Fan horsepower requirements

From the known liquid rate L and the value of L /Gg assumed, calculate the needed value of the air rate, Gg. This value converted to CFM at the fan inlet, together with the calculated pressure drop gives the fan horsepower requirements. 

Minimal pressure drop during adsorption, which reduces fan horsepower requirements... 

Fan Horsepower Required. A good rule of thumb for induced draft cooling towers is that each 8,000 cfm of air requires one blower horsepower. 

It is desired to strip chlorinated hydrocarbons from 2,700 gpm of water that is at a temperature of 50°F. The principal contaminant is 890 ppb by wt. trichloroethylene, which must be reduced to 8 ppb by wt. The stripping air is at a temperature of 55°F and 65% relative humidity therefore, litde change in water temperature will occur during stripping. In order to minimize fan horsepower requirement, which will provide a low operating cost, Intalox Snowflake packing will be used. 

Fan horsepower is obtained from Fig. 12-15. Connecting the point representing 100 percent of standard tower performance with the turning point and extending this straight line to the horsepower scale show that it will require 0.041 hp/fr of actual effective tower area. For a tower area of 1000 ft" 41.0 fan hp is required to perform the necessary cooling. 

Suppose that the actual commercial tower size has an area of only 910 fr. Within reasonable hmits, the shortage of actual area can be compensated for by an increase in air velocity through the tower. However, this requires boosting fan horsepower to achieve 110 percent of standard tower performance. From Fig. 12-15, the fan horsepower is found to be 0.057 hp/fr of actual tower area, or 0.057 x 910 = 51.9 hp. 

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. 

This method will approximate required fan horsepower. 

If the system differential pressure requirements are low, sometimes a fan can be used. A fan is limited to a maximum of about 65 inches of water (just over 2psi) in vacuum service or 77 inches of water in pressure service (just under 3psi). To estimate fan horsepower use the equations found in Chapter 6 in the section entitled Horsepower Calculation. In lieu of manufacturer s data use an adiabatic efficiency of 50% for initial work. 

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. 

As an example, consider the speed of a fan that is increased by 10%. Let s determined the revised horsepower requirement. 

The appropriate drive ratio is selected for each motor so that it is fully loaded when in operation- a distinct advantage over two speed, variable torque motors where the available horsepower is proportional to the square of fan speed while the required fan horsepower varies as to the cube of fan speed (from the fan law). As an example, a fan motor that can produce 40 hp at high speed can produce 10 hp at low speed while a fan that requires 40 hp at high speed only requires 5 hp at half speed. As a result, 1800/900 rpm motors are always 100 % oversized at low speed. And, since the motor usually operates at low speed most of the time, the... 

Economical tower sizes usually require fan horsepower between 0.05 and 0.08 hp/ft of ground plan area [19], and motors larger than 75 hp are not often used due to... 

The minimum number of the tube rows recommended to establish a proper air flow pattern is 4, although 3 rows can be used. The typical unit has 4-6 rows of tubes, but more can be used. Although more heat can be transferred by increasing the number of tubes, the required fan horsepower will be increased however, this balance must be optimized for an effective economical design. Tubes are laid out on transverse or longitudinal patterns however, the transverse is usually used due to the improved performance related to pressure drop and heat transfer. The tube pitch is quite important for best air-side performance. A typical representative tube arrangement for design optimization is for hare-tube O.D., tinned-tube O.D., and tube pitch ... 

The following procedure steps are given to calculate the required transfer bare tube surface, the number of tubes required, the number of tube passes required, the required airflow, and the fan horsepower. First, however, certain data are required, which are itemized in the following. 

Approximate Horsepower of Propeller Fans.—The horsepower required to drive any propeller fan may be represented by an expression of the form,... 

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