[Vwdiesel] Hydraulics and heat: was, Mileage test 84 Rabbit. --- ( hagars Pickle Jar day ?

[Lee Hillsgrove]

  OK, time to hit the books. I'm not a scientist, but I do have plenty of
practical experience and a selection of reference material. No slight meant.




> I would be inclined to argue otherwise.  There is little energy
> released when a rather incompressible fluid is decompressed.
> This much is calculable.  The number will be small.  And this is
> why hydraulic systems are so neat.




 From 'Using Industrial Hydraulics', T.C. Frankenfeld, ISBN #0-932905-01-3,
page 1-17:

 "Unfortunately, the newcomer to hydraulics, until he gets burnt, so to
speak, with his first system, really has no way of realizing just how
efficient a heater a hydraulic system can be. We must remember that flow
multiplied by pressure is the hydraulic equivalent to horsepower. Wherever
we have flow from point A to point B in a system, with any loss in pressure,
there is an equivalent loss of horsepower between points A nad B. Since, a
pressure loss occours without doing useful work (causing mechanical motion)
a percentage of the input horsepower is wasted. This wasted horsepower shows
up in the hydraulic system in the form of heat. Of course, the higher the
pressure loss and the higher the flow, the more we waste horsepower. Some
typical examples where pressure is lost without doing useful work are
summarized as follows:

 Heat is generated when there is a drop in pressure as oil flows:
- through pipes & fittings
- through flow controls
- over relief valves
- across counter balance valves
- internal leakage of pumps and motors

 If we know the flow rate (GPM), and the loss in pressure (PSI) across any
device that does not produce mechaniical motion, we can calculate the heat
generated.

 Since the hydraulic system either does work, or creates heat, the units of
heat may be expressed in any units of work or energy. However, we more often
see heat expressed in BTU (British Thermal Units) than we do in ft-lbs of
heat.

1 BTU=778 ft-lbs

 In effect, this shows that one BTU cntains enough energy to move a one
pound object 778 feet."

 There's some more, but it just explains the formulae they use to explain
this formula:

1 HP = 42.4 BTU/min = 2545 BTU/hr  or BTU/HR = 1.5 x GPM x PSI, where PSI is
the loss in pressure that does no useful work.

 If you want, I can supply formulae for heat dissapation, pressure loss
through pipes, etc.



> I won't begin to dispute your experience with your pumps.
> However, I can tell you that my tractor hydraulic pump,
> which runs about 3500 psi bypass, doesn't begin to get
> hot.  It's a small pump, and it doesn't seem to get
> much warmer than the surrounding engine area.  I could
> measure it sometime, but I doubt if it's 50F hotter than
> the surrounding area, even under heavy load.
>
> What DOES get hot is the bypass valve area.  That's not
> because of the pressure differential.  It is because of the
> turbulent flow, and the heating secondary to that turbulent
> flow.



 I disagree; the reason it gets hot IS because of the pressure differential.

 The only thing that prevents your positive displacement tractor pump from
melting down is that your system is designed to use open-spool hydraulic
valves. There is essentially zero pressure at the pump unless you move one
of the valves and direct fluid to a cylinder, motor, or whatever. Otherwise,
all of the pump flow simply flows through the open center spool in the valve
back to tank with no pressure generation and thus no (or at least
negligible) heat generation.

 Try this: run a hydraulic cylinder to it's limit, deadheaded,  and hold the
lever there, so that the fluid bypasses continuously over the bypass in the
pump. (Don't do this for more than a minute or so, or you will begin burning
up the fluid!) Now, go and VERY carefully feel the pump. You might want to
wear gloves for this.

 That should be a good practical lesson in how much heat is generated
passing over a relief valve where no work is being done.


>
> The VW injection pump has less flow than the tractor hydraulic
> pump.  And the tractor hydraulic pump creates higher pressure.
> So there's more power put into it.  But the hydraulic pump is
> cooler than the VW injection pump.
>
> Why?
>
> I submit that it is because of turbulent flow.  Yes, some from
> the release of pump blow-by.  But likely more from the stirring
> of the fuel with the flyweights.

  See above.  :-))



>
> Look, I really don't mean to be argumentative.  I'm just sharing
> some of my reasoning, and "common sense" aquired through life.
> Just like your electrically driven pump story.  Perhaps this
> question would be a good one to have a ME student work out,
> and give us some sample numbers.  Can anyone give a textual
> description of the flywheels, including an estimate of their
> wetted area, and rotational velocities?


 Nor do I mean to be argumentative. Read over what I copied verbatim and see
if that changes your mind. If it doesn't, feeling your tractor pump bypass
after deadheading it for a while, that  should.



> I'm a scientist by profession, and we work lots of things out
> by numbers.  But we also try to shortcut the numbers with
> intuition and other experience.  I may be real wrong in guessing
> the source of the heat, but my best guess is the turbulent flow,
> vs. the pressure release.  There just isn't much energy to
> derive from simply releasing pressure on a liquid.


  Agreed, I know hydraulic fluid is about .5 to 1% compressible, primatily
due to entrained air. But, we're not releasing pressure generated by the
compressibility of the fluid; we're passing fluid that is held at pressure
by the pump over an orifice.

 Interesting side note: Brand-new hydraulic fluid right out of the drum is
"lazy". It contains sigificantly more entrained air than used fluid does.
You need to "work" the fluid to drive the air out.


>
> Guessing isn't really necessary, because a student experienced
> with hydraulics, fluid flow, and the like can readily calculate
> the different components involved.  We can then see if his or
> her prediction comes close the the heat gain we see in Hagar's
> test environment.


  I guess we need to gather and plug in numbers to the equations I provided
above.



>
> Val
> [COM-ASMEL, CFI-AI, N2EPO (work HF at all?)]



  No, not much HF. Most of my hamming lately is done VHF/UHF mobile on my
hour-long commute or when I volunteer at the many public service events my
club supports as well as others that they don't, like the Boston Marathon
or, coming up, the Maine Forest Rally. Can't wait for that one, I love to
watch rally racing and this one is run mostly on paper mill company roads
out in the puckerbrush of northern Maine.
 All I have for HF equipment is an old, borrowed Swan 350 with no antenna
tuner. I had a Windom dipole strung up but it didn't work too well. I dunno
if the tubes are shot or what but I was only seeing a few watts out. I need
to bring that to a club member who is up on Swans and see if he can tune it
up for me. I was able to work Europe occasionally, though, that was kind of
fun.

 ....Jealous of the COM-ASMEL, CFI-AI......

73 DE Lee KB1GNI


  Lee
  Oo-v-oO
  PP-ASEL
  KB1GNI