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2) Multi-Speed Fan Control

It is possible to further reduce total annual energy consumption by the fan drive equipment, but over a wider range of wet-bulb temperature conditions. To do so requires increased operational flexibility of the tower. Thus the specification of multi-speed motors must be considered. The most commonly used such motors in cooling towers are of the two speed type with either 4/8 pole or 4/6 pole. Such motors are capable of operation at full and half speed (4/8) or full and two-thirds speed (4/6).

The majority of cooling tower installations incorporate multi speed fan drive motors. These, all too often, operate the fans throughout the year in accordance with ambient wet bulb temperature, heat load capacity, or required cold water temperature. Without a special additional cost, the energy conservation is very large if the speed selection is automatically controlled by using a microprocessor or the operator has full control discipline. This control is most effective to apply to the cooling towers having 3 or more cells installations.

It must be pointed out that the yearly average cold water temperature is approximately the same, and therefore so is plant production, for all modes of operation. Obviously then, considerable thought should be given to several modes of operation before one is chosen. Again, it must be pointed out, that the yearly average cold water temperature will be higher than if the fans had been allowed to run constantly at full speed. This may reduce plant production somewhat. The benefits of reduced energy consumption must be weighed against any loss in production.

To achieve the power savings, requires continuous monitoring of the cold water temperature and manual operation of the fans. This may not be practical, and may reduce the overall cost effectiveness of the installation. For large, twin and three cell installations simple direct control by fan thermostats, may be suitable. Possibly one for each fan or a multiple unit depending on the range covered, triggered by cold water temperature, would be suitable.

As with any such system, safeguards must be built into the thermostat logic to avoid exceeding the maximum number of starts a motor may make in any one hour. Such a situation can occur during fairly stable but fluctuating ambient conditions especially if the system lag times are small. Further, if multi-speed motors are considered, care must be taken to prevent the fan from operating at any of its low critical speeds. If attention is not paid to this aspect of design, the fan could suffer damage and possible complete failure. To assure long fan life, the cognizant engineer should contact the fan engineer and obtain the necessary information and recommendations.

Below tables show the trace of cold water temperature varying per the entering wet-bulb temperature under the same thermal conditions in the range (23.4 ^oF or 13.0 ^oC) and the 12.8 degree of constant pitch angle.

Note that the fan power is increasing as the ambient wet-bulb temperature entering into the cooling tower is gradually decreased. This is due to the decrease in the exit air temperature (increase in air density and actual static pressure.

The fan operations can be controlled to save energy considering the required cold water temperature above. Of course, the actual cold temperature could be varied when to operate the fans below sequence because the isolation of individual cell air streams is ineffective due to non-partition between cells. The full speed fans will receive a substantial amount of air from the cells that are OFF or at 2/3 speed due to the differential pressure between cells.

3) Variable Speed Fan Control

The most efficient method of energy saving is obtained by varying the speed of driven machine. The speed of motor shall be automatically adjusted by the use of adjustable frequency power. A variable frequency controller simulates the normal AC voltage sine wave. The adjustable frequency drive converts 60 hz (or 50 hz) power to 0 - 100 hz power which, when frequency controlled, determines the synchronous speed of the standard induction fan motor. This is accomplished with solid state electronics. Any standard AC fan motor may be speed regulated, manually or automatically, in the 0 - 60 hz speed range for air flow rate control.

It is obvious that the power could be most significantly saved with the variable speed control. However, most careful consideration does need to be given to the resonant or critical speed of the fan assembly. That is, one would not like to operate the fan for any length of time at its critical speed which might cause damage. Due to these dynamic characteristics, cooling tower fans are not suitable for variable speed service. Of course, the motor speed is restricted with the minimum speed (generally 500 to 600 RPM) of gear reducer, since the internal parts including the bearings can not be lubricated at the low speed.

4) Variable Pitch Control

The previous discussion was based on the adjustable pitch fan, which is delivering a constant amount of air flow at the preset pitch angle and the pre-selected fan speed. Power consumption changes only with changes in air density. The previous tables show the actual fan power is varying the ambient wet-bulb temperature. This is only due to the changes in the air density.

The pitch of variable pitch fan is automatically changed in operation as per the heat loads and air density. This fan provides the precise amount of airflow to control the outlet cooling water temperature and save substantial amounts of energy at the same time. In the years of low energy cost, the impetus to use the variable pitch fans has been for precise temperature control, generally to within four percent or less of the set point. However, there is new interest in variable pitch fans for the energy saving potential as well.

Another one of the major advantages of the variable pitch fan is constant speed. Constant speed control eliminates the problem if the fan speed is at least ten (10) percent away from a critical speed and also eliminates the frequent stopping and starting normally associated with manually pitched fans under fluctuating loads. This constant speed increases the service life of drive components such as motors, couplings and gear reducers. The inrush current when starting the standard fan motor is usually 600 - 700% of full load current. This means that the motor will experience 6-7 times the normal mechanical stresses and approximately 36 times the normal heating during the starting period. If the starts are frequent and/or prolonged due to the fan inertia, the motor will fail prematurely. This is due to the break down in electrical insulation caused by these abnormal thermal/mechanical stresses. This same peak stress is also transmitted through out the coupling shaft and the gear reducer. This cyclical stress pattern will eventually reduce the fatigue life of these components.

An automatically adjustable pitch fan is a very simple mechanical device. The blades are attached to cammed shafts which are rotated through a ring when a central actuating rod is moved up and down. A main load spring moves the rod upward and compressed air, operating against a diaphragm, overcomes the spring pressure and moves the rod downward. Compressed air is introduced to the diaphragm by a rotating air union. A force diagram is shown in below.

As the blade moves the air, the aerodynamic moment (clockwise) tries to feather the blade. The hub spring creates an opposing moment (counter clockwise) to make the blade do work. This is a fail safe mechanism so that if the air pressure on the diaphragm fails, the fan operates as a fixed pitch fan providing design air flow. Fans can be assembled to move to maximum or minimum air flow on loss of air signal. The initial spring preload has to be sufficient to keep the blade from feathering.

To reduce the air flow, or even reverse the air flow direction, air pressure is exerted on the diaphragm to oppose the hub spring and decrease blade pitch. When the blade pitch is about minus 10^o, no work is done and essentially "zero" flow is attained. Minimum air velocity obtainable is approximately 50 - 100 fpm. If the hub has its pitch stops adjusted for reverse flow, the air is directed downward and can be as much as about 60% of the upward flow at the same horsepower. The decrease in flow capability is because of poor efficiency in the reverse pitch mode.

A typical fan hub mechanism consists of Hub Spring, Diaphragm, Piston, Blade shafts with eccentric actuator, Rotary air joint, Valve positioner, etc. The blade shafts or axles hold the fan blade and have an eccentric engage the groove in the piston. As the piston moves up or down a twisting motion is imparted to the blades, changing pitch. The rotary air joint is the static/dynamic interface between the rotating fan and its control air system.

A typical variable pitch hub requires a 3- 15 psi control signal and operates the blades from some maximum pitch down to "zero" air flow. It typically fails to maximum flow if the control signal is interrupted but can be made to fail to minimum or negative flow. Most hubs are capable of 45^o total pitch travel and perform as shown in below figure.

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