Example 13.2

A rotary vanes blower works between the pressure limits of 1 bar and 1.8 bar, and gives 5 m³/min of free air delivered when running at 240 rpm. Determine the power required to drive the blower when (a) ports are so placed that there is no internal compression and (b) when the ports are so placed that there is 50% pressure rise due to internal adiabatic compression before backflow occurs. Assuming mechanical efficiency to be 98%, calculate the blower efficiency.

Solution

The p−V diagrams without and with internal compression are shown in Fig. 13.6(a) and (b), respectively.

Figure 13.6 p−V diagrams: (a) Without internal compression, (b) With internal compression

1. Without internal compression, 
2. With internal compression, For minimum power requirement, the whole compression should be a reversible adiabatic process.Actual I.P. = Ideal I.P. × η_mech    = 5.333 × 0.98 = 5.226 kWIsentropic efficiency without internal compression = Isentropic efficiency with internal compression =