Blowers positive-displacement

Blowers, positive displacement, rotary—by-passed for starting 30 25 150 3-8... 

Air scour blowers Positive displacement blowers supply low pressure air to the feed side to scour the membrane fibres during pre-backwash. 

Lobe-type blowers (positive displacement type) three lobe-types give less pressure pulsations on discharge side than twin lobe-type. 

Positive displacement blowers Positive displacement blowers are probably the most commonly used type of compressor for dilute phase conveying systems. They provide an ideal match, in terms of pressure capability, with the conventional low pressure rotary valve, and are a typical working combination on many plants. They can be used as vacuum pumps, or exhausters, as well as blowers. The principle of operation is illustrated in Figure 4.26. Twin rotors are mounted on parallel shafts within a casing, and they rotate in opposite directions. As the rotors turn, air is drawn into the spaces between the rotors and the casing wall, and is transported from the inlet to the outlet without compression. As the outlet port is reached, compression takes place when the air in the delivery pressure pipe flows back and meets the trapped air. 

Many users consider rotaiy compressors, such as the Rootes -type blower, as turbomachines because their behavior in terms of the rotor dynamics is very close to centrifugal and axial flow machineiy. Unhke the reciprocating machines, the rotary machines do not have a veiy high vibration problem but, like the reciprocating machines, they are positive displacement machines. 

FIG. 10-79 Two -impeller type of rotary positive-displacement blower. 

Vacuum systems (Fig. 21-12h) are characterized by material moving in an air stream of pressure less than ambient. The advantages of this type are that all the pumping energy is used to move the product and that material can be sucked into the conveyor line without the need of a rotaiy feeder or similar seal between the storage vessel and the conveyor. Material remains suspended in the air stream until it reaches a receiver. Here, a cyclone separator or filter (Fig. 21-12c) separates the material from the air, the air passing through the separator and into the suction side of the positive-displacement blower or some other power source. 

Piston, or positive displacement pumps, are well known and much used. Centrifugal pumps are not as well understood. Consequently, piston pump performance is sometimes expected from centrifugal blowers. The main difference is that positive displacement or piston pumps generate flow, whereas centrifugal pumps produce pressure. With a piston pump, the pressure will increase to the level needed to maintain the flow set by the piston volume and stroking speed. In contrast, centrifugal pumps produce pressure the flow will increase until the pressure drop, produced by the flow, matches the pressure produced by the pump. 

For higher differential pressures (or lower differential pressures where it is preferred not to use a fan), rotary positive-displacement blowers are used. These are excellent for conveying systems since they provide... 

Figures 12-106, 12-107, and 12-107A show an exploded view of a typical positive displacement blower. The impeller lobes rotate in opposite directions on parallel mounted shafts. Figure 12-107. One shaft serves as the drive shaft and drives the other through the gears. A timing hub allows for adjusting the timing angle of the lobes. The rotation may be either for upward, downward, or side gas flow. When liquid is entrained in the gas, the downward flow is preferred to assist in case drainage.

Rotary blowers are of the positive displacement type, and a typical lobe-type of machine is shown in Figure 8.34. The rotors are driven in opposite directions so that, as each passes the inlet, it takes in gas which is compressed between the impeller and the casing before being expelled. Machines of this type are capable of developing pressure differentials of up to 100 kN/m2 they are made in a wide range of sizes, with maximum throughputs of up to 20.000-30,000 m3/h. 

If air is used as the transporting medium, before it enters the system from the surroundings it is usually cleaned by a filter, and it may also be dried. The energy needed to move the air is provided by a blower. A positive-displacement blower is usually specified, since it can provide enough head to move the material in case the line plugs up.36... 

Figure 5.6. Components of pneumatic conveying systems, (a) Rotary positive displacement blower for pressure or vacuum, (b) A rotary airlock feeder for fine materials (Detroit Stoker Co.), (c) A four-compartment receiver-filter (Fuller Co., Bethlehem, PA), (d) A two-stage cyclone receiver, (e) The Fuller-Kinyon pump for cement and other fine powders. Powder is fed into the aeration chamber with a screw and is fluidized with compressed air (Fuller Co., Bethlehem, PA).

Figure 7.19. Some rotary positive displacement compressors, (a) A two-lobe blower, (b) Performance of a two-lobe blower (Rools-Connersville Co.), (c) A screw pump with one power and two idle rotors (Kristal and Annett, 1940). (d) Performance of 3.5" screw pump handling oils at 1150 rpm against 325 psig (Kristal and Annett, 1940). (e) Principle of the liquid ring seal compressor (Nash Engineering Co.). (0 A sliding vane blower (Beaeh-Russ Co.).

Typical auxiliary equipment is shown on Figure 5.6. The most used blower in pneumatic conveying is the rotary positive displacement type they can achieve vacua 6-8 psi below atmospheric or positive pressures up to 15psig at efficiencies of about 65%. Axial positive displacement blowers also are used, as well as centrifugals for large capacities. Rotary feeders of many proprietary designs are... 

Vacuum blowers (vacuum pumps) are used to draw waste nitrogen, carbon dioxide, and water vapor from the adsorber vessels during the evacuation step. There are two rotary-lobe blowers operating in series. They are positive displacement machines. A timing gear arrangement maintains the close tolerances that are essential for ef cient operation. 

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