Use Vacuum With Discretion

By Andy Lovell
PIAB USA Inc.

Hingham, Mass.

Figure 1. Decentralized vacuum systems place the vacuum source — usually a vacuum generator — close to where it is needed. This not only improves productivity by improving response, but can save energy as well.

Figure 2. When evacuating a volume, maximum flow of a vacuum generator occurs initially, near atmospheric pressure. Flow then decreases as the volume evacuates and vacuum level increases. For most applications, a multistage vacuum generator proves most practical by producing high vacuum flow while consuming less compressed air.

Figure 3. A vacuum switch and shutoff valve keeps vacuum within a prescribed range and saves energy by reducing or eliminating the amount of compressed air used.

Designing or implementing a vacuum system for an important and often costly expenditure can cause many decision makers to encounter mixed messages, making the right choice unclear. In order to ensure a profitable investment, engineers should consider reliability, product safety, efficiency, response time, flexibility, and maintenance issues. Although several different types of vacuum systems exist, this article examines and compares the different characteristics of centralized and decentralized systems.

Centralized and decentralized systems
A centralized vacuum system consists of one vacuum source — usually a mechanical piston or vane type vacuum pump — mounted remotely and providing vacuum to multiple use points. This setup often serves as a plant-wide supply of vacuum provided by the single vacuum pump. Other examples include individual machines having their own dedicated vacuum pump, or a system where a machine may have pockets, or cells of multiple vacuum cups operating from a single vacuum pump. A centralized system is a common, traditional solution that is easy to integrate into a design and is easy to install.

A decentralized system places vacuum sources closer to the points of use. The typical vacuum source for these systems is a vacuum generator, which produces vacuum by routing compressed air through a venturi. Decentralized vacuum systems can range from a zoned network, where groups of vacuum cups that work together are isolated, Figure 1, to a system where each cup operate independent of each other. Advances in pump design allow for a vacuum generator to be installed at each vacuum cup. Decentralized systems also are easy to incorporate into a design and are easy to install. In many applications they can boost productivity through quicker response and save energy at the same time.

Go with the flow
Vacuum flow is an important concept that often is ignored or misunderstood when choosing and designing a vacuum system. As shown in Figure 2, a vacuum source achieves highest flow as it operates at or near atmospheric pressure. Vacuum flow then decreases as vacuum level increases — which occurs as air is evacuated from the system.

When a vacuum cup first makes contact with a part, it is the flow that creates the initial grip that securely grabs the part. Leaks created in the system — such as those produced when handling porous parts, or materials with a textured surface — rely on flow to maintain enough vacuum to hold the part. Professionals in the packaging-industry recognize the importance of vacuum flow and view it with greater importance than vacuum level. It is also the flow of a vacuum system that determines response time, and, therefore, cycle time.

Although it is detrimental to restrict vacuum flow, many technicians restrict it in material handling applications. For example, most companies that use centralized systems channel vacuum flow through tubing and manifolds. Restrictions from tubing and fittings are probably the greatest factor in reducing system performance and reliability.

Flow restrictions also create the need to oversize vacuum pumps to compensate for line losses. Of course, installing larger pumps increases the energy consumption of the system with no additional benefit at the vacuum cups. Tubing and manifolds in a centralized system also create additional volume that needs to be evacuated each cycle, then vented to atmosphere for each cycle. The combination of evacuation time and release time determines the cycle time for a material handling application.

Therefore, it may seem logical that smaller diameter tubing would produce faster cycle times by decreasing the system volume.

However, this practice impairs system performance by creating pressure drops and reducing available flow, both of which increase cycle times.

Actually, larger size vacuum tubing increases system performance by allowing greater flow. With a pressure drop through the system caused by restrictions, the vacuum level seen at the pump may not be the same as that seen at the vacuum cup, especially when part surfaces are porous.

This means the vacuum level present at the pump is not necessarily the same as the vacuum level present through the entire system. Assuming otherwise causes erratic performance and troubleshooting problems. In fact, the vacuum flows and levels at each cup will likely be different if tubing lengths are different.

A decentralized network, with little or no vacuum tubing, minimizes or eliminates the effects of line losses and pressure drops. Higher flows are realized at the vacuum cups, and cycle times are decreased due to the increased flow and smaller volume that must be evacuated and repressurized. The increased flow adds to the reliability of the system, and the pumps often can be downsized to provide similar performance while using less energy.

Making systems reliable
A vacuum system's reliability and safety relate to developing enough vacuum to pick up parts successfully and not drop them. With a centralized design, all vacuum cups are tied together into one volume. Therefore, when a low flow or low vacuum situation occurs, it affects all the cups in the system.

