SCFM...anyone?

Anyone see any issues with this? I need to display SCFM on RSView32, this is what I am going to try...

Raw input 4-20ma scaled to inches of H20 =Inches in H20 (IN/WC)

1.325*Barometric_Pressure/(460+AirTemp) =Air Density (LB/CF)

1096.2*POWER(InchH20*0.84/AirDensity,0.5) =Velocity (FPM)

3.1416*dia*dia/4/144 =Duct Area (sf)

Velocity*0.9 =Velocity Average (FPM)

Average_Velocity*Area =Air Flow (ACFM)

Air_Flow*530/(AirTemp+460) =Air Flow (SCFM)



PLC=SLC504

Many thanks...

Seems ok as a rough measurement but probably no better than 5% accurate.

This document should confirm your calculations.

You are making this harder than it needs to be. Your pitot tube or venturi should have a constant that will give you actual CFM (ACFM) proportional to the squart root of the differential pressure. ACFM = k x DP^0.5

Your flow meter transmitter should be able to extract the square root directly, so you may be able to get the ACFM directly so 4 mA = 0 ACFM and 20 mA = max ACFM. You can then convert to ACFM directly in the PLC by scaling this linear signal. (If not, you will have to do the square root of DP and calculate the ACFM yourself.)

SCFM is a mass flow rate, corrected to standard conditions. This is usually, but not always, 14.7 psia and 68F, which is a density of 0.075 pounds per cubic foot. Once you have the ACFM, you can convert to SCFM by using this simplified formula:

SCFM = ACFM x 2.69388 x psia / ((F + 460) x ).075)

2.69388 = (460 + 68) x 0.075 / 14.7 If you are using 60 F or another temperature for standard conditions substitute accordingly.

I think GIT is trying to do this with a simple static/velocity differential pressure reading (i.e. static pressure from a straight tapping into the duct and velocity pressure from an open pipe pointing into the airflow) rather than a pitot/venturi/orifice plate specifically sized for the job which can be expensive.

If you just need a rough indication of flow then this is probably the cheapest way to go.

burnerman said:

I think GIT is trying to do this with a simple static/velocity differential pressure reading (i.e. static pressure from a straight tapping into the duct and velocity pressure from an open pipe pointing into the airflow) rather than a pitot/venturi/orifice plate specifically sized for the job which can be expensive.

If you just need a rough indication of flow then this is probably the cheapest way to go.

Click to expand...

What you describe is essentially a pitot tube.

In that case he can combine all of the calculations for area etc. into a single constant k, extract the square root and scale to ACFM in the PLC.

True Tom, most of those equations quoted by GIT can be worked out into one constant.
I thought you were talking about an averaging multi holed pitot (e.g. Torbar) which gives the average velocity pressure across the duct and can be reasonably accurate when sized correctly but, as stated, can be expensive.
With a simple pitot you are always assuming the velocity profile across the duct, like other flow measuring devices this can lead to gross inaccuracies if the flow is not reasonably uniform at the point of measurement.

more info...

By nature all temperature variables will drastically change

System: Incinerator with
1. Preheater, this preheats the in coming air to 400deg f, the incinerator sp is 1400deg f it recovers the heat from the incinerator and diverts it into the pre-heater

2. Oil heater, the heat is recovered from the incinerator and diverted into the oil heater and or into the by-pass and/or the pre-heater


What I am trying to accomplish, this reading will be displayed on a HMI screen, the measurement is differential pressure sample taken from a single ‘mid’ (pitot/peto tube(EDIT SPELLING PITOT) point from the duct, the sample is after the induction fan and before the pre-heater, this will determine the air flow (SCMF), after a base point is established, I will be able to determine when the pre-heater is clogged or when the air is restricted to the point that it will need to be addressed

Also just prior to the id fan there is a collection chamber, this chamber is set with a 0 static sp, depending on the pressure, this is where the set point is generated for the id fan

So by your comments the calculations will work? But I will no better then 5% accuracy? That would be fine, I will look more into Tom’s calculation

Thank you very much for your replies

geniusintraining said:

What I am trying to accomplish, this reading will be displayed on a HMI screen, the measurement is differential pressure sample taken from a single ‘mid’ (peto tube) point from the duct, the sample is after the induction fan and before the pre-heater, this will determine the air flow (SCMF), after a base point is established, I will be able to determine when the pre-heater is clogged or when the air is restricted to the point that it will need to be addressed

Click to expand...

Should be ok, we normally take a dp measurement across anything in the line (e.g. bag filter, heat exchanger) that may be prone to blockage and flag up a potential blockage when the measurement exceeds the preset design limit.

Also just prior to the id fan there is a collection chamber, this chamber is set with a 0 static sp, depending on the pressure, this is where the set point is generated for the id fan

Click to expand...

We usually control the ID fan to maintain a slightly negative pressure (say -5mm w.g.) in the primary combustion chamber to ensure that we don't pressurise and smoke out the building.

burnerman said:

We usually control the ID fan to maintain a slightly negative pressure (say -5mm w.g.) in the primary combustion chamber to ensure that we don't pressurise and smoke out the building.

Click to expand...

We have several ducts that are positive that are going to the chamber, these are also monitored with DP switches that will shut the system down incase of back pressure

Thanks again