Al, do you remember this message?
>> Paul;
>> I think we need some clarification on this oil cooler info. Seems that
your
>> saying that a 3.5" thick oil cooler can't work because the air side
pressure
>> drop based on a specific case that is not applicable. The pressure
drop
is
>> dependent on much more than thickness; at the very least we need to
know
fin
>> and tube density, cross sectional area and whatever.
>>
Yes; I downloaded all three charts and did the math. A typical design
point
for that specific cooler is an oil flow of about 1.5 gpm and air flow of
about 20 lbs/min. That airflow results in a air side pressure drop of
about
4" water; and for that cross-sectional area, an air velocity of 22 mph.
Obviously we come to different conclusions for the same data; and it is
pointless to spend time disputing it.
I don't think it is pointless for our readers at all.
OK here are both the air flow and the heat rejected rate charts.
You choose 20 pounds per minute mass air flow rate.
Entering the calculated heat rejected rate chart
below at about 20 pounds per minute mass airflow gives
only 260 BTU/per minute. Obviously nowhere near enough.
Paul,
1 Btu/min = 17.56 watts
(You put the 60 in the wrong place, so are off by a factor of 3600)
So, 74,600 watts = 4246.206 Btu/min
175 Btu/min = 3074.509watts = 4 HP
Bill Schertz
KIS Cruiser #4045
Now if that is the correct conversions factor
from BTU/min to watts 260 BTU/min is only 4565.6 watts!!
Since one HP is 746 watts that is 6 HP! No where near enough
to cool the waste oil heat from a 13B let alone a 20B.
Checking the pdf file for the size of the cooler that corresponds to these
two attached charts it is roughly 4 inch by 5 inches by 3.5 inches thick.
This is too small by a factor of ten at the dynamic pressure that
corresponds to 22 MPH. Obviously this cooler is not going to work.
But in any case, it is not correct
for you to delete ( withhold from the other readers) the more important
part
of my last message; i.e.;
Sorry. I deleted it to simplify and make my point. My mistake.
I get caught [up] in my points from time to time. Forgot to include the rest
of your message. Sorry again.
"I am getting a custom oil cooler from Griffin for my 20B installation. It
has a core thickness of 3.35", commonly used by NASCAR. The MEASURED air
side pressure drop for the core for the mass flow per unit area that I
need
is 1.48" of H2O. Perfectly acceptable."
And, that with a core size of 5" x 20.4" (x-sectional area of 102 sq in)
get
the
needed heat rejection rate of about 2350 BTU/min for takeoff power on 100
F
day (total air flow 1600 cfm, oil flow 16 gpm, air velocity through the
core
of 26 mph). This is a very compact, effective heat exchanger, and; yes -
it
is nearly 3.5" thick.
Al Gietzen
Since one third of the heat rejection is by the oil and your engine is
roughly 300 HP you will need to reject roughly 100 HP or 74,600 watts
to the oil alone. Waste heat is roughly equal to net HP out.
Converting that to btu/min gives us 4200 BTU/min
The area of your chosen cooler is 100 square inches or five
times larger than this cooler represented by the attached charts.
Looking at a more realistic pressure drop of 6 inches of water or
about 100 MPH with a 80% efficient duct the mass airflow rate
would be 30 pounds per minute.
Entering the calculated heat rejected rate chart again
at about 30 pounds per minute mass airflow gives us 350 btu/min
at 25 pounds per minute oil flow.
The area of your chosen cooler is 100 square inches or five
times larger than this cooler and about the same thickness.
This gives us 1750 BTU/min.
This is is still too small by a factor of 2.4 at the six inches of
H2O dynamic pressure that corresponds to 100 MPH.
No way will it work at 1.48" of H2O.
If you have data for the custom oil cooler from Griffin
that differs greatly from the charts in the SW pdf file please by
all means share it with us.
Paul Lamar
Yes; I recall that message, and in my opinion it is still valid. I've been
swamped with other stuff, so I'll admit to not having followed through your
argument again in detail; but I think it best to just present the data on my
case.
With my prop speed limit, my power is about 265 hp - that is SAE hp
normalized (is it always is) to a standard day at sea level. So the only
time I'd get that is on a 60F day, WOT while flying low over the ocean. My
design point is more like a 90F and a density altitude of maybe 2500 ft. At
that point in a WOT climb I'll get about 5600 rpm, and that is about an
actual 220 hp.
The heat rejected to the oil then is about 2400 Btu/min. On the dyno, we
measured the oil flow rate to be about 16 gpm at 5600 rpm. That flow rate
requires about 30-35F temp drop in the oil through the cooler; which right
in a good range. Assuming a 60F air-side temp increase; my calcs say I need
about 2000 CFM through the cooler to remove the heat. If the air side temp
change is different, the CFM required is in direct proportion.
I measured the pressure drop across the core of my cooler in flow tests I
did here. Unfortunately, the blower I had would only give me about 1000 CFM
through the cooler. So I used the 5 data points I had up too that level, did
a regression analysis to fit to a quadratic; and extrapolated on up to 2400
CFM. That data is in the attached XL file.
I'll admit that extrapolating from a 1000 CFM to 2000 CFM with the quadratic
fit could introduce some error; but the data point that I got from Griffin,
which they claimed to be measured, was 1.48" H2O at 1580 CFM - which falls a
bit above; but quite close to my curve. What this data suggests is that to
get the cooling I need from this cooler, the air side pressure drop through
the core need be about 2 - 2.5" H2O.
This assumes a uniform flow distribution through the core; which my pressure
measurements suggest is nowhere near the actual case. So what I conclude
from all this is that my cooler design is quite right, but the air ducting
is not - which brings us back to the problem at hand. What I need, and what
the vane in the entrance may do for me; is get that air flow more uniform,
and increase the total air flow through the cooler.
Btw; I did have foam over the end of the tubes for the last set of
measurements, and the numbers I gave were again at an airspeed of 160 mph.
Hopefully I haven't made any serious departures from reality in all this;
and that I can get the 20-30% increase in air flow I need without major
mods.
Al
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