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Chiller Plant Optimization
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 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!
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