This book has been written to serve a very distinct purpose. As the title
suggests, it was written to illustrate the control of mechanical systems, using
practical means and methods. Yet in order to serve that purpose, a good deal
of the book was devoted to developing insight into the design of mechanical
systems, in terms of how they are intended to operate and be controlled. A
prerequisite knowledge and understanding of mechanical systems can be
followed up with an education in controls and control systems. Hopefully this
book has covered some ground in both of these areas.
The content of this book is meant to serve as a resource, a reference guide if
you will, for future applications. There are some key issues, however, that the
reader should come away with upon finishing this book. This short chapter
touches upon those key issues that should remain with the reader, and tries to
summarize the basic intents of the author.
Chapter 1 introduced the reader to the definition of an HVAC system, as being
a mechanical system plus the control system required for its proper operation.
The chapter also introduced the term “Sequence of Operation”, and
demonstrated the importance of it.
The distinction between packaged and built up equipment was a key issue put
forth in Chapter 2, as was how a packaged piece of equipment can be a part
of a built up “system”. “Hierarchy of control” was the term introduced to
describe how packaged equipment with factory controls can be integrated into
a higher tier system of control.
In general, three methods of control exist for the operation of mechanical
systems. They were introduced in Chapter 3 as two-position control, staged
control, and proportional control. This chapter made special reference to
proportional control, and included P+I and PID control, as well as floating
control, as types of proportional control. The chapter also formally introduced
the term “setpoint”.
Chapter 4 defined the functions of sensors and controllers, and made the
distinction between the two. In short, a sensor itself is not a controller, but can
be an integral part of a controller. A controller’s purpose is to gather
information from a sensor, process it, compare it with a setpoint, and affect an
action upon the “controlled variable”, in preferential accordance with the
setpoint. The chapter also touched upon DDC, networking, and Building
Automation Systems, illustrating that technology plays a powerful role in HVAC
control. Yet though technology will continue to simplify efforts and improve end
results, the basic concepts of control will tend to remain.
Chapters 5 and 6 finished off the general topic of controls. Chapter 5 gave a
detailed overview of end devices, their relation to the three methods of control,
and the role that they play in an HVAC system. And Chapter 6 introduced the
reader to some of the more common control schemes applied in this day and
age, and set out to instill a familiarity with the terms describing these schemes.
The rest of the chapters really got into the bulk of what the book is all about.
This is where the book becomes more of a reference manual, for the design
and evaluation of HVAC control systems. However, there are key issues
located throughout, some of which are reiterated here.
When it comes to our business, we seem to have our own language. The
terminologies used throughout the industry are far from standardized, and
nowhere is this more prevalent than with controls. Terms are used and abused,
and one individual’s or manufacturer’s lingo may be quite different than another’
s. The key issue here is to be aware of the terminologies, and understand their
use and their context. While it may be acceptable to talk in simple concepts
and “technically” misuse common terms, it must be done so with each side
understanding the general ideas behind the terms, so that misinterpretation is
avoided and expectations are met.
As for “rules of thumb”, we tend to sometimes overuse and even misuse these
handy little tools. So much of what we do has been done so many times
before, that there is generally no reason to “re-invent the wheel” every time we
embark on a new project. Hence we use these rules of thumb to assist our
efforts and accelerate our design processes. They are extremely useful, and
should be among the contents of every designer’s toolbox. The key issue is
that they are to be used with caution, with an understanding of where they
originated and how they relate. In other words, know the concepts behind the
rules.
Also among the control system engineer’s tools should be a solid
understanding of the formulas that govern many of the processes that
constitute a mechanical system. Strong analytical skills are an invaluable
attribute of any designer. To know how to manipulate the equations and
formulas that characterize any mechanical system is a powerful skill to
possess. It is the controls engineer’s responsibility to not simply design a
control system, but to also know the criteria used in the design of the
mechanical system, so that he knows how to design his control system, in
alignment with the mechanical engineer’s intended method of operation for the
mechanical system.
It must be known: controls can’t work magic. A control system is only as good
as the mechanical equipment that it is controlling, and cannot compensate for
mechanical malfunctions and inadequacies. An example of the flipside of this
“conventional wisdom” is the occupant who turns the thermostat controlling his
rooftop unit down from 70 to 60 degrees, because it’s 80 in the space. Of
course, when it’s 80 in the room and the thermostat is set at 70, turning it down
any further will do no good. The thermostat has done all that it can do. This
example demonstrates in simple fashion that not all environmental control
problems can be blamed on the control system. Yet the controls are frequently
the first to be blamed. This is seen more and more as digital controls
propagate the industry, where the simple temperature control system is
replaced with “black box” controls that people can’t tell by looking at whether or
not they’re doing their job. For the vast majority of circumstances, the properly
commissioned control system will be doing what it’s supposed to be doing,
and the failures will be mechanical in nature.
In line with the above train of thought, controls cannot compensate for a poorly
designed mechanical system. And problems that surface upon startup are not
always attributable to the control system. The reliance on the control system
should not be such that the controls will be able to overcome mechanical
system shortcomings. Sure, a slick control systems engineer may be able to
help out an ill-conceived mechanical system. But the control system in general
will not be a panacea for all mechanical design problems, and should not be
looked upon in that manner.
The importance of the control system should be evident, in what it brings to the
HVAC system as a whole. It must be solid in concept and in design. And it
must allow the well designed mechanical system to operate as intended. If
properly applied and implemented, it can bring value to any HVAC system,
even the ill-conceived system, to an extent. Yet if poorly designed and
inadequately commissioned, the control system can wreck even the most well
thought out mechanical system.
The final chapter to this book (prior to this one) closed by touching upon the
advantages of “distributed DDC”. The technologies available in this day and
age open up a whole new world of control system options and flexibility.
Distributed control systems have become commonplace, and will continue to
grow both in size and in number. As we as an industry move away from simple
controls and control systems to integrated microprocessor-based systems and
networks, we must hone our own skills and stay up with the trends and the
technologies. Yet in the end, our installations must still appeal to the end user.
A sophisticated control system must be made to “appear” simple, at least to
the recipient of the system. That may be our biggest challenge for the future,
because when all is said and done, the average Joe will still continue to hold
on to the value of simplicity.
If there was any one particular purpose of writing this book, it would be not
necessarily to teach the technical aspects of controls and mechanical systems,
but perhaps more so to provide the reader with insight into the proper
operation of these systems. And also to hone that skill, in hopes that the reader
can perform his own evaluations and come to his own conclusions on how
mechanical systems should operate and be controlled. To the individual that
has gotten this far and completed the book, hopefully you have acquired at
least a bit of that insight. And if you understand the key issues as well, then this
book has succeeded in its goal.
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