Gary, et al,
There are many ways to skin a cat. I take cabin heatmuff intake air
from the lower horizontal baffling aft of the prop flange via a 1.5 in scat
tube. There is no restriction on the air that is dumped into the lower
cowl if the cabin heat valve is closed. This intake location does not
alter the cooling air distribution to the cylinders.
Now, let's talk about temperatures. In the colder months, engine
cooling is less of a problem because the intake air is cooler and the separate
intake for the oil cooler is partially closed to keep the oil temp
indication up close to 180. This is cooling neutral. Let's also
remember that the pressure in the cockpit is much lower than that in the
upper cowl. Thus, there is no problem in getting hot air into the cockpit
and I seldom have the heater control fully open when the sun is also helping
with keeping the cockpit warm.
In the warmer months, a restriction plate is attached to
the intake air opening that changes it from the 1.5" to about .6" and,
if the cabin heat valve is closed, that lesser amount of air passes
through the muff and is dumped in the lower cowl. Thus, no super heating
of the pipe surrounded by the muff nor will the muff itself radiate high
heat in the lower cowl. This may also allow more of the upper cowl
air to be routed over the cylinders to compensate for the warmer
cooling air coming in to the upper cowl. The oil cooler intake air
door is open wide and the oil contributes to overall engine cooling.
This is also cooling neutral.
Try not to let the unknown unknowns get ya......
Hmmmmm.... Never mind.
Grayhawk
PS: A 320 located in the upper midwest and able to experience the
extremes of winter and summer (except for this
winter).
In a message dated 2/3/2012 7:02:59 A.M. Central Standard Time,
casey.gary@yahoo.com writes:
Peter,
What you mention is certainly the "common knowledge" with respect to
cabin heat and every plane I've seen since the dawn of (my) time has been
configured to route the hot air overboard when the heater is turned off, not
shut it off completely as you describe. Will that portion of the exhaust
pipe overheat if no air is flowing through the system? I doubt it, but
I'm not sure. How much hotter would it get? I don't know, but I'm
sure not a lot. After all, the rest of the exhaust system doesn't have
deliberate cooling - it just exists in whatever air flow goes around it, and
I've never seen any one worry about that. I would think it more likely
that the flex hose attached to the heat muff would overheat and fail
prematurely. Yes, the air dumped does reduce the air available to cool
the engine. I find it ironic that many (including me) go to great
trouble to reduce all leakage around the shroud, including the use of RTV
sealant, and then ignore the air flow through the heater valve. Maybe I
should put a restricter orifice in the "dump" line to reduce, but not
eliminate, that air flow. The answer to your question is that I'm not
sure anyone knows the answer to your question.
Gary Casey
From Peter:
I am routing my cabin heat hoses and the follwoing was braought to my
attention.
Presently I take the pressured air from my plenum, than to the heat
exchanger mounted on the exhaust and routing the heated air to the valve which
I can open for cabin heat or close it. If I close it, the air gets blocked at
the valve as I thought it is better to block it rather to dump the heated air
to the cowling. Reason for that is that I get the air from the plenum and
dumping it to the plenum reduces the available air to cool the engine and
reduces the pressure difference between the plenum and the cowling.
Now a friend mentioned that I might get a heat blocking at the heat
exchanger with undefined consequences. So to say better dump the air to the
cowling rather than getting a cracked exhaust.
What is your experience / opinion ?
Thanks in advance,
Peter