Rate in Fluid Systems
3.2
Volume Flow Rate

Change in volume per unit time
 Volume flow rate = change in volume/time
interval

volume is in m3, cm3, ft3, in3, liters or
gallons
 volume flow rate is in m3/s, ft3/min, liters/h

Mass Flow Rate

Mass of fluid moved per unit time
 Mass flow rate = change in mass/time
interval


Mass flow rates are in slugs/s and kg/s
Measuring Fluid Rates
Easiest method is to collect fluid and
measure the amount of fluid collected
over a time interval
 Average flow rate—volume or mass of
fluid collected divided by the time interval
 Flowmeters measure indirectly and do
not significantly interrupt the flow. (They
measure pressure or velocity.)

Measuring Fluid Rates

Volume flow rate = cross-sectional area x
fluid speed



Mass flow rate = fluid density x crosssectional area x fluid speed
Processes that Use Rates of Fluids
Bottling machine
 City water supply system
 Air compressor to tool

Conversion factors to know:



1 gal = 231 in3
1 gal = 0.1337 ft3
1 gal = 3.785 x 10-3 m3