April 2007

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Practical Controls: A Guide To Mechanical Systems

Steve Calabrese, Project Engineer
Automated Logic, Chicago

Practical Controls: A Guide To Mechanical SystemsA relative newcomer on the scene of HVAC control systems publications, Practical Controls: A Guide To Mechanical Systems endeavors to teach, in an easy-to understand manner, the role of controls in mechanical (HVAC) systems. This 469-page hardcover book is published by The Fairmont Press (2003), and contains 24 chapters, 90+ diagrams and illustrations, and an 18 page glossary.

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The book begins with a Forward, which describes the purpose of the book and how it attempts to achieve it. The book is written from a mechanical contracting viewpoint, and the title, Practical Controls, sets the tone. “The approach that it takes is one of practicality”, and the book’s objective is to try to convey the practical methods of control as seen through the eyes of a mechanical contractor. The Forward also defines suitable candidates for the book, and suggests that the content is geared more toward the working professional; someone with at least some measure of industry experience. “Although written from a mechanical contracting perspective, the book hopefully appeals to all corners of the HVAC industry, from consulting engineers to controls contractors.”

The first few chapters of the book are not unlike the beginning chapters of many basic controls books, however the style in which the subject matter is addressed does lend itself to be read and understood perhaps more easily. That concept is in fact the driving intent throughout the book; take an esoteric subject like HVAC controls and make it more approachable by presenting it in simple terms and terminologies, interjecting humor here and there to keep it from getting “too heavy”.

Chapter 1 offers an introduction to mechanical systems and controls, and defines an HVAC system as a “mechanical system plus the associated controls and control system required to operate it.” Chapter 2 provides an overview of mechanical systems, and categorizes them accordingly. As the reader will find, this categorization plays out in the later chapters.

Chapter 3 introduces the reader to controls and provides the fundamentals, while Chapters 4 and 5 cover the physical hardware that is used to build control systems. And Chapter 6 scratches the surface of practical control systems by offering up some popular control schemes that are commonly implemented in today’s mechanical systems.

The book takes a break at this point, with a clever chapter entitled “Intermission”. This chapter recaps the previous chapters and provides insight into the direction taken with the rest of the book. It’s a nice “breather”, included so as to give the reader a chance to sit back and absorb the content presented in the previous chapters. “It is important to note that all discussions up to this point have been necessary, in order to comprehend the material in the upcoming chapters. A ‘prerequisite’, if you will, to the rest of the book. It is these chapters to come that are the prime reason for writing this book, yet in order to understand the content, there needed to have been some groundwork laid…[the following] chapters are meant to provide a solid understanding of how these systems and equipment should operate, and how they can be controlled to meet that end, with methods that are ‘practical’ and controls that are ‘available’.”

What’s evidently being said to the reader is to understand the content previously covered, and get ready for the chapters to come. Indeed, this is where the book departs from the common offering and begins to showcase its uniqueness. The chapters follow the categorical format of mechanical systems that is first presented in Chapter 2. Within each chapter is a broad description on the physical aspects of the mechanical equipment making up the system, as well as a discussion on how/why each system is applied. Building upon that is a narrative on how the individual systems should operate, from a practical standpoint, and how they should be controlled to meet that end. The controls and control systems that are described run the gamut from simple electrical and electromechanical controls, to stand-alone electronic control systems, to microprocessor-based controls and full-blown Building Automation Systems.

As an example, the chapter on fan coil units begins with a description of the physical components of a fan coil unit and how they are arranged within the piece of equipment. The section also touches upon the application range of this type of equipment, and discusses the numerous variations of fan coil unit configurations that are possible. Finally, each typical variation is studied, with the intention of discovering how the piece of equipment is most suitably controlled, and how it’s equipped with controls (sensors, motorized valves and dampers, controllers, etc.) in order to make that happen. For the case of fan coil units, the descriptions range from simple thermostatic control to DDC, using an “application specific” controller tailored for the application. This chapter, as with all chapters in the book, has plenty of diagrams to help illustrate the concepts behind the words.

[an error occurred while processing this directive] Also included in each chapter are common practices, equipment guidelines, rules of thumb, pros and cons, do’s and don’ts, etc., all presented under the premise of “real world” practicality. And while the book steers clear of detailed discussions on the design concepts behind mechanical systems, and of the “inner-workings” of packaged HVAC equipment, enough information is given in order to accomplish the task at hand: to teach the practical aspects of HVAC control.

The final chapter of the book touches upon the advantages of distributed DDC. While not a book strictly on DDC, it is heartily acknowledged that most of what is done in this day and age is enhanced through technology, and the industry as a whole will continue to move away from simple control systems to integrated microprocessor-based control systems and networks. “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.”

The book closes with a short Conclusion, which describes in retrospect the purpose of the writing, and offers the book up as a practical resource and reference guide for future applications. Some key issues are presented as well, a few of which are reiterated here:

“…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.”

“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…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.”

“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.”

Practical Controls: A Guide To Mechanical Systems contains a wealth of practical information, and is a worthy addition to the myriad of books written on the topic of basic controls. The publication is supported with a website that provides written excerpts, illustrations, and a link to the publisher. You can view these items at www.pcs-engineering.com.

About the Author

Steve Calabrese holds a BS degree in electrical engineering from the University of Illinois at Chicago (UIC), and is a member of several professional associations, including ASHRAE and AEE. He has spent his entire post university career, which spans over 15 years, working for HVAC contracting companies, in various roles including control systems design, project management, and department management. His present role is with a large controls company serving the Chicagoland area. Mr. Calabrese couples his broad mechanical knowledge and experience with a strong background in the area of electricity and electronics. His control systems expertise includes electrical and electronic stand-alone controls, as well as microprocessor-based direct digital controls (DDC) and networked



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