It must be appreciated that the available power for a combined positive and negative pressure conveying system has to be shared between the two parts of the system (see Figure 2.5). If a positive displacement blower/exhauster is used, the pressure capability on both the vacuum and blowing sides will be lower than that which can be achieved with an equivalent machine used for the single duty. With a blower, for example, a pres- sure ratio of 2:1 is generally considered to be the upper operating limit, regardless of the application.
This means that for a positive pressure system the maximum delivery pressure is about 1 bar gauge (2 bar abs/1 bar abs = 2). For a negative pressure system the maximum exhaust pressure is about -0.5 bar gauge (1 bar abs/0.5 bar abs = 2). For a combined system the limit on pressures is approximately 0.4 bar gauge on blowing and -0.3 bar gauge on vacuum (1.4 bar abs/0.7 bar abs = 2).
A sketch of a typical system is given in Figure 2.5 and velocity profiles through such a system were presented in Figure 9.7. Even though a common air mover is used for both parts of the system, the diameter of pipeline employed for the vacuum side of the system is generally larger than that for the positive pressure side. If an improvement in performance is required or there is an imbalance in conveying distances between the two sections, two separate systems and a dedicated air mover for each would be better. By this means the pressure rating and air flow rate can be chosen to match the require- ments of each section more closely.
As a result of the performance characteristics of fans, conveying air velocities will be high at low material flow rates, and low at high material flow rates. A comparison of the operating characteristics of fans and positive displacement machines is shown in Figure 6.3. If a fan system is overfed, the pressure demand on the fan will increase. This will cause a decrease in volumetric flow rate and it is possible that the pipeline will block.
The ideal characteristics for an air mover, for a pneumatic conveying system, are those that result in no change in volumetric flow rate with increase in pressure. Positive displacement machines come close to this, as shown in Figure 6.5, and hence this type of air mover is widely used for pneumatic conveying systems. They are almost exclusively used for high material flow rate and long distance conveying duties.