[Amps] Coupling a blower to an air system socket

[John Lyles]

Good summary of the actual mechanism of cooling finned anodes in tubes, 
Ian. I might mention that hams designing with these tubes should always 
consider the air density where they plan to operate their amplifier. 
Makes a significant difference if it is going to be 3000 meters up on a 
mountaintop or at sea level. Fans must be selected with this in mind. 
When I test them with amplifiers, I make a test bench where I run the 
blower into a circuit, all the while observing the airflow with 
anemometer and pressure meter through a straight laminar flow section. 
Then plot the operating point on the fan curves, and see if it is close 
to a bad regime. Its best to operate in the zone where a small pressure 
change doesn't cause a drastic fluctuation in flow and vice versa. It 
helps to have a variable speed blower for the first tests, as you can 
optimize your fan speed, noise to get exactlym the flow that Eimac or 
others want without overblowing with subsequent noise.
73
John
K5PRO


> Message: 4
> Date: Sat, 16 Mar 2013 22:56:58 -0000
> From: "Ian White" <gm3sek at ifwtech.co.uk>
> To: "'Jim Garland'" <4cx250b at miamioh.edu>,	<amps at contesting.com>
> Subject: Re: [Amps] Coupling a blower to an air system socket


> 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