On Page 3, under the Heading "Compressor Inlet Flow" the paper says (**emphasis mine**): "Conversely, **increasing compressor suction pressure** will significantly increase gas density and **reduce the gas volume**. The lower the suction pressure the larger the effect of pressure changes on compressor capacity. For example, increasing pressure from 18.7 psia to 20.7 psia decreases the inlet gas flow rate by 10.6% for the same mass flow rate."

What this is saying is this. For a given mass flow rate (not volumetric flow but mass flow which flow really is, that is, the flow of mass) the mass will occupy a smaller volumetric flowrate if it is at a higher pressure and density. Since a compressor produces volumetric flow and volumetric flow is a constant for a given polytropic head (proportional to differential pressure P2-P1) and say you are flowing at the first operating point on the curve which appears to be 13,200 ICFM at 26,600 FT head. Well all other things being same (such as differential pressure say by also increasing the discharge pressure simultaneously) and the you just change density then the actual flow required will be lower by 10.6% per the example above. So now you are flowing the same mass you were previously flowing but at a higher density/pressure and a lower volume = 0.894 (13,200) = 11,800.8 ICFM. So if you are still putting in 26,600 FT of head of work into the compressor you still are going to get 13,200 flow because that is the operating point on the curve. Therefore you get an extra 1399.2 flow capacity to flow additional mass.

However if you increase the suction pressure but do not increase the discharge pressure then the differential head (work) will required will be less, so the operating point would move down the curve and you will be operating on the lower point on the curve at lower head and higher flow. Therefore you get a double advantage by increasing the suction pressure - one due to increase in density, and another due to decrease in differential polytropic head which shifts the operating point on the curve to the right.

Then, at the end of Page 3, under the Heading "Compressor Head," the paper says (**emphasis mine**): "Either **increasing suction** or

decreasing discharge** pressure** can be used to reduce polytropic by 1500-feet and** ****increase the compressor inlet flow capacity** by 6%."

This is saying that as I said in last paragraph above, reducing the differential head output of the compressore increases the flow (to understand why is due to the fact that the impeller has backward curved vanes which changes the resulting velocity vector of the fluid existing the compressor for lower flows which result in changing the head for a constant RPM). So the polytropic head is proportional to the differential pressure between the suction and discharge, if you reduce the differential pressure (by increasing the suction pressure or reducing the discharge pressure) the operating point on the curve shifts to the right and more flow is developed by the compressor. So a compressor is creating flow by increasing the suction pressure to the discharge pressure and this discharge pressure is lost in friction loss as the flow travels down the piping system. The polytropic compression in the compression process also adds internal integy to the fluid in the compression process.

I can't understand the seemingly contradictory claim in the same paper. I looked for other sources, but I encounter the same conflict as well. Some paper explains that increasing suction pressure increases the gas density and lowers its volumetric flow.

I think this was a poor way to state this. An increase in suction pressure does not reduce the volumetric flow of the compressor, it just reduces the volume of the flow stream per unit mass, with the volume flow of compressor same at the same differential pressure per the compressor curve, it is just that now since the volume has shrunk for the given mass flowing you have more volume capacity of the compressor to flow more mass.

This makes sense, but on the flip side, isn't it true that low suction pressure also lowers volumetric flow?

If you lower the suction pressure but keep the discharge pressure same then the differential polytropic head required increases which shifts the operating point on the curve to the left and lowers the capacity output of the compressor.

You can call me confused, and I admit that I am. Please teach me how a centrifugal compressor performs, preferably step by step from when the suction pressure is low, to when it reaches normal range, to when it reaches a high range.

It is difficult to explain all at once - feel free to asks questions that you have a little at a time and I will respond. However basically a centrifugal compressor operates same as a centrifugal pump - which is on the curve and only on the curve shown in the diagrams above. At a given differential head you get a given flow - period (I can explain why but it would take me a while). So you can see if you decrease the differential head (equivalent to saying decrease of differential pressure) by increasing the suction pressure or decreasing the discharge pressure or both, the operating poing of the system will stay on the curve but shift to the right.

**Edited by snickster, 31 May 2023 - 04:55 PM.**