Have you ever wondered where the Turbo Blower manufacturers obtained their data for their wild claims of 30-40% increase in efficiency over PD blowers? While pursuing a recent project we found that a turbo blower manufacturer (which we will keep unnamed) took the owner out to an existing WWTP plant in the area to compare the currently installed PD blower to their turbo machines. The owner was amazed at how quiet and efficient the turbo blowers were and wanted his engineer to use them for their plant upgrades! What no one discussed that day, and what the turbo blower manufacturers hope no one discusses period, is that the PD blower packages used in the comparison were 20+ years old! Think about that for a moment. Technology makes leaps and bounds every 3-5 years, let alone 20! Existing technology is modified and improved upon constantly in a competitive market!
Today, PD blower systems are much more efficient than those installed in the past. There are multiple reasons why this is true, but the most significant reasons are as follows: improved blower efficiencies, improved motor efficiencies, direct driven systems. We will touch on each one of these in the attached paper. There are even comparisons at the end for you to evaluate!
What are your thoughts after reading the document?
You miss two areas of slip in your discussion of rotary lobe positive displacement blowers. 1) Between the rotors. 2) End of the rotor and the case.
ReplyDeleteI can appreciate better machining techniques will allow closer manufacturing tolerances, but you still have to contect with thermal growth of the rotors and the case.
Does it follow that the running tolerances have to be large enough to compensate; defeating much of the potential benefit of your expensive machining?
On another point. Why did you twist the rotor? Why not just twist the case inlet and outlet.
Has the same effect, but would be a lot cheaper to build straight rotors and machine a slanted inlet an outlet.
John,
ReplyDeleteThanks for the reply! You are correct. There are additional contributors to slip, but I was trying to keep it simple. We’ll probably be more detailed in future discussions.
As for thermal expansion, yes that’s a limiting factor. However, any increase due to thermal expansion must be added to rotor torsion under maximum load. If you compare a straight lobe with a helical lobe (of let’s say perfectly equal thermal properties, approximate geometry and capabilities) the thermal expansion within the impeller chamber would be equal, but the clearances within the straight lobe machine’s impeller chamber must be greater to account for a higher torsion under operation than the helical machine. There is more rotor deflection for a given pressure and the clearance is built into the design to prevent interference. We would not gain this with a slanted inlet port. However, you are on to something. Aerodynamic efficiency and noise performance can be increased by optimizing the inlet and outlet ports. There are many bare blower models available that employ this feature.