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Three Proven Strategies to Improve Mission Critical Chiller Performance in Existing Facilities


by Kenny Reed, PE, CxAP, CEM, Senior Commissioning Agent – HDR

This commissioning study of a clean research lab demonstrates how planning, investigation and targeted commissioning implementation can improve chiller performance in a mission critical environment. Learn how trend analysis, sequence of operations investigation, and ongoing performance measurement have led to a significantly more reliable chiller operation.

A Case in Point

This 200,000 square-foot laboratory, located within a multi-building campus facility, is considered mission critical due to the nature of research conducted within. The facility serves several different lab types including metrology, high accuracy instrument and cleanroom areas. Environmental requirements for the cleanroom labs require relative humidity to be maintained at 45% +/- 2 percent. Zone temperature set points are maintained around 68°F throughout the labs. The lab operates 24/7 and maintains aggressive space temperature and relative humidity requirements.

 

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Photo courtesy of HDR Architecture, Inc.; ©2010 Rion Rizzo / Creative Sources Photography, Inc.

 

The building was experiencing chronic issues related to a low temperature chilled water plant that serves dedicated air handling systems for the cleanroom laboratory facility. Facility staff was plagued with user complaints, and component replacements resulted in reduced reliability of the chilled water system.

A targeted commissioning approach was developed for this project to improve the chilled water system’s operation and reliability. Project scope included field investigation, trend review, recommendations and implementation design and planning. Energy savings was secondary since the primary objective was to provide adequate research conditions for cleanroom users.

Project goals focused on the reliability of the low temperature chilled water system. Our solution had to be flexible and stand the test of time to increase the life of the existing chillers. The cost-effective solution required a clear path to implementation and a plan for ongoing performance monitoring.

System Description

Low temperature glycol water (LTGW) serves 18 of the 22 HVAC units within the facility. Air handling units receiving LTGW include four makeup air and 14 recirculating air handling units. Heat rejection from the low temperature glycol chillers is a closed loop condenser water (CHRW) system used as hot water reheats, for a total of 22 air handling units. The existing CHRW loop is able to divert return water to a plate and frame heat exchanger using the site chilled water return as a heat sink. The low temperature glycol system consists of two 400-ton centrifugal chillers operating in a lead lag configuration.

Typical makeup air handling units consist of a glycol pre-heat coil, campus chilled water coil, LTGW coil, humidifier and reheat coil. Typical laboratory recirculation air handling units consist of a hot water preheat coil, campus chilled water coil, LTGW coil, reheat coil and humidifier. Central plant chilled water is supplied at 44°F, while LTGW is supplied at 32°F.

Gathering Data Without Disrupting Operations

When analyzing critical systems a definitive implementation plan is essential to obtain reliable facility data—without disrupting critical operations. For mission critical systems it’s important to consider these procedures as part of the investigation plan:

  • Drawing Review
  • BMCS trend request
  • Sequence of operation review
  • O&M record review
  • Operator interviews
  • Manufacturer discussions
  • A focus on root cause of operational issues
  • Documenting unknown sequence parameters

In this project, drawing review and field investigation uncovered several operational deficiencies that had an ultimate effect on the reliability of the low temperature chiller operation. The original indoor environment design requirements were set at 40 percent peak relative humidity during the summer and 30 percent for winter conditions. Existing lab specifications for relative humidity were 45 percent with a tolerance of +\- 2 percent. The total connected cooling capacity was 2588 KW, while the total low temperature glycol chiller capacity was 2800 KW.

Our maintenance records review revealed several service records related to refrigerant leaks, replaced circuit boards and a new purge tank. By reviewing the sequence of operation and building automation trends, we uncovered operational deficiencies at the AHU level. The heat recovery sequence had been overridden on several air handling units and multiple temperature sensors appeared to be out of calibration. Historic alarms were reviewed for the glycol chillers, including several alarms related to high-lift conditions and frequent short cycling during shoulder seasons.

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Photo courtesy of HDR Architecture, Inc.; © 2000 Tom Kessler Photography

Deeper investigation into the sequence of operation and building automation system trends uncovered inefficient staging of LTGW across the air handling units during mild seasons. We determined that LTGW served as the primary cooling source for only two of the 18 makeup air handling units throughout the year. After further reviewing the chiller and coil capacities we found that this only accounted for 27 percent of a fully loaded chiller, or 105 tons, assuming the glycol valve was operating fully open, which actually was not the case.

