Calculating pressures from flow rate & pressure differential

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Discussion Overview

The discussion revolves around calculating the pressures of incoming and outgoing lines of a 24" gas line flowing into a dryer, based on flow rates, temperature, and pressure differential. The context includes practical considerations of gas flow and pressure drop in piping systems.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents a scenario involving a 24" line with wet gas flowing into a dryer, seeking to calculate pressures based on given flow rates and pressure differential.
  • Another participant questions whether the flue gas exhausts to the atmosphere, prompting clarification on the gas flow path.
  • A later reply suggests that the pressure drop over a long exhaust pipe may be significant, complicating accurate pressure calculations.
  • Some participants propose that the outlet line could be treated as atmospheric pressure, while the inlet pressure could be derived from the differential pressure gauge reading.
  • One participant raises a concern about the relationship between gas velocity and pressure, questioning if subtracting the differential from atmospheric pressure would yield a vacuum.
  • Another participant suggests fitting pressure gauges to measure system pressures directly for more accurate results.
  • A final suggestion involves a method to determine inlet pressure by allowing the gauge to read atmospheric pressure on the low-pressure side, contingent on specific conditions being met.

Areas of Agreement / Disagreement

Participants express differing views on the impact of exhaust pipe length on pressure calculations and the methods for determining pressures. No consensus is reached on a definitive approach to the problem.

Contextual Notes

Participants note the uncertainty in calculating pressure drops in exhaust systems, particularly with long pipes and chimneys, indicating that assumptions about flow characteristics may affect outcomes.

Who May Find This Useful

Individuals interested in fluid dynamics, gas flow calculations, and practical applications in engineering or HVAC systems may find this discussion relevant.

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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Does the flue gas exhaust to the atmosphere ?
 
Last edited:
Yes
 
Directly or via a short generously sized duct or via a long exhaust pipe/chimney stack ?
 
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
 
Jason Reid said:
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.
 
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?
 
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 .
 
Thanks for the help y'all
 
  • #10
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.
 

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