Issue link: https://www.e-digitaleditions.com/i/1212417
March 2020 / 27 STATIC VERSUS DYNAMIC VELOCITY CONTROL: WHICH IS MORE BENEFICIAL IN DILUTE- PHASE PNEUMATIC CONVEYING? This article focuses on the positive and negative reactions that effective air velocity has on a dilute-phase pneumatic conveying system. This article compares using static velocity control versus dynamic velocity control in a pneumatic conveying system and the impacts each control type has on blower speed, blower power, and effective velocity. Jonathan Thorn, Schenck Process Duty and process operation The pneumatic conveying dilute-phase design process ensures that the air mover, usually a positive-displace- ment (PD) blower like the one shown in Figure 1, is operating faster than required to perform the duty. The term duty is used to describe the value operators use to define the requirements for conveying a particular material at a certain rate across a given distance. As any of these variables change, the pressure and airflow required to operate the system change as well. However, in the design process, generally only the most-extreme W hether you're processing pet food, human food, or plastic pellets using a pneumatic conveying system, air velocity plays a large role in how effectively and efficiently your system operates. In dilute phase, as part of the design process, it's important to determine your particular material's minimum entrainment velocity (also known as pickup velocity), which is the minimum air velocity at which the material and air move within the system. Then effec- tive air velocity, which is a value greater than the mini- mum entrainment velocity, is applied to the system. The generated airflow from a blower or other air-movement device combined with the cross-sectional area of a pipe and line pressure create the effective air velocity that interacts with the process material. The effective air velocity picks up and transports each particle while it's suspended in the air, creating a dilute- phase flow. Pneu- matic conveying systems also move material via dense- phase flow, but for the purpose of this article, we will be focusing on dilute-phase flow only. Dilute-phase flow has been used over many years on thousands of systems. Each time dilute-phase flow was used, the most important aspect to ensuring proper sys- tem operation was that the system's effective air velocity was greater than the material's minimum entrainment velocity. It didn't matter if the effective air velocity was 5 percent or 50 percent greater; the system would work just as well at either rate with the user unaware of the excess. A system operating at an excessively high velocity is a possible unintended outcome when using this pneu- matic conveying design method, unfortunately, but we'll go over that issue later. FIGURE 1 Typical pressure dilute-phase conveying system Equipment 1) Positive-displacement blower 2) Clean air line 3) Feed airlock 4) Conveying line 5) Filter-receiver 6) Discharge airlock Material in Material out 1 2 3 4 5 6