Cast iron centrifugal air and gas blowers are designed specifically for each application. When an engineer specs a blower for a particular job, she must identify the volume required at a specific pressure, temperature range, elevation, and inlet vacuum. All of this information is used by the manufacturer to design a blower and create a "Performance Curve". 

Generally all of the blowers in a particular application would have the same performance, but not always.  There could be 2 blowers designed for a peak flow, and 2 identical size blowers, with a different impeller combination, designed for a minimum flow.  You can't tell they are different just by looking at them.  Which is why when someone has a blower to sell, the first thing I need to see is a performance curve. Knowing the blower Model will get me close to the probable performance(1000 cfm vs. 2000 cfm), but most likely it will not operate in parallel with any other blower unless they are of the same design.   

Blowers are usually designed to operate at one point on the curve-say 1000 cfm @ 6.4 PSI, 100^OF inlet temp, and 500 ft elevation.  To be the most efficient blower, it needs to use the least horsepower. This is where manufacturer's differ.  One manufacturer may be able to achieve this performance with a 5 stage blower using 50 HP, while another manufacturer might have to build a 6 stage blower using 60 HP.  And then you might have the High Speed Turbo blower manufacturer's come in and offer to provide the same performance with a 30 HP motor!

The blower curve is the key.  The design point on the curve also needs to be as close to the full horsepower as possible in order to be the most efficient.  A blower that operates at 48 HP at design flow is more efficient than a blower that operates at 45 HP at design flow.  Performance curves provide much more information than I show here, especially to the trained application engineers. 

One important note, all of the manufacturers design their centrifugal blowers at one speed - 3600 rpm.  Changing the speed drastically changes the performance.  In fact, each change in speed, would actually require a different curve!  The original design point may, or may not, be on the "new" curve.  Just like throttling the inlet with a butterfly valve changes the blower performance, increasing or decreasing the speed changes the performance.

Operating way out on the curve could cause the blower to overload the motor. Operating with minimum flow, back up on the curve, could lead to surge and blower failure.  The curve tells you where the blower can be safely operated.    

If you have a new blower, be sure to keep that performance curve in a safe place, so when you are ready to dispose of the blower, I will be better able to help you. Invest in PM, it will save you money!