Here=92s an engine dehumidifier you can build to increase the
probability that
your engine will make it to TBO.
An Engine Dehumidifier
EAA Chapter 79 Spokane, WA ---Dave Barker
The only owners likely to reach TBO targets are those who use their
aircraft
on a nearly daily basis. Why? The reason is not the flying. It is the
parking! The primary culprit for premature aircraft engine overhaul is
corrosion caused by condensation within the engine cavity that occurs
after
shutdown. Aircraft engines that are used daily frequently reach their
rated
TBO because liquid condensate is boiled off on a regular basis. Low use
results in reduced engine life. As the engine cools and the internal
temperature drops below the dew point, liquid moisture condenses out of
the
vapor and clings to internal engine surfaces. This liquid water then
resumes
its ongoing process of eating up your engine from the inside out.
However,
if the dew point can be made sufficiently low, then liquid water will
never
form. The engine dehumidifier provides a continuous positive pressure
injection of extremely dry air (dew point approximately -100=B0F) on a
24/7
continuous flow basis.
How it works:
The dehumidifier is connected the engine as soon after engine shutdown
as
possible before it cools. It is then run on a 24/7 basis. A small
aquarium
air pump forces ambient humid air thru a Plenum bottle containing Silica
Gel, the stuff used in shipping and storing aircraft engines and
electronics. The Silica Gel has a great affinity for moisture and
literally
sucks it out of the air. The dried air is filtered and injected into the
engine crankcase.
Any moisture inside the engine vaporizes with the incoming dry air and
is
displaced overboard by the constant positive pressure from the air pump.
At
some point in time, the Silica Gel will absorb all the moisture it can
hold
and turns blue.
At that time:
1. Remove the Blue saturated Silica Gel from the bottle.
2. Spread it out on a cookie sheet.
3. Heat in oven at 350=B0F until it turns pinkish again.
4. Cool and return to the plenum bottle.
That's' it.
The frequency of this recycle rate will depend up the humidity of the
local
environment varying from six months or more in dry regions down to just
a
few weeks in deeply humid climates. Adding more Silica Gel to the Plenum
will extend the service interval.
Connection hookup:
Connect the drier output via Tygon plastic tubing to a convenient engine
crankcase access port usually the crankcase blow-by vent. Alternately,
a
short standpipe may be added oil filler cap. I adapted a custom oil
filler
cap by drilling through the length of a 1/4" -20 carriage bolt. (I used
a
lathe to cut off the threads on the leading =BD=94 of the bolt. This
permits a
slip fit of the Tygon dry air supply hose. The hollow bolt was then
installed on the oil fill cap. Additionally I made a 11/4=94 -20
threaded
Delrin plastic plug to cap this little standpipe for flight.
Please NOTE: If you chose to use the crankcase blow-by vent pipe as the
input port you will have to also devise a temporary plug for the
freeze-emergency blow-by slot located few inches up the blow-by vent
pipe
inside the aircraft engine nacelle. If this slot is not blocked~ then
you
will be dumping the dried air out of this slot instead of into the
engine
crankcase.
Dehumidifier Components consist of:
1. Vibrating reed type aquarium air pump.*
2. 2-liter plastic pop bottle.
3. Airstone aquarium air bubbler
4. Six feet of 1/8=94 bore Tygon plastic aquarium tubing.
5. 12=94 of 3/16=94 o.d. (1/8=94 i.d.) rigid plastic tubing.
6. 1 pound of silica gel pellets.
*Note: The low cost aquarium pumps do have an irritating 60 Hz buzzes
caused
by their vibrating reed design. So-called =93silent=94 pumps are of the
same
design but are supported in a manner the will minimize noise. If you
spend
allot of time in the hangar get the =93silent=94 pump.
Tools Required:
Exacto Knife
3/16=94 drill
Hot Glue Gun
Parts List:
Aquarium Air Pump (* Optional Silent Air Pump)
2 liter clear plastic pop bottle with screw on cap.
6=92 Tygon aquarium tubing
l2" rigid 1/8"plastic tubing
Aquarium bubbler Air Stone
1 lb. Silica Gel Pellets or desiccant pellets or if you are at an
absolute
loss in scrounging Silica Gel, it can be purchased from the Michel's
Craft
Stores Chain. They sell it for drying flowers. It is highly overpriced.
One
of our chapter members scored 50 lbs of Silica Gel from our local
airport
engine conversion shop that installs P&W turbines on the Piper Malibu.
The
new turbines containers are packaged with Silica Gel.
Fabrication:
Drill 2 ea. 3/16" -1/4" off the center in the top of the bottle cap
close
enough to enter to allow easy tube clearance of the bottle neck interior
wall. For the pump inlet input, insert a 2" length of the rigid tubing
in
one hole and hot glue in place.
Insert the remaining 10" rigid tube in the other hole and hot glue it so
the
bottom end of the tube is positioned about 2" from the bottom of the
bottle.
Use a 1" length of the Tygon flex tube to connect the aquarium bubbler
Airstone to the end of the longer rigid tubing. The Airstone is used as
a
dust filter to keep Silica Gel articles out of the engine. The Airstone
should lie on the bottom of the bottle.
Fill a clean dry 2-liter pop bottle with 1 pound of Silica Gel pellets
or
Granules. Insert Airstone/tube assembly, work it to the bottom of the
bottle and tighten the cap. Use Tygon tubing to plumb the air pump to
the
short air input stub. Connect 5 feet or so of Tygon tubing to the
Airstone
equipped exit port and to the crankcase breather port or any other
convenient access to the engine crankcase. Note: This must be a leak
tight
fit. Mating to a typical crankcase vent tube usually located near the
firewall may be done by inserting a piece of the rigid tubing through a
closed-cell-foam ball or a tight fitting rubber stopper.
|