Article - November 2001
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Chilled Water Plant Control

Bellevue Corporate Plaza

LOOP control manages valve opening and pump speed to automatically maintain minimum flow at all times a chiller is in operation.

Tom Hartman, The Hartman Company 
Thomas Hartman, P.E.
Contributing Editor

After nearly 25 years of operation, the chiller plant at Bellevue Corporate Plaza, a multi-tenant mid-rise office building in Bellevue, WA needed to be upgraded. The plant employed a phased out refrigerant, and the air cooled centrifugal chillers were near the end of their useful life as demonstrated by the dramatic increase in maintenance and sporadic failures in the last several years. Furthermore, the plant lacked the capacity to meet growing tenant internal heat loads at peak conditions. The configuration of the existing system is shown in Figure 1.

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Figure 1: Bellevue Corporate Plaza Existing Space Cooling System Diagram

Figure 1: Bellevue Corporate Plaza Existing Space Cooling System Diagram

Upgrading The Plant 

After exploring several replacement options, the building owner, Hallwood Commercial Real Estate, adopted an upgrade approach that employs new "LOOP" demand-based chiller plant operation technologies. The Hartman Company provided the LOOP operation sequences and assisted Hallwood in selecting and pre-purchasing equipment that best suited the new technologies being implemented. A design/build installation contract was then let to remove the existing chiller system and install the new equipment. At startup, The Hartman Company helped in commissioning and implementing LOOP ultra-efficient operating control sequences to bring the new system on line. The LOOP sequences were installed in the building's existing DDC system. A diagram of the upgraded system is shown in Figure 2.

Figure 2: Bellevue Corporate Plaza Upgraded Space Cooling System Diagram
Figure 2: Bellevue Corporate Plaza Upgraded Space Cooling System Diagram

distech The upgrade consisted of replacing the existing 270 and 210 ton air cooled centrifugal chillers with two new 270 ton air cooled screw chillers. The change to screw chillers was necessary because air cooled centrifugal chillers are no longer manufactured. All balancing valves were removed and variable speed drives (VFDs) added to the distribution pumps. The three way modulating valves serving each cooling coil were replaced with line size two way modulating butterfly valves. Because LOOP sequencing employs network based controls, this variable primary flow distribution requires no bypass valve. LOOP control manages valve opening and pump speed to automatically maintain minimum flow at all times a chiller is in operation.

The new system provides additional capacity with a reduced design chilled water temperature so that the existing cooling coils can be used. On a budget of approximately $500,000, the pre-purchase/design build process with technical support reduced construction costs about 20% below the best of the conventional replacement proposals that had been received.

Benefits of LOOP Control 

The new LOOP demand based control technologies have significantly improved the chiller plant operating efficiency by using the control network to coordinate cooling supply with the actual demand. Conventional chiller plant design requires extra energy to supply chilled water cold enough to serve any load at any time and distribution pressure high enough to overcome the high head loss of small modulating valves. With LOOP control, the cooling components are now operated at optimal levels of power use to maintain space temperature and humidity conditions rather than to maintain a specific chilled water temperature or distribution pressure setpoint. This control strategy is called "Demand Based Control." When cooling is called for, the chiller plant and distribution pumps are optimally operated together such that at least one cooling coil is just satisfied with its valve fully open. Under normal operations there is no direct control of chilled water temperature or distribution pressure. At low load conditions, chilled water temperature floats upward and pump head requirements for the distribution system fall, resulting in very stable operation and large energy savings. Even at full load conditions, the chilled water temperature remains above the design minimum and pumping head is nearly 50% lower than it was before the retrofit.

Another significant element of this upgrade that has paid large dividends is the manner in which LOOP demand based control reduces peak demand. Operation of the chillers, distribution pumps and main supply/return fans are coordinated for highest overall cooling and distribution efficiency. This automatic network optimization is especially useful during periods of demand limiting. In conventional systems, demand limiting may be applied only to chillers. The chilled water distribution pumps and air distribution fans often speed up during demand limiting because their loads are not met when the chillers reduce loading. This results in more power use by pumps and fans which reduces the demand limiting function of the chillers. At the Bellevue Corporate Plaza building , the LOOP sequences coordinate the operation of all equipment and provide more effective demand limiting with less impact on the occupants because the system efficiency increases during the demand limiting so that the cooling effect remains high.

Despite the larger cooling system and lower design operating temperatures, this building's summer peak electrical demand has been reduced by about 20% and the building's daily energy use during cooling system operation has been reduced about 22% as a result of the chiller plant upgrade.

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