Consulting-Specifying Engineer - February 2008 - (Page 24)

Cover Story Are engineers losing control of sequences of operation? Over the past few decades, there has been a tendency to provide less detail in the development of a sequence of operation. It is interesting to reflect on what might have contributed to this trend. First, over the past couple decades, we have transitioned from discrete pneumatic logic to direct digital controls (DDC) in the HVAC business. The fundamental differences in these hardware platforms had a profound effect on the design of these systems. Pneumatics consisted of individual components that typically had a discrete function (like a switching relay, a high-signal selector, or a controller). So, one actually could lay out the logic using the functions of these devices on paper or reverse-engineer it from tracing how things were connected in the field. With DDC, the control logic is buried in software—it’s not as “visible.” As this transition occurred, we also transitioned from our pneumatic control technicians to DDC technicians who now require HVAC, controls, electrical, computer, and networking skills. Second, with a focus on increased liability, it argues that performance specifications are safer then more prescriptive sequences. While more general sequences are safer from a liability perspective, they are not in the best interests of the projects or the ultimate inheritors of the systems. to review it from an overall performance perspective. If you’ve done a good job of defining the basic processes, included all the required limits and conditions, and identified all the supervisory control logic, then the system should be well coordinated with the other control processes of the system. This “inter-loop” control action is some of the more complex logic and easily overlooked during the design process, but this logic usually impacts energy efficiency and operability the most and as such needs careful consideration. Check and check again It is not unusual to rewrite sequences or re-do logic diagrams two, three, or four times before one gets it right. But consider the alternative—a general sequence, leaving out numerous details and relying on the installing contractor to make these design decisions. This process may or may not work out depending on the circumstances. As a useful exercise, take a sequence of operation that you use and attempt to develop a logic diagram for the sequence. Typically, you’ll find numerous instances that the ver- Don’t forget the networks Although this article is focused on the process of the development of sequences, it is worth noting that there are hardware and networking implications of sequences that need to be considered. Many similarities in hardware and networking platforms are in our industry, but there are many differences as well. Certain systems are able to handle a complex sequence of operation of an air handler or a variable air volume box in a single controller. Others will require multiple controllers to do the same logic or require the use of higher end supervisory controllers to perform some of the functions. This is not as robust as if all the logic was resident on a single device. In critical applications this should not be allowed except for the supervisory logic that originates from elsewhere in the system. The engineer needs to be aware of the limitations that certain products have in this regard. Also, if the control sequence requires communication across the network by design (for example, the highest demand air handler communicates to the chilled water distribution system, or air terminal demand reset mode or setpoints in the air handler), then particular attention needs to be paid to the architectural robustness of the system. In these cases, one is relying on the network for basic control function and its speed and robustness becomes critical to the basic function of the control logic. This type of sequence cannot be required “blindly” without specifying networking performance criteria. The engineer will need to specify performance criteria for the system while fully loaded (all trending and alarming features operational). This may require limitations on the number of devices allowed on sub-networks in a building depending on the manufacturer. bal sequence doesn’t contain clear direction on specifics on control of the system. Each time this occurs, rewrite the sequence more clearly to fill in the details. You’ll see that the verbal sequences get much longer. You’ll also appreciate all the decisions that are being made (without you) by the installing contractor that will affect the system performance. The process of writing good control sequences or developing detailed logic diagrams for HVAC systems is not trivial. The logic that gets programmed into the direct digital controls system is the brain of the HVAC system and as such determines the performance of the system. Developing good engineering direction in this area is key to the success of the installation. The development of these sequences deserves more time in the design of HVAC systems than it is typically given. The above process is one way to organize thoughts in preparation of sequences or logic diagrams. Jay Santos is a principal and co-founder of Facility Dynamics Engineering. Santos has directed the development of DDC/control master plans for numerous institutions and organizations. He teaches DDC courses for the University of Wisconsin, North Carolina State, and the PGE Energy Center in San Francisco. The original idea for the process discussed in this article came from course material developed by Robert Schultz, PE, chief engineer, TAC Americas, and it was presented at various DDC Courses taught by Santos and Schultz. 24 Consulting-Specifying Engineer • FEBRUARY 2008

Table of Contents Feed for the Digital Edition of Consulting-Specifying Engineer - February 2008

Consulting-Specifying Engineer - February 2008 Contents Viewpoint News M/E Roundtable How To Write Control Sequences Mentoring Engineers: Myths, Motivations, and Models Keep Young Electrical Engineers Grounded Protecting a Vulnerable Population Codes & Standards Case Study New Products Equipment Lifecycles Advertisers Index Green Space

Consulting-Specifying Engineer - February 2008

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