9 replies to this topic

[Said Salim]

Dear Members,

 

A four stages CO₂ compressor datasheet is attached. However, I am having a doubt in the datasheet

 

calculations of both polytropic head and efficiency if I used datasheet values of compressibility factor (z) and

 

specific heat ratio (k) to calculate polytropic head and efficiency respectively the will not give the same as what

 

stated in the datasheet and hence the power consumption will be different. My question wether I have calculated

 

them wrongly or the supplier is using a different  method of calculations other than what I am using.?

 

I have attached my calculations along with datasheet. I will appreciate your response.

Attached Files

•  compressor datasheet.pdf   404.98KB   78 downloads
•  CO2 compressor.xlsx   72.77KB   153 downloads

[Bobby Strain]

You should use a process simulator to check the calculation. Average properties are not suffiently accurate for CO₂ compression.

 

Bobby

[himant]

as per excel sheet, calculations look fine to me... 

 

but what's with varying molecular weight, mass flow rate, etc? 

 

and i hope it doesn't have anything to do with "Difference between Operating point & Surge"

 

Regards

Himant 

[breizh]

said ,
I confirm the data sheet values for Z , using Redlich & Kwong model , except for the suction 4 stage ( Z1= 0.6124 )  .To me there is a typo on the data sheet , Zs and Zd are different.

 

For Polytropic head , Z value @ suction should be used (refer to relation attached with your table) , not the average value.

 

Hope this helps
Breizh

Edited by breizh, 19 September 2014 - 04:48 AM.

[PingPong]

Breizh is correct that 4^th stage Z on datasheet is too high.

Using the NIST webbook for pure CO₂ one would find for 4^th stage that Z_in = 0.563 and Z_out = 0.672 @ 110 ^oC (so average Z = 0.618) and the enthalpy increase is 48.9 kJ/kg.

For a flowrate of 81032 kg/h (22.5 kg/s) that gives a 4^th stage gas power of 1100 kW. Add 20 kW for mechanical losses and one gets 1120 kW.

 

When using the standard textbook formula to calculate Head and Power one should not recalculate the polytropic efficiency from T₂/T₁ but use the vendor efficiency. In that case the calculated powers and polytropic heads would be much closer to the vendor data than Said calculated. But for the 4^th stage one should then of course also use the correct value for Z.

Edited by PingPong, 19 September 2014 - 05:50 AM.

[himant]

can you please tell me what's with varying flowrate and varying molecular weight? I am confused!

 

Is it part of refrigeration cycle? 

 

regards

himant

[Said Salim]

Breizh is correct that 4^th stage Z on datasheet is too high.

Using the NIST webbook for pure CO₂ one would find for 4^th stage that Z_in = 0.563 and Z_out = 0.672 @ 110 ^oC (so average Z = 0.618) and the enthalpy increase is 48.9 kJ/kg.

For a flowrate of 81032 kg/h (22.5 kg/s) that gives a 4^th stage gas power of 1100 kW. Add 20 kW for mechanical losses and one gets 1120 kW.

 

When using the standard textbook formula to calculate Head and Power one should not recalculate the polytropic efficiency from T₂/T₁ but use the vendor efficiency. In that case the calculated powers and polytropic heads would be much closer to the vendor data than Said calculated. But for the 4^th stage one should then of course also use the correct value for Z.

 Dear PingPong,

You meant to say that the vendor has determined the polytropic efficiency practically not by the normal calculation procedures.?

Said Salim

[Said Salim]

can you please tell me what's with varying flowrate and varying molecular weight? I am confused!

 

Is it part of refrigeration cycle? 

 

regards

himant

 

Dear himant, 

 

It is not a part of a refrigeration cycle. It is a CO₂ feed to a urea reactor. However, variying of Molecular weight and flow is due to the addition of passivation air in the 1^st satge discharge and taking into consideration the removal of moisture in interstage seperators.

 

Said Salim

[Said Salim]

Dear Members,

 

Thank you all of your response I have checked the 4^th stage values of Z and k as well using both HYSYS and NIST webbook and they were different than what is available in datasheet. Actually, I used to consider whatever given by a vendor datasheet is accurate and no way to have such typo mistakes.

Said Salim

[PingPong]

I am sure that the vendor simply copied the values for Z and k from the purchase order of the buyer, but did not use them as the vendor used their own software to calculate Discharge Temperature, Head and Power.

 

You meant to say that the vendor has determined the polytropic efficiency practically not by the normal calculation procedures.?

Vendor calculates (or better: predicts) discharge temperature using known polytropic efficiency, not the other way around.

If you like to calculate something back from T₂/T₁ then you could calculate (n-1)/n and use that calculated term in the rest of the formula for H_p. Or even better: use vendor value for H_p to calculate back (n-1)/n value.

 

Note also that stage 4 works in the dense phase area of CO₂ where properties change differently with changes in P and T.