The quantity of air delivered at any given speed by a Roots-type blower, as shown in the illustration, decreases as the pressure ratio increases. This is due to the __________. See illustration MO-0082.
• Roots-type blower operation with constant displacement per revolution • How clearance between rotors and casing/lobes affects internal leakage • Effect of higher discharge pressure on backflow or leakage paths inside the blower
• When pressure on the discharge side increases, what happens to the tendency of air to flow backward through any small clearances in the blower? • In a positive-displacement blower with fixed geometry, do the physical clearances between metal parts change significantly as pressure ratio changes, or does something else change? • Looking at the illustration, identify where high‑pressure and low‑pressure regions meet; how would increased pressure difference affect flow across those regions?
• Remember that in a Roots blower, displacement per revolution is fixed by geometry; speed is constant in this question. • Decide whether clearance size itself is changing, or whether flow through existing clearances is changing as pressure ratio rises. • Visualize air paths in the illustration and determine whether the main loss at higher pressure ratio is more or less leakage past the rotors/lobes.
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