Chiller Plant Investigations Result in Major Energy-efficiency Savings
By Jean O'Brien Gibbons, P.E., LEED AP, and Eric Peyer, P.E., LEED AP
The facility staff at a corporate campus in the western suburbs of Chicago had experienced various problems related to the control and operation of the chilled water system, including low temperature differentials, excessive online chiller capacity and cooling-capacity shortages. Grumman/Butkus Associates, energy consultants and design engineers in Evanston, Ill., was retained to identify the cause of these deficiencies and determine corrective actions.
The 2.5-million-square-foot corporate campus comprises buildings A through G, a conference and training building, and food-service building. The buildings are interconnected through interior corridors. Major functions include offices, a conference center, cafeterias, data centers and telecommunication rooms.
The buildings are cooled by two chilled-water cooling plants that are interconnected. The D Plant was built in 1990 and serves buildings B through F and the conference and training building. The A Plant was built in 1996 and serves buildings A, B and food-service building. Each cooling plant is part of a primary/secondary chilled-water system. When the corporate buildings are at or near their peak cooling load, the chiller plants operate independently from one another. At lower loads, A Plant is shut down and D Plant provides cooling to the loads typically served by A Plant via interconnecting piping.
Each cooling plant is part of a primary/secondary chilled-water system. The primary side of each system consists of the chillers and primary chilled-water pumps. The secondary side consists of secondary pumps, control valves and air-handling-unit cooling coils.
The chilled-water flow rate through each chiller is held relatively constant. Flow in each primary circuit varies in increments depending on the number of primary pumps online.
Flow in the secondary systems varies to meet cooling loads. Secondary system flow variation is determined by the position of automated two-way control valves serving the cooling coils plus the flow that bypasses through a system bypass valve. Chilled-water distribution from the plant to the cooling loads is handled using a mix of constant- and variable-speed secondary chilled-water pumps.
During winter the chillers are not operated; a water-side economizer system utilizing a plate- and frame- heat exchanger transfers heat energy from the chilled-water system to the condenser-water system where heat is rejected through the winterized cooling towers. The D Plant water-side economizer system is used in conjunction with the interconnect piping to serve the A Plant winter cooling needs.
Chiller Plant Problems
At the project’s onset, the facility staff identified the following primary concerns:
1. Secondary chilled water-flow rates were frequently higher than the primary chilled water-flow rates.
2. Low temperature differential between the secondary chilled-water supply and return; typically 6 to 8 degrees F lower than the design value of 14 F.
3. Short cycling of chillers.
4. Excess online chiller capacity.
5. Shortage in capacity through the piping system interconnecting the two independent chilled-water plants.
Early in the analysis, it was determined the overall cooling system had sufficient chiller and heat-exchanger capacity, heat-rejection capability, and chilled- and condenser-water-pumping capacity to satisfy the cooling demands of the entire facility.
The excessive online chiller capacity and lack of capacity in the interconnect piping were all suspected to be the direct result of the less-than-design chilled-water supply to return temperature differential—a problem often referred to as low Delta T syndrome. When chilled water Delta Ts are low, the chillers hit their limit of allowable flow prior to attaining their capacity limit. Thus the chillers are flow limited rather than load limited. This results in the operation of more chillers than are necessary to satisfy the cooling load.
When chilled water Delta Ts are low, the chillers hit their limit of allowable flow prior to attaining their capacity limit. Thus the chillers are flow limited rather than load limited. This results in the operation of more chillers than are necessary to satisfy the cooling load.
In D Plant, two 940-ton chillers were always operated—even when cooling loads were less than the 940 tons. Similarly, in A Plant, the second 500-ton chiller had to be brought online any time the cooling load was in excess of 350 tons.
Low Delta T may also explain the cooling-capacity deficiency of the interconnect piping between A and D plants. If the temperature differential is half of its design value, the cooling capacity of a gallon per minute of water is cut in half.
Grumman/Butkus Associates’ investigation identified the following two main contributors to low Delta T syndrome:
1. AHU supply-air-temperature setpoints were significantly lower than the design value (they were at high 40 F instead of 54 F) causing excess water flow through cooling coils without added cooling benefit.
2. The original chiller sequencing strategy allowed the chilled-water-supply temperature to climb to 48 F, which was 6 F above setpoint, before another chiller was activated. When the entering water temperature at an AHU’s chilled-water coil rises above the coil-design value, the amount of chilled water needed to maintain the discharge air setpoint increases.
These issues increase chilled-water flow and cause low Delta T. In addition, the high flows increase pump energy, making the system less energy efficient.
