Regarding volumetric efficiency.

[vinsen]

Hi,

i am looking to calculate volumetric efficiency of a screw compressor,

i have pressure ratio and temperature at outlet and inlet and comp speed and power.

how to find volumetric efficiency with volume flowrate
that is actual volume flow rate / theoritical volume flow rate.

your suggestions will be so much of help to me.

thanks

vinod.

 

[25362]

Visit thread407-82026, it may be of help.

 

[vinsen]

Hi,

thanks for the info,

but it is just to find the comp speed,

i would like to know how to calculate the theoritical mass flow rate and the volumetric flow rate

and also its difference from actual, how to calculate.

it would be nice if someone can suggest,

then i can put them to find the volumetric efficiency.

 

[pmover]

how about asking the compressor mfg?

or

using "volumetric efficiency screw compressor" as search criteria via google returned many results.

good luck!
-pmover

 

[dcasto]

You'll need to ask the vendor because they have all the data that they use to create an emperical volume capacity of thier machines. If the screw has a slide valve, that changes the Bite the gas takes and the volume capacity. If the screw is flooded with oil, the oil takes up space or capacity depending on how much oil is circulating. If it a volume ration valve, that will affect the volumetric effiency.

The good news is most vendors will give you thier program. One screw packager has even modified vendors data with dat they have from actual operations. For one freebie go to

http://www.arielcorp.com.

They have reciprocating compressors and they use to market a screw compressor and the model for their old line is in there so you can see the relationship of capacity versus screw size.

What will be interesting is to look at the overall effiency, HP/mmscf. The screw compressor will be real bad as compared to a recip.

 

[crjones]

You need to know the theoritical volume per revolution. This is number based on the profile of the rotors, so its vendor specific. A slide valve effects the unit's ability to produce pressure (adjusts the internal compression ratio)the cfr (cubic feet per rev) is the same.

One thing in comparing compressors, you need to look at usage intent. A screw compressor has a fairly flat eff curve over a given speed range. But it works better running flat out not choking the inlet flow or modifying the leakage path with slide valves. A recip is a part load unit, if you want 100 cfm the sales folks size it at 200 cfm. Running flat out wears the piston rings too fast and spoils the gains in eff.

 

[vinsen]

Hi friends,

thanks for your infos,

i have comp speed,pressure at inlet and outlet and temperature at inlet and outlet,

can you tell me what else i need to get the volumetric efficiency using volume flow rate.

etta = actual volume flow rate / theoritical volume flow rate.

actual volume flow rate is - velocity * area

can i use the speed for velocity as it is scalar, for area i can take the dia and calculate area of compression.

what abt the theoritical volume flow rate, how can i calculate its value.

 

[dcasto]

Why do you need to know volumetric effiency, it has no meaning on an operating system.

 

[vinsen]

Hi guys,
again i would like to calculate volumetric efficiency of a screw compressor,

i have mass flow rate at inlet and also temperatures and pressure at both inlet and outlet measured, and also i have compressor speed and power measured.

apart from that i also have the theoretical volume and volume ratio of the compressor for the calculations.

can you please tell me how to calculate the actual volume flow rate so that i can come at volumetric efficiency

waiting for your replies.

 

[dcasto]

VE = actual ACFM/theroretical CFM ACFM is the mass in lb/min / flowing density in lb/CF