I like to use conventional cooling.
BUT with the blower mounted inside of the amplifier compartment. With
strategically placed intake holes, I achieve cooling of the tank and caps where
I think it's needed. I make sure to put in plenty of intake holes so there is
more input volume than output. I also bring in air into the “meter compartment”
to cool the WD7S triode board components and associated transformers as well.
Pictures here:
http://s900.beta.photobucket.com/user/w7ry/library/8877%20Amplifier
73
Jim W7RY
-----Original Message-----
From: Paul Hewitt
Sent: Saturday, March 16, 2013 8:10 PM
To: Ian White ; 'Jim Garland' ; amps@contesting.com
Subject: Re: [Amps] Coupling a blower to an air system socket
Greetings Ian
Besides the lower back pressure advantage of blowing into the anode
compartment, this method also cools the tank components. This helps reduce
themal tuning drift in very hi-Q tanks.
73, Paul
PAUL HEWITT
WD7S PRODUCTIONS
QRO HOMEBREW COMPONENTS
http://home.earthlink.net/~wd7s
----- Original Message -----
From: "Ian White" <gm3sek@ifwtech.co.uk>
To: "'Jim Garland'" <4cx250b@miamioh.edu>; <amps@contesting.com>
Sent: Saturday, March 16, 2013 2:56 PM
Subject: Re: [Amps] Coupling a blower to an air system socket
> >From: Jim Garland [mailto:4cx250b@miamioh.edu]
> >Sent: 16 March 2013 13:43
> >To: 'Ian White'; amps@contesting.com
> >Subject: RE: [Amps] Coupling a blower to an air system socket
> >
> >> Another very effective method of cooling is to blow air directly into
> >> a sealed anode compartment. Most of the air flows upward through the
> >> anode cooler and is vented directly to the outside through a chimney
> >> ABOVE the anode cooler. There is NO chimney between the base and the
> >anode cooler.
> >> Meanwhile 25-30% of the air flow is allowed to bleed downward through
> >> the tube socket to cool the base seals. This method reduces the back
> >> pressure on the blower, and allows it to deliver much more air than
> >> the conventional base-upward layout. It has been used very
> >> successfully for decades in VHF and UHF amps - so much so, it is
> >> regarded as "the normal method".
> >>
> >>
> > Very interesting concept, Ian. I'm wondering how the 25-30% downward
> flow past
> >the filament pins is adjusted? Presumably one needs to size an outside
> vent on
> >the underchassis to exhaust that air. Also, I don't quite understand
> why the back
> >pressure on the blower is reduced. Seems like most of the air has to
> flow up
> >through the anode cooler, which presumably is the largest flow
> impedance. The
> >air flowing past the filament pins has to have a bottleneck where it
> vents from the
> >enclosure to keep the flow down to 25-30%. . The combination of the two
> vents
> >(the filament vent and anode cooler vent) presumably reduces the back
> pressure
> >slightly, but I wouldn't think the effect would be very great.
> >
> >On a related topic: In my experience, a problem with blowers is often
> that the
> >motor rpm is too high, causing turbulence in the airflow. As noted by
> somebodly
> >else, turbulent flow is less effective at cooling an anode than laminar
> air flow. This
> >fact was known by the Collins engineers who designed the 30S-1 cooling
> system.
> >They mounted a low speed blower directly under the tube socket, powered
> to give
> >laminar airflow through the 4CX1000A. Some hams (misguidely, in my
> opinion)
> >swap the orignal 4CX1000A for a 4CX1500B, in the hope that the 1500W
> plate
> >dissipation of the latter tube willl provide a larger safety margin.
> Unfortunately, the
> >reverse happens, because the fins in the 4CX1500B are much more densly
> >packed, which inserts additional flow impedance into the air path and
> causes
> >turbulent flow. The actual net effect is to reduce the cooling and,
> hence, decrease
> >the amplifier performance.
