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Thursday, March 4, 2010

Why Optimize a Chiller Plant?

Chiller Plant Optimization

The chiller plant can now be optimized for maximum efficiency at part load as well as full load operation. Annual operating savings of $5,000 - $25,000 and more per chiller are easily attainable.

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.

The classic chiller plant design only uses a control algorithm based on a simple temperature measurement, set-point, and control mechanism to control cooling tower operation.

Properly matching all the component operations under various load and operating conditions will save significant amounts of energy. Total average annual energy savings of 25% are possible with many systems. Combining these savings with a variable drive chiller and expected savings may exceed 40%.

The following control strategies, properly applied, will increase your operating savings. The Chiller Plant Optimizer utilizes a patented methodology to apply sophisticated adaptive control routines used in modern robotics to easily realize significant savings. The Chiller Plant Optimizer TM is configured so that real savings can be monitored.

Cooling Tower Temperature Relief

A chiller plant is designed for 100% operation with the condenser pumps and cooling towers. With operation 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. Under these conditions if 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 provide a smaller cooling tower or the same cooling tower with smaller motor and a smaller condenser water pump.

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

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, its restricted control orifice, and the evaporator. The design matches the condensing pressure to this pressure requirement. This then sets the design condensing temperature for maximum load.

At maximum load the condensing temperature, which is pre-established, defines the required condenser water temperature. So you find condensing water requirements in the range of 80 to 85°F. If the condensing water temperature is to low then the capacity of the chiller is reduced. At the same time 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 control strategy must be carefully applied in a "fool proof manner, since there are situations that could result in more energy usage if a simple control sequence is used. This is also why manual adjustment of condenser water temperatures as practiced by some facilities, can be easily misapplied. More fan energy is required to provide the lower condenser water temperature therefore it is necessary to find the right balance for the greatest energy savings. It is even conceivable that with small chiller loads, the fan energy usage could be greater then the chiller energy savings if not properly compensated.

The Chiller Plant Optimizer’s unique method easily 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, or distributed control unit, systems costs are kept low and do not require a customized design for every chiller plant.

Variable Condenser Water Flow

Varying the flow rate of the condenser water is a 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 which 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. Many real world chiller plants have much larger condenser water pumps then the theoretical design requirement. When the individual pumps are paralleled on a common distribution they are sized, and balanced to provide flow and pressure significantly higher then in a single circuit systems.

The Chiller Plant Optimizer is a pre-designed system, the engineering costs are already paid for, so it is truly cost feasible to include variable condenser water flow in many situation.

Chiller Capacity

Chiller capacity is a function of the heat transfer capacity (enthalpy difference) and rate of flow of liquid refrigerant. Referring to a pressure-enthalpy diagram and plotting a typical refrigeration cycle on the diagram one notices that lowering the condensing temperature increases the heat transfer capacity of the refrigerant. For R-123 a 25 degree reduction of condenser temperature can increase in heat transfer capacity on the order of 10%. But don't interpret this as a direct capacity increase for the chiller.

Here is a short explanation of what really happens to capacity and efficiency when the chiller tries to load up, but the condenser water temperature remains low. First refrigerant begins stacking in the condenser, reducing the condenser heat transfer effectiveness which causes the condenser pressure to rise. As approach temperature between the leaving condenser water temperature and the saturated condenser temperature increases it indicates that the condenser heat transfer is poor. If the chiller components are oversized, it is possible that the pressure will increase enough to achieve near full system capacity, but with a truly unacceptable decrease in efficiency.

Therefore, if condenser water reset is poorly applied it is possible to substantially reduce overall system efficiencies rather than increase system efficiencies.

Try this experiment, make like you are sizing the flow orifice (or a valve) between the condenser and evaporator, and calculate the orifice (valve wide open) flow coefficient required for full flow at full load with normal system design parameters. Then using the calculated flow coefficient, recalculate the flow rate of refrigerant at reduced condenser temperature/pressure and note the reduced capacity in flow rate and refrigeration effect.

So why is it that some claim that reducing condenser water temperature does not reduce capacity and even claim an increase in capacity? The explanation for the increase in capacity claim is due to the wrong interpretation of the pressure-enthalpy diagram and/or misunderstanding of the marginal capacity built into the condenser/evaporator selection, plus total disregard for the effects on efficiency losses!

Why Optimize a Chiller Plant
Cooling Tower Temperature Relief
Condenser Water Temperature Reset
Variable Condenser Water Flow
Chiller Capacity

Chiller Plant Optimizer
Email: Bill Henry


DM water for chilling plant
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