This article will focus on optimizing the demand-side so the centralized “supply-side” (the vacuum pumps and controls) can then run at a lower energy and maintenance cost. First, I will start with a simplified model of a vacuum pump system demands. See Figure 1 for a one-pump/one-demand simplified system. See Figures 2-6 for some typical controlled and uncontrolled demands. The symbol with the three lines is an orifice, a hole essentially. I am defining three types of system demands adding up to the total demand on the vacuum pump.
If you want to understand vacuum systems, you have to get out of the ruts, and slog through the mud and bounce over the rocks a bit. If you’re a “compressed air person”, think outside the box for a few pages with me. I am going to borrow some terms from the “pump people” to explain how vacuum systems are similar, yet different from compressed air systems. There are several ruts to get out of. Remembering what changes and what doesn’t, what is controlled, and how to design systems for optimal energy consumption.
Industrial process operating loads and optimal set points are not usually accurately known at the time of design, so often there is significant mismatch between equipment and the process it serves. To overcome this uncertainty, designers typically oversize equipment. Over time, process changes and equipment efficiencies decline, so equipment might be operating less efficiently than at start-up. Or, equipment can be undersized, thereby hampering the entire system and causing other inefficiencies to compensate. For instance, too much steam usage in the dryer section of a paper machine can occur because of inadequate vacuum at the wet end.
Multiple vacuum pumps can be running mostly “dead-headed” in the many production systems that don’t require constant flow. Any system that evacuates a small volume and then holds a product down while it is being machined, or sucks a bag shut to seal will spend the majority of its time not moving much mass of air. This type of operation is found everywhere in secondary wood processing, machining, food packaging, and many other industries. Anywhere vacuum is used as a motive force or to evacuate a small volume repeatedly. This article will apply to any of these types of systems- and not apply to constant-flow vacuum applications in the process industries.
Industrial vacuum systems are a challenge to optimize. They have more distribution system variables to balance than a compressed air system does. Vacuum systems conveying particulate are sensitive to velocity. If the velocity is too high, pressure drop results. If it’s too low, particulate doesn’t stay in suspension, and there can be compliance and safety problems. For instance, when conveying wood or other explosive dust, dropping below “critical” velocity allows dust to accumulate in the bottom of the duct, creating an explosive hazard. Entire plants have burned to the ground, and lives have been lost due to these types of incidents. According to OSHA1, wood is not the only explosive material in dust form.
Vacuum systems are considered “black magic” by most plant engineers, even more so than compressed air. Terms like icfm, cfm, torr, and Nm3/hr get bandied around and confuse us all. What plant engineers know is what works. If they run vacuum pump X at vacuum level Y, everything works. That is a hard thing to change if there are inefficiencies in the system, even when an audit is recommending change. One of the biggest opportunities I run into for savings is the consolidation of multiple vacuum systems running at a lower absolute pressure (higher vacuum) than is really needed. Therefore, educating the customer is critical.
