Optimizing The Chiller Plant – A White Paper

Why Optimize a Chiller Plant?

The chiller plant can now be optimized for maximum efficiency at part load as well as full load operation. Annual operating savings are significant and wear and tear is reduced with lower pressure operation.

The typical design only optimizes a chiller plant for maximum load with the cooling towers and the condenser water pumps sized and set-up for maximum load operation and design conditions. While the chiller may have good part load performance, the rest of the plant is not effectively “re-tuned” to operate at part load, even though cooling tower fans do turn down based on a fixed condenser water temperature set point.

This less then optimum chiller plant design only uses a control algorithm based on a simple temperature measurement, set-point, and control mechanism.

Properly matching all the component operations under various load and operating conditions will save significant amounts of energy.  Total average annual plant energy savings of 25% are possible with water cooled chiller plants.

The following control strategies, properly applied, will increase your operating savings. The Chiller Plant Optimizer(TM) utilizes a patented methodology to apply sophisticated adaptive control routines to continuously adjust / tune control parameters for optimal energy demand.

Condenser Water Temperature Reset

A chiller’s maximum capacity is effected by a pressure requirement to “push” a given mass flow rate of refrigerant from the compressor through the condenser to the evaporator.  Typically there is a limiting orifice or valve between the condenser and evaporator.  The chiller design matches the condensing pressure to this pressure requirement.  This is what defines the design condensing temperature for maximum load.

At maximum load the condensing pressure / temperature which is designed as noted above defines the required condenser water temperature.  So you find condensing water requirements in the range of 80 to 85°F.

During operation of the chiller, If the condensing water temperature is below the design temperature, the capacity of the chiller is reduced because the mass flow capability of refrigerant is reduced.  On the other hand, the efficiency of the chiller increases with lower condensing water temperature.

A control strategy that reduces the condensing water temperature when the chiller is operating at part load will significantly improve the chillers operating performance at that load and can lead to significant savings. This control strategy is defined as Condenser Water Temperature Reset.

This savings is not pure, that is to say more fan energy is required to provide the lower condenser water temperature under most operating conditions due to the outside wet bulb temperature.  It is conceivable that with small chiller loads, the fan energy usage could be greater then the chiller energy savings.

The Chiller Plant Optimizer(TM) has a unique method that readily achieves the right balance between cooling tower and chiller operation for greater savings benefit. Using advanced logic and adaptive control computer routines prepackaged in a stand alone control unit installation costs are minimized and unit configuration to match the specific chiller plant is accomplished offsite.

Cooling Tower Temperature Relief

A typical chiller plant is designed for best efficiency when operating near or at 100% operation with the condenser pumps and cooling towers.   When operating at part load capacity the cooling tower and pump still have the maximum operation capability.

A simple example; the chiller plant is 85% loaded but the outside conditions (humidity) are maximum. The cooling tower fan will be required to operate at full speed to achieve the desired condenser water temperature set point. With lower part load situations and lower outside conditions, the cooling tower fan will still be required to work harder then necessary to achieve the desired set point. But, if under these conditions the maximum fan speed is limited, the savings in cooling fan energy will outweigh the slightly increase energy usage of the chiller.

A control strategy that is designed to achieve this balance is called Cooling Tower Temperature Relief.  An intuitive example would be to consider a plant that is designed to operate at a lower load and note that the design considerations could provide a smaller cooling tower or the same cooling tower with smaller motor and a smaller condenser water pump.

The patented Chiller Plant Optimizer(TM) uses a unique method including advanced control logic and adaptive control routines to determine the ideal limits for cooling tower fan operation.

Chilled Water Temperature Reset vs Condenser Water Temperature Reset

It is readily recognized that re setting or raising the chilled water temperature, will save some energy. The typical savings for this strategy is limited, and  there are many situations where it is not possible, nor wise to use this strategy.

Raising the chilled water one degree results in nearly the same efficiency improvement as lowering the condenser water temperature one degree.  Condenser water temperatures can be lowered down to 60 degrees F.  for many chillers while the opportunity to raise chilled water temperature is about 5 to 7 degrees F. at max.

Still, when it is feasible applying  Chilled Water Temperature Reset  should not be overlooked. Several different strategies are available from various vendors, and the results can be mixed. Generally when it is clear chilled water reset is not problematic, and savings will be achieved, then the implementing strategy is straight forward and uncomplicated.

Variable Condenser Water Flow

Varying the flow rate of the condenser water can be important addition with the above strategies for a total energy savings. Published papers of actual performance tests by a major chiller manufacturer show us that condenser water flow rates can be significantly reduced without impairment to the chiller.  ASHRAE publishes minimum recommended condenser water flow rates to prevent mineral buildup.  Design compromise (remember price to performance ratio) dictate the typical design flow velocities that range for 2 to 3 times the ASHARE minimum.

Some engineers feel that to design a control strategy to save energy by varying the flow rate and motor speed is not worth the effort.  a) Many real world chiller plants have much larger condenser water pumps than the theoretical design requirements.  b) When the individual pumps are paralleled on a common distribution header they are sized, and balanced to provide flow and pressure significantly higher then in a single circuit system.  c) Also, it is not always feasible to have a short run between the plant systems.   d) Finally, since the Chiller Plant Optimizer(TM) has been designed for additional optional algorithms, implementation costs are kept to a minimum.

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Optimizing chiller plant to improve efficiency and reduce energy.