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Looking at it from Q = MDt*cp. We know that the coolant mass flow (
M ) through the system (both coolant and air) is going to be a
constant at some specific power setting and air speed. With an oil/coolant
heat exchange as part of the system - then the waste heat (Q) must be exchanged
with the air through the radiator. Therefore the efficiency of the
radiator in discharging heat to the air is an important consideration. The
more efficient - the smaller the radiator required and lower cooling drag
possible.
So since we want to get rid of the same total Heat Q and
since mass flow and Specific heat is constant that only leaves Dt as a factor to play with. So all else being the
same, the configuration that produces the greatest Dt
between coolant and air will be the most efficient and require the smaller
radiator.
So if the coil cooler exchange is placed on the Hot side
of the radiator, that would increase the temperature of the coolant going into
the radiator and provided a larger Dt (more efficient)
than if it were placed on the Cool side of the radiator.
If placed on the cool side there is a greater transfer of oil heat to the
coolant , since the coolant has already passed through the radiator and is
lower temp, but that results in a higher temperature coolant going back into the
engine. To keep the coolant temperature at the same level prior to
inserting the oil/coolant exchanger into the circuit requires (as Tracy
pointed out) you to lower the coolant temp even more before it goes into
the oil/coolant exchanger by using a larger
radiator.
Since the objective is to remove heat most efficiently
from the engine (and hopefully keep cooling drag down), it would appear that the
oil/coolant heat exchanger being placed on the hot side of the radiator offers
an overall advantage from that perspective. Clearly either way, it can be
made to work.
Ed
Edward L. Anderson
Anderson Electronic Enterprises
LLC
305 Reefton Road
Weddington, NC
28104
http://www.andersonee.com
http://www.eicommander.com
Sent: Sunday, December 19, 2010 3:39 PM
Subject: [FlyRotary] Re: Fw: Water temps
"no matter which
leg of the coolant system you pick up the heat from the oil cooler, you raise
the temp of the coolant loop by the amount of additional heat; so the rejection
temp of the rad is higher either way."
Not so fast! Here's my take on
it.
Yes, the total heat
rejection of the rad has to end up being the same. BUT with the cold side
oil cooler, to keep the engine coolant inlet temperature (and temperature of the
engine block) the same requires a 33% larger rad (compared to what we needed a
separate air to oil cooler). Actually, the rad has to be even bigger
than this because we have to have the rad outlet temp even lower to compensate
for the rise in coolant temp due to temp rise of oil cooler. That
means the median temp of the rad is lower and therefore less
efficient.
If you cool the oil on the hot side of the coolant
circuit, the inlet temp of the rad is now higher than in the cold side
scheme and the required rad size and/or the airflow through the rad size
increase is smaller due to the higher delta T between air and rad. This
results in less cooling drag.
That's my story & I'm stick'n to it :
)
Tracy
On Sun, Dec 19, 2010 at 12:13 PM, Al Gietzen <ALVentures@cox.net> wrote:
For maximum cooling
of the oil from an oil/water HX (heat exchanger) you’ll want to plumb the oil
from engine out to the cooler; and plumb the cooler between the radiator and
the engine return. And I’m not quite sure about Tracy’s point because no
matter which leg of the coolant system you pick up the heat from the oil
cooler, you raise the temp of the coolant loop by the amount of additional
heat; so the rejection temp of the rad is higher either way. The
assumption is, of course, that you have sufficient capacity to keep the
coolant exiting the engine below boiling for extended high
power.
On my
Velocity 20B installation I
have a primary radiator in the cowl, and a secondary in the wing root; plumbed
in parallel. The wing root rad has an in-line 170F thermostat, and generally
only comes on line during extended climb. No thermostat in the engine that
would restrict flow and add a potential failure mode.
I have an oil/air
cooler and an oil/coolant HX, also plumbed in parallel. The oil/coolant HX is
plumbed to the exit of the primary rad. The oil temp runs about 20F
higher than the coolant (measured at oil return and coolant out). I could
probably reduce that difference by restricting flow to the oil/air cooler
forcing more through the oil/coolant HX; but I’m quite happy with the way
things work.
Over 200 hours and
lovin’ my rotary more all the time.
Al
G
----- Forwarded Message
----
From: "CozyGirrrl@aol.com" <CozyGirrrl@aol.com>
To: keltro@att.net
Sent: Mon, December
13, 2010 4:05:31
PM
Subject: Re: Fw: [FlyRotary] Re: Fw:
Water temps
Thanks
Tracy for the adjustment
in my thinking. I don't want to target temps unrealistically.
Where does that
leave us with our pick up and return points for the oil/water cooling system?
If the oil were being cooled with water only and we wanted the best possible
chance at that, wouldn't we want to feed it the cooler water from the high
pressure side of the pump prior to entering the block?
Whatever space we
would use for an oil/air cooler takes away from potential coolant radiator
space. Putting the two side by side becomes less efficient spacewise for both
due to losses for structure etc. If it is not possible to cool the oil
adequately with water alone then we'll need to back up a little and make other
layout plans. I remember Richter's Cozy III with three P-51 scoops, it got the
job done but was like dragging a parachute in drag.
Kelly, to your
comment below, while I am sure we'd have no problem putting adequate heat into
the oil, I am very concerned about keeping any more than just enough out of
it. Much easier going one way then the other =)
In
a message dated 12/13/2010 2:49:02
P.M. Central Standard
Time, keltro@att.net
writes:
My own opinion is that after warm up and in flight the oil temp leaving the
engine before the
coolers (air or
water to oil) will almost always be well above
160-180 F..............If it is not this
high then
the oil to water cooler will actually help warm it to a more efficient
temp.........IMHO
Somebody correct me if this is a fallacy
!!.......................<:)
Kelly
Troyer
"DYKE DELTA
JD2" (Eventually)
"13B ROTARY"_
Engine
"RWS"_RD1C/EC2/EM2
"MISTRAL"_Backplate/Oil
Manifold
"TURBONETICS"_TO4E50
Turbo
From: "CozyGirrrl@aol.com"
<CozyGirrrl@aol.com>
To: keltro@att.net
Sent: Mon, December 13,
2010
12:52:06
PM
Subject: Re: Fw: [FlyRotary] Re: Fw:
Water temps
This is where I
get confused:
-ideal water temp
= 160*~180*F
If we are cooling
oil with water that is at best hotter than the ideal temp of the oil, then
are we not adding heat to it rather than removing it?
If we are trying
to cool oil, why would we feed the highest temp water to the oil/water
cooler rather than the coolest temp water by tapping into the pump housing
where it enters the block?
Based on feedback,
the water entering the block may be as low as 150*~160*F, would this be cool
enough to do an adequate job of cooling the oil?
Also, which model
of Mocal is being used?
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