5 replies to this topic

[gosooners]

Recently i have been working on compressor performance calculation, and this is first time i am working on compressor.
I know that increase in suction pressure increases compressor capacity,acfm. As suction pressure increases, we are on left side of the performance curve so actual volume going to compressor is low.(am i right!)So, actual cfm going to compressor is low and compressor will have more capacity available.

now, i am confused for how decreasing polytropic head will increase compressor capacity ? Because when head goes down, we are operating on right side of the curve and compressor doesn't have capacity available to compress more gas? Am i going on right direction?

please explain with engineering equation.

thanks
gosooners

[ankur2061]

Hi,

The inlet volume flow for compressor in m³/h can be calculated as follows:

M*0.08314*(T1+273.15)*Z1 / P1*MW

where

M is mass flow of gas in kg/h
T1 is inlet temperature in deg C
Z1 is inlet compressibility, dimensionless
P1 is inlet or suction pressure in bar(abs)
MW is molecualr weight of gas, kg/kg-mole

The polytropic head is:

Hpoly = (8314/MW)*T1*Zavg*(n/(n-1))*((P2/P1)^(n-1)/n-1))

where
H_poly = polytropic head, N-m/kg
Z_avg is average compressibility (Z1+Z2/2)
P2 is discharge pressure in bar(a)
n is polytropic exponent

The polytropic exponent is calculated as follows:

n/n-1 = (k/k-1)*η_poly

where

η_poly is 75-78%

and k = Cp/Cv the specific heat ratio of the gas or gas mixture to be compressed.

It is obvious from the above equations that as the head decreases the capacity increases and vice-versa.

You need to study the equations carefully to understand the relationship of head versus capacity of the compressor.

Hope, the above equations and explanation will help your understanding.

Regards,
Ankur.

[ankur2061]

Hi,

I have made a comprehensive spreadsheet for calculating polytropic head, no. of stages and shaft power considering a polytropic process for centrifugal compressors. The spreadsheet has been peer reviewed and I can provide you answers for the above mentioned parameters using my spreadsheet. The spreadsheet is not yet meant for open distribution. I would need the following inputs to do the calculations:

mass flow of gas

Composition of gas, mole fraction or percentage

Inlet or Suction Pressure, gauge or absolute units

Discharge pressure, gauge or absolute units

Inlet or Suction temperature

Let me know if you would like to know some answers.

Regards,
Ankur.

[gosooners]

Thanks Ankur,

I understood your comments, but i am confused in grasping few things.

(1) Suppose you are decreasing head from 15000 to 12000 Nm/kg, and your capacity increase by 5%.
This means that 5% more gas will go through the compressor. Is this right ?
(2) Now how this decrease in head is in different with operating in far left side of the curve(not on surge point)? I was reading one article regarding compressor capacity, and it says that when you operate on left side of the curve, compressor have plenty of free capacity available to compress the gas. When you move to right side of the curve(say, by decreasing head), the free capacity available to compressor decreases as more gas is now going through the compressor.

So, operating on left side of the curve(not close to surge point) is good OR operating on right side(not close to choke condition) is good ? Which capacity will be more ? And are we talking about free capacity OR capacity ?

i hope you understand what i am trying to say.

[gosooners]

All,

Can anyone attach paper/articles available on centrifugal compressor capacity control and parameters that affect centrifugal compressor ?

thanks

[Zauberberg]

Gosooners,

This subject is quite simple. If you look at the compressor operating curve (assuming fixed-speed, motor driven centrifugal compressor), you can observe that increase in actual volume flow at compressor suction decreases polytropic head developed by the compressor.

The head itself cannot be altered by some other means except varying compressor capacity, unless you apply mechanical changes to an existing compressor design (e.g. number of wheels, wheel diameters etc). How the system will behave depends on the actual control scheme - whether you're throttling compressor suction or recirculating certain amount of gas in order to maintain the set pressure difference (differential head) between the suction and discharge points.

There is one good article at PCS website, I think it will give you more clear understanding of how centrifugal compressor works in the process: Understanding centrifugal compressor performance in a connected process system