Fan power requirements

Figure 9-121. Fan power requirements for components of a typical counterfiow induced draft cooling tower. Used by permission of Groitein, E. E., Combustion, Nov. (1957) p. 38.

In the composite boiler, a watertube chamber directly connected to a single-pass shell boiler forms the combustion space housing the fluid bed. In order to fluidize the bed the fan power required would be greater than that with other forms of firing equipment. 

FIGURE 7.17. The increase in fan power requirement due to particulate fouling... 

Relatively low oxygen requirements result in lower gas volumes and as a consequence the plant size and the fan power requirements are minimised. 

Heuristic 56 For an air-cooled exchanger, the tubes are typically 0.75-1.00 in. in outside diameter. The ratio of fin surface area to tube outside bare am is large at 15-20. Fan power requirement is in the range of 2-5 Upper million Btu/hr transferred, or about 20 Hp per 1,000f of tube outside bare surface (fin-free) area. Minimum approach temperature is about 50°F, which is much higher than with water-cooled exchangers. Without the fins, overall heat transfer coefficients would be about 10 Btu/hr ft °F. With the fins, U = 80-100 Btu/hr fF °F, based on the tube outside, bare surface area. 

The 9-ft 0-in. ID column design clearly is preferred. Not only does it have a slightly lower capital cost, but its fan power requirement is much lower than the 9-ft 6-in. ID column. Because the 9-ft 6- in. ID tower used 16,200 SCFM of air at 2.5 in, H2O pressure, and the 9-ft 0-in. ID tower uses only 10,800 SCFM of air at 1.8 in. H2O pressure, the smaller-diam-eter column requires less than one-half the fan horsepower. 

Fan Rating. Axial fans have the capabiUty to do work, ie, static pressure capabiUty, based on their diameter, tip speed, number of blades, and width of blades. A typical fan used in the petrochemical industry has four blades, operates neat 61 m/s tip speed, and can operate against 248.8 Pa (1 in. H2O). A typical performance curve is shown in Figure 11 where both total pressure and velocity pressure are shown, but not static pressure. However, total pressure minus velocity pressure equals static pressure. Velocity pressure is the work done just to collect the air in front of the fan inlet and propel it into the fan throat. No useflil work is done but work is expended. This is called a parasitic loss and must be accounted for when determining power requirements. Some manufacturers fan curves only show pressure capabiUty in terms of static pressure vs flow rate, ignoring the velocity pressure requirement. This can lead to grossly underestimating power requirements. 

When the mean annual temperature is 16.7°C (30°F) lower than the design dry-bulb temperature and when both fans in a bay have automatically controllable pitch of fan blades, annual power required has been found to be 22, 36, and 54 percent respectively of that needed at the design condition for three process services [Frank L. Rubin, Tower Requirements Are Lower for Air-Cooled Heat Exchangers with AV Fans, Oil Gas J., 165-167 (Oct. 11, 1982)]. Alternatively, when fans have two-speed motors, these dehver one-half of the design flow of air at half speed and use only one-eighth of the power of the full-speed condition. 

Appendix Rules of ihumb for every day use 12/323 Power requirements for pumps 12/323 Power requirements for lifts 12/323 Power requirements for fans 12/323... 

Flue gas recirculation (FGR) is the rerouting of some of the flue gases back to the furnace. By using the flue gas from the economizer outlet, both the furnace air temperature and the furnace oxygen concentration can be reduced. However, in retrofits FGR can be very expensive. Flue gas recirculation is typically applied to oil- and gas-fired boilers and reduces NO, emissions by 20 to 50%. Modifications to the boiler in the form of ducting and an energy efficiency loss due to the power requirements of the recirculation fans can make the cost of this option higher. 

The power consumed to operate a wet electrostatic precipitator is much less than that required by most other methods of control. There are four areas in which power is consumed (1) electrostatic power, (2) fan power, (3) insulator heating power, and (4) pump power. The total electrostatic power input required for operation is 0.8 to 1.0 kW/1,000 ft of collection area. A comparable piece of equipment is a venturi scrubber with 50-in.wg pressure drop. The power required for this installation would be 6 to 7 kW/1,000 cfm. This would mean that approximately seven times the power would be needed to achieve the same amount of cleaning with a venturi scrubber as opposed to using a precipitator. 

For isentropic or nonisentropic flow, the fan shaft power is greater than the power required for the flow through the impeller. 

Leakage may require an increase in the fan power in order to ensure that the required design air distribution to all distribution points is achieved (See Table 9.12). 

Rubin, F. L., Power Requirements Are Lower for Air-Cooled Heat Exchangers with Variable-Pitch Fan Blades, Oil and Gas Jour, Oct. 11, (1982). 

Regardless of type, overall fan action must depend on a rate of change of gas momentum in a tangential direction. Without this change in momentum, no resisting torque can exist, and no fan power input is required or absorbed. ... 

Propeller fans are of low capital cost for the volume moved but are used in applications where the resistance is very small such as non-ducted openings through partitions. The power required increases with resistance. Their pressure-volume characteristic changes with the relative position of the blades and mounting plate. 

Large fans are required for boiler plant, furnaces and large air-handling units. These fans can now be fitted with variable-speed motors to reduce electricity consumption and maximum power, albeit at additional cost. 

Select a fan and calculate the power requirements from the known air volume and the pressure drop characteristics of the selected pack. 

The forward-curved fan blade increases the tangential velocity considerably (see Figure 27.5b). As a result, the power required will increase with mass flow, although the external resistance pressure is low, and oversize drive motors are required if the system resistance can change in operation. The backward-curved fan runs faster and has a flatter power curve, since the air leaves the blade at less than the tip speed (see Figure 27.5c). 

Filtration apparatus is available to remove any size, but the very fine particles require a deep, bulky and expensive filter, which itself sets up a high resistance to air flow and therefore requires high fan power. A practical balance must be reached to satisfy the requirements ... 

Example 28.3 Refrigerant For the same duty, liquid R.22 enters the expansion valve at 33°C, evaporates at 5°C, and leaves the cooler at 9°C. Fan power is 0.9 kW. What mass flow of refrigerant is required ... 

It is known that the power required to drive a fan depends upon the impeller diameter (D), the impeller rotational speed (co), the fluid density (p), and the volume flow rate (0. (Note that the fluid viscosity is not important for gases under normal conditions.)... 

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