You cannot make compressor 1, or 2 run at a lower head than the other into the same discharge manifold. If you don't reach the manifold pressure with compressor 1, compressor 2 will fill up compressor 1 from the discharge side. It does not matter what type of compressor you have, never mind what the drive type is.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

BIGINCH IF IT WERE PUMPS, COMPARING INCOMPRESSIBLE TO COMPRESSIBLE, WHAT WOULD BE THE DIFFERENCE IF THE HIGHER FLOW HEAD PUMP PUSHES BACK THE LOWER FLOW HEAD ONE, WHERE APPROX WOULD BE THE COMBINED OPERATING POINT INTERSECTION OF THE SYSTEM HEAD CURVE AND PUMP CURVE? WOULD THE SMALLER PUMP STILL KEEP RUNNING OR WILL IT JUST STALL IF IT DOES STALL, THAT WOULD NOT BE GOOD FOR THE MOTOR COULD YOU KINDLY COMMENT ON THE ABOVE?

THANK YOU

Tomato5,

A fundamental principle for rotating machinery is that the operating point is at the intersection of system curve (i.e. resistance) & the machine curve(i.e. pump or compressor) keep this in mind when inspecting or analyzing rotating machinery performance/operation.

hope this helps. good luck.
-pmover

There isn't much of any difference if the fluid is slightly compressible or very compressible, except for the mass flowrate at the given moment. A highly compressible fluid will contain more mass in the same volume. As far as the equipment is concerned there is essentially no difference between responses. If one compressor controls the system curve, the other compressor will be forced to match it, back up on the curve similar to a pump. If it reduces flow, maxes out on power and overheats, or breaks a rod in the case of a recip, things will either balance to match the prevailing conditions at a lower flow, or quickly come to a halt.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

There is always an option to use control/throttling valve at discharge of the compressors. In this way the system curve is modulated for any operating scenario you need.

http://antwrp.gsfc.nasa.gov/apod/astropix.html

And hope your CVs don't fight each other, or don't wind up continuously in motion. In any case, pressure/head delivered at the discharge manifold must be equal from all sources.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

Tomato5,

Centrifugal compressors are constant pressure, variable flow machines. The inlet valves will be modulated to maintain constant pressure in the event of drop in capacity.

For multiple installations, if the capacity reduces, then the control valves (either inlet or BOV) will be controlled to maintain the pressure. The drivers usually will not influence the compressors for capacity changes.

Every compressor manufacturer will have control systems designed for multiple installations. Load sharing is much simplified if you use these programs (modules). The programs are so efficient that they can shut-off compressors depending upon the consumption of air.

Rakuday,
"centrifugal compressors are constant pressure, variable flow machi
nes" is simply not true. you should clarify.

-pmover

Well they certainly are ... when running on pressure control

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

Not "all" centrifugal compressors are "pressure controlled" units. thre are many variable speed flow control units in operation.

back to OP question, the variable speed unit "could" be operated at lower flow, same head condition, but likely at lower speed. but this mode of operation needs careful analysis of compressor curves to ensure the proposed mode of operation is possible. keep in mind that operating at conditions described, the var-speed unit is operating closer to surge control.

hope this helps.

i'm using a teeny-tiny smartphone wich is not easy to navigate/use and it is annoying at times.

--pmover

Of course, hence the smiley.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

Pmover,

At design point (maybe I should have stated this in my first post) centrifugal compressors operate at constant pressure & variable volume. When VSD’s are used, they vary the speed, but still deliver the design pressure. In VSD compressors, CV’s are generally not used as the motor can control the capacity. In some cases where capacity modulation is needed below 65% of the rated capacity, then CV & VSD can be used together. As you have rightly mentioned in your second post, “the variable speed unit "could" be operated at lower flow, same head condition”.

As seen in the performance curves of centrifugals, above the design point, when flow reduces, the pressure increases and the machines will surge, in such cases the CV’s are modulated to stabilize the compressor.

a simple way to run two centrifugals in parallel:
set one machine with a pressure set point approximately 5psig higher than the other machine. the unit with the higher set point will eventually be running full out, trying to reach the higher pressure set point. the other machine will be throttling trying to keep the pressure at the lower pressure point (which is the pressure you desire the system to run at).

this set up works fine if it is known that the demand for flow is less than both compressors running full out, but more than one running full out and the other throttled down below the surge line.

if the demand drops further, the lower set point compressor will start to run with recycle in operation which is not efficient. at which point, you will want a system balancing control that the OEM or other companies can provide.

Rakuday,

quoted
"centrifugal compressors are constant pressure, variable flow machines"
unquoted

As already mentioned, this is not correct.If you are on production you would maintain a given constant pressure against huge variation of suction pressure, MW, and flow whatsoever. In a downstream environment, the discharge pressure can be floating (e.g refineries process) whereas in this case the suction conditions are well known. That really depend on the application.

quoted
In VSD compressors, CV’s are generally not used as the motor can control the capacity. In some cases where capacity modulation is needed below 65% of the rated capacity, then CV & VSD can be used together. As you have rightly mentioned in your second post, “the variable speed unit "could" be operated at lower flow, same head condition”.

As seen in the performance curves of centrifugals, above the design point, when flow reduces, the pressure increases and the machines will surge, in such cases the CV’s are modulated to stabilize the compressor.
unquoted

Control valve are there anyway, at least for anti-surge protection, and quite commonly are used nowadays for continuous recycle except where special considerations apply (heavy duty recycle, etc) imposing some further checks or whereas an operator does not want recycle and anti-surge protection done by the same valve. On some situations you would omit completely without the control valve (hydrogen recycle service is an example). So it is not only a matter of VSD or not VSD.

As late reply to the OP... I would recommend to have a look on CCC documentation.
There is an algorithm for load sharing patented by CCC which is often adopted as standard compressor manufacturers.
some further doc maybe here:
http://www.cccglobal.com/sites/default/files/docum...

Hope that help

Correct definition of centrifugal compressor " variable volume delivery, fixed head capacity (for a certain flow), self limiting, is flow sensitive to system resistance, is flow sensitive to gas density...."