Several more problems were identified:
1. Chillers were staged by the building-automation system based on three arguments:
a. The first argument used a conventional comparison between the cooling load and nominal online chiller capacity. The calculation for the cooling load in the chiller staging sequence was scientifically incorrect, resulting in mixing of warm secondary return water with cold supply water.
b. The second and third arguments for staging chiller capacity made comparisons between primary and secondary chilled-water temperatures. Their function was an attempt to fully load and limit short-cycling of chillers. These parameters are useful in determining whether there were too many chillers online. However, the sacrifice made by implementing this control was loss of the chilled-water-supply temperature.
2. Chilled-water flow meters and temperature sensors in D Plant used for calculating load and staging chillers, were inaccurate.
3. The secondary chilled-water pump sequence used a floating differential pressure setpoint to maintain chilled-water temperature at the cooling coils. As the chilled-water temperature rose above setpoint, the differential pressure setpoint was incrementally increased and vice versa. By commanding secondary pumps to increase differential pressure when chilled-water temperatures exceed setpoint, the flow in the secondary circuit increased—increasing the supply water temperatures and worsening the low Delta T condition.
4. Several cooling-coil control valves in the chilled-water system were 100 percent open, indicating damaged valves, inaccurate signals from the valve actuator, poor chilled-water distribution and/or starved coils.
The following modifications have been made to the system (corrective-action items and numbering correspond to the problems identified in previous section):
1. AHU discharge-air setpoints were increased to the original design setpoint of 54 F during the cooling season, resulting in increased chilled-water temperature differentials.
2. The chilled-water plant now functions to maintain a chilled-water-supply temperature of 42 F. Lowering the chilled-water-supply temperature reduced secondary chilled-water pump energy and raised chilled-water temperature differentials without significantly increasing chiller energy use.
3. The cooling-load calculation has been corrected.
4. Chillers are now staged to maintain a supply-water temperature setpoint, and the primary chilled-water flow rate always remains equal or greater than that of the secondary circuit. Inaccurate control sensors have been recalibrated or replaced.
5. The pressure differential setpoint reset used to control the secondary chilled-water pumps was eliminated. Pumps now are controlled based on a single adjustable differential pressure control setpoint.
6. Non-functional control valves have been replaced.
After implementing recommendations outlined in the study, chilled water Delta Ts are generally at or above the design value--sometimes as high as 18 F. In addition, supply-water temperatures are consistently at or below the design setpoint of 42 F, and an individual chiller now can be loaded to 100 percent of available cooling capacity.
Many times the basics are lost through the complications of the application.
The changes have resulted in increased chilled-water temperature differentials, greater primary water-flow rates than secondary flow rates, elimination of short cycling of chillers and elimination of the need to operate excess chiller capacity. In addition, the higher Delta T results in more heat transfer through the interconnect piping, reducing the capacity shortage.
A Return to Basics
This project utilized fundamental engineering principles and best temperature-control practices. Although the engineering community would hope this was standard practice, many times the basics are lost through the complications of the application. This return-to-basics approach coupled with the fact that the modifications were implemented and monitored b the existing service and in-house facility staff are innovative in their simplicity.
Colder chilled-water temperatures and increased chilled-water Delta Ts resulted in reduced fan and pump energy use, respectively. These modifications were completed in 2006. Based on electric bills from May 2001 through September 2008, normalized using cooling-degree days, the annual facility energy savings was determined to be 696,000 kWh, corresponding to an annual energy-cost savings of approximately $64,000 (using present-day energy costs). The energy savings reduce carbon-dioxide emissions by 832 tons annually, which is the equivalent to 138 less automobiles, 1,756 less barrels of oil or 10 less tanker trucks of gasoline. And because all modifications were relatively minor and performed as part of existing service contracts, implementation costs were kept to a minimum. The consulting fee resulted in a simple payback of 10.3 months based on energy-cost savings.
|Energy-consumption Reduction||696,000 kWh|
|Carbon-dioxide-emissions Reduction||832 tons|
Although the project was focused on the chilled-water system, cooling-system operation changes resulted in unintended benefits to indoor air quality and thermal comfort. Increasing the AHU discharge-air-temperature setpoints did not degrade thermal comfort while colder, more consistent chilled-water-supply temperatures maintains more uniform temperature humidity levels throughout the entire facility.
Jean O'Brien Gibbons is vice president and Eric Peyer is a project manager at Grumman/Butkus Associates, energy consultants and design engineers located in Evanston, Ill.
This corporate campus was not designed to be “green”. However, the case study demonstrates that any building can achieve energy savings through simple processes.