For example, if one cup is damaged, or leakage is otherwise present, the resulting vacuum loss will be seen at every cup in the system. Troubleshooting such an event is more difficult because the symptoms occur throughout the entire system, making it difficult to identify the source of the problem.

Conversely, with a decentralized system, each pump and cup combination works independently. Therefore, a leak at one cup has no influence on any other. Furthermore, a leak at any cup can be quickly identified and fixed.

Flexibility and energy efficiency
Decentralized systems allow users to individually control cups and pumps to adapt to changing requirements. If users choose not to operate specific cups in an array, they can simply shut down the generator that corresponds to that cup, thereby reducing operating energy.

One the other hand, if vacuum cups are linked together in a centralized system, a vacuum valve could be used to isolate each cup. However, this does nothing to reduce the energy consumed by the vacuum pump powering the entire system. Restrictions often are installed in the fittings in applications such as this, so that any cup open to atmosphere is limited in how much leakage it will induce. Again, the energy consumption remains constant, and the effect of this leakage is felt throughout the system.

Regardless of the layout of the vacuum system, other application concerns should be evaluated as well. It is important to consider the operating vacuum levels as they relate to the performance of the system and the amount of energy needed to operate at these levels. You should also give careful consideration to the size and style of vacuum cups and the effect they will have on the system.

Back to basics
Boyle's Law states that an inverse relationship exists between pressure and volume for a gas at constant temperature — the product of pressure multiplied by volume is constant if temperature remains constant. Imagine pulling a piston in a cylinder in order to create a vacuum in the cylinder by increasing its volume. As the internal vacuum level approaches a perfect vacuum, the volume, and therefore the distance that the piston needs to travel, increases exponentially toward infinity. This means to produce a deep level of vacuum, considerably more energy must be expended for only a small increase in vacuum level.

As a result, the relationship between energy input and vacuum level is not proportional, it is exponential. For example, it may require one unit of energy to pull vacuum to 18 in.-Hg, four units to pull 27 in.-Hg, and 400 units to pull 29 in.-Hg — a lot of energy for a small gain in vacuum level.

Theoretically, it takes 10 times the energy or more to receive only minimal gains in holding force at the cup with a sealed vacuum system. For an application with porosity or leaks, the effort needed to overcome leakage at deeper vacuums requires significant vacuum flows, and often is cost prohibitive.

Working at lower vacuum levels where a vacuum pump naturally generates more flow is a better option. On any system, the response time and release time will be enhanced working at lower vacuum levels, and the life span of the vacuum cups will be increased because of the lower stress from more moderate vacuum levels.

Inevitably, limitations on the design of a vacuum gripper require using smaller vacuum cups and higher vacuum levels. This is unavoidable in some cases. When possible, the preferred solution is to use larger vacuum cups and operate the system at lower vacuum levels. For example, a 2-in. diameter flat cup may have a rated holding force at 18 in.-Hg of roughly 16.5 lbf. By instead using a 3-in. diameter flat cup, which has 64% greater holding capacity, a much lower vacuum level could be used and still achieve the same holding force.

New pump technologies
Consider also using new pump technologies that improve vacuum system design. Vacuum generators are being produced today that are designed for optimum performance from lower supply ( compressed air) pressures. Operating at lower pressures saves energy by reducing the compressed air flow through the nozzles. Using a lowpressure generator builds safety into the system by effectively eliminating the impact of fluctuating supply pressures as seen in most manufacturing environments.

Using this type of vacuum source in a decentralized system can produce high levels of performance with a minimum of energy. Vacuum generators are also available in various sizes, with different characteristics to suit the vacuum and flow requirements of applications.

Energy-saving devices
Energy saving devices, such as shutoff valves, can save energy by eliminating consumption of compressed air by the vacuum generator. If a particular circuit has no leaks, vacuum can be maintained while consuming no energy. This is accomplished with a valve controlled by a vacuum switch. The vacuum generator operates until a predetermined vacuum level is reached. The switch then closes, which holds vacuum in the line and terminates compressed air flow to the generator. If leakage causes vacuum to drift beyond the predetermined setpoint, the switch senses this condition and again routes compressed air through the generator. This setup is recommended for use with sealed systems and should not be used in applications where part surfaces are porous. Porosity would introduce leakage that would cause the system to continually cycle on and off.

Justifying costs
Designing or redesigning a vacuum system should be done with the expectation of achieving some benefits to the performance of the system. Some of these benefits include higher efficiency, greater reliability, and improved safety. Each project should be evaluated to determine cost justification and payback on investment. Some justifications may be unique to each application, depending on the function of the machinery and the objectives of the customer.

Although few applications share performance requirements and operating parameters, a careful analysis of the vacuum system can help you build an optimum system for the application or improve an existing one.