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Sigma F Series Cooling Tower
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Engineering Data: Energy Management
10
CAUTION
The cooling tower must be located at such distance and
direction to avoid the possibility of contaminated tower
discharge air being drawn into building fresh air intake
ducts. The purchaser should obtain the services of a
Licensed Professional Engineer or Registered Architect to
certify that the location of the tower is in compliance with
applicable air pollution, fire, and clean air codes.
Cooling towers are usually selected to produce a specific
cold water temperature at the higher summertime wet-bulb
temperatures. During the remainder of the year, the cooling
tower is capable of producing much colder water. Unless your
system will benefit from the coldest possible water tempera-
ture, you should consider controlling cold water temperatures
to higher levels. You’ll also save energy by using such control.
For greater insight on cold water temperature control, please
read “Cooling Tower Energy and its Management”, Techni-
cal Report #H-001A, available from your Marley sales represen-
tative or on the web at spxcooling.com.
Always control leaving water temperature by manipulating
the quantity of air that the fan moves through the tower. Vary-
ing the quantity of water flow is not normally recommended
and can be harmful in freezing weather. You can alternately
start and stop single-speed motors to maintain water tempera-
tures within an acceptable range. However, exceeding a total
acceleration time of 30 seconds per hour can overheat the mo-
tor, causing the insulation to fail. Limiting the number of motor
starts, on the other hand, can produce significant variations in
the temperature of the water delivered to the process.
Increased flexibility can simplify your operating procedures
and save you money in the long run, both on operation and on
maintenance. Here are two of the more popular options.
Two-Speed Motors
Two-speed motors improve operating flexibility by increas-
ing the number of potential operating modes. Users in northern
climates will find that the tower can carry winter loads at
half-speed; reducing fan power requirements by 85+% during
that time. Two-speed motors also help to control icing during
wintertime operation. See Marley Technical Report #H-003,
“Operating Cooling Towers During Freezing Weather,”
available from your Marley sales representative or on the web
at spxcooling.com.
Normally, two-speed motors are provided in 1800/900 RPM,
single winding configuration, which is the least expensive two-
speed option. They are also available in other combinations
including the more expensive double winding.
Variable Speed Fan
Frequency modulation devices work well on induced draft,
propeller fan cooling towers such as the Sigma. However, their
design must include the capability to lock out any critical fan
speeds and the very low fan speed ranges.
Marley VFD drive systems are designed to combine abso-
lute temperature control with ideal energy management. The
cooling tower user selects a cold water temperature and the
drive system will vary the fan speed to maintain that tempera-
ture. Precise temperature control is accomplished with far
less stress to the mechanical equipment components. The
improved energy management provides fast payback. Indeed,
many utilities offer generous rebates for users having installed
VFD drives.