Issue link: https://www.e-digitaleditions.com/i/1344205
March 2021 / 33 Using this information, we can estimate the pressure loss from air only in the system and solve Equation 4. ΔP air = fLeqρ air u air 2 = 77 ft 2 2D s (0.023) (139 ft) 0.075 lb m ft 3 2 3.55 in 12 in/ft = 2,404 lb m ft ⋅ s 2 4 of 4 and an equivalent length of 15 diameters for each, this results in each elbow being 4.5 feet or a total of 9 feet extra conveying equivalent length. Therefore, the total equivalent length, Leq, becomes 50 feet + 30 feet + 50 feet + 9 feet (from elbows) = 139 feet (with the equiv- alent line length, L, of 130 feet without the elbows). An excellent reference for equivalent lengths for pipeline elbows and their various forms can be found in D. Shrikant, P. Solt, G. Klinzing's paper "Understanding bends in pneumatic conveying systems." 5 There are numerous terms describing air (or gas) and solids flow and pressure conditions within a pneu- matic conveying system, and understanding some basic terminology before embarking on designing a system is necessary. Here are some commonly used definitions and other information that can be helpful. Conveying systems can be configured and classified in a number of ways depending on their function, operating pressure (positive, negative, combined, or closed-loop), and operating pressure magnitude. Perhaps the most common, and often misunderstood, descriptions are dilute-phase convey- ing and dense-phase conveying, which refer to the solids concentration inside the pipeline. Conveying can occur over a wide range of conditions bounded on one end by gas alone with no entrained solids and on the other end by a completely full pipe where the solids are essentially extruded through the line. Dust collection systems are an example of conveying systems that operate at very low solids loading where the performance is governed almost entirely by the gas flow. Dilute-phase conveying. This occurs when parti- cles are conveyed in the gas stream at a velocity that's greater than the saltation and choking velocities. This type of system is often referred to as a stream-flow sys- tem because particles are fully suspended in the gas stream. Most bulk solids can be conveyed in dilute- phase mode. However, potential adverse effects, such as pipeline wear or particle attrition, could result due to high conveying velocity. The term "dilute phase" also refers to the solids loading within the conveying line. Typically, a dilute-phase line conveys less than 15 pounds of solids per pound of gas at a relatively low pressure (generally less than 15 psig). Dense-phase conveying. This occurs when parti- cles are conveyed in the gas stream at a velocity that's less than the saltation velocity. Two general modes of flow can result in dense phase and are referred to as plug or piston-flow and moving-bed (dune) flow. Only certain bulk solids can be efficiently conveyed in these two dense-phase modes. In plug flow, coarse, permeable bulk solids like pellets or coffee beans can be reliably conveyed, whereas with moving-bed flow, fine, impermeable bulk solids like cement or pulver- ized fuel ash can be effectively conveyed. The term "dense phase" also refers to the solids loading within the conveying line. Typically, a dense-phase line conveys greater than 15 pounds of solids per pound of gas at a higher pressure (generally greater than 15 psig) than a dilute-phase system. Major advantages of a dense-phase system versus a dilute-phase system include reduced conveying gas velocity (yielding less pipeline wear and particle damage), lower gas requirements, lower operating (energy) costs, and the ability to convey long distances. Pressure versus vacuum systems. A positive- pressure conveying system uses gas pressure above atmospheric pressure to entrain the bulk solids and transport the material to either one or multiple des- tinations (often at atmospheric pressure). Conversely, a vacuum conveying system picks up solids at near atmospheric pressure, often from multiple locations, and discharges the material into a vessel that's at a pressure less than atmospheric (hence the name "vacuum"). Positive-pressure systems can operate at high-pressure levels and convey materials over long distances, such as up to a few thousand feet. Vacuum systems are typically limited to transporting mate- rial less than 300 feet, though some systems have greater capabilities. Vacuum systems also are well- suited for handling dusty or toxic materials, given that any leakage in the pipeline will be inward. There are specialized systems that incorporate features of both positive- and negative-pressure con- ditions, such as pull–push systems used in ship and railcar unloading equipment. Pneumatic conveying terminology