AHU type Percent Total Load
Cleanroom MAU 14
High Accuracy MAU 6
Lab AHU 80

Minimum winter cooling load capacities were calculated, assuming all available AHUs were operating with LTGW as the primary cooling source. Outside air temperatures were selected to represent a mild winter day requiring slight demand for chilled water cooling and stable indoor relative humidity conditions. The total estimated load was calculated to be 787 KW or 56 percent capacity of a single glycol chiller. Given the way the chiller had been operating previously, it was easy to see why the chiller experienced constant short-cycling and high-lift conditions.

Recommendations to Upgrade Performance

After analyzing trend logs, O&M data and better understanding the sequence of operation, our two primary recommendations involved modifying existing chilled water coil control sequences, and installing a heat exchanger on the LTGW supply. Sequence modifications could then be made immediately by in-house staff and staged as operations proved successful.

  1. Modify AHU chilled water controls. During normal modes of operation the LTGW is used as a primary source of cooling for the cleanroom makeup air units. All other air handling units use campus-chilled water as the first stage of cooling coil operation. To improve the operation and reliability of the glycol chillers, we recommended modifying the as-built sequences of operation to operate the glycol coil first, during winter conditions on all air handling units will. The scope of work involved revising the sequence of operations for all air handling units to accommodate the proposed summer and winter modes of operation.
  2. Install Heat Exchanger on LTGW Loop. This recommendation involved installing a plate and frame heat exchanger on the LTGW return loop, using chilled water as the heat rejection medium. The heat exchanger will be sized to provide an additional cooling load of up
    to 840 kW on the chiller to prevent the chiller from short cycling during the shoulder and winter seasons. The scope of work involved programing for LTGW valve control.

Implementation Planning is Key for Successful Commissioning

Preparing a detailed implementation plan is key for any existing building commissioning project to be successful. For the projects we recommended, a detailed scope of work was developed, in addition to the necessary design drawings, equipment schedules and revised sequence of operation.

Verification testing involved seasonal performance testing using trend reviews and obtaining input from facility operators. Chiller performance and trends should be reviewed on a monthly basis to track and benchmark. Ideally these checks are included as part of the monthly preventive maintenance schedule within the computerized maintenance management software.

A Targeted Project Outcome

Reduced energy consumption and prolonged equipment life are typical project outcomes when performing existing building commissioning. This project is a case where energy savings was considered secondary, when compared to the importance of providing increased system reliability and ensuring occupant thermal requirements are maintained. For complex projects it is important to continually assess long-term performance and to not abandon strategies, based solely on initial results.

Best Practices Enhance Existing Building Commissioning

When undertaking an existing building commissioning project, it is important to begin with the end in mind. It is critical to right size the scope based on the current facility needs and balance the scope with a reasonable budget—to achieve maximum success. Leveraging technology when capturing historical data can help expedite the investigation and lead to more streamlined solutions. Lastly, it is important to track the progress and benchmark with a plan to maintain ongoing performance.

 


Kenny Reed, PE, CEM, CxAP is a Senior Commissioning Agent with HDR who is well-known and respected by owners, design and construction teams, and building operations personnel for his hands-on approach and focus on building performance. Passionate about helping building owners ensure their buildings perform as intended, he is skilled in managing projects and optimizing building controls. Kenny has managed commissioning projects for building types that range from higher education to mission critical government facilities.

Within the professional community, Kenny shares his knowledge of the field by serving as the chair of the Building Commissioning Association (BCxA) Professional Development Committee. He is also a member of both the Nebraska ASHE Education Committee and the Advisory Council for Creighton University’s Energy Technology Program in Omaha, Nebraska.

  1. David L Lewis
    Reply

    Fantastic – great job Kenny. Everyone who is in the business of commissioning can come up with an article like this. BCA has the staff/volunteers who can edit and jazz up your Blog item.

  2. Alex
    Reply

    At what minimum ambient temperature can a chiller with a remote air cooled condenser work?

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