[Amps] Coupling a blower to an air system socket

[Roger (K8RI)]

On 3/17/2013 4:05 AM, Ian White wrote:
>> 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
>
> That is also true - the complete opposite of layouts like the Dentron
> DTR-2000 where the 8877 blasts HOT air at the tank circuit.
>
>>
>> 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.
>>
>
> Back to physics. The only fundamental requirement for cooling the tube
> is the air flow rate in cfm. Data sheets also quote the associated back
> pressure, but that is not a fundamental requirement - it also depends on
> the manufacturer's chosen air-system layout. In Eimac data sheets this
> was always for blowing upward through the specified air system socket
> and chimney, which places all the flow resistances in series and greatly
> increases the back pressure.
>
> Some history: the original Eimac air system socket was designed for the
> 4X150A, which had  fragile glass seals. The recommended method of
> blowing upward was entirely appropriate for the 4X150A because that tube
> certainly did need the full blast of air directly onto the base wafer.
> The 4X150A was very soon superseded by ceramic-metal tubes starting with
> the 4CX250 - but Eimac still specified the same cooling method.
>
> Even when faced with the evidence of the K2RIW and similar amplifiers,
> Eimac continued to insist that only their own specified cooling method
> would do, because the base seals of a metal-ceramic tube required just
> as much air flow as the anode. Until it suited them to change, that is.
> Enter the 8877 and the 3CX800, with a wafer socket that completely
> covers the base seals. In other words, Eimac's rejection of the K2RIW
> cooling method was a simple case of "Not Invented Here".
>
> All the work with has been done to show that the K2RIW cooling method
> does provide more than adequate cooling of the base seals. I and several
> others still have the temperature sensitive paints from the 1970s, and
> thousands more amplifiers using this cooling method have been built over
> the following 40 years.
>
>> OTOH  is relatively easy to raise the back pressure required for a
> given flow with
>> obstructions
>>
>> 73
>>
>> Roger (K8RI)
>>
> It's very easy to raise the back pressure, but not so easy to notice.
>
> Amplifier builders have a good understanding of "resistance" in
> electrical circuits, and most of us can understand "flow resistance" in
> hot-water circuits as well. Air flow circuits really aren't all that
> different - the fundamental requirement is to drive a sufficiently large
> current of air through the tube.
>


Given a specific tube design, dissipation rating, and max temperatures 
it is relatively easy to calculate the air flow required to carry away 
the required calories.  For a given anode structure this requires a 
specific back pressure.  IOW the anode structure behaves the same as a 
resistor in electronics or restriction in liquid flow.

The three are direct equivalents, not just merely analogous.

This is not arguing whether some other method is adequate, or even what 
is adequate, but that to get that air flow "Through" the anode requires 
that specific  delta T across the anode cooler.

What the manufacturer says is needed and careful calculation may result 
in answers that are quite different.  Still this does not change the 
fact that to get X airflow through a given anode structure requires Y 
pressure just as to get X current through a given resistor takes Y 
voltage (or pressure/potential difference)

whether the wind is blowing across the tube or along its axis makes 
little difference to the tube as long as there is enough to keep the 
seals with in their ratings as well as carrying away the heat from 
dissipation.

This has been accepted since the early days, long, long ago

73

Roger (K8RI)



>
> 73 from Ian GM3SEK
>
>
>