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Words of Wisdom
Prudent advice from the pages of Practical Controls
Steven R. Calabrese
This month I take the opportunity to share some insights from the pages of my book Practical Controls: A Guide To Mechanical Systems. Hence I depart from the usual technical stuff and wax philosophical for a bit. Part fact, part opinion, and perhaps a little self-indulgence. Cut me some slack though! After a year of writing this “Practical Controls” column, I simply wanted to take a break from the practical hands-on aspects that normally fill this column, and engage in more thought-provoking rhetoric. So with that in mind, allow me to share some of my views of the industry and of how it’s perceived.
…You can call it what you want, but the HVAC industry is primarily a “mechanical” industry. The equipment and components that make up a typical HVAC system are mechanical in nature. Fans, pumps, dampers, valves, ductwork, and piping. Put ‘em all together and you have yourself a mechanical system. The designers of these systems, the mechanics that install them, and the technicians that work on them, are all trained in the mechanical vein. How these systems are controlled, however, might very well fall under a different discipline. An HVAC system can be described as being a mechanical system plus the control system that is required to properly and efficiently operate it.
…As the designer of a mechanical system, an engineer may choose to write a description of how he/she would like the system to operate. This description, commonly referred to as a Sequence of Operation, should describe in detail how each piece of equipment and each subsystem should operate, so that the system as a whole is properly functional. With the Sequence in hand, the control systems designer has a clear direction to follow, and can design his or her control system in accordance with the Sequence. During the design phase, the designer may also choose to elaborate on this given Sequence of Operation, so as to include important operational details, temperature settings, equipment specific information, etc.
…A distinction should be made between “packaged” equipment, and equipment that must be “built up”. A packaged piece of equipment is one that is furnished with some amount of factory mounted and installed controls devices. These controls may partially or even completely allow the equipment to operate, without any other added controls. Contrast this with a piece of equipment that must be “built up”, i.e., comes from the factory with no integral controls, and must be equipped with controls upon installation. Regarding packaged equipment, it’s interesting to note that just because a piece of equipment comes with factory controls, it doesn’t mean that it can’t be part of a built up “system”. “Hierarchy of Control” is a term that can be used 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: two-position control, staged control, and proportional control. P+I and PID control, as well as floating control, can be categorized as types of proportional control.
…A distinction is to be made between sensors and controllers. 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 rampant growth of DDC, networking, and Building Automation Systems illustrates 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.
…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. 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 is to use these 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/she knows how to design his/her 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.
Tip of the Month: No matter what your role in this industry, remember that, amidst all of the rhetoric, technical data, theories, opinions, and misinformation, in the end it’s simple common sense that prevails…okay, enough of that! The real tip here is to tune in next month, when we get back to business and probe the anatomy of a typical Building Automation System
…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 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.
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