Pressure/Volume Accumulator

[alexit]

I have an older machine cooling system in the shop (not recirc but open to drain). Infeed is 5/8" dia line (~40psig) to T and valve that splits off feed to fill open top reservoir. After T fitting system is all ~3/4" to cooling system (then maybe 2.5" at exit from cooling system to drain.) Before inlet to cooling system there is Y to accept either direct from infeed or to take from a 2.5gpm pump (~40psi outlet) emptying open reservoir. This gets us cooling (for volume of reservoir) when no infeed available (>40min.) Simply power pump and keep running.

Now we need more cooling sometimes. By trial and error, IF we run the pump when there is an infeed, the cooling system outlet pressure is higher. Maybe we are also pushing back water from reservoir into the infeed line?

Could we be flowing at much reduced rate because only 5/8" infeed line?

I think to try an accumulator tank where the Y is, to take 40psi flow from 5/8" line and add % of 40psi flow from 3/4" pump outlet, to get more pressure/flow to the cooling system (cooling system was rated at 125psi.) How do I keep from pumping reservoir into infeed line, check valve?

I cannot change 5/8" line size, it is mostly under concrete and runs ~1000ft to main, so no more infeed is possible...

Thanks for your ideas,
Alex

 

[swainw]

Your problem intrigues me, but without knowing the fluid being piped, the pipe properties, and the pump characteristics it would be difficult to determine your losses through the system, the pressure drops, the effect of the resevoir head, etc.

I think you are working with a liquid so the accumulator stored energy would be no greater than the original feed pressure when full. At this point the accumulator will mostly act as a viscous dampener to fluctuations. It will not be able to increase your pressure head or flow rate without itself being pressurized higher than its source.

Thoughts for now to get you thinking. I think a pump change would be your best bet.

Bill

Bill Swain
Ultra Electronics Precison Air Systems
Technical Coordination Manager-US Programs
swainw@asme.org

 

[alexit]

Bill,

Thanks for the thoughts. I was afraid this would be the case, but hoped that I was seeing flow restriction causing my limit. The fluid is filtered tap water (infeed to system connected to public mains with large rope-wound filter.) Pressures above measured after filter, which is a must for water conditions are not best.

All exposed pipe is 5/8" or 3/4" Nylon (Legris) tubing. There is metal pipe in the cooling system, either SS or Copper but this is after I am measuring...this does not seem to have any restriction I can tell with 2.5" exit. All fittings are nylon compression-type, and the pump is 24VDC impeller-type with fixed speed for 2.5gpm and pressure relief fixed at 40psig.

If I treat the accumulator as flow storage, with 2x 5/8" inputs (infeed and from reservoir) and one 3/4" output this would dampen the variation in flow, but not increase flow if my limit is from 5/8" infeed? (I know steady-state is not possible, but for period of time before accumulator is empty?

Thanks,
Alex

 

[swainw]

Alex,

Your pipework isn't the problem. The losses due to the pipework will be what they are. The acummulator will soften the effect of any pressure fluctuations such as from the rapid opening at the end for service. But, it looks like you will need to increase your pump outlet pressure to sustain the losses inherent in the piping system.

The increased cooling capacity will come from an increased pressure head that will drive the flow rate to enable faster heat exchange at the cooler end. There will probably be limitations to the upper end of the heat exchanger's capacity to transfer the heat to the liquid. You may wish to talk with the heat exchanger manufacturer for some transfer numbers.

Your description is a bit difficult to diagram in thought to understand the role of the open resevoir vs. pump.

Good Luck


Bill

Bill Swain
Ultra Electronics Precison Air Systems
Technical Manager-USA
swainw@asme.org

 

[alexit]

Bill,

I had originally hoped the open reservoir was to be used like toilet with leak. From infeed we get steady stream to cooling system, but when we need we can activate pump to put additional water into system *flush*. However, the infeed pressure and pump pressure are close to same, and with the small infeed pipe I was trying to make another solution hence the accumulator before the cooling system.

I will try to measure infeed flow closely with pump on also and see if I can find any restrictions other than pressure to limit my flow.

Thanks,
Alex

 

[swainw]

Alex,

I think I understand it now. You effectively have a two branch feed system going into one that supplies the volumetric flow rate, i.e. GPM, at the cooler. Instead of increasing the source pressure you are trying to combine two streams into one to add more "GPM" through the system.

Have you done a mass balance on the system to see what are your driving forces, such as gravity head for the "flush" plus pumping, etc. then subtract your losses. Also, I can now see where your concern is about the pressure going back to the resevoir.

It seems to me you have to do a systems analysis of your layout to see what are the driven requirements to achieve the cooling desired. Then you'll be able to see what flow velocity you need to achieve that and the subsequent pressure heads and effects of that velocity on your pipe losses. I don't think you can approach it from a components or sectioned point of view as it is all linked together.

Bill

Bill Swain
Ultra Electronics Precison Air Systems
Technical Manager-USA
swainw@asme.org