6 replies to this topic

[panagiotis]

Hello everyone! I would like to bring again the topic of the compressor head:

 

 

Generally, it is stated that the the head developed by a centrifugal compressor is the same regardless of the nature of the gas being compressed. This sentence is referring to what we called actual head? Please confirm if you agree. This actual head depends on the compressor characteristics (impeller, etc.). This actual head is the ratio of the polytropic head divided the polytropic efficiency.

 

The next topic is the polytropic head, which definitely depends on the molecular weight, and generally the characteristics of the gas, which is going to be compressed. The gas inlet conditions affect the volumetric flow, which results in a different polytropic head.

 

If we think in terms of centrifugal pump, then there are two basics equations for head:

 

See attachment 1 (equation 1)

 

 See attachment 2 (equation 2)

The 2nd equation can explain the very true statement that head does not depend on the liquid characteristics. 

 

If we try to find an analogy between compressors and pumps, then

the meaning of polytropic head (compressor) is similar to the equation 1 for the pump.

the meaning of the actual head (Compressor) is similar to the equation 2 for the pump.

 

After this long post, my main questions are:

1) Where I can find in the books, literature explanation about the meaning of the actual head. Is the actual head what we call the head produced by compressor?

2) The derivation of the equation 1 for the pump is based on the Bernoulli's equation, simply looking at the inlet and outlet of the pump without applying the Bernoulli equation for the entire system of suction and discharge lines? If we apply the Bernoulli for the entire system then we calculate what we call the system curve.

 

 

 

 

Please share your opinion on this topic.

 

Thank you in advance for your time.

 

Kind regards,

Panos

 

Attached Files

•  attachement 1.PNG   9.17KB   5 downloads
•  attachement 2.PNG   11.22KB   0 downloads

[breizh]

Hi,

Consider the documents attached to support your work.

Good luck

Breizh 

[panagiotis]

Thank you for the documents!

[Muhammad Hamid Khan]

At Our plant has three screw type propane refrigeration compressor.
since the past 6 months, we have been using a compressor with low calculated differential oil pressure which is causing tripping to compressor due to low frick oil differential pressure.
When we did the oil test found ok, we also changed the frick oil cartidge filter, but so far the problem has been that of low-calculated oil differential pressure.
which is causing tripping of propane refregeration compressor.
Please guide us

[breizh]

Hi ,

To me you should report the problem to the manufacturer to get advices. What about your preventive maintenance program ? 

For a better follow up you should have started a new topic .Your query is not related to the existing post 

My 2 cents.

Breizh 

[panagiotis]

I thought that centrifugal compressors are more common for refrigeration systems.

[ctrend.app]

As far as centrifugal compressors are concerned, the head coefficient (work input factor) can be impacted by the gas molecular weight (more generally the tip Mach number). So at a given speed, if the head coefficient drops (or increases), it impacts the head of the compressor (i.e., in terms of matching the duty point or not).

 

Another point is that the total head (= enthalpy differential) is defined by the start and final state of the compression path but also depends on the gas composition. Indeed, based on real gas equation of state, the enthalpy is of the form h=(P, T and gas composition). P and T are respectively the total pressure and temperature of the gas at the condition being considered.

 

Polytropic head is the total head times polytropic efficiency. In theory, a perfect machine which has zero irreversibilities and is adiabatic (not a realistic scenario), would have a polytropic efficiency equals to 100% which means the real efficiency of the compressor equals the isentropic efficiency. In reality this cannot be the case as there are always losses be it mechanical, thermal, etc.

 

Please note that gas composition effects the compressibility factor and the isentropic volume exponent which impacts the polytropic head and also effects the isentropic temperature exponent which impacts the temperature ratio between suction and discharge.

 

 

As practical exercise, I suggest a tool like https://ctrend.somee.com/; try to sensitivize on molecular weight to understand the effect on gas properties and head calculation.