Benefit of two-stage supercharger?

[Achi]

Hello members and guests.I cannot understand the benefit of a two-stage blower in comparison with a greater single-stage blower.The impellers of the two-stage blower are firm on one shaft and cannot drive seperately.The energy to archive X bar ma.-pressure must be the same-or not?I am sure that I oversee something.Thank you for answer.Regards,Achi

 

[Colin1]

Compressors increase the pressure of the air at the cost of also increasing its temperature. This sets a limit as to how far the air can be compressed as it would get to the point where air temp was so hot, the fuel would ignite as soon as it was mixed (pre-ignition). The two-stage device sought to overcome this by compressing the air up to a point in the first stage then passing it through an intercooler radiator before being fully compressed in the second stage. It would then be further cooled in another section, the aftercooler radiator. Aftercooler techniques could be air-to-air or air-to-coolant radiator.

 

[Shortround6]

The energy required is the same in theory but not in practice.

Centrifugal compressors are delivering two things at the same time. Volume and pressure. A single stage is limited as to the pressure it can deliver and no amount of extra power (or size of the compressor if it is the same design) will really increase the pressure.
Centrifugal compressors are NOT positive displacement like a piston pump. They can 'stall', they can 'choke' and since the only thing that keeps the working fluid (air) moving through the compressor (pump) is momentum once they hit the limit of the design any excess 'flow' simply bleeds back around the edges or walls of the passages.
This is actually a gradual process as the limit is approached and shows up as the rapidly decreasing efficiency of the compressor as it's limits are reached.
This also brings in the advantage of the two stage compressor. Two compressors acting in series, each operating at a relatively high efficiency will need less power to compress the same amount of air to the same pressure as a single compressor operating at a low level of efficiency.
Since the difference in power between the actual work done in compressing the air and the power needed by the compressor shaft mostly shows up as extra heat in the charge air, using two stages also means a lower intake charge temperature compared to a single stage supercharger even if you could get one that would give you the pressure desired.
For most of WW II no single stage centrifugal compressor, even in the laboratory, would give the required pressure for high speed flight in the 20,000ft and up range.

Inter and after coolers are necessary to get anywhere the full benefit from high pressure ratios as Colin has said.

 

[Achi]

Many thanks for reply!Regards,Achi