Vacuum Conveying: Two Methods to Use as Vacuum Pump Energy Source

Vacuum conveying technology is a proven method of moving dry bulk solids and powders in a range of applications. It is widely used in food, pharmaceutical, chemical, and industrial environments. Frequently noted benefits of vacuum conveying systems include cleanliness due to the closed conveying lines, reliability, low noise, simple operation, gentle material transfer with dilute to dense phase conveying, easy-low cost maintenance, lower operating cost, and safety.
  The most common vacuum conveying systems use a batch process. The transfer line is sealed, a vacuum pump is activated, and the process material (powder or granules) is conveyed to a receiving vessel (Item C in Figure 1). When the batch is complete, vacuum is released, and the material is dropped or transferred from the receiving vessel to the desired destination. The system is again sealed, and vacuum is reintroduced to get the product flowing again. (See Figure 1)
  The selection of the vacuum pump is one of the most important elements of the system design. The vacuum pump needs to be able to quickly evacuate air from the line, pull and maintain enough vacuum to induce product movement, and provide a volume of air that is sufficient to transfer all the material needed in a given time.

Figure 1: Typical Vacuum Conveyor Process. 1. Bottom valve (B) is closed 2. Vacuum draws product from feed source (E) into the receiver/volume module (C) 3. Product collects in receiver module (C), while air is drawn through filter (A) and discharged to environment. 4. Vacuum is stopped (A); product is released via the bottom valve (B) as the filter cleaning pulse (G) is applied. 5. Close bottom valve- repeat process.

COAX Air-Driven Vacuum Pumps
The standard solution for many vacuum conveying applications is the multistage COAX vacuum pump. (See Figure 2) The COAX pump uses compressed air as the input, and through a Venturi effect, is able to generate a deep vacuum (approx. 22-in. Hg) while driving air volumes (SCFM) that are three times the volume of the compressed air. Through the use of multiple stages, a high vacuum is generated, as air flow is quickly ramped up. The COAX is built in cartridge form, which allows simple modular construction of many pump sizes. (See Figure 3) The modular pumps are small, lightweight, efficient, and easy to clean or change as needed. There are no electric components within the COAX pump, insuring long life and inherent safety in dusty or hazardous environments.

Figure 2: COAX Cartridge: three-stage operation uses Venturi effect to convert compressed air in to vacuum and high-volume flow for vacuum conveying applications.

Figure 3: piFLOW p vacuum pump, cross section view. Multiple COAX cartridges are stacked to provide energy efficient, increased air volume.


Regenerative Blowers
Regenerative blowers are electro mechanical devices used to move air by way of a non-positive displacement method. A positive displacement device uses an impeller design that will tightly trap a quantity of air and force it to move a given distance in a given direction. A regenerative blower also moves the air, however, the impeller design allows some air to pass over the blade tip and slip past it. This air is moved forward by another blade. The impeller is enclosed in a housing which has a gap between the tips of the blade and the housing. Air flows into the blower through an inlet port and out through an exhaust port. The rotation of the impeller is normally driven by a three-phase induction type motor, which may be controlled via a VFD (variable frequency drive) and sized to match the needs of the particular application.
  For vacuum conveying applications, the air inlet/suction side of the regenerative blower is connected to the head of the vacuum conveyor system receiver-post air filtration. In this, layout, the suction from the Regenerative blower takes the place of the COAX vacuum pump. The batch process is still used and the conveying cycle is similar. The primary difference between the two systems is that a Regenerative blower runs continuously, while the COAX pump is intermittent and shut off when no vacuum draw is needed. The regenerative blower runs continuously in order to provide air flow for cooling of the motor. A valve assembly/air diverter (Figure 4) is added to the regenerative blower system for this function. When product is being moved, air comes through the process, filter, and then goes through the regenerative blower. When the vacuum is not needed, and the receiver discharge is complete the diverter valves open both the blower suction side, and the receiver vessel to atmosphere. This allows the bottom valve to move freely, and provides a path for ambient air to flow through the regenerative blower for cooling. (See Figure 4 for the regenerative blower driven schematic.) As with all motors, periodic maintenance like bearing service or other services may be required.
       
