inlet guide vanes, 1 or more?

[cryotechnic]

In our plant we have different compressors, used for air compression and compression of our products.

We have a few integrally Geared Compressors. They have all different discharge pressures, capacity and they are used for different products (air, oxygen, nitrogen)they are all from the same mgf. But they're from different years.

Now we've noticed that some have Inlet Guide Valves on every stage, and some only at the inlet.

Can someone explain why this is? Based on what theory will a mgf choose to use 1 IGV of multiple IGV?

Thanks,
Cryo

"Math is the ruler of your potential succes...."

 

[gtsim]

What is stated here is general and applicable to any axial compressor operating at a relatively high pressure ratio and trust will be useful to you. At high pressure ratios there is a significant difference in air/gas density between the HP or latter stages compared with the LP or front stages of the compressor. At design conditions the air/gas angles in the rotor and stators are matched and do not cause a problem. At off design conditions (i.e. lower compressor speeds) the decrease in pressure ratio results in a decrease in the density in the HP stages which causes the velocity to increase in these stages. If the difference in density between design and off design conditions is large enough then the increase in velocity results in the HP stages choking. This choking of the HP stages forces the LP stages to stall and may even surge the compressor. It is also thought that the kink in the compressor characteristic is largely due to this effect. It is possible for the running line to interest the surge line making starting, low speed operation or accelerating the compressor impossible.

This problem may be resolved by employing variable stator (VSVs) and variable inlet guide vanes (VIGVs). These variable stators (usually fitted to the front or LP stages) are closed at lower compressor speeds thus reducing the loading on the front stages hence preventing stalling of theses stages. Furthermore, the closure of these stators also reduces the flow capacity through the compressor thus reducing the velocity in the HP stages of the compressor hence preventing the choke conditions in this part of the compressor. However, at maximum compressor speeds when maximum capacity is needed the stators are fully open. The movement of each stator may be different and the largest movement occurs in the VIGV and the front stage VSV.

For low pressure ratio machines the stalling discussed above is unlikely to occur and variable stators are not present. However, a variable inlet guide vane may be fitted to control the flow particularly for a constant speed machine.

 

[cryotechnic]

Thank you GTSIM,

Our compressors are all running at constant speed. The capacity is controlled by the IGV's.
The compressors have all different work ranges. (flow and pressures)

For example, we have 2 Nitrogen compressors. Both have a discharge pressure of 80 bar.
1 has a working range between 30000Nm3/h-55000Nm3/h, 8 stages, and IGV's at all stages.
the other one has a range of 55000Nm3/h-80000Nm3/h 7 stages, and only a IGV at the first stage.
They're both integrally geared type compressors.


Greetings
Cryotechnic

"Math is the ruler of your potential succes...."

 

[gtsim]

What is the suction pressure for each compressor?

VBR
GTSIM

 

[cryotechnic]

suction pressure is between 80 and 100 mbar for both machine.

Cryotechnic

"Math is the ruler of your potential succes...."

 

[gtsim]

80 to 100 mbar (i.e. 0.08 to 0.1 barA) suction pressure and discharge pressure of 80 barA gives a pressure ratio varying bewteen 800:1 to 1000:1. This seems far too high for a axial compressor? I don't know, I am sorry.

VRB
GTSIM

 

[cryotechnic]

GTSIM,
sorry, now I'm getting it, you was talking AXIAL compressors ONLY.
Your right, the compressors in my last post are not from an AXIAL type.
The compressors I'm talking about are radial type compressors.

In fact, we do have a axial/radial compressor in our plant. This one has the variable stator you're talking about ont he axial part of the machine. The machine is a single shaft axial/radial type, we use this one to compress air to 5,5bar. (max 180000Nm3/h)

Thanks
Cryotechnic

"Math is the ruler of your potential succes...."

 

[gtsim]

Cryotechnic,

Thanks for the update. I am not very familiar with radials however I understand that they can use variable stator in the discharge side to improve the turn down. I trust this helps.

VBR
GTSIM