Customers in today’s combination sewer cleaner market have two choices with which to generate vacuum: centrifugal fan or positive displacement blower. This column is not intended to sway, discount or condemn either method, but rather dispel the myths long associated with both vacuum methods and components. Neither fans nor PDs were originally intended for the sewer cleaner market, but both were designed to convey. To better understand how they ended up in this market you must first learn a little history.

 

Positive displacement

The positive displacement machine was invented around 1860 as a “more efficient” water wheel. Made of wood, it was put into the millstream and as water flowed through it, the rotors turned and power was generated from the output shaft. Over time, wood components swelled and ruined the clearances, causing the mill to stop. The unit would then have to be taken out of service and dried out, shrinking the components to their original dimensions. As soon as the rotors would turn, a power source was connected to the output shaft to speed up the drying process. Someone noticed that this arrangement could move a significant quantity of air, and the use as air blowers began.

Cupola blowers became the first application for this newfound technology. Each rotation of the rotors represented a specific amount of air being forced into the furnace, thus allowing the furnace master to develop the theory that the more rpm the blower turned the hotter the fire became. The positive displacement (PD) machine was originally intended for pressure applications, which led to the original nickname, “blower.” Only in the last 30 years has the machine been adapted to provide negative pressure in truck-mounted vacuum applications.

The PD machine is constructed with two lobes that rotate in a cast steel housing in opposite directions of each other. The design of the PD is such that each lobe traps a specified amount of air – depending on the size of the PD – for each rotation. PD machines rely on close tolerances – as small as .009 of an inch – to trap the air. Unless the PD is designed to run fully blanked off, relief valves are necessary to protect the unit from building too much vacuum, which creates too much heat and subsequently causes the internal components to expand and collide with each other. Generally, the input speed of a PD blower is approximately 2,200 rpm, which produces 16 to18 inches Hg while creating 2,400 to 4,200 cfm.

One common misconception of PDs is that no matter how deep, no matter how far, a PD can “pump” the water or material. This is untrue. A PD machine has a finite lifting capability; once a PD reaches its lift capacity an operator must fluidize (add air) to continue conveying material to the holding vessel. PD machines work well in liquids, sludge or thick materials and vacuuming over a long distance horizontally.

 

Centrifugal fan

The centrifugal fan was designed in or around the 1960s for conveying grain. The conveyor’s original design had a centrifugal fan powered by a diesel engine, piping in which the material was conveyed, a cyclone, and an airlock. The technology was aimed at offloading grain from ocean-going cargo ships more efficiently than the shovel and bucket method. This machine could vacuum material from the berths of a cargo ship and lift it into a cyclone. Grain would drop out of the cyclone via an airlock and the exhaust or pressure side of the blower would then push the material to the storage vessel on shore. The unloading process, which previously took several days, could now be completed in hours.

The centrifugal fan uses rotating “fan” blades compressing air in compound stages. This device in its most powerful configurations is capable of lifting a column of water 200 feet, the equivalent of approximately 14 inches Hg (vacuum). It is fully capable of vacuuming material under water up to its rated lift performance, either without air induction, or with air induction. The closest tolerance in a fan is roughly 1/4 of an inch. The air is drawn to the inlet of the fan blade, and as the blade spins, it moves and compresses the air.

One common misconception of the centrifugal fan is that it cannot vacuum under water. That is absolutely untrue. A fan can vacuum to its rated lifting performance whether it’s under water or not. If a fan is rated at 200 inches, it can lift an 8-inch column of water to 16.67 feet. If the level of the water is less than 16.67 feet from the top of the boom, the fan can vacuum under the level of the water with no attachments. If the water level is greater than 16.67 feet, a fluidizing nozzle is used to add air into the air stream. Centrifugal fans work well in moving liquids and solids at over 100 feet vertically or horizontally.

This timeless topic has been and will continue to be the single most argued point in the combination machine industry. The fan vs. positive displacement argument rivals the Ford vs. Chevy debate. Books have been written regarding the benefits of fan over PD and vice versa. Each of these vacuum generators has applications where it outperforms the other. The single most important thing to consider is the application for which the machine is going to be used the most. If you are still unsure of which machine to purchase, be sure to contact your local sales engineer who is trained in the operation of both of these machines.