Consulting-Specifying Engineer - February 2008 - (Page 20)

Cover Story control sequences Developing new and complex sequences of operation has been a challenge in the design of an HVAC system. BY JAY SANTOS, PE, Principal, Facility Dynamics Engineering Inc., Columbia, Md. he development of prescriptive, detailed sequences as part of the design documents is now more critical than ever. As newer, more complex systems are applied to achieve greater energy efficiency, there is a need for equally new and complex sequences of operation. Additionally, with commissioning becoming commonplace, detailed sequences are necessary for the commissioning engineer to develop proper functional testing. Without detailed prescriptive sequences, the commissioning engineer must referee various interpretations of a sequence of operation. Altogether, the responsibility for developing the sequences rests with the engineer and it is critical to be well documented early in the building process. How to write T This article presents an organizational process to help manage the thought process needed to write and create good sequences of operation. Divide and conquer An effective way to organize and prepare control logic is to categorize the logic into two distinct types—process- and supervisory-level control logic. Process-level control logic is the control logic associated with the direct control and response of the controlled device. In a typical control loop it consists of the interaction of the process variable, the control response, its setpoints, and any special limits and conditions associated with that loop. The supervisory level logic is more similar to overhead logic. It involves the collection of data from multiple sources, analyzing this data and sending a specific instruction to the process level control. It also may involve special alarming, complex reset strategies or the logic for operating a redundant system. There are no specific rules for discriminating between these two levels of logic but it is a convenient way to think about control sequences. An eight-step process An overall process for the development of control logic involves the following basic eight steps listed below and then described in turn: 1. Identify the devices to be controlled in each system 20 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

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.