> >73,
> >Jim W8ZR
>
> Sorry, Jim, but that is exactly backwards. Laminar flow is good for
> aerodynamic design where the objective is to minimize drag, and
> turbulence is your enemy. But in cooling applications the objective is
> to maximize the heat transfer from the hot metal into the cool air...
> and for that purpose, turbulence is your friend.
>
> Laminar flow is slow, smooth and orderly. A defining feature of laminar
> flow is that all of its streamlines (the lines that you'd see traced out
> by thin streamers of smoke) are parallel. The highest velocity is in
> the middle of the duct, tapering away to zero in the "boundary layer"
> alongside the walls of the duct. Laminar flow with a static boundary
> layer is great if your objective is to minimize drag; but laminar flow
> is bad for air cooling because that stagnant boundary layer acts as an
> insulating blanket.
>
> Turbulent air is the exact opposite - quick, swirling and chaotic. The
> turbulence breaks up the blanketing boundary layer and is far more
> effective at transferring the heat away from the surface and into the
> flowing air.
>
> The air flow into a blower is generally quite laminar; if you trail a
> streamer of smoke into the air intake, you can see that the streamlines
> hold together and remain substantially smooth and straight. But once it
> enters the blower, the air is stirred up violently by the high-speed
> blades and comes out highly turbulent. This turbulent air at the blower
> outlet is the most efficient means of cooling available, so ideally the
> blower should always be just upstream of the tube.
>
> The finned anode coolers of tubes like the 4CX1000 and 1500 are a form
> of heat exchanger, and the fins are intended to increase the surface
> area available for heat transfer. But this creates a large number of
> very thin airways, which force the air to flow straight and parallel to
> the fins - no matter what's happening outside of the anode cooler, the
> air flow inside is *always laminar*. We'd like it to be turbulent, but
> the spaces between the fins are simply too small to allow any whirlpools
> and eddies to form.
>
> The reason why it doesn't work to swap a 4CX1000 for a 4CX1500 is that
> the 1500W dissipation rating requires more air to be forced through the
> narrower gaps inside the cooler. If you don't change the blower as well
> as the tube, that isn't going to happen.
>
> Another major part of the problem is that small blowers are not very
> good at generating the pressure that is needed to drive a sufficient
> volume of air through the close-spaced fins of the anode cooler. A small
> increase in back pressure can cause a disproportionately rapid reduction
> in air throughout, which is known as "choking".
>
> The traditional cooling method is to blow air into a sealed grid
> compartment and then upward through the base, chimney, anode cooler and
> exit chimney. The problem is that each of these items creates some
> back-pressure and they are all connected in series so the back-pressures
> add together. You are constantly fighting against the characteristics of
> the blower and its tendency to choke.
>
> The method of cooling by blowing air into a sealed anode compartment was
> first popularized by the revolutionary K2RIW amplifier design for
> 432MHz. It was then exploited by Fred Merry, W2GN, whose amplifier
> designs for 50 through 220MHz are detailed here:
> <http://www.newsvhf.com/w2gn.html>
>
> As I said, this method of cooling is completely normal in the world
> above 50MHz. The advantage of this system is that it places the flow
> resistances of the anode cooler and the base in parallel. Back-pressure
> drops dramatically and the same blower can push a much larger flow rate
> through the anode cooler. The blower characteristics are now working in
> your favor.
>
> Jim, you were quick to notice the need to regulate the fraction of the
> total airflow that is directed downward to cool the lower part of the
> tube, but this is surprisingly non-critical. If the tube is mounted in a
> conventional base, that limits the downward air flow so all you need to
> do is seal the grid compartment and provide a screened vent of a few
> square inches. It's actually quite hard to get this wrong - if you make
> the vent too large, the blower will compensate by delivering more air
> without "robbing" the upward flow through the anode cooler.
>
>
>
> 73 from Ian GM3SEK
>
>
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