Cast iron centrifugal air blowers are each specifically designed for a given installation. An engineer determines the volume and pressure required for a particular application, taking into consideration the ambient temperatures, jobsite elevation, and numerous other parameters. Usually it is a range, such as 900 - 1100 cfm @ 8 PSIA. But sometimes they may request more than one "design performance" such as 500 cfm @ 6 PSI and 1000 cfm @ 8 PSI.
When the manufacturer reviews the engineering spec, they must find a blower in their selection options that will best provide the requested blower performance. Often they decide how large the blower will be-6" diameter, or 12" diameter, or even 16" diameter, and what the horsepower would need to be. One manufacturer might select a blower from their arsenal that would have say 5 impellers and use 50 HP, while another manufacturer might select a 5 stage blower and require 60 HP to operate. Both would satisfy the engineer's spec, but one would cost more to operate! The same size blower would have different combinations of impellers.
Each manufacturer has maybe 15 different "styles" of impellers. There are the standard radial vane impellers, and the standard backward curved (BC) impellers, and then variations of these 2 types. An impeller could have more vanes, or fewer vanes, than the standard. Or there could be variations in the size of the vanes(1/2 vane, ¾ vane, etc...).
Then there are variations in the configuration of each blower. It could have 3 radial vane impellers, and 2 BC impellers. Or 4 BC and only 1 radial. All of these variations are used to obtain the engineer's design performance. Even changing the order of the impellers in a blower-such as putting the radials closer to the outlet of the machine vs. toward the inlet of the blower, will change the performance(volume and pressure).
So when a plant closes, or is upgraded, and they have a blower or 2 to get rid of, to find another jobsite that requires the exact same performance, and HP, can be very difficult to match. Usually, they find one that is "close enough", or simply settle for the change in performance. If there is more than one blower feeding a single header, those blowers must be designed exactly equal in performance to be able to operate properly.
There are differences in head positions to connect to the system piping. Inlet or outlet driven, side inlet or outlet, even directly down through the floor inlet or discharge is possible. A standard aeration blower cannot be used in a landfill gas application-the gas would be leaking all over the place.
All these variations makes it very difficult to find a "pre-owned" blower to fit your exact design and application needs.
