Chemical Engineering June 2020 - 28

and temperature data
are required to convert
the volumetric measurement
into a mass
flowrate. Controlling
volumetric flowrate
may be the end goal,
but to do a system
analysis, you will need
to know the mass
flowrate for fundamental
continuity analysis.
while measuring
volumetric flow is a
more direct process,
there are also measurement
tools that
yield mass flowrates,
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12
8
4
10
20
30
Mass flowrate, pound mass per minute (lbm/min)
Figure 3. Compressors supply energy in the form of flow (kinetic energy) or pressure (potential
energy). This concept can be illustrated with a compressor curve versus a system curve with
manual valves
such as a thermal mass flowmeter, which
determines a mass flowrate based on heat
transfer. however, these tools are a less direct
way of measuring flowrate and require
more careful calibration. both types of flow
specifications provide useful information as
long as the implications of each are understood,
especially when designing process
control schemes.
to remain consistent with the fundamentals,
the rest of this discussion will focus on
mass flowrate.
process controls all usually have a similar,
high-level pattern. a measuring device
reads a certain parameter, and a signal is
sent to compare the actual reading with a
desired setpoint. the difference between
the two causes an actuator to trigger a
change in the process. in flow controls specifically,
there is communication to a valve
actuator that will open and close the valve
to adjust the fluid flow.
while this is a common operation, the reason
for why this works is not always obvious.
if a compressor runs at a constant speed,
you might expect the flowrate to also be constant.
according to the law of conservation
of mass, a constant flowrate will remain no
matter what other changes are made to the
system. by closing a valve, the area through
which the fluid flows is made smaller, and
thus, an increase in velocity is observed to
keep that same flow.
but it is important to understand that centrifugal
compressors can generate different
steady-state flowrates with one motor
speed. they operate on a curve, similar to
a pump. by closing the valve, you have increased
the frictional losses in the system,
and thus the compressor will need to supply
more pressure to overcome those losses.
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this increase in pressure is coupled with a
decrease in flowrate. that is why compressor
curves are downward-sloping. a compressor
supplies energy in the form of flow
(kinetic energy) or in the form of pressure
(potential energy). increasing one decreases
the other (figure 2).
it is easiest to show this concept with a
compressor versus a system curve for a
simple system (figure 3). the system curve
represents the pressure demanded by the
system at a range of flowrates. the intersection
of the compressor and system curve is
the operating point. increasing the frictional
losses in a system will move the system
curve leftward (System curve - valve 60%
open) to a new operating point. there is
more pressure demanded by the system to
ensure that flow occurs because now there
are more losses. this example is presented
as the control of a manual throttling valve,
to help simplify the system behavior and understand
the role of component losses and
flowrate. (a control valve similarly controls
flow based on its pressure-loss information,
but the generation of a system curve
to visually represent flow control becomes
difficult because the valve changes its loss
information across the varying flowrates to
meet its setpoint.)
of course, centrifugal compressors are not
the only way to move gases. reciprocating
compressors are also common, and they operate
at a single flowrate on a near-vertical
compressor curve. in the case of reciprocating
compressors, the flowrate is not controlled
via typical means. rather, you will only
change the operating pressure by the changing
losses through the system. if you want to
change the flowrate, you will have to modify
the compressor directly.
it is also common for steam systems to
ChemiCal engineering www.Chemengonline.Com June 2020
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50
System curve, valve 100% open
System curve, valve 60% open
Compressor curve
Discharge pressure (psig)

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Chemical Engineering June 2020

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