# Calculating pressures from flow rate & pressure differential

#### Jason Reid

It seems there must be a way, but I cannot seem to wrap my head around it. Here's the scenario...

I have a 24" line flowing with a wet gas (combustion flue gas) into a dryer. It comes in at 90 degrees F, at a rate of 180 SCF/M. It leaves the dryer at 73 degrees F at a rate of 124 SCF/M. I have a gauge that shows me the pressure differential between the 2 lines as 20.5 inches of water column. Is there a way, given the information provided, to calculate the pressures of the incoming and outgoing lines?

Any help would be appreciated.

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#### Nidum

Gold Member
Does the flue gas exhaust to the atmosphere ?

Last edited:

Yes

#### Nidum

Gold Member
Directly or via a short generously sized duct or via a long exhaust pipe/chimney stack ?

#### Jason Reid

Sorry, my mistake, the wet gas does not vent, it enters the dryer, the dried does eventually, it's horizontal for about 50 feet, then it turns up for about 100ft

#### jer300

Sorry, my mistake, the wet gas does not vent, it enters the dryer, the dried does eventually, it's horizontal for about 50 feet, then it turns up for about 100ft
Without doing a full calculation, 124 cfm of gas through a 24" line for 150 ft isn't going to have much of a pressure drop. Depending on the level of precision you need, you can probably treat the outlet line as atmospheric and the inlet as whatever your differentual pressure gauge is reading.

#### Jason Reid

Hmm, would not the faster moving gas have a lower pressure? If I subtract the differential from the atmospheric, would that give me a vacuum?

#### Nidum

Gold Member
If the exhaust path was direct or very short then a simple solution was possible . Unfortunately this solution can't be used reliably with a long exhaust pipe and tall chimney as well .

The pressure drop over the exhaust pipe and chimney length may be significant for an accurate calculation of the pressures in this system . Calculating pressure drops for exhaust gasses in pipework - and in chimneys in particular - can be a rather uncertain process .

Realistically it would be easier to fit some pressure gauges and find out what the system pressures are for certain .

#### Jason Reid

Thanks for the help y'all

#### DoItForYourself

Hi,

If you can force the gauge read atmospheric pressure in the low pressure side, you can find the pressure of the inlet pipe (by subtracting the atmospheric pressure from the result).

For example, you can disconnect the tubing that connects the pressure gauge to the side of the lowest pressure and let it open to the atmosphere. This method can work only if there is a block valve in the tubing.

Please let me know if I am wrong.

"Calculating pressures from flow rate & pressure differential"

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