Without getting into the engineering detail of these 2 common types of equipment, I'd like to give a simple primer on the differences between the uses of these blowers. There are many who don't realize they cannot be used interchangeably. A blower system can be designed to use positive displacement(PD) blowers, or be designed to use centrifugal blowers. Although it may be possible, almost all systems cannot use both. The characteristics of each are very different.
Cast iron, centrifugal blowers are used for high volume of air applications(thousands of cfm-cubic feet per minute), and relatively low pressure(less than 15 PSI-pounds per square inch). PD air blowers are used for high pressure applications(20 - 75 PSI), and usually lower volumes(1-2 thousand cfm max). Of course, there are exceptions to these numbers, but this is a general guideline.
PD blowers usually have rotary lobes with very tight interior clearances(.010" - .020") between the rotating mechanical components. Centrifugals have clearances measured in fractions of an inch(.100"- .200") between the rotor and the casing. Due in part to the tight clearances, PD's must have better air filtration, and no moisture present in the air/gas being passed through them. This makes centrifugals more adaptable to use in landfill gas applications. You won't see very many PD's pushing landfill gas.
You also don't see PD's being used in vacuum system applications. Vacuum blowers draw the air through filter bags, along with the "dirt" being transferred. This dirt would create havoc with the PD blower.
Although there are many varying operational characteristics between them, an important difference is the PD will keep pushing air as the system pressure increases. A unit may be designed for 20 PSI, but if there is a blockage, the PD blower may build up 80 PSI trying to overcome it. The system must be able to handle it, or have safeguards to prevent a catastrophic failure. The centrifugal blower will only push its' near designed pressure of say 5 -10 PSI, and the blockage will actually cause the air/gas to flow in reverse! Usually the blower will "surge" as it tries to overcome the increased pressure. More than once I have seen centrifugal blowers turning backwards, even while the motor is trying to rotate forward!
If a system is designed to use PD blowers, you cannot install a centrifugal blower unless changes are made to the system design. PD's can be 'reconditioned", with rebuild kits, but more often than not, the cost to repair is over half the cost to purchase new. When PD blower fails, it is best to buy a new one. Centrifugals, on the other hand, can have their bearings & seals replaced at relatively low cost, and the unit is good for another 10 years of service!
Preventative maintenance is the key to keeping you equipment operational. Invest in PM, it will save you money!
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