2014-10-15 Consulting Specifying Engineer
Correctly setting controls for pumps and circulators is key to achieving energy savings.
By: Randy Schrecengost, PE, CEM, Stanley Consultants, Austin, Texas
Control sequences are a key element in achieving energy management and savings goals. Most water systems’ control sequences are straightforward and easy to use, but the control schemes for the system will vary with the size and complexity of the system, and especially with the type of pumping scheme chosen. Most chilled water (CHW) systems’ flow rates can be controlled from differential pressure. This control option provides some reliability but at times may have limited flexibility for operational changes, and can waste energy in overpumping. In the past, this differential pressure has been measured at the chilled water plant, in the piping distribution system, and/or at the hydraulically most remote location. ASHRAE Standard 90.1 indicates it should be measured at or near the most remote heat exchanger or the heat exchanger requiring the greatest differential pressure.
Pump pressure optimization can be used. This method looks at the control valve positions within the system and varies the setpoint control of the pumps (i.e., differential pressure) to force one control valve to go nearly wide open. This range might be 80% minimum to 95% maximum open, but will vary based on the application to avoid hunting on pressure setpoint adjustments. This methodology requires feedback from any/all local building controls back to the central plant. This is easy if the plant is part of a building, but harder if there is more than one building such as in a campus environment.
Other methods are chilled water temperature reset and the Hartman Loop. The Hartman Loop is best suited for an “all variable speed” plant but can be used on plants that are not all variable speed. This method uses the natural curve of the system and evaluates the efficiency of the entire system. It then operates individual components at the most efficient system operating point, which may not be the most efficient device operating point.
Randy Schrecengost is a project manager/senior mechanical engineer with Stanley Consultants. He has extensive experience in design and in project and program management at all levels of engineering, energy consulting, and facilities engineering. He is a member of the Consulting-Specifying Engineer editorial advisory board.
- ASHRAE Handbook, Chapter 44 – HVAC Systems and Equipment 2012.
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- ASHRAE Standard 90.1-2013: Energy Standard for Buildings Except Low-Rise Residential Buildings.
- “Pump Basics,” a presentation by W.A. Liegois, PE, Jon Bovenkamp, PE, Stanley Consultants Inc.