Figure 4: Vacuum conveying system with regenerative blower as vacuum source

Comparative Analysis- COAX Vacuum Pump vs. Regenerative Blower for Vacuum Conveyors
The COAX vacuum pump or the regenerative blower can both be used in a wide range of applications. Selecting the vacuum source for a specific application normally involves the comparison of several criteria, such as:
a. Production rate: How much product is to be moved?
b. What are the characteristics of the materials? (bulk density, particle size, moisture levels, flow ability, is it fragile/friable)
c. What is the layout for the process flow path? What are the vertical and horizontal distances? How many pipe bends?
d. Is regulated compressed air available at the rate required to drive a Venturi pump system?
e. Is three-phase power available for a regenerative blower? Are noise, sparks, and exhaust air a major concern in the area where the system will be running?
f. What are the installed costs, and the long-term operating costs?

Conveying Phase (Dense or Dilute)
Your answers from questions a, b, and c will, in large part, help inform what conveying phase will be best for your application. In general terms, dilute phase means that there is a large portion of free air flow, compared to the amount of material flowing through the process pipe. This is normally used for material that is easy to pick up, is free flowing, and can travel at high velocity without any damage or ill effects. Conversely, dense phase means that the process pipe will be densely packed with product, which will move at lower velocities, and will often move in slugs though the line.
    COAX vacuum pumps draw a much deep vacuum, meaning they are the best choice for product that will be transferred in a dense phase. Note, the COAX vacuum pumps can also be operated throughout the dilute/dense spectrum. The phase can be changed by the way the product is fed into the process pipe. Aspiration at the product pick up point can be adjusted to optimize the flow characteristics for each material.
  With regard to the amount of material that can be conveyed, vacuum pumps can easily move up to 15,000 or more lb/hr, depending on the material density and the conveying distance and routing. Regenerative blower systems may be able to move even more material per hour, assuming that the material can be moved in a dilute phase, which is well matched with the electro mechanical blower systems.

Utility Availability (questions d and e)
Compressed air usage at industrial plants can be a complicated issue. The range of tools, controls, and equipment that can be driven pneumatically is virtually unlimited- due to such importance, its use is closely monitored by most plant and operations managers. If a COAX vacuum pump is selected for the vacuum conveying system- it is necessary to verify that peak plant air availability. If a plant does not have any compressed air, or if the legacy compressor is already near operating limits, then the Regenerative blower may be a practical option. Keep in mind that the Regenerative blower requires three-phase power, and that noise levels for the motor when running at full speed could exceed 80 dB. Because of this issue, regenerative blowers are often mounted in a remote location, with process lines running a distance. Additionally, if explosion-proof ratings are needed, an additional charge will be incurred for the special purpose built blower motor. COAX vacuum pumps are air driven and require no modifications for explosion proof areas.

Total Cost of Ownership (question f)
The purchase price for a vacuum conveying system using a regenerative blower is generally about 10-15% higher than the price for a complete COAX vacuum system of similar performance/same application. The three-way valve package and blower motor account for most of the difference. Operating costs are also competitive when calculated on an operational usage. Bear in mind, COAX pumps only consume energy when operating compared to the total run time requirement of a regenerative blower system.

Conclusion
Vacuum conveying is an excellent choice for much dry bulk, powder, and granulate conveying applications. It is a proven method, with thousands of successful applications in food, pharmaceutical, chemical, and general industrial sites. Selecting the right type of vacuum pump is an important part of the design process. For a majority of applications the COAX vacuum pump is a good choice. Simple operation, lower maintenance, operating and capital cost, low noise, and easy installation are common factors that favor the COAX vacuum pump. In applications where compressed air is limited, or not available, the regenerative blower driven systems are an excellent option.
    Timothy Rohr is technical sales manager, Piab USA, Material Handling Group. For more information, visit www.piab.com.

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