[Amps] Coupling a blower to an air system socket

Sun Mar 17 02:03:07 EDT 2013

On 3/16/2013 11:10 PM, Paul Hewitt wrote:
> 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

One thing to remember about external anode tubes. They  often have 
cooling requirements listed as so many cfm at a given back pressure,

I do not know of any way to achieve the required air flow at a reduced 
back pressure other than an exhaust fan reducing the exhaust pressure 
thus making it a little easier to get more cooling air through.
I believe Emtron and OM both use this approach on some models.

OTOH  is relatively easy to raise the back pressure required for a given 
flow with obstructions

73

Roger (K8RI)

>
> PAUL HEWITT
> WD7S PRODUCTIONS
> QRO HOMEBREW COMPONENTS
> http://home.earthlink.net/~wd7s
> ----- Original Message -----
> From: "Ian White" <gm3sek at ifwtech.co.uk>
> To: "'Jim Garland'" <4cx250b at miamioh.edu>; <amps at 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 at miamioh.edu]
>>> Sent: 16 March 2013 13:43
>>> To: 'Ian White'; amps at 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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