ASHRAE Journal - July 2022 - 58

REFRIGERATION APPLICATIONS
COLUMN
Andy Pearson
Confessions of a
Control Freak
BY ANDY PEARSON, PH.D., C.ENG., FELLOW ASHRAE
When I was an undergraduate I found, to my surprise, a one-to-one correlation
between the subjects I enjoyed most and the ones I found most difficult. Chief among
the challenges was the concept of dynamics. I was fascinated by the way that this, at
least for me, was not intuitive and how instinctive attempts to improve performance
of a control system could easily make it worse.
For example, if a system seems to be slow to respond
to changes and is constantly " hunting " it might feel
appropriate to speed up its response to change, but this
will just increase the overshoot. The correct
adjustment would be to slow the reactions
and allow it to approach the desired value
gradually. In one case I was faced with a large
variable air volume air-conditioning system
with 12 steps of capacity control and was permanently
ramping up to full load and back
down before switching off and restarting.
Stable operation was achieved by increasing
the time delay between steps of loading and
unloading from 45 seconds to 75 seconds.
Refrigeration also has its fair share of these nonintuitive
systems. A float valve on a liquid refrigerant
storage vessel will open as the liquid level falls, but one
of the consequences of this action is to raise the pressure
in the vessel slightly. If the liquid is frothy because of gas
bubbles mixed in, then the immediate response to the
rise in pressure is for the liquid level to drop, increasing
the opening of the make-up valve. A thermostatic
expansion valve where the degree of opening is adjusted
by the temperature of the gas at the evaporator outlet
relative to its pressure is also difficult to model due to
inherent time delays built into the mechanism.
Over many years the control systems in refrigeration
have become more and more complex, with motorized
electronic expansion valves, variable speed fans
58
ASHRAE JOURNAL ashrae.o rg
J U LY 2022
Geppetto wished that he had a
bit more control of his unpredictable
creation.
and many options on compressor capacity control
becoming common. Now that we have a great array of
control options, often built into sophisticated electronics
including auto-tuning of proportional,
integral and derivative control, it's easy to
lose sight of the odd behaviors of cooling
systems. This is unfortunate because the current
global shortage of electronic components
means that many systems are having to be
operated " by hand " as a short-term expedient
while awaiting parts for new builds or service
replacements.
Proportional control varies the size of corrective
steps taken depending on how far
from the setpoint the measured signal sits. Integral control
helps to home in more accurately on the setpoint
by noting how long the measured signal has been away
from the setpoint and nudging it in the right direction.
Derivative control adds a further adjustment by considering
how quickly the measured signal is changing and
making an adjustment to the size of the corrective step
accordingly. On paper this can be a fantastic benefit in
reducing overshoot, but it is difficult to do well in practice,
particularly in refrigeration plants when the system
characteristics are changing all the time. I remember
a wise old hand telling me " Don't you worry about
derivative control-it's not for the likes of you. "
Andy Pearson, Ph.D., C.Eng., is group managing director at Star Refrigeration in
Glasgow, UK.
http://ashrae.org
ASHRAE Journal - July 2022

Table of Contents for the Digital Edition of ASHRAE Journal - July 2022
