Positive pressure systems

Although positive pressure conveying systems discharging to a reception point at atmospheric pressure are probably the most common of all pneumatic conveying systems, the feeding of a material into a pipeline in which there is air at a positive pressure does present a number of problems. A wide range of material feeding devices, however, are available that can be used with this type of system, from venturis and rotary valves to screws and blow tanks. 

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Dilute Phase Conveying. Dilute conveying systems, sometimes called disperse conveying or stream conveying, operate as positive pressure systems at pressures up to 100 kPa (14.5 psig), or as negative pressure systems (vacuum conveying) at pressures up to —50 kPa (—500 mbar). 

Positive pressure system normally used when conveying from one pickup point to several discharge points. 

In the positive pressure system air is blown into the pickup duct often up to a cyclone with atmospheric vent. 

Even though 15 psi is possible from a blower, most positive-pressure systems are limited to 10-12psi differential pressure because of limits of the rotary air lock feeder valves (deflections in shaft and bearings and increased blowback air). 

Figure 4-4 A positive pressure system which can deliver product to many receivers.

The pressure drop in the ductwork is first figured on the basis that no solids are present. This result is then corrected using Figure 8-1 1 to obtain the true pressure drop. The weight ratios of solids to air vary from 1 1 to 20 1. The flow rate of the solids in the positive-pressure system is much greater than for the typical negative system.37 The pressure drop in the fiber filters is generally about 4 in H20 (100 kg/m2), and that in cyclone separator varies from 1 to 3 in HzO (25-75 kg/m2).37... 

A positive-pressure system will be used because the delivery is to a number of different points. 

This will be a positive-pressure system, since each dryer will feed 5 extruders. It will be designed to run at the same rate as each dryer, 9,210 lb P.S./hr. The vertical rise is 30 ft and the maximum distance traveled is 130 ft. Because powder is being conveyed a cyclone separator will be specified for each of the extruders. The blowers should be designed to have an LHP of 7.2 and a BHP of 8.4. The power required to operate both is 12.6 kw. 

Rotary valves have been specified for the receivers of the vacuum pneumatic conveying systems (RV-601 and RV-602) and for the feeders (RV-40 1) and receivers (RV-501) of the positive-pressure system transferring powder from the dryers to the extruders. Perry72 gives some recommended sizes for rotary valves. The power required by all these valves will probably not exceed 2 kw and will be ignored. 

The ability of people to obtain information about private-good risks and take cost-effective precautions against them is illustrated by the public response to radon exposure. Once information about radon exposure risks became public and geologists asserted that the northeastern area of the United States was prone to radon release, companies were quickly started to measure residential radon levels and to install positive pressure systems to keep radon from seeping into basements. 

Linear velocities, carrying capacity as cuft of free air per lb of solid and power input as HP/tons per hour (tph) are listed in Table 5.1 as a general guide for a number of substances. These data are for 4-, 5-, and 6-in. lines for 8-in. lines, both Sat. and HP/tph are reduced by 15%, and for 10-in. by 25%. Roughly, air velocities in low positive pressure systems are 2000ft/min for light materials,... 

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