Operating Curves vs. Fluid Temp

[Sefrick]

Hey guys (gals)

I'm considering a condition where your fluid temperature varies with temperature. I know ( I think) when going from warm to cold your operating point shifts down/right.

I'm trying to prove to myself why!

I dusted off and cracked open my Smits book and considered the energy equation for pipe flow

(Pin/? + vin2/2 + g*hin)-(Pout/? + vout2/2 + g*hout) = W

With Wshaft set based on motor parameters and constant, when going from a more dense to less dense fliud I can see that velocity would increase. Seems to explain why the point shifts to the right. But what about the pressure? And thus far am I using the right approach? Are there other equations to consider?

Thanks for any input
Sef

 

[BigInch]

You have there a variation on the Bernoulli equation that does not consider frictional loss from flow in the pipes. Viscosity will change the operating curve more than the density, if the bulk of pressure loss in the system is due to friction flow losses, rather than static head lift.

Add head loss due to pipe friction "-hl" and determine that from a Darcy friction factor or similar pipe flow loss equation. I recommend Churchill.

If flowrate is held constant, more mass would be moved as a fluid becomes more dense, thus requiring an increase in power, or a slow down in velocity (towards the left on a pump curve).

As a fluid becomes more viscous (usually with decreasing temperature) more power is required to move it, if power is held constant, velocity slows.

As fluid becomes less dense with increasing temperature, the operating curve tends to move towards the right, as the same power (moving the same mass) would do so at a faster rate thereby increasing velocity (towards the right direction on the curve).

As a fluid becomes less viscous (usually with increasing temperature) the same power would tend to move it along faster, thus velocity increases.

So both effects combine to normally cause left movement on the curve with a decrease in temperature or right movement with an increase in temperature, given constant power.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/