Estimating dPg/mass flow from compressor maps? 1

[CWitkowski]

Hello,

I am trying to estimate the effect of changing the CAC (charge air cooler) on a given generator set. I want to replace the current CAC with one which has different tanks and therefore a lower pressure drop across the cooler. The cores are identical.

I have pressure drop curves vs. mass flow for both coolers, and I have the compressor map for the given engine. Is there a way that I can plot the information together (ie. plot the pressure drop curves onto the compressor map) and then use the information on the map to interpolate from one Pd curve to another at engine operating conditions, enabling me to estimate the new mass flow with the new CAC?

Regards,

Chris

 

[Bribyk]

I think it's going to be difficult to predict what's going to happen. What are you trying to achieve? More power? Greater efficiency?

Depending on where the engine and turbo are operating you could reduce the compressor efficiency by reducing your pressure drop (pressure ratio) which would increase heat into the pre-cooler air and you might end up at the same place (same manifold temp and mass flow rate into cylinder), within measurement error.

 

[CWitkowski]

Basically what I need to estimate is the new mass flow through the low restriction CAC instead of the original (assuming all other parameters are the same). Once I know the mass flow with the new CAC, I can calculate my new manifold temperature (as the mass flow will increase, manifold temp. will increase)

What I'm essentially trying to achieve overall is CAC commonality between two generator sets (eg.eliminate use of the old CAC).

 

[TDIMeister]

http://www.not2fast.com/turbo/glossary/turbo_calc.shtml

 

[CWitkowski]

Just to re-iterate what I'm trying to do...

I have a set of pressure drop curves for my CAC's, showing pressure drop (mbar) against mass flow (kg/min). I want to plot these onto my turbocharger compressor map, to give me a set of system resistance curves.

I can convert the mass flows into the correct volumetric flow units required by the compressor map, but I'm unsure how to convert the pressure drop units into a pressure ratio.

Do I need to divide intercooler inlet pressure by outlet pressure (giving me a pressure ratio) and plot that on the compressor map? If I do this, the CAC PR values seem too low, ranging from 1 to 1.4. Am I missing something? Do I need to calculate the CAC outlet pressure relative to ambient?

Cheers,

 

[BigInch]

System curves should be converted to absolute pressure against mass flow.

Now take the absolute system inlet pressure at one flowrate and divide by the inlet pressure of the compressor.

That's the compression ratio needed for that flowrate to run in the system.

Now, since its compression ratio, you can plot it on the compressor map.

Do it again for another flowrate.


**********************
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http://virtualpipeline.spaces.live.com/

 

[CWitkowski]

Thanks for your reply, but I'm not sure I fully understand.

Please see my attachment.

Assume all pressure values are absolute, and that P1 and Pambient are constant for all flow rates. The only thing that varies is volumetric flow and P2 (due to varying pressure drops across the intercooler).

Can you run me through a calculation using these values, to give me my system resistance plot point? I'm trying to get my head round it but having a little difficulty.

Thanks.

 

[BigInch]

It sounds like the compression ratio is held constant at 2500/1000 = 2.5

So apparently compressor speed is changing to vary flow while the 2500 mbar discharge pressure (your system inlet pressure) is held constant.

Holding the intercooler inlet pressure constant essentially makes the system curve that the compressor sees flat. Holding constant compressor inlet and outlet pressures constant makes your compressor ratio constant, hence the map is a flat line at CR = 2.5 through all flowrates, with some vertical marks indicating rpm corresponding to various flowrates.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/