 28 posts
TimePosted 19/01/2010 20:40:27
Saprina says

### Normal Cubic Meter

Hey;

I want to know why do gas flow rate in cement industry measure by Nm3 (normal cubic meter) not by m3?

This is the conversion formula:

Ideal Gas Law: PV = nRT,

Let n= 1

R = Pact.Vact/ Tact                      At given temp & pressure

R = P0.V0/ T0                            At normal condition (0 °C & 1.0 atm)

Pact.Vact/Tact = P0.V0/T0

è V0 = Vact . (Pact/ P0) . (T0/ Tact)

OR:

Q0 = Qact . (Pact/ P0) . (T0/ Tact)

Let;

Q0 = QN = Gas flow rate at normal condition (0 °C & 1.0 atm)

Qact = Measured flow rate in m3/h

Pact = Pmeasured + Pambient

Pambient = 1.01325 bar

P0 = Pressure at normal condition (1.0 atm

T0 = Temperature at normal condition (0 °C = 273°K)

Tact = Measured Temperature in °K

Then;

QN = Qact . ((Pmeasured + Pambient)/ P0) . (T0/ Tact)       [Nm3/h]

Significant deviation from ideal gas behaviour appears when the empty space between molecules is reduced as e.g. high pressures or low temperatures. Compressibility factor (Z) uses to determine how much a gaseous system deviates from ideality. For ideal gas Z=1.0, any deviation of the compressibility factor from 1.0 is a non-ideality (real gas).

Z is not a constant factor for real gas but changes with pressure and temperature and can be determined from the Compressibility Factor table (or graphic) where K (compressibility number) = 1/Z.

Then;

QN = Qact . ((Pmeasured + Pambient)/ P0) . (T0/ Tact) . (1/K)       [Nm3/h]

Example: (Incoming Natural Gas)

QN =? Nm3/h

Qact = 2956 m3/h

Pmeasured = 5.4 bar

Tact = 1.3°C+273 = 274.3 °K

Z (=1/K) = 1/0.99 = 1.01

Then;

QN = 2956 x ((5.4 + 1.01325)/ 1.01325) x (273/ 274.3) x 1.01 = 18804 Nm3

2956 x ((5.4 + 1.01325)/ 1.01325) x (273/ 274.3) x 1.01 = 18804 Nm3

Best Regards;

Saprina

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TimePosted 21/01/2010 13:37:31

### Re: Normal Cubic Meter

With any specification or comparison the conditions must be defined. A car may be quoted to do 20km per litre, but it will depend how fast it is driven. The same with gas volumes. By stating the normal cubic metres we are correcting for any temperature and pressure differences to a standard base. The conversion to actual volume is made using the Ideal Gas Laws. 57 posts
TimePosted 25/01/2010 13:30:28

### Re: Normal Cubic Meter

Normal Cubic Meters are in fact closer of being a measure of mass than a measure of volume as they are strictly equivalent to a measures of moles of gas. You only need one factor, the average molecular weight, to convert to a true measure of mass (kg), while you need at least two, pressure and temperature to convert to a measure of volume (actual cubic meters).

As Nm3 are (almost) a measure of mass, the conservation of mass does apply, which makes them handy for process calculations. Actual cubic meters are not conserved as they will already vary with temperature.

Some people do prefer to express the gas flows in kg/hr though.