centrifugal compressor efficiency 1

[hhjlep]

I need to calculate the energy use of a large centrifugal compressor installation at a gold mine. I do not have any curves, nor can I make any measurements.

I have the following specs:

Inlet pressure: 85 kPa
Outlet pressure: 550 kPa
# of stages: 4
mass flow: 25 kg/s
Inlet temp: 18 C

Assumptions:

Compression index = 1.3
Perfect intercooling between stages

Based on the info above and thermodynamic losses only, I get an isothermal efficiency of roughly 90%.

I'm pretty sure that my calculations are right, so the questions are:

1)What other inefficiency exists in centrifugal compressors that I did not consider?
2)What effect does slip etc. have on efficiency?
3)What is a typical efficiency for a 4MW compressor as described?

Hope somebody can help

Thanks

 

[pmover]

well, short of obtaining operational data from the compressor, one can reasonably determine HP by obtaining driver data (electric or engine driven). but, that does not tell you much about the compressor.

as far as determining compressor efficiency or performance, it is necessary to obtain operational data. namely, inlet/outlet pressures and temperature and fluid properties. I suggest contacting the compressor mfg, give them the fluid, serial #, make, and model data and request performance curves be provided for each stage/impeller!

good luck!
-pmover

 

[hhjlep]

Thanks, I didn't state the question all that clear, so let me try again.

I have to use a theoretical approach in order to determine the compressor efficiency, thus I cannot rely only on supplier information. I want to know if it is possible to determine the overall compressor efficiency based on the information that I have and a few assumptions?

As an example, the efficiency of 90% stated in the original post, only considers the thermodynamic loss, in other words the loss due to the non-isothermal nature of the compression process.

Other than that, I believe that there are inefficiencies in the compressor itself, namely the slip factor due to impeller clearance, friction loss etc.

How do I string all these parameters together to obtain an overall efficiency, and what are the parameters that influence the efficiency the most?

More info:

Working fluid: Air
Compression ratio / stage: 1.6535

 

[zdas04]

"Effeciency" only has meaning relative to something. Are you wanting to compare actual conditions (which you don't have) to manufacturer's published performance (which you don't have)? or are you trying to compare actual to theoretical isothermal performance? In the latter case you have to do it on a stage by stage basis and then multiply the results (so if each stage is 90% or so, then a 4 stage machine is about 65%).

On the other hand it is sometimes useful to compare the input energy (thermal content of fuel to a IC driver or total kW of electricity at the meter) to the actual power output going from suction conditions to discharge conditions. This number tends to be much lower.

"Effeciency" is not just one thing.

David

 

[milltir]

As zdas04 said, you need to estimate the power on a stage by stage basis (I normally use polytropic equations, but as it is air, then adiabatic is OK). This gives you the "gas power". You need to add on an allowance for mechanical losses (bearings, seals). This will give you an estimate for compressor shaft power. If there is a gear in the system you need to make an allowance for this as well.

 

[hhjlep]

Thanks guys. I'm more interested general guidelines from guys that know large compressors.

Let's start from what I mean with compressor efficiency, and how I understand the whole compression process.

A) There is a loss due to the non-isothermal nature of the compression process. This is based purely on thermodynamic principles, and I have calculated this on a stage by stage basis, using equations for polytropic compression. Name this "thermodynamic efficiency" or eff1 (answer is 90%. Sounds high, but the compression ratio per stage is only 1.654)

B) It is not possible to achieve a compression efficiency calculated in A, because there are mechanical losses to consider as well. I would guess the mechanical losses are due to gears, motor inefficiency, slip as a result of impeller clearances, pressure drop due to large air velocities etc. These losses will be used to calculate the "mechanical efficiency" or eff2.

C) I define compression efficiency as: The isothermal compression energy required to compress the air divided by the actual energy required to compress the air.

The energy calculation is thus as follows:

Isothermal energy = Actual energy * eff1 * eff2

Given this framework, I have two questions:

1) How do I go about calculating eff2? I realise that this is specific to every compressor ever built, but surely there must be some sort of a standard. As with water pumps, we know that single stage centrifugal pumps can give efficiencies of 80- 90% depending on the size and head. I just want to be in the right ball park.

2) Am I on the right track, or did I completely miss the mark?

Further info. the reason that I cannot measure the current or motor power input, is because there are seven, 4.2 MW electrical compressors connected to a large network that delivers air to operations over a distance of 5 kilometres. I'm only interested to calculate what energy is required to compress 25 kg/s of air that is used in a specific operation.

The total capacity of the 7 compressors are much higher, but I do not know the total capacity. I'm still trying to find operation curves from the supplier, but in the meantime I want to get as close as possible. Going through the whole process on a theoretical basis also helps one to understand what impacts the efficiency of a compressor the most, and what the physical limitations are.

 

[HPMach]

Efficiency is not an independt term and defintiely it will be linked with equipment (with in the reasonable band). If the operating parameters established appropriate, it is possible to reach around 80-85% on centrifugal machines. I am not sure why you are saying 4stages in your case. Each stage can go about 3-3.3 compressor ratio if the temperature is maintained well at suction. Best way is deal with some vendor or online softwares to get reasonable efficiency information. Goodluck.