Specifications
temperature entering on start-up must not exceed 90°F
(32°C).
Evaporator Leaving Water Temperature
Warmer leaving chilled water temperatures will raise the
compressor’s suction pressure and decrease the lift,
improving efciency. Using 45°F (7°C) leaving water instead of
the typical 42°F (5.5°C) will signicantly reduce chiller energy
consumption.
Evaporator Water Temperature Difference
The industry standard has been a 10°F (5.5°C) temperature
drop in the evaporator. Increasing the drop to 12°F or 14°F
(6.6°C or 7.7°C) can improve chiller efciency and reduce
pump energy consumption.
Condenser
Reduced Condenser Fluid Flow
Several popular chiller plant control practices also advocate
reducing the condenser uid ow rate as the chiller load is
reduced. This practice can signicantly reduce the condenser
pumping power, but it may also have the unintended
consequence of signicantly increasing compressor power
since the leaving condenser water temperature is directly
related to compressor lift and power. The higher compressor
power will typically be larger than the condenser pumping
power reduction and will result in a net increase in chiller plant
energy consumption. Therefore, before this strategy is applied
for energy saving purposes it should be extensively modeled
or used in an adaptive chiller plant control system which will
take into account all of the interdependent variables affecting
chiller plant energy. If it is decided to use variable condenser
uid ow, the model WMC chiller can operate effectively as
long as the minimum and maximum tube velocities are taken
into consideration when selecting the chiller.
Condenser Entering Water Temperature
As a general rule, a 1°F (0.5°C) drop in condenser entering
water temperature will reduce chiller energy consumption by
two percent. Cooler water lowers the condensing pressure
and reduces compressor work. One or two degrees can make
a noticeable difference. The incremental cost of a larger tower
can be small and provide a good return on investment.
When the ambient wet bulb temperature is lower than design,
the entering condenser water temperature of Magnitude
®
WMC chillers can be lowered to improve chiller performance.
Chillers can start with entering condenser water temperatures
as low as 40°F (4.4°C). For short periods of time during
startup, the entering condenser water temperature can even
be lower than the leaving chilled water temperature.
Depending on local climatic conditions, using the lowest
possible entering condenser water temperature may be more
costly in total system power consumed than the expected
savings in chiller power would suggest, due to the excessive
fan power required.
In this scenario, cooling tower fans would continue to operate
at 100% capacity at low wet bulb temperatures. The trade-
off between better chiller efciency and fan power should
be analyzed for best overall system efciency. The Energy
Analyzer
™
II program (available from your Daikin Applied
sales representative) can optimize the chiller/tower operation
for specic buildings in specic locales.
Even with tower fan control, some form of water ow control,
such as tower bypass, is recommended.
Figure 8 and Figure 9 illustrate two temperature-actuated
tower bypass arrangements. The “Cold Weather” scheme,
Figure 9, provides better startup under cold ambient air
temperature conditions. The bypass valve and piping are
indoors and thus warmer, allowing for warmer water to be
immediately available to the condenser. The check valve may
be required to prevent air at the pump inlet.
Figure 8: Tower Bypass- Mild Weather Operation
Figure 9: Tower Bypass- Cold Weather Operation
(Bypass Indoors)
Condenser Water Temperature Difference
The industry standard of 3 gpm/ton or about a 9.5°F (5.3°C)
delta-T works well for most applications.
Condenser Water Temperature Control
The standard MicroTech
®
II controller is capable of four stages
of tower fan control plus an analog control of either a three-
way tower-bypass valve or variable speed tower-fan motor.
Stages are controlled from condenser-water temperature.
The three-way valve can be controlled to a different water
temperature or track the current tower stage. This allows
optimum chilled water plant performance based upon specic
job requirements.
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CAT 602-4 • MAGNITUDE
®
MODEL WMC CHILLERS 14 www.DaikinApplied.com
applICaTIon ConsIderaTIons