how to find out the actual CFM of ahu

[azrim07]

hello guys i am mechanical engineer i want some information related to hvac.
how i can find out the acutal cfm of running ahu? which parameters are required for this purpose?

[chemi-cool]

You simply need one of these...

[Brian_UK]

True Chemi but a pitot-static tube with manometer would give more accurate readings.

http://www.imeko.org/publications/wc...03-TC9-004.pdf

[azrim07]

well is there any other method with which we can find its cfm without this device?i mean anyother formula ??

[Sridhar1312]

Measure velocity across the Filters or best cooling coil and Measure the area of the cooling coil or filter whereever you have measured the velocity. Multiply velocity feet per minute and area in sq.feet of coil /filter as the case may be to get air quantity in cfm.

[Brian_UK]

well is there any other method with which we can find its cfm without this device?i mean anyother formula ??

NO, the correct tool for the job is what is needed.

If you don't want to buy one then hire some test kit instead or hire a commissioning engineer to do it for you.

[dougheret0]

A rough measurement of AHU air flow can be found by simply measuring the static pressures at the inlet and outlet of the unit. If you have the manufacturer and model, you can go to the manufacturer's web site and find the AHU blower performance in cfm vs "external static pressure", either as a table or graph. For large air handlers with belt drive, you will need also to know rpm. For small three speed air handlers you will need to know the speed tap, hi, med, or low. For inverter (variable speed) air handlers, you will need to know the control tap. If it is a Variable frequency drive unit, you will need it to be on 100% speed, and again know the rpm at that speed.

This is just a rough measurement, since static pressure - pressure normal to the air stream - may be affected by ductwork conditions. The only accurate way to know AHU air flow is with a pitot rake in a straigt section of duct in the supply or return, ahead of or after all branches. The rake has several radial spokes, each with an array of pitot ports, each port having one total facing into the air stream and one static normal to the air stream. This will give you the average duct velocity and hence the operating air flow without having to know anything about the air handler itself.

Buy yourself a copy of the National Standards of the Associated Air Balance Council (AABC) to learn all about HVAC measurement. As a mechanical engineer, you will need this to effectively work as an air conditioning engineer.

Another source is my book HVAC Design Manual – A Mechanical Designer’s Guide to Design of Small Commercial and Institutional HVAC Systems which can be downloaded free from my web site www.nettally.com/doughert

[moideen]

If know the tonnage of A/c system can be find out the cfm by using this formula

CFM= Btuh / Air density*60*∆enthalpy

Moideen