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Volume of Gas Through a Line (Again)

Volume of Gas Through a Line (Again)

Volume of Gas Through a Line (Again)

My apologies in advance for creating another gas flow thread and I promise not to use any text-speak! It has been 32 years since I took fluid mechanics so there has been a lot of corrosion in my memory and I need some assistance in pigging that portion of my brain. Thanks in advance for any help!

I am designing a process flow control system for sending various sample gases through a heated line and need to create a calculator for the Volumetric Flow Rate based on the following:

-Input Pressure: 40-60psi
-Line Length: 65-200 feet
-Gas Type: Nitrogen (This varies among 10 different gases and gas mixes so we will change this in the calculator as needed.)

-Line ID: 3/8" (0.375")
-Line Material: Teflon
-Coefficient of Kinetic Friction: 0.04
-Outlet: Open to atmosphere
-Inlet Temperature: 75F
-Outlet Temperature: 375F
-Height Delta: 15-65'

If this is not enough information to make the calculator, please let me know what else I need.


RE: Volume of Gas Through a Line (Again)

This problem really requires knowledge of your goal.
  • If you are trying to heat a fixed size sample to a specified temperature then you need the heat transfer characteristics of the system to know how long the gas must be in the heat to acquire enough heat to reach the desired temperature. That soak time will determine your flow rate--usually the velocity is so slow that the friction is not a significant consideration.
  • If you are trying to get a fixed volume to flow from a process into an analysis then you need some sort of regulator since each gas species will give you a different friction value (and your "coefficient of Kinetic Friction" is kind of meaningless without knowing the gas species and pipe relative roughness)
  • Some other goal
Without knowing the goal, the path is difficult to assess.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Volume of Gas Through a Line (Again)

This is for an emissions analysis system. Currently we flow various dilution gases into a chamber varying distances from our source gases, usually 65' or 100', that mixes with exhaust gas allowing us to measure the makeup of the exhaust gas using an FTIR. The current gas flow is at 40psi through a 3/8" teflon line, wrapped in heat tape, insulated, and is 375F when it enters the chamber at the aforementioned distances. Our objective is 2-fold. First, we want to install a mass flow controller to automate the process gas selection. Second, we want to be able to determine how much we can lower the flow rate to possibly reduce our consumption and therefore cost of process gas. This will also allow us to determine the effect of lower flow on response times. Our IT guys need a volumetric flow rate formula to use in programming the PLC.

RE: Volume of Gas Through a Line (Again)

I don't think you're going to get a simple formula that the PLC will understand.

You have, IMO, too many variables - length of line, type of gas, inlet pressure and variables which change along the pipe, i.e. density caused by reduced pressure and also density changes fro the increase in temperature to get any sort of reliable calculation.

how do you measure flowrate now? Or don't you?

A simple strap on ultrasonic meter would give you volumetric flow when you can determine the ID of the tube and a pressure tapping would allow you to correct this flow to standard conditions.

Just measure the flow and feed it back into the PLC.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Volume of Gas Through a Line (Again)

Typically the end use (equipment on the recieving end) defines the flow required.

First you regulate the pressure for the required flow, cool it as necessary, remove any condensibles, and tackle the rest of the design....

RE: Volume of Gas Through a Line (Again)

Thanks for the replies! Decided it was easier to pull the line to the top of the office building, turn on the temperature controller, open the regulator, and measure the flow at the outlet line then reverse engineer the formula.

Much appreciated!

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