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 tempera- tures 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”, Technical Report #H-001A, available from your Marley sales representative 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 tem- peratures within an acceptable range. However, exceeding a total acceleration time of 30 seconds per hour can overheat the motor, 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 north- ern 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 expen- sive two-speed option. They are also available in other combi-
nations including the more expensive double winding.
Variable Speed Fan Drive
Frequency modulation devices work well on induced draft, propeller fan cooling towers such as the Sigma Steel cooling tower. 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 tem- perature. 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.
