Flow Rate Calculations

This is  a technical article on calculations pertaining to volume and mass flow rates of fluids.
Mass flow rate is
It is the product of volume flow rate (m3.s-1) and specific mass (kg . m-3), where the cubic metres simply cancel each other out.

(kg.s-1) = (m3.s-1) . (kg . m-3)

Using the units this way enables you to determine the formula (rather than remember it).

For example, if you want Volume flow rate (m3.s-1)
and you know the specific mass to be 1.205.kg.m-3, then you only need look at the units to see what it is you need to measure.

we have (m3.s-1) and (kg.m-3), and it’s clear that if you divide (m3.s-1) by (kg.m-3) , the m-3 and m3 will cancel leaving kg and s-1 which is (kg.s-1) — which is mass flow rate.

If you want to determine the Volume Flow Rate for ventilating a room, and you do not have specific mass or mass flow rates to hand, you can improvise, using the reasoning of units.

You would be likely to know the space’s volume (floor area multiplied by height), and the air changes per hour, and this is all you need:

Space volume is m3, and air changes are “per hour” or hr-1. To get per hour to per second, divide air change rate by 3600. s.hr-1

Room volume x air change rate/3600 = Volume Flow rate

m3 . ( a . hr-1/3600. s.hr-1) =>

m3 . ( a /3600. s.) =>

m3 . ( a /3600)  . s-1 =>

m3. s-1 . = Volume Flow rate.

By changing the subject, you can determine the air change rate for the room if you know the volume flow rate, or you can figure out the room volume if you know the air changes and volume flow rate.

Obviously by mixing the equations you can determine the mass flow rate if you know a room volume an air change rate and specific mass.


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