Cooling tower fans

Other energy considerations for cooling towers include the use of two-speed or variable-speed drives on cooling-tower fans, and proper cooling-water chemistry to prevent fouling in users (see Water, industrial water treatment). Air coolers can be a cost-effective alternative to cooling towers at 50—90°C, just below the level where heat recovery is economical. 

Cooling-tower fan horsepower can be reduced substantially as the ambient wet-bulb temperature decreases if two-speed fan motors are used. Theoretically, operating at half speed will reduce air flow by 50 percent while decreasing horsepower to one-eighth of full-speed operation. However, ac tual half-speed operation will require about 17 percent of the horsepower at full speed as a result of the inherent motor losses at lighter loads. 

Recent developments in cooling-tower fan energy management also include automatic variable-pitch propeller-type fans and inverter-type devices to permit variable fan speeds. These schemes involve tracking the loaa at a constant leaving-water temperature. 

While the foregoing examples have dealt with applications on centrifugal pumps, the same sort of analysis can be made for reciprocating pumps, reciprocating compressors, or other rotary users hke cooling-tower fans. 

Figure 5.12 Severely corroded galvanized steel pipe supplying water to a cooling tower fan-bearing system. Where the galvanized metal has been consumed, only a brown rust patch is visible.

The condenser water pump will not start unless the cooling tower fan is already running. 

To simulate the next summer s condition the plant was run at the desired production rate and two cooling tower fans were turned off. It turned out that the cold water temperature rose to slightly above that predicted for the next summer. A thorough inspection of critical temperatures and the plant s operation indicated that the plant would barely make it the next summer. Process side temperatures were at about the maximum desired, with an occasional high oil temperature alarm on the large machines. 

Cooling tower fan motors must be properly selected for long life and trouble free operation. In addition, it is important to match the motor s enclosure to the application. Motors in draw through applications that are mounted directly in the air stream, for example, experience tough duty and must be of the totally enclosed type. Motors are categorized as follows ... 

In addition, the small motor fan attempts to blow air down over the motor and is no match for the much bigger cooling tower fan drawing air up and around the motor. The small fan in this case is useless- even counterproductive. This application needs a TEAO motor. Cooling tower manufacturers sometimes purchase special motors tailored for their application... special grease, seals, slingers, weep hole locations, epoxy coatings, etc.. Such features make an OEM replacement more desirable than an off the shelf replacement. 

Large towers often have the motor mounted horizontally connected to a right angle gear drive. The motor can be closely coupled- in the air stream- or connected with a drive shaft with the motor outside the air stream. Maintenance personnel typically prefer the external TEFC motor- when available- for its easier access. Cooling tower fans- like all fans- operate in accordance with the fan laws one of which states that the horsepower required to drive a fan increases to the cube of fan speed. 

The plant was running smoothly when a common trouble alarm on the cooling tower fans sounded. The outdoor operators investigated, discovered a fire, called the fire department, and initiated a plant shutdown. The fire was extinguished in about 2 hours. 

Cooling tower fans move large volumes of air therefore, they must be designed to do it economically. In addition, fan operation must be smooth. Vibration and air pulsation can be detrimental to mechanical equipment and tower structures. The materials of construction not only must be compatible with their design, but also capable of withstanding the corrosive effects of their environmental. 

Improving Cooling Tower Fan System Efficiencies Monroe, R. C. 

Cut Costs. . . Balance Cooling Tower Fans in Place Blake, M. P. 

Optimization minimizes the unit cost of cooling by minimizing the operating speeds of both the cooling tower fans and pumps. 

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