7 replies to this topic

hello to everyone,
as we know the characteristics curve (Q flow rate vs H head) for centrifugal compressor,is it anything like that for positive displacement compressor curve (Q flow rate vs H head) ..

if it so available for positive displacment compressor also then how it looks..can anyone explain this in detail,so i grateful to you..

[Art Montemayor]

Rame:

There is no Performance Curve associated with reciprocating compressors that is generated by manufacturers. You may, of course, construct one but it would be an almost vertical line when plotted with the abscissa being the capacity and the ordinate being the total developed head.

I've never seen a curve for a screw compressor either.

A positive displacement machine is just that: it acts as a virtual metering device, passing on a metered amount of fluid with each repetitive action. Of course, with wear and tear there will be increased leakage. However, this is never a subject to track or to trend. When it starts to be a problem, you cure the problem by maintenacne or repair. In the case of a reciprocating compressor, you change out the piston rings or re-machine the cylinder liner. In the case of a screw compressor you have a more difficult and expensive problem. Do you feel you have a need for a Performance Curve?

[AlertO]

the curve look like the pump curve. See in the attached picture.

Attached Files

•  Comp_Curve.JPG   20.65KB   215 downloads

[raghav]

hi,

good input, valuable one, thanks

raghav

First, I should thank you for your valuable information. As Mr Art said, there is no characteristcis curve offered by vendors in the market. But factually the shape of the curve will form as you have given in the reply..so i want to raise one more question from the same concept:

In the case of centrifugal compressors, the characteristics curve contains two section of curves - flat curve where compressor produces almost constant head and diffrent flow rate, and steep section where the compressor head curve changes drastically with flow rate..

I want to know "how the flowrate of the fluid changes with head?" as per the characteristic curve..

I expect Mr. Art will resolve my queries, so i grateful to all who reply for this..

[djack77494]

if it so available for positive displacment compressor also then how it looks.

i want to know "how the flowrate of the fluid changes with head?" as per the characteristic curve.

rame,
I don't know what you're looking for here. Mr. Montemayor did answer your question when he stated that the characteristic curve would be very nearly a vertical line. devilchicken then went on to illustrate this with his very general curve shapes plot, showing a PD Compressor curve was nearly a vertical line. So, trying once more to answer your question, the flowrate is not very much impacted by changes in the head.

[iyer]

Listen.
A PD compressor, in case of a reciprocating compressor, the volume is "fixed". The discharge pressure is regulated by down stream pressure. The flow curve is a vertical line for P vs Q with P on the y axis. So how do we adjust flow, normally by loading the compressor suction valves/ clearance pocket, we can go from 0-50-90-100%(It differs for system to system). This hopefully ought to bring up another question from your side. What about in between these values. There are spill back control values for this. These are set to maintain inter stage pressure.
Hope I have not confused you too much. Please feel free to get back, if you need more clarification.

[kkala]

A PD compressor, in case of a reciprocating compressor, the volume is "fixed". The discharge pressure is regulated by down stream pressure. The flow curve is a vertical line for P vs Q with P on the y axis. So how do we adjust flow, normally by loading the compressor suction valves/ clearance pocket, we can go from 0-50-90-100%(It differs for system to system). This hopefully ought to bring up another question from your side. What about in between these values. There are spill back control values for this. These are set to maintain inter stage pressure.

In several cases reciprocating compressors operate intermittently (at full capacity when operating), discharging into an air chamber through a check valve. Distribution line starts from this chamber, the pressure of which vary between set pressures P1 & P2 (P1>P2). The compressor automatically stops when pressure = P1 and automatically starts again, when pressure = P2. So the supply pressure varies between P1 & P2, which may not annoy e.g. for plant or instrument air. And by increasing the air chamber volume (V), difference between P1 & P2 can be less and less.
Contrary to centrifugal compressors, a reciprocating compressor is usually installed with a standby spare.

Edited by kkala, 29 April 2010 - 03:22 PM.