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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