lb/min to cubic ft/min (Steam Flow)

[CaptBissell]

Could someone help me with a steam flow calculation problem I'm having. First, can lb/min be converted to cubic feet/min ? Second, the manual I am using states that:

steam rate(lb/hr)= ACFM * (60 min/ hour) * (1/specific- volume)
is this an accurate formula?

My problem is that I have an uncorrected steam flowmeter(lb/hr) value inputted to a mass flow computer and need to have a corrected value displayed and recorded(if needed). The flow computer also has a temperature input(Farenheit) and a PSI input.
For some reason the analog output of the computing device is giving a bogus value. I want to be able to calculate the corrected value myself and then impliment the formula into either/or mass flow computer and recorder.
Any help will be greatly appreciated.

 

[Montemayor]

Capt.:

Go to:

http://webbook.nist.gov/chemistry/fluid/

This website gives FREE thermodynamic data on Thermophysical Properties of Fluid Systems and you can easily obtain accurate thermophysical properties for several fluids – including water (or “Steam”).

To answer your questions:

1) Lb steam/min can easily be converted to cu. ft. steam/min. Just go to the above website and obtain the specific volume of the steam you are dealing with (is it saturate? is it superheated?).

2) Your formula is correct for converting ACFM to lb/hr. The reason is shown by the following…

(ft3/min) (lb/ft3) (60 min/hr) = lb/hr

Note how the units cancel to yield your product desired. But you must obtain the steam density value (lb/ft3, which is the inverse of the specific volume) at the operating temperature and pressure of the process. And, as I state above, you obtain the density (or the reciprocal of the specific volume) by using the above website.

This is too easy. Something else must be wrong...right? Anyway, I know the conversion is correct -- now your instrumentation is something else. Good luck.


Art Montemayor
Spring, TX

 

[JStephen]

If the steam is superheated, you're in good shape. If saturated, you could be getting some liquid that would throw things off.

And pardon my ignorance, but what is the "A" for in ACFM?

 

[quark]

Mr Montemayor is right. The calculation is as simple as you stated but the trick is in feeding the correct values from the steam tables. As you are measuring both temp. and pressure, I presume this is a superheated steam application. Just speak to the supplier about the authenticity of the steam tables which he fed into the flow computer.

Regards,

 

[katmar]

Hi Capt Bissell,

Quark and Mr. Montemayor have given good advice (as they always do) but I think you might be missing a trick here. You refer to your flow meter as giving an uncorrected reading in lb/hr. By their very nature, most steam meters give their raw output in volumetric terms and not as mass flow. This is often converted to a mass flow using the known density at the flowing conditions - this is the reverse calculation of your initial question.

However, if the temperature and pressure change then the steam density (or specific volume) changes and your flow computer can compensate for this based on the temperature and pressure inputs. If you want to check your meter then I suggest you go back to the base data and work forward from there.

If you are using an orifice meter find out the orifice bore and the tapping locations. Find out what temperature, pressure and flowrate it was designed for. Calculate the expected differential pressure and check this against the actual value. Once you are satisfied the meter is correct at this level, then you can start debugging the downstream calcs.

regards
Katmar

 

[pmover]

jstephen,

ok, i'll respond...

ACFM - actual cubic feet per minute. flow quantity at point of measurement.

good luck!
-pmover

 

[CaptBissell]

Thanks to all who responded to my question on lb/min to cubic ft/min and steam rate calculations. Oh, for what it's worth, it seems the problem I was having was a bad analog output on a FC900 Series Mass Flow Computer. Go Figure.

 

[unclesyd]

Had the same problem, lost 37,000 pounds/hour of 650 superheated steam with a new super duper meter was installed to replace an orifice run after some changes to a boiler that couldn’t do anything except increase the output. It took two months to resolve the problem even though the flow was obviously higher due increase noise level and increased in the valve openings. Computer error that corrected itself one weekend while the Provox people were upgrading