Inlet vane controls

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...

Figure 12-135A. Power savings comparison between inlet vane control and outlet dampers. (Used by permission The Howden Fan Company.)...

Plot the power-input curve AFED for inlet-vane control on the forced-draft fan or inlet-louvre control on induced-draft fans. The data for plotting this curve can be obtained from the fan manufacturer. 

Inlet vane controls should be inspected for linkage or bearing looseness, and adjusted or replaced before they cause more trouble. The minimum air flow should be reset to 10% of maximum to proteet recuperators and any other air-cooled devices. 

Inlet vane controls on blowers and fans should be checked for looseness of linkages and bearings, and corrected or replaced before they cause more trouble. Make sure that inlet screens, filters, and silencers are in place, tight, and cleaned. Do not use any flammable cleaning compound. 

The hood pressure control system functions as follows When the pressure in the hood rises, which may occur for example as a result of a decrease in the exit gas flow or an increase in the cooling air flow, the controller increases the volume of air delivered by the exhaust air fan. It does this by adjustment of a damper or an inlet vane control unit or by varying the fan drive motor speed. Conversely, when the pressure in the hood goes down, the air delivery rate of this fan is reduced by the control system. Thus, with the aid of the hood pressure controller, the exhaust air fan performs the function of a pressure relief valve. 

As the air pressure to be developed by the fans varies greatly with the specific load of the cooler, fans having not too flat a characteristic curve (pressure-volume curve) should be chosen. Simple damper control is uneconomical it is preferable to use inlet vane control. 

An inlet vane control system (Fig. 29) comprises a static guide-vane unit which is installed in front of the fan s impeller and whose radial vanes can be swivelled by means of a control device so as to vary the inlet air flow conditions. These vanes deflect the inflowing air in the direction of rotation or in the opposite direction. As a result of this preliminary guidance, the entry losses are substantially less than those associated with ordinary damper control. 

The conditions for fans with inlet vane control which are, respectively, well adapted and unfavourably adapted to the operating requirements are indicated in Figs. 31 and 32. In these diagrams the static pressure of the fan has been plotted against the volume flow for various guide-vane settings. The dot-dash lines indicate the efficiency of the fan. 

On variable-air-volume (VAV) systems, install variable-frequency drives (VFDs) to control supply fan and return fan speed as a function of system pressure set-point in place of inlet-vane control. This is applicable for airfoil or backward-curve fans, not forward-curve fans, which have relatively efficient unloading with inlet-vane control. 

Vane control Fan inlet vanes alter fan perform.ance by controlling the swirl. 

Inlet vanes Specially-designed adjustable vanes inserted in the airstrearn entering a fan inlet to control fan performance by producing a swirl of the gas in the direction of the rotation of the impeller. 

Stationary or moveable inlet vanes, if used, attach to the inlet beU. These are also dieformed for uniformity and are welded in place for the stationary type and attached to a spider that is attached to the beU for the movable type. Figures 12-127A-B. The discharge damper. Figure 12-127C, is used for system volume and pressure control and is discussed later in regard to comparative performance with inlet vanes. 

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.

CONTROL (SPEED SUCTION VALVE. INLET VANES) ... 

Steam turbines with compressors are used for providing process gas flow at a required pressure in high throughput processes. The process demand is determined by a pressure controller, which adjusts the setpoint on the turbine speed controller. In smaller processes, fixed speed compressors may be used by adjusting either an inlet or discharge valve to achieve pressure control. It is more energy efficient to adjust an inlet valve, or better yet to adjust inlet vanes which provide a pre-rotation to the gas. However, adjustment of speed is the most energy-efficient method control. 

Figure 19-10 Performance curve of vapor compression evaporator with inlet guide vane control.

Speed control for the compressor of an evaporation system with linear characteristics is shown in Figure 19-11. Inlet guide vane control permits greater capacity turndown end allowence for evaporator fouling before the surge condition is reeched. 

The rate of air delivery by the cooling fan can be adjusted by manual control of the inlet vanes. Such adjustment, however, is necessary only in the event of major changes in the rate of clinker output from the kiln. If the kiln is to be run at reduced output for a long period, one or more compartments of the gravity cooler can be shut off in order to save electric power consumption. 

High temperature in the mill may, however, make it necessary to cool the interior before it can be entered for inspection. In that case the fan will have to be switched on again, but taking care that it is started with its control damper or inlet vanes closed and that these are subsequently opened up only to such an extent as is necessary to lower the temperature sufficiently. 

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