rod load

[Venezueladq]

In the simulation program ARIEL, the graphs show that as I increase the suction pressure, the compression load on the piston rod increaases. The cylinders are single-acting. Can anyone tell me why?

I think that in this case, the force exerted on the cylinder head is given by the clearance volume at the head end. Since there is no increase in discharge pressure, the force exerted on the piston is smaller, so it will generate less compression force on the piston rod.

2) Reply (from forum member abeltio):

Hello, I think it's only a matter of overlapping effects.

If the suction pressure is Ps and the pressure on the piston is provided by the ^p (delta p), the discharge pressure (Pd) is:

Pd = Ps + ^p

Being single action, the backpressure balances the load on the piston. Hence, the load increases with the suction pressure

 

[zdas04]

Abelito didn't get it quite right. It is more a function of the way the program works than a physical phenomena.

Work done by a compressor (and therefore rod load) at constant rpm is a function of: (1) gas composition (which did not change); (2) compression ratios (which went down); and (3) mass flow rate (which went up if you didn't lower the rpm in the simulation). At the end of the calculation the mass flow rate has the largest contribution to work. If you look at the report from Ariel Performance instead of the graph, you'll see that the volume flow rate at standard conditions is quite different on each of your runs and neither one matches your input flow rate.

I really like Ariel Performance, but you have to watch carefully to make sure that a "small" change in operating conditions didn't put you way outside the flow rate range that you are interested in. The program always honors your input rpm, suction pressure, and discharge pressure so that only leaves flow rate for it to adjust and it does. My approach is to make a change, run the report and look at the flow rate, then go back to the compressor picture and change the rpm up or down to get my target flow rate and THEN look at the rest of the report. If you do that, then you'll find that for the same flow rate the rod load goes down when you raise suction pressure at a constant mass flow rate just like your intuition expected.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[DRWeig]

Venezeuladq, zdas04's translation is here:

http://www.eng-tips.com/viewthread.cfm?qid=339303

Best to you,

Goober Dave

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