Speed , fans

Near top speed, a fan may operate at a speed that is near or above the natural frequency of the wheel and shaft. Under such conditions, the fan can vibrate badly even when the wheel is clean and properly balanced. Whereas manufacturers often do not check the natural frequency of the wheel and shaft ia standard designs, many have suitable computer programs for such calculations. Frequency calculations should be made on large high speed fans. The first critical wheel and shaft speed of a fan that is subject to wheel deposits or out-of-balance wear should be about 25—50% above the normal operating speed. 

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. 

Inverters make it possible to control a variable-speed fan by changing the frequency modulation. Standard alternating-current fan motors may be speed-regulated between 0 and 60 Hz. In using inverters for this apphcation, it is important to avoid frequencies that would result in fan critical speeds. 

Variable-speed fan drives permit conditioned air distribution to be matched more closely to a building s needs. High-efficiency electric motors are used to drive the fans, saving as much as half the energy once used. Both variable-speed and high- efficiency motors are being applied even in residential air conditioning systems. 

Constant speed fans have the lowest first cost, quickest response, least efficiency, greater noise, and hardest start. Variable-speed fans have more efficiency, less noise, an easier start, greater first cost, and slower response. 

A dispersant fan sprayer has been built and tested statically on land and demonstrated offshore on a supply vessel while spraying water. Coverage rates of 4 miles /day are possible by boat, using high-speed fans that create a... 

Figure 7.23. Performances of fans with several kinds of controls (American Standard Co. Inc.), (a) A damper in the duct with constant-speed fan drive, (b) two-speed fan driver, (c) inlet vanes or inlet louvers with a constant-speed fan drive, (d) multiple-step variable-speed fan drive, and (e) hydraulic or electric coupling with constant-speed driver giving wide control over fan speed.

For one- or two-cell cooling towers the water flow is normally maintained, and to limit the fall in water temperature the air flowrate is controlled through the tower. The simplest method is an on/off control of the air fan by means of a thermostat with a sensing element in the water leaving the tower. Another relatively simple method of control is to use a two-speed motor together with a two-step thermostat to have high-speed or low-speed fan rpm, depending on the water temperature. 

Microprocessors that control processing improve the precision of the product (Chapter 3). It optimizes the process by controlling the machine cycle based on temperature and time, rather than time alone by using an infrared thermometer to monitor the mold s outer temperature. The molding cycle data for the oven and cooling chambers can be stored for different products and recalled when needed. Cycle time, oven temperature, major and minor axis speeds, fan and water spray times are typical of functions under complete computer control. 

Multiple-step variable-speed fan control, type d. is best applied with steam-turbine drives. In aplant with ac auxiliary motor drives, slip-ring motors with damper integration must be used between steps, making the installation expensive. Although dc motor drives would be less costly, few power plants other than marine propulsion plants have direct current available. And since marine units normally operate at full load 90 percent or more of the time, part-load operating economics are unimportant. If steam-turbine drive will be used for the fans, plot the power-input curve LMD, using data from the fan manufacturer. 

These sprayers usually include the same components as low-pressure or high-pressure sprayers, plus a high-speed fan. Nozzles operating under low-pressure dehver spray droplets directly into the high-velocity airstream. The air blast shatters the drops of pesticide into fine droplets and transports them to the target. The air blast is directed to one or both sides as the sprayer moves forward, or it may be dehvered through a movable nozzle. 

Variable-speed fan tor chamber pressurization (proportionally controlled)... 

Thus, these three laws may be used to determine the effect of fan speed, fan size, and gas density on flowrate, developed static pressure head, and horsepower. For two conditions, where the constants k remain unchanged, these equations become... 

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