the replies fellows, I just recently became an EIT...
Thanks again.
RE: Calculating gpm from a Pressure Diff reading?
As you note, you can also do this from water side data.
If you can't get the pressure vs. flow characteristic from the coil manufacturer perhaps there is some other device - a valve, orifice plate or whatever in the circuit with a know pressure vs. flow characteristic that you could use.
Or you could rent a non-contact flow meter.
http:
Or hire a local T&B contractor who probably has one.
RE: Calculating gpm from a Pressure Diff reading?
Q=(p/V) x Cp x AT
Q=cooling load capacity
p=density of water
V=water flow rate
Cp=Specific heat of water
AT=difference of entering-leaving water temp.
try to familiarize with this because you will need it when your dealing with HVAC.
if you can give me the total air flow rate (cfm) of ahu, i can help you to get actual tonnage & water flow rate.
RE: Calculating gpm from a Pressure Diff reading?
Thanks for your help!
The cfm ratings from the manufacturer are 28,000 - 30,000 cfm. Though in-house we usually assume 30,000cfm.
I will check to see if I can get my hands on a non-contact flow meter... I think that can come in real handy at various sites we have where we don't have flow meters for individual AHUs. Any info on their accuracy (+/-%)?
Thanks again to all!
RE: Calculating gpm from a Pressure Diff reading?
Ultrasonic time-transit flowmeters and magnetic types are among the most accurate flow devices out there.
±1.0% at higher flow rates is typical.
Good on ya,
Goober Dave
RE: Calculating gpm from a Pressure Diff reading?
if you want to get the ACTUAL tonnage, you need to get the ACTUAL cfm too not using the manufacturing cfm rating.
get the size of the duct and get the air velocity so that you can get the air flow rate thru the main duct.
RE: Calculating gpm from a Pressure Diff reading?
If you want to see what tonnage is being delivered, you'll need both hydronic and air side. That is no different than any other heat exhanger-calculating one side only doesn't do much good.
If you don't have a flow meter, then a low tech approach might be to get a 55-gallon drum, stop watch, and hose to fit up to the coil drain. That won't provide velocity, but the dP for supply and return will indicate whether you have plating/clog at the coil. An IR thermometer would probably tell you as much (or contact thermometer).
For the air side, you'll need to have OA flow and temperature (and preferably RH) as well as return. Space or return RH would also help. Hard to overlook condensate when considering a cooling coil. Is it humid there? Does much condensate go through the air gap? With that, you can use the equations from the ASHRAE Fundamentals Chapter 6. Even easier, go to the Trane web site and use the mixed air psychometrics tool. No need to memorize equations if the program is free on line.
RE: Calculating gpm from a Pressure Diff reading?
Go to : http://www.bellgossett.com/literature/files/22.pdf:
RE: Calculating gpm from a Pressure Diff reading?
gpm = scheduled gpm x (coil pressure drop / scheduled pressure drop)^0.53
RE: Calculating gpm from a Pressure Diff reading?
RE: Calculating gpm from a Pressure Diff reading?