1 reply to this topic

[Dtan76]

Hi all

I'm back! Long time never posted due to overload task in here...

Ok, recently we just encoutered HYSIS problem handle more than one inlet stream feed to a screw compressor... This project is provision cooling function to dew point the natural gas feed temp from 43 to 20deg c, prior discharge to pipeline. Eng study has conducted and concluded that refrigerant system using R-134a will be more viable.

The problem here during our simulation, one return stream about 5.4% of refrigerant mass flow is "split" and return to the screw compressor...

Hence in our simulation, we can't really modeled the screw compressor but instead, we replace it using "Balance" logic. Assigned the inlet stream and outlet stream with temp...

See attached PDF file you will understand ...

My questions:

1) We understand many experience industrie refrigerant vendor don't really rely on simulation package to analyst the system.. could someone here share how they generate the heat & mass balance flow sheet?

2) As the various equipment sizing basis don’t match, we input the air cooler (vendor's data) sizing flow rate and the return flow is adjusted accordingly. This results in higher return stream flow rate than in the screw compressor sizing (2,881 kg/h vs. 1,734 kg/h total for both compressors) since the vapor to the compressor is less than compressor's vendor.

3) Basically the vendor's data and HYSIS doesn't match at all !!!!!

I'm totally confuse......

Attached Files

•  Screw_Compressor_in_HYSIS.xls   939KB   269 downloads

[Dtan76]

Hi all

I got reply from ASPENTECH technical support, see below message:

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Unfortunately, there are no unit operations that can directly model a screw compressor in HYSYS bacause in HYSYS application compressor is designed to model centrifugal or reciprocating compressors.

However there are a couple of workarounds, although all are approximate rather than rigorous so that you are getting heat and mass balance problem.

With a screw compressor, both gas and oil enter the compressor (the oil acts as a sealing agent and coolant). Regarding the presence of two phases in the Compressor, HYSYS will generate a warning although the calculations will normally be performed. The temperature increase across a screw compressor is not as high as for a conventional centrifugal compressor; the compression is closer to isothermal. The calculations performed by the HYSYS Compressor are as follows:

* Isentropic flash for calculating the ideal conditions (here the program uses the inlet entropy and the specified outlet variable (either pressure, temperature, etc.) to calculate the rest of the "ideal" outlet conditions).

* Apply efficiency (adiabatic or polytropic) to obtain actual conditions (efficiency = (work required ideal / work required actual)).

So even by using an efficiency of 100%, what is being calculated is always the isentropic compression which will give a higher outlet temperature than operating close to isothermal compression conditions. For example, a P-V diagram of the two different paths followed during the compression will show the isothermal curve goes right to the isentropic one. The work HYSYS predicts will also be less due to thermodynamic reasons (the area under the P-V diagram curve will be smaller because the final state will be different, higher final volume for the same final pressure in the case of isentropic compression). Therefore, isothermal compression as such cannot be calculated by any HYSYS operation.

One rough approximation to the isothermal compression is to build a series of compressor-cooler groupings to reach the final state at an almost isothermal condition, however in this case the work consumed by all operations will be higher than the real isothermal compression work.

Another option would be to map the performance data from the vendor into curves (in the format used by the default Compressor) and represent the screw compressor with a centrifugal compressor.

Alternatively if, from vendor data, you know the outlet conditions you expect to achieve you can use a heater operation to represent the compressor, leave duty and delta P empty, then specify the outlet conditions and HYSYS will calculate the pressure rise and the required duty.

Another alternative is Alternatively, the vendor's data can be used in conjunction with a Balance Unit Operation since the screw compressor has a lower discharge temperature than centrifugal or reciprocal compressors.

I hope this answers your question.

Let us know if you need any further information.
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