Variable speed

A powerful stirrer, driven by a flexible driving shaft between the motor (I h.p.) and the stirrer, is depicted in Fig. II, 7, 3. The motor may be placed at a distance from the stirrer head and reaction vessel, thus enabling the assembly to be used for inflammable, corrosive or fuming liquids without damage to the motor. Furthermore, any laboratory retort stand and clamp may be used since the stirrer head weighs only about 250 grams. A variable speed control (500-2000 r.p.m.) is provided. 

Mention must also be made of magnetic stirring. A rotating field of magnetic force is employed to induce variable speed stirring action within either closed or open vessels. The stirring is accomplished with... 

Variable Air Flow Fans. Variable air flow fans are needed ia the process iadustry for steam or vapor condensing or other temperature critical duties. These also produce significant power saviags. Variable air flow is accompHshed by (/) variable speed motors (most commonly variable frequency drives (VFDs) (2) variable pitch fan hubs (J) two-speed motors (4) selectively turning off fans ia multiple fan iastaHations or (5) variable exit louvers or dampers. Of these methods, VFDs and variable pitch fans are the most efficient. Variable louvers, which throttle the airflow, are the least efficient. The various means of controlling air flow are summarized ia Table 3. 

Variable Flow Rate Conventional variable clearance volume and valve lifting devices are impracticable at high pressures and, should it be necessary to vary the flow rate, use has to be made of variable speed electric drives or magnetic clutches. Integral steam and gas engines have been used and Burckhardt (168) developed an hydrauhc drive to provide an integrated variable capacity machine, but its efficiency is less than that of a straight mechanical drive. 

In 1840 a hydrauHc power network, which involved large reciprocating pumps that were driven by steam engines, suppHed fluid power to London. However, concurrent technology in steam (qv) turbines and the electric generators outmoded such networks until hydrauHc systems were improved with the use of rotary pumps and oil. The rotary piston pump marked the transition from use of water to oil as the hydrauHc fluid (4). The use of vacuum-distilled, refined mineral oils were instmmental in the success of rotary axial piston pumps and motors such as the Waterbury variable speed gear... 

Pig. 6. Lurgi pressure gasifier A = variable speed drive B = coal distributor C = rotating grate. HP = high pressure. 

The Calcimatic is a patented kiln of Canadian origin that is radically different from other kiln types. It consists of a circular traveling hearth of variable speed, supported on two concentric tiers of rollers. Kiln feed of 12.7 mm is fed onto the hearth in a 2.5—10 cm bed from a preheater chamber. The kiln is usually fired with natural gas or fuel oil, although the option of using pulverized coal has also been developed. After great interest, resulting in sales of many units throughout the world, the popularity of the Calcimatic has ebbed because of disappointment in the unit s mediocre thermal efficiency. 

Speed Devices. Many displacement pumps are connected by variable speed drives. When these pumps are used as a time device on a homogenizer, the setting is fixed, ie, the maximum speed is limited in order to meet the requirements of pasteurization. 

Screen drive motor Variable speed coupling... 

If the allowance for control can be reduced, it should be. One option is the use of variable-speed drives. This eliminates the control valve and its pressure drop and piping. Its best appHcation is where a large share of the head is required for friction and where process demands cause the required flow to vary. 

Other energy considerations for cooling towers include the use of two-speed or variable-speed drives on cooling-tower fans, and proper cooling-water chemistry to prevent fouling in users (see Water, industrial water treatment). Air coolers can be a cost-effective alternative to cooling towers at 50—90°C, just below the level where heat recovery is economical. 

A variable-speed drive is usually used on the feed and cross-belt drives to exercise control in separator operation, although the speed is not usually changed once the optimum operating condition is estabUshed. Feed rates and the selection of the number of magnetic poles are usually deterrnined by preliminary laboratory tests. The mineral types involved in the feed largely determine the number of poles selected. High intensity cross-belt separators are frequendy used in combination with induced-roU or electrostatic separators. 

After the SO converter has stabilized, the 6—7% SO gas stream can be further diluted with dry air, I, to provide the SO reaction gas at a prescribed concentration, ca 4 vol % for LAB sulfonation and ca 2.5% for alcohol ethoxylate sulfation. The molten sulfur is accurately measured and controlled by mass flow meters. The organic feedstock is also accurately controlled by mass flow meters and a variable speed-driven gear pump. The high velocity SO reaction gas and organic feedstock are introduced into the top of the sulfonation reactor,, in cocurrent downward flow where the reaction product and gas are separated in a cyclone separator, K, then pumped to a cooler, L, and circulated back into a quench cooling reservoir at the base of the reactor, unique to Chemithon concentric reactor systems. The gas stream from the cyclone separator, M, is sent to an electrostatic precipitator (ESP), N, which removes entrained acidic organics, and then sent to the packed tower, H, where SO2 and any SO traces are adsorbed in a dilute NaOH solution and finally vented, O. Even a 99% conversion of SO2 to SO contributes ca 500 ppm SO2 to the effluent gas. 

Dried coal is pneumatically conveyed to feed bins, pressurized through lockhoppers, and fed to the refractory-lined fluidized-bed gasifier vessel by variable-speed screws. The gasifying agent, mixed oxygen and steam, is fed near the bottom of the gasifier. The bed operates at 2.5 MPa (25 bar) and... 

Many improvements have been made to streamline performance and to reduce machine operation labor. Some of these are tensionless jigs using variable speed electric motors with built-in drag for brakes, automatic reversing equipment, and automatic temperature and level controls. These machines are widely used for goods that are easily creased, such as fabrics consisting of filament acetate, heavy filament nylon, or cotton duck. They are also convenient for small dye lots and for sampling purposes. 

V ri ble Frecjuency Drives. An important energy by-product of soHd-state electronics is the relatively low cost variable speed drive. These electronic devices adjust the frequency of current to control motor speed such that a pump can be controlled direcdy to deUver the right flow without the need for a control valve and its inherent pressure drop. Eigure 11 shows that at rated load the variable speed drive uses only about 70% as much power as a standard throttle control valve system, and at half load, it uses only about 25% as much power. 

Eig. 11. Power saving for variable speed drives. Power input for variable speed adjusts with flow to naturally match the frictional losses. FIC = flow... 

In addition to energy conservation, the variable speed drives offer better control because of a faster response, ie, reduced dead band. They are also sometimes chosen for safety reasons because of elimination of the control station and accompanying valving. The capital saved by use of a smaller motor and elimination of the control valve partially compensates for the cost of the drive. 

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