Positive Displacement Blowers

Positive Displacement Definition: To lift a given volume for each cycle of

operation.

History

• The machine was first invented as a more efficient water wheel. Made of wood, it was put in the millstream and as the water flowed through it, the rotors turned and power was generated for milling from the output shaft. It worked well except for a minor flaw; the wood would swell up and ruin the clearances and the mill would stop. The unit would be taken out of service and dried out, shrinking the parts to their original dimensions.

• In an effort to speed up the drying, as soon as the rotors would turn, a power source could be connected to the output shaft and the rotors spun. Someone noticed that such an arrangement would move a quantity of air and the use as an air blower was begun.

Warranty

• The blower is warranted by the manufacturer (Roots, Hibon) and supported by a blower dealer for a period of 1 year. The warranty covers defect of material or workmanship. A failure of any type, the blower must be removed from the truck and shipped to the blower dealer or manufacturer

Failure to repair cycle and typical down time

Customer

Vac-Con Dealer

PD blower dealer

Blower manufacturer

Customer

1 week minimum

Determine cause of failure and submit findings to manufacturer for approval

3 weeks typical

Manufacturer approves warranty and dealer orders parts for repair.

Intake rotation

Must have tight tolerances

Must maintain tight tolerances to move material

Exhaust rotation

P.D. Blower Construction

Drive gears

Driven bearings are shimmed to 9 thousandths (.009) of an inch to center the lobe in the housing

Drive gearOil slinger

Shim plate

Bearing

Spring washer

Fluid seal

Vacuum seal

Drives the lobes

Lubricates gears and bearings

Centers lobes in housingSupports and allows lobes to rotate

Keeps lobes centered

Keeps oil in gear housingKeeps vacuum in housing and stops air leaking in

The land is responsible for maintaining the tight tolerance between the lobe and housing. Any material ingested

will wear the land down increasing the distance between the lobe and the

housing decreasing efficiency

Clearance between land and housing is 9 thousandths of an inch

About the thickness of 2 business cards

Close tolerances

• Absolutely must not have any carry over.• Debris body screens absolutely must be in

place.• Final strainer must be in place• Absolutely nothing larger than 9 thousandths

of an inch (.009) can pass through the blower.

• Special care must be taken, if the blower is not going to be operated for long periods of time oil must be applied to the lobes to keep them from rusting together.

Potential causes of damage

• Carryover running through the blower for long periods of time

Severe erosion

Potential causes of damage

• Large debris causing the blower to stop rotation instantly

Housing cracked because of lobes stopping while running

Head plate broke in two

Potential causes of damage

Over heating caused the lobe to grow (expand) too much causing lobe to head plate contact. Over heating is caused by a restriction or vacuum relief (kunkle) valves failing.

Vacuum relief valves