Instruction book
6996 0224 30 13
Flow rate definitions
There are 2 common but different ways to denote flow rate in vacuum. The first one is based on the
displacement or volumetric flow rate and the second one is based on the throughput or mass flow rate.
Atlas Copco vacuum pumps use volumetric flow rate to denote performance, the unit being actual m³/h.
Displacement/volumetric flow rate
Over the relevant pressure range, a GVS A pump operates at constant motor speed (rotations per minute)
and since the compression chambers have fixed dimensions, the same volume of air is pumped from inlet
to outlet with falling pressure level. Over the relevant pressure range, this makes the volumetric flow rate
practically independent of the vacuum level. It is the expression of the flow rate inside the piping at the
governing vacuum level (in actual m³/h) and is always higher than the standard flow rate (in Nm³/h).
Standard flow rate
Although the volumetric flow rate remains practically constant with decreasing (absolute) pressure, the
number of molecules in that pumped volume is not. By definition: the deeper the vacuum, the lower the
amount of molecules in the same volume. This means that the mass flow will decrease with decreasing
(absolute) pressure. It is clear that a flow rate must be stated at a certain vacuum level when using this
denotation.
To Next Page
Loading...
Need help?
General
