Calculation of Power vs. Flow Rate for Air (call MHI for other gasses).  Use the formula given below to obtain an approximate value for the power (kW) required for a selected exit temperature and flow rate.  However, it is best to use the graphs which MHI supplies with the Airtorch for a more accurate number.  A sample graph is shown below.  Please register with MHI for more graphs.

For a refresher for power and energy please click here.

The literature uses both SCFM (standard cubic feet per minute) and MCFM (sometimes called ACFM- actual flow)) to quantify flow. Often SCFM may be calculated at the line pressure and temperature (as in the MHI graphs) or may be more strictly defined at 14.7psia and 32°F. Please read the legend of all flow graphs very carefully as SCFM and ACFM can be very different.

Conversion to SCFM (at a given pressure and temperature) from Actual CFM (MCFM)

SCFM(at 14.7psia and 32°F) = MCFM x([(Pg + Patm)/(Patm)] x [(Tref +460)/(Tact +460)]) note T here is in °F

SCFM = Airflow in standard cubic feet per minute (i.e. corrected for temperature and pressure) Normally SCFM is taken at 14.7psia and 32°F, but not always.

Note: 1 cubic foot = 28.316847 liter and 1 cubic foot = 0.028317 cubic meter
MCFM = Measured Airflow in cubic feet per minute.

Tact = Actual air temperature (°F)
Tref = Reference air temperature (say 32 °F or room temperature - please check your flow meter)
Pg = Gage pressure (psig)
Patm = Atmospheric pressure (normally 14.7psia at sea level elevation)

1 bar= 105 Pa=0.1MPa=100KPa

1 atmos= 101,325 Pa= 1.01325 bars

1Kgf/cm2=9.807 N/cm2= 9.807 x10**4 Pa= 0.9807 bar=0.96788 atmos

At any location where the gravitation constant is 9.807m/s2 and temperature is 0C ( often called standard atmosphere). 

1 atmosphere=760mm Hg= 29.92" Hg= 10.3m of pure water= 760 torr

1torr=133.3Pa

The standard atmosphere changes from 101.3KPa at sea level to 54.05KPa at 10,000ft.

For general engineering conversions please click on Conversions.

The power required, assuming full conversion is (mass rate) x (specific heat) x (temperature increase) if there is no conversion of enthalpy of gas to other types of energies and only the electrical work is transferred to the gas for an enthalpy (temperature) increase.  As gas property values are generally known for standard conditions, an approximate equation (approximate because average properties are assumed) for this calculation is:

Theoretical power in KW= SCFM (at 14.7psia and 0°C) x (T2-T1)°C /~2650  for air  (approximate only follow the curves for a better estimate).

kW = Power i.e. Energy/sec (Kilowatts) required to heat the air.
SCFM = Air flow rate in standard cubic feet per minute 14.7psia and 32°F. (in psia a stands for absolute pressure- in psig the g stands for gauge pressure which is less than psia. 

• (psia= psig+atmosphereic pressure) for Pressure above atmosphereic pressure
• (psia= atmosphereic pressure- Pvac) for Pressure below atmosphereic pressure

T2 = Temperature at Airtorch exit in Centigrade.
T1 = Temperature at Airtorch inlet  in Centigrade.

Note that each airtorch may also have heat losses through the nozzle or body.  Please consult MHI for the efficiency of your particular model. Although MHI Airtorches are highly efficient in power conversion, it is prudent to plan for a slightly higher power than that determined by the equation.  In addition, if excess heat is lost from the exit nozzle (typically if a metal nozzle is used) then plan for a higher power unit than what the equation above indicates.  Note: loss in efficiency normally depends on the use.  Please contact MHI for assistance.

Note Density of Air at 14.7psia and 32°F is 1.293 Kg/cu. m 

For all other conversions please see Conversions

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Last Updated: May 23, 2016