Raising Chilled Water Set Point

March 6, 2018

Raising your chilled water supply temperature, while still meeting the cooling demands of your Data Center, can reward you with energy savings by reducing the work of the chiller’s compressor. The compressor after all is the largest user of electrical energy when compared to the pumps and cooling tower fans that make up the rest of the chiller plant.

 

ASHRAE Standard 90.1 2016 includes a requirement for resetting CHWST (chilled water supply temperature). Part 6.5.4.4 of the Standard requires systems larger than 25 Tons (300,000 BTUH) to include controls that automatically reset supply temperature by representative building loads. Automatic reset is a very important component in evaluating the part load performance of a system across a wide range of annual weather data for a given location. For a Data Center, evaluating the Mechanical Load Component (MLC) using the calculation described in ASHRAE Standard 90.4 2016, the Energy Standard for Data Centers, this systems evaluation approach is an ideal way to compare systems that are being proposed for a new or renovated Data Center.

 

Even Data Center loads, which are typically constant year round and effected very little (if at all) by outdoor seasonal changes, can benefit by automatic reset of CHWST. For instance, since the cooling system is typically sized for the final ITE load, it will be oversized until the ITE load is fully populated. This sometimes takes several years. Until that time, the cooling plant will operate at part load. A good plant design will take advantage of this and will have the components designed to operate efficiently at these partial loads while designing the entire HVAC infrastructure with the final load in mind or as modular components with redundant modules. The 90.4 Energy Standard allows energy modeling to include operation of the redundant components as long as part load efficiencies of this equipment are used. Apart from the chiller plant, additional cooling terminals (CRAH units) will typically be incorporated into the Data Center design to provide redundancy in the white space. It is common to operate all of the CRAH units simultaneously including the redundant ones, all using variable speed fans to provide good underfloor air distribution as well as part load efficiency.

 

Keep in mind that when incorporating automatic chilled water reset, CRAH unit chilled water coils must be sized for cooling the final load with warmer chilled water supply temperatures. Taking advantage of chiller energy savings by resetting CHWST will not work unless the terminal coils are designed properly with sufficient coil surface area for cooling the design load using the warmer, reset temperature.  This would all need to be determined during the design phase with upper limits set, based on coil sizing, as to how high the chilled water temperature can be raised.

 

High Humidity Issues

Problems with high humidity can result during portions of the year while delivering too high a CHWST. High humidity issues are usually caused by humidity migration into the Data Center from an outside source and not directly related to warmer chilled water supply temperatures. Humidity will migrate from areas of higher vapor pressure to areas of lower vapor pressure similar to temperature seeking to equalize from outdoors on a hot summer day through an insulated wall by conduction to the indoors of a cool air conditioned building. The insulation slows the conduction of temperature just as a vapor barrier slows the migration of vapor. Not all vapor barriers are perfectly sealed and gaps in the vapor barrier let moisture into the building. This should not discourage you from raising your chilled water set point. Even in most of the Northeast United States, where the climate is considered cool/humid to cold/humid, there is still a large portion of the year that you will be able to cool the Data Center with elevated chilled water temperatures. Problems with high space humidity may only present itself during summer months.

 

Automatic Reset Control

Resetting the CHWST should be performed based on feedback from the actual cooling load or demand for cooling. This can be accomplished through monitoring the room temperature with sensors located at the rack inlets. Chilled water return temperature can also be used since that represents the demand for cooling. The preferred method is monitoring the actual real time position of the chilled water control valves at the CRAH units. This method can only be performed through the use of DDC based building automation controls. Space relative humidity and dew point temperature can be monitored and used to override the reset schedule and lower the CHWST when space humidity becomes an issue.

 

Many factors need to be considered when deciding to raise the chilled water temperature setpoint.   It is not as easy as a couple clicks in the Building Automation System.   Even slight changes in the chilled water supply temperature could have a ripple effect on the system and could result in a number of space condition issues, and potential failures.

 

If there is a desire to operate your system at an elevated chilled water setpoint then it needs to be designed to do so right from the beginning.  On an existing system, it would need to be properly analyzed to determine if 1) it is even possible with the existing equipment’s coils, 2) how high of a chilled water temperature can the Data Center tolerate, and 3) can my control system safely accommodate such a system without any reduction of reliability. 

 

The control system is the brains of the operation and would need to be evaluated to make certain it has the correct logic to automatically control a resettable system.  Sure, it is not a problem to manually change the chilled water setpoint from 45F to 50F, but what if there is an issue in the Data Center.  Are there enough ‘smarts’ in the control system to recognize an adverse condition and automatically make the necessary, “safe” modifications to bring the system back under control.

 

There is a continued design effort among engineers and now a gentle push (from ASHRAE Standard 90.1 and 90.4 2016) to be a better steward of our electrical consumption, but the entire system needs to be reviewed to make sure that saving energy does not come at a cost of reduced reliability.

 

 

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