----- Original Message -----
From: "Curt Nixon" <[email protected]>
To: "Damien Mannix" <[email protected]>; "drakelist"
<[email protected]>
Sent: Thursday, December 16, 2010 8:41 AM
Subject: Re: [Drakelist] AC4 heat Fan Summary
Hi Damien:
I perhaps got a little too annoyed at reading some of the
cr*p that gets passed off on line as worthwhile technical
information. Anyway...too issues going on really.
1. Do the AC4 or any other Drake or similar units need
fans?
2. How to implement fans..especially blowing in or out.
For 1. Do they need fans. I would say, technically
no..most have lasted over 40 years without them....
The fact is however, that the service life of electronics
is extended greatly if they are kept cooler. In the 60s,
adding a fan to a rig was an expensive proposition..thats
why many manufacturers made them optional. I am sure
Drake never had an engineering discussion about "lets make
sure these rigs are still working fine in 2010 !! They
did however have discussions about maximizing performance
at chosen price points. Today, adding a quiet, small fan
is trivial...so I opt to do it..as do many others. The
only real issue with the AC4 is that holes must be drilled
in the cover. I got over that once I replaced all of the
"original" filter caps, rectifiers and bleeders with the
Heathkit shop upgrade board. I use my Drake rigs; a lot.
When they are on, they stay on for 12-16 hours at a time
and frequently I run RTTY contests, or CW contests with
them. If a collection just sits there and never runs more
than a couple hours a month, I'm sure there is not much
need for a fan--anywhere in the system. Even the PS7
however, has an optional location for an 80mm exhaust
fan. And it is a large open enclosure compared to the AC4
and also has perforated covers.
It is interesting to note that the AC3 had no cover. I
suspect that Drake felt some safety pressure to cover the
PS. Also...the fact that the cover on the AC4 is aluminum
I suspect is not just coincidence but a thermal decision
as well. This could have been perferrated steel like the
rest of the line. Why not? I suspect that the solid
aluminum cover is better thermally..aluminum is MUCH more
conductive than steel. Its a big heat radiator.
As for question 2, all of these small fans are designed to
move air in only one direction. They also are designed to
work with a very small negative pressure on the draw
side...there ability to function drops to nearly zero if
they work against a positive pressure on the exhaust side.
SO...unless there is zero restriction in the path, they
should always be run pulling air and not pushing it. A
simple experiment can prove this. Put a 40W lightbulb in
a cardboard box and put a fan on it pointing in. seal it
up with a thermometer and turn on the fan and lite and
watch the temp. Poke a few small holes in the box and
watch wat happens....not much. Now turn the fan around so
it pulls air out..air will be drawn into the holes you
made..the temp will drop fast.
Or just remember what happens when the bag (creating
pressure on the fan) in your vacuum cleaner gets plugged.
Not much air flow going in then.
Confusion occurs I think because people think about things
like there furnace or a linear with forced air cooling.
There are blowers that ARE designed to work against
positive pressure. Squirrel cage blowers for example. In
this case, the blower is able to pressurize the enclosure
and the warm air is expelled thru vents in the box. The
fundamental design of the blower is different.
In fact, small fans can't operate very well with much
suction either but really die with pressure. They are
designed to operate in a moving stream of air. Restrict
the stream, and efficiency drops drastically.
The AC4 is really a excellent setup for cooling. The
slots around the cover low and high offer a fair amount of
total cross section open area, and when the fan draws air
out of the box, the ingoing fresh air is distributed
thruout the enclosure and exhausted via the fan. No dead
air and constant, low level flow.
A great setup would be the fan exausting from one end and
inlet holes in the opposite end. I looked at doing that
but then the PS would not fit inside the MS4.
Across Yaesu, ICOM, and the Drake TR7, ALL of the fans,
standard and optional, draw air out of the enclosures.
This maximizes the efficiency of the fan operation,
reduces the chance of dead air hot spots.
So..I am staying with my approach to utilize fans on all
of my drake and other equipment..I'll have the R-4 fan
install pix added soon to the picture album..as noted
before, it keeps the r4 audio tube heat from the
transformer and in the process, improved the initial vfo
drift performance as well.
Will the AC4 drop dead without a fan? No, surely not.
Will it last longer with one? Absolutely.
In the interest of closure, I will post this to Drakelist
as well Damien.
As always, questions or comments are welcomed.
Curt
KU8L
I don't want to trim the above discussion.
Much commercial equipment uses positive pressure
cooling, i.e. the fan blowing it. This has the advantage
that if a proper filter is placed over the fan the amount of
dirt entering the enclosure is substantially reduced. Where
negative pressure is used dirt can enter at any air leak
even when the enclosure has a specific entry point for air
with a filter on it. But, the use of positive pressure is
practical only where the enclosure has been designed
intentionaly for it. The main problem with forced air
cooling, whichever direction the air flows, is that one must
be careful that air which has been heated by one component
is not carried to where it will heat rathr then cool some
other component. Actually, this condition exists for either
air flow scheme. In the case of the AC-4 the enclosure is
sealed to begin with and must be modified to allow fan
cooling. If the location of the fan and air entrance are
chosen correctly either positive or negative pressure
cooling will be equally effective but if positive pressure
is chosen it has the advantage noted above, namely that
reduction of dirt accumulation is somewhat easier. If the
case were completely sealed except for the air openings this
would not hold true but there will be some air leaks and
with negative pressure dirt can get through them.
I am not sure about the effect on cooling of the
enclosure. It stops cooling by convection around the
components and direct radiation which would take place
without it. The case becomes heated and must itself
dissipate the heat either by convection or radiation. The
amount of heat dissipation will depend on the exposed
surface area of the cabinet. While the thermal conductivity
of the case material will have an effect I don't think it
would increase cooling unless it was made to have a very
large area, i.e., fins or some such arrangement. Further,
because the heat from the internal components is contained
in the enclosure the distribution of heat load inside is
different than it would be if the components were directly
exposed to the air. I have seen commercial equipment where
some components were severely overheated because the heat
generated by them, or by nearby components, could not
escape.
I think the main heat source in the AC-4 is probably
the power transformer. So, it should be closest to an
exhaust fan or closest to the air exit where a positive
pressure fan is used. The circuit board of the Heathkit Shop
mod reaches across the end of the cabinet and would
effectively block air flow around it where the chassis
mounted capacitors of the original design provide form much
freer air flow so that the design for cooling will be
different for the two arrangements. Forced air cooling of
either type would have little effect on under-chassis
components but I don't think much heat is generated there so
that may be of little concequence.
I also agree that where one wants additional cooling
but can not control the air flow well a negative pressure,
i.e., sucking, fan may be best as I think it is for cooling
the finals in T4 series transmitters where a blowing fan
would send hot air where its not wanted.
I do agree that operating the supply in the further
enclosure of the loudspeaker cabinet reduces the convection
around the case and probably increases the internal
temperature significantly.
I also agree that, in general, the cooler electronic
stuff runs the better.
If any one actually reads through all this they've got
to be serious about it:-)
--
Richard Knoppow
Los Angeles
WB6KBL
[email protected]
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