Centralized vs Decentralized Vacuum Systems

By Nick Kroupa, Vac-Cubes 

One of the many choices, when using  vacuum for handling materials, is whether to use a centralized or decentralized system. Centralized vacuum systems use one main vacuum pump or generator to operate multiple suction cups. A decentralized vacuum system uses a smaller pump or generator for each suction cup.

Each system has its advantages and disadvantages. One advantage of a centralized system is the ease of implementation due to using only one pump. It’s not necessary to have plumbing for extra lines or find space to put many smaller generators. The pump will need to be sized to accommodate an increase in required flow when suction cups fail to make contact or fail to establish a good seal. Adding a limiting orifice to the suction cups reduces the amount of air that can be drawn through the cups and can help reduce the size of the vacuum pump. 

A centralized system benefits from placing the vacuum generator further from the work. Another benefit is that there is just one part to install and maintain.

Some centralized vacuum systems can provide vacuum to multiple points on a large packaging machine. These may use large mechanical vacuum pumps or regenerative blowers. They require valving on the vacuum side to control when each is supplied vacuum and sometimes compressed air to “blow off” the part faster.

Centralized systems can also indicate that several vacuum generators are being used, with each operating a clustered group of cups. One group could pull a cardboard box out of a hopper while another group is used to pick and place multiple items inside that box.

There are some possible downsides to a centralized vacuum system. The larger pump may have a high cost and require the use of more energy such as electricity or compressed air. A pump failure may cause the whole system to go down.

With a decentralized vacuum system, each suction cup is paired with its own single-nozzle, venturi-type vacuum generator. Bernoulli’s Principle states that faster moving fluids correspond to lower pressures. The venturi sends compressed air through a nozzle, thereby increasing the speed and decreasing the pressure.

 These generators are small, supplying vacuum flow to a single cup, resist clogging, and are easy to place at the point of use. This eliminates the problem of losing the grip on all parts because one cup failed to seal. Each generator needs only supply vacuum flow for one cup and is sized much smaller than a centralized vacuum pump.

A downside of the decentralized system comes from the additional number of vacuum generators. Each generator requires plumbing and adds weight to the cup. Some applications, like End-of-Arm Tooling (EOAT) and high-speed pick and place, need to keep the weight as low as possible. The installed cost of multiple venturi generators may exceed the cost of a centralized pump.

A real-life example is detailed below. 

Near the start of the COVID-19 pandemic, companies were cranking up their usage of personal protection equipment (PPE) and related items. I had a customer who needed help with a packaging line for COVID-19 nose swabs. His problem was that the fibers from the nose swab came loose and were dispersed everywhere, a consequence of working with certain materials. They ran a few new machines 24 hours a day but had to stop them multiple times each day to spend valuable time cleaning out clogs in the vacuum line. 

The machine used a decentralized vacuum system. A vacuum head with a venturi over each of the 18 grooves for swabs, would come down and pick them up. The swabs were aligned inside a half-round cavity with small holes behind to allow the vacuum to pull the swabs into the cavity. It is difficult to get a proper seal on the swabs, but picking them up is not difficult. Since the swabs are lightweight and the surface area is sufficient, low vacuum levels and higher flow will hold the swabs in place. This is an effective way of ensuring immediate vacuum at the point of use and eliminating the possibility of one dropped swab affecting the vacuum on the others. However, the high flow causes lots of loose fibers to make their way into the vacuum tubing and, eventually, into the vacuum pump. If a venturi vacuum generator has the airflow obstructed at all it will lose vacuum, so any debris accumulation creates dropped swabs. Every time fibers built up and clogged the system, it did so at 18 separate points, and cleaning each out was difficult. They had nine of these machines running three shifts, which made maintenance almost a full-time job. 

Filtration was needed in the line between the vacuum head and the vacuum generator. However, having 18 small filters was not very practical. I suggested one multi-venturi vacuum generator with a single filter to operate the whole head. This made sense for several reasons. One large, easy-to-clean filter would prevent the vacuum generators from clogging, making maintenance far easier. Having a clear bowl makes it possible to visually check to determine cleaning needs. The multi-venturi vacuum generator also makes sense because it can produce the low vacuum/high flow necessary to hold the swabs. 

A multi-venturi vacuum generator uses four nozzles in series. These four nozzles act as three sets. The first set has compressed air entering through the first and smallest nozzle. This turns high-pressure air into fast-flowing air. In between the first and second nozzle is a gap where ambient air is drawn into the lower pressure flow. Next, the air flows into the second nozzle where the air is controlled by the slowly expanding shape of the nozzle. Again, it flows between nozzles where air is drawn in, then one more time before the air has slowed too much to be of use. The first set of nozzles creates a high vacuum level with low flow. The second set is a medium vacuum with medium flow, and the third is a low vacuum with high flow. Inside the pump are flapper valves that will close off the last two sets of nozzles to the rest of the vacuum system as the vacuum level rises above the maximum vacuum each set is capable of making. This causes loss of vacuum flow from that set when the vacuum level is higher than it can make. When there is enough airflow past the swabs that the vacuum level cannot build, the advantage of using all three valve sets all the time will present itself. 

The 18 individual pumps had a combined vacuum flow of 300 lpm @ 0mm Hg (10.6 CFM @ 0”hg), or a likely working vacuum level of 127 lpm @ 127mm Hg (4.5 CFM @ 5”hg). With our intermediate-sized multi-venturi pump, we provided 464 lpm @ 0mm Hg (16.4 CFM @ 0”Hg) and 184 lpm @ 127mm Hg (6.5 CFM @ 5”hg). This extra 57 lpm (2 CFM) of vacuum flow, a 44% increase, would more than compensate for a potentially-dropped swab. 

This example shows how the common practice of decentralizing to protect against dropping of all parts failed to take into account more important factors. It actually created more dropped parts and called for frequent maintenance to keep the machine running. A decentralized system with better filtration usually means many filters. In this example, 18 filters are needed. 

In many instances, it makes sense to use smaller centralized systems. One example is when a few suction cups pick up a larger item like a cardboard box. It’s never advisable to have four cups with four pumps picking up 1 box. The extra wasted compressed air and unnecessary components do not add security to that system. 

When there is more complexity in what is being picked up, e.g. odd-shaped items or placement lacking in uniformity, then better options are necessary. It’s best to think through the advantages and disadvantages of each system and consult with a professional for expert advice.