Cooling tower fan motors can be operated more efficiently and at less
cost if they are controlled by inverters. Malcolm Staff compares the
traditional methods of control with modern inverter control and
illustrates the sort of energy savings that can be expected
Water cooled chillers provide the most efficient means of creating
chilled water, being as much as 20% more efficient than their air cooled
counterparts; and depending on climate, cooling towers are often the most
energy efficient method of cooling the condenser water from chillers.
Condenser water is sprayed into the cooling tower 'fill' (used to
increase its surface area) and a fan blows air through the fill and
sprayed water to aid evaporation, and hence the cooling effect. Cooled
water is collected at the foot of the tower and is pumped back into the
chiller condenser to repeat the cycle.
Traditionally, in order to conserve energy, fans are run in on/off mode,
driven by two-speed motors or sometimes fitted with variable pitch
blades. The temperature of the condenser water leaving the cooling tower
is the 'on/off' control parameter, and usually a wide temperature band is
established, at the upper part of which the fan is cycled 'on' and at the
lower end 'off'. The load on the chiller and the outside wetbulb
temperature determine the cooling tower's operation profile. As the
wetbulb temperature or system load decreases, the speed of the fan can be
decreased (two-speed motor example) or the blade pitch adjusted
accordingly.
With inverter control, cooling tower fan speed can be precisely adjusted
to maintain condenser water temperature, as well turned on and off. Some
inverters (including products supplied into the HVAC industry by the
author's company) are dedicated to this type of duty. As the fan drops
below a certain speed, the effect it has on cooling the water becomes
small; moreover, when a gearbox is incorporated, a minimum speed of
40-50% may be required. The customer-programmable minimum frequency
setting of the inverter is available to maintain this minimum frequency,
even if the feedback or speed reference calls for a lower speed.
These inverters can also be programmed to enter a 'sleep' mode, which
stops the fan until a higher speed is required. Similarly, undesirable
system resonances can be avoided by programming the drive to bypass the
offending frequencies.
Calculating the saving
The load profile indicates the amount of flow the system requires to
satisfy its needs and must be studied carefully in order to calculate any
savings accruing from an inverter installation. Figure 2 shows a typical
load profile for a cooling tower, which will be used for the purpose of
our calculation. Assuming a 30kW fan motor installation, the energy
consumption during one year for a two-speed motor is compared with that
of an inverter controlled motor (in this case a Danfoss VLT 6000 HVAC).
The figures are shown in the table below and show energy savings in
excess of 86%.
Sensor location for optimum inverter fan control
The temperature sensor should be located in the basin of the cooling
tower of the return line to the condenser pump. The efficiency of a water
cooled chiller varies with the condenser water return temperature; the
cooler the return temperature, the more efficient the chiller. However,
the energy consumption of the chiller needs to be compared with the
energy consumption of the tower fans and condenser pumps required to
optimise system efficiency. The ideal temperature varies with each
installation and should be calculated. Once the optimum temperature has
been determined, the drive can be used to maintain this temperature as
system load and conditions modulate.
At this point, its is worth comparing installation and maintenance costs
associated with both methods of control. The two-speed motor system
requires a pole-changing motor and appropriate con