Chemical Engineering June 2020 - 27

Figure 2. Centrifugal compressors can generate different steady-state flowrates with a single motor
speed
involve compressible flow, they are
complicated by sets of coupled variables,
among which are flow and
pressure. gas handling processes
require precise control of flow and
pressure. to understand the process
control for these parameters,
you need to understand the relationship
between the two.
Pressure as driving force for flow
the discipline of chemical engineering
has a unique focus on transport
phenomena. Chemical engineers
understand the unifying cornerstone
that any transfer of mass,
energy and momentum materializes
from a driving force. in the case of
industrial gas handling, pressure is
that driving force for momentum
transfer. it helps to remember that
pressure is dimensionally consistent
with momentum flux. a pressure differential
causes a process fluid to
spontaneously move from high to
low pressure, in accordance with
the second law of thermodynamics.
in other words, flow occurs when
a difference in pressure is present.
when talking about flow and pressure
control as separate controlled
variables, we might ask " are we really
controlling one or the other? "
the answer is that the two parameters
are not independent of one
another; we really control and affect
both. it is just a matter of what the
objective is, and the reference point.
in operations involving gas handling,
there is often a desire to control
one parameter over the other: either
the goal is a certain flowrate, or
a certain pressure. however, there is
commonly a secondary goal to keep
the counterpart in check - for example,
to control the flowrate while
ensuring that the pressure does not
exceed the maximum allowable operating
point. in these situations, it
becomes important to remember
pressure and flow are not independent
of one another.
Flow measurement and control
first, let's discuss cases where flow
measurement and control are the
primary objectives.
it is common to specify flowrates
as mass flowrates when discussing
compressible flow. the conservation
of mass is a fundamental principle,
while the conservation of volume is
not. however, it is quite common for
engineers to measure and handle
volumetric flow because of the ease
of direct measurement at a single
point. in gas-handling systems with
changing fluid densities, engineers
need to pay careful attention to the
type of measurement taken.
flow readings may come from
measuring the pressure drop across
an orifice plate, the displacement of
a ball flowmeter, or by measuring the
velocity with a pitot tube. however,
the volumetric flowrate at this single
measurement point is not constant
down the rest of the pipe. pressure
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Chemical Engineering June 2020

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