Fan power

Fig. 7. Control of fan performance with inlet vane control. SoHd lines marked A and N show normal performance without vanes (vanes wide open). As vanes are progressively closed, static and power curves are modified as indicated by dashed lines. Intersection ( - ) of the system resistance curve with these reduced pressure curves at points B, C, D, and E shows how imparting more spin to the inlet air reduces flow. Projecting points A to E vertically downward to the corresponding power curve locates fan power points A through E7 Power savings achieved over throttling control can be estimated by projecting points B through E vertically downward to the A power curve and comparing the value with that from the proper reduced power curve. To...

The cross-flow-tower manufacturer may effec tively reduce the tower characteristic at very low approaches by increasing the air quantity to give a lower L/G ratio. The increase in air flow is not necessarily achieved by increasing the air velocity but primarily by lengthening the tower to increase the air-flow cross-sec tional area. It appears then that the cross-flow fill can be made progressivelv longer in the direction perpendicular to the air flow and shorter in the direction of the air flow until it almost loses its inherent potential-difference disadvantage. However, as this is done, fan power consumption increases. 

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. 

Class Maximum leakage as % of total flow Increase in fan power... 

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). 

To give the designer some advice, many recommendations exist in literature. Some general recommendations can be found in prEN 13779 for designing the air distribution system for low energy consumption. These include giving a certain target level for power consumption [the so-called SEP class. SEP means specific fan power, the ratio between the input power of fan mo tors and the total airflow, in W/(m s" )]. 

This equation shows that the operating pressure drop is proportional to the square of the filtering velocity. For a fixed set of operating conditions, increasing filtering velocity to reduce the size of the collector will result in increases in pressure drop, fan power costs, and penetration and probably reduction in bag life. 

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.

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. ... 

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. 

Ducts convey conditioning air from point to point at a variety of speeds. Slow speeds result in large ducts, costly in themselves and in building space. High speeds result in noise and the need for high fan powers. A good basis for air conditioning is 6-7 m/s adjacent to the plant but, as discussed below, less at distant points. 

An approximate total system resistance can be estimated from the design average duct loss and the maximum duct length, adding the major fittings. However, this may lead to errors outside the fan power and it is safer to calculate each item and tabulate as shown in Table 27.1 for the system shown in Figure 27.8. Only the longest branch need be taken for fan pressure. 

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 ... 

Air coolers Tubes are 0.75-1.OOin. 00, total finned surface 15-20 sqft/sqft bare surface, U = 80-100 Btu/(hr)(sqft bare surface)(°F), fan power input 2-5 PIP/(MBtu/hr), approach 50°F or more. 

Recently much attention has been directed at the development of devices to control mosquitoes by using products in ambient temperature devices because of their increased safety and ease of use, especially during outdoor activities. This development has resulted in a variety of fan powered mosquito vaporizers and associated formulations which are now being marketed. These devices have limitations in performance that are imposed by the insecticidal activity of the active ingredient used. In order to overcome some of these limitations, we undertook extensive research to find new pyrethroids with higher vapor action which were highly active against mosquitoes. 

Another special application of adsorption in space is presented by Grover et al. (1998). The University of Washington has designed an in situ resource utilization system to provide water to the life-support system in the laboratory module of the NASA Mars Reference Mission, a piloted mission to Mars. In this system, the Water Vapor Adsorption Reactor (WAVAR) extracts water vapor from the Martian atmosphere by adsorption in a bed of type 3A zeolite molecular1 sieve. Using ambient winds and fan power to move atmosphere, the WAVAR adsorbs the water vapor until the zeolite 3A bed is nearly saturated, and then heats the bed within a sealed chamber by microwave radiation to drive off water for collection. Tire water vapor flows to a condenser where it freezes and is later liquefied for use in tire life-support system. 

Fan power consumption is the major operating cost and can be counterbalanced in part by greater investment in natural draft construction. In the majority of process applications, fen-operated towers are preferred. Very large installations such as those in power plants employ chimney assisted natural draft installations. A limited use of atmospheric towers is made in areas where power costs are especially high. 

Plant operations personnel generally purchase cooling towers rather than construct them themselves. The philosophy behind this policy is that it makes available to operators a wealth of practical knowledge directly applicable in the field. The operator must specify the amount of water and the temperature range required to handle a specific set of process conditions. It is the fabricator s responsibility to propose a system that will meet the operator-furnished conditions for the 5% wet-bulb in the plant locality. This also means that the fan power with which the operation will be accomplished will be guaranteed. 

A low air rate requires a large tower, while a high air rate in a smaller tower requires greater fan power. Limitations in air velocities are typically 300-500 fpm in counterflow towers, and 350-650 fpm in crossflow towers. 

The paper discusses operating limitations imposed by the turbine exhaust element and the alternatives presently available to the electric utility industry. It also presents tools for estimating dry tower plot area, fan power and circulating pump power requirements. It shows the savings in fan power which can be expected with a decrease in turbine-generator load and ambient air temperatures. It discusses expected maintenance costs and the owner s possible exposure with a large 1000 MW dry cooling tower system. The paper ends with an evaluation of the potential for lower dry tower system costs in the future. 12 refs, cited. 

---