November 2015
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Central Energy Plant Control and Integration

Proper design of the controls and integration to key equipment is one of the best options to improve not only efficiency but also reliability and maintainability.

Paul Ehrlich, Ira Goldschmidt & Angela Lewis
Building Intelligence Group

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Engineered Systems 
November Issue - BAS Column

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Central energy plants, consisting of chillers, boilers and associated towers, pumps, and accessories, require a large capital investment and are energy intense.  The goal of these plants is to provide a reliable source of heating and cooling for a building or group of buildings, while minimizing the costs for energy, operations, maintenance, and equipment replacement. 

To achieve a reliable, efficient plant, there are many decisions to be made including careful selection of equipment as well as plant layout and pumping strategy.  Proper selection and design of the controls system can help to dramatically improve efficiency, and through integration, can also help to manage reliability and maintenance.  There are several key elements to keep in mind when designing controls for a central plant:

Efficiency:
The goal of the plant is to provide the required output BTU’s (chilled water, hot water, steam, etc.) using the minimal amount of input energy (electricity, natural gas, fuel oil, etc.).  An efficient plant starts with chillers and boilers that are highly efficient, but this is not enough.  To optimize the plant, control sequences are needed that look at the plant as a whole.  Examples of optimization strategies include use of variable flow pumping at minimum static pressure, measuring plant load and running the most efficient equipment to meet the load, efficient tower operation, and resetting condenser water based on plant load and outdoor air conditions.  Each of these strategies needs to be carefully developed for the plant, implemented, commissioned and documented.  Integration is also key to efficiency, since data from variable frequency drives, flow meters, chillers and boilers can all be used to calculate and document plant efficiency in real time.

Reliability:
Central plants require a high level of reliability.  To achieve this, most plants are designed with a certain level of redundancy. Controls can help assure reliability in several ways.  The most basic is to utilize sequences that monitor for faults and automatically start back up equipment.  For example in the event of a pump failure, a back up pump should automatically be brought on.  Beyond this, controls algorithms and analytics can be used to monitor key parameters to look for problems before they become serious.  For example, monitoring the approach temperature between the refrigerant and the evaporator at a specific load is a good indicator of tube fouling.  Integrating data from key equipment including chillers, boilers, variable frequency drives, etc. is key to being able to do good monitoring and improve reliability.

Reliable Controls Maintainability:
Finally, in order to properly maintain the plant assets, good maintenance procedures are required.  Controls and integration can provide critical data to assist in developing and supporting a maintenance plan. 

Central plant design is both challenging and rewarding.  Proper design of the controls and integration to key equipment is one of the best options to improve not only efficiency but also reliability and maintainability.



About the Authors

Paul and IraPaul and Ira first worked together on a series of ASHRAE projects including the BACnet committee and Guideline 13 – Specifying DDC Controls. The formation of Building Intelligence Group provided them the ability to work together professionally providing assistance to owners with the planning, design and development of Intelligent Building Systems. Building Intelligence Group provides services for clients worldwide including leading Universities, Corporations, and Developers. More information can be found at www.buildingintelligencegroup.com  We also invite you to contact us directly at Paul@buildingintelligencegroup.com or ira@buildingintelligencegroup.com


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