Thanks Carl,
Yes - 8877 (not 8873) and yes I was using slang (heater) when referring to
the filament.
There was another post I read regarding actual seal temperature using color
sticks, which is really what we are interested in. At the risk of boredom
(grin), it may also be of some added interest that while proportioning the
forced airflow to the power dissipation is valid (because of conservation of
energy), it is probably not valid with respect to the assumption that the
tube seal temperature remains constant in the process. This is because the
convective heat transfer coefficient (h-value) is a mild function of the air
velocity (Reynolds #) over the cooled surface, and h-value decreases with
lower air velocity. This in turn means that the surface actually runs
hotter at lower velocities, when power and airflow are both lowered in
equivalent proportions (which satisfies conservation of energy). By "mild
function", h-value has often been shown empirically to be proportional to
airflow raised to some power well less than unity. Since the theoretical
evaluation of h-value is difficult for complex geometries, it is frequently
determined by experimentation. If the relationship for the tube is assumed
proportional to Re^.5 (square root of airflow), then the surface temperature
rise for such a large reduction in airflow as 3:1 (as in the case examined)
might mean that seal temperature rise almost doubles (x 1.74) at the
limiting power case with this airflow reduction - which is not good. The
actual limiting power would therefore not necessarily be proportional to the
reduction in airflow, but perhaps more accurately be proportional to the
reduction in airflow raised to something like the 1.5 power - at least under
continuous dissipation. And that leads to another consideration - what is
the transient temperature response of the seal temperature with respect to
the thermal mass of the tube itself? Probably much more valid, and more
reliable to get an indication of the actual seal temperature as the post
suggested.
Would be interesting to talk to one of the tube engineers as to how they
evaluate this in R&D...
Matt
KM5VI
-----Original Message-----
From: Carl [mailto:km1h@jeremy.mv.com]
Sent: Tuesday, February 04, 2014 8:48 AM
To: Matt; amps@contesting.com
Subject: Re: [Amps] FW: Air temps leaving the amp.
I suppose you mean that the 8877 filament is 10A.
Carl
KM1H
----- Original Message -----
From: "Matt" <maflukey@gmail.com>
To: <amps@contesting.com>
Sent: Monday, February 03, 2014 10:56 PM
Subject: [Amps] FW: Air temps leaving the amp.
> Air temp rise = (total watts dissipated x 3.41) / (1.08 x cfm)
>
> Total watts dissipated = heater watts + (1-efficiency) x (input power +
> exciter power)
>
> 8873 heater spec is 10A @ 5.0 = 50w
>
> Assuming your tube is biased fully off at idle, then back calculating your
> fan cfm is as follows:
>
> Cfm = (50w x 3.41) / (1.08 x (82-68 F) = ~11.3 cfm
>
> On a good SSB pileup assume you are on the key ~50% of the time and your
> modulation duty cycle is ~50% running ~60% plate efficiency:
>
> Total dissipated power = 50 + (1500 x .5 x .5 x .6) = 275w
> (assume 1500 PEP at the output here includes the exciter)
>
> Expected temp rise would then be:
>
> Rise = (275 x 3.41) / (1.08 x 11.3) = ~76 F
>
> 68 + 76 = 144 F, whereas you measured 142 F, 11.3 cfm seems to check
> out....
>
> So to answer your question, given the Eimac spec....
>
> 1500w x (11.3 cfm / 38 cfm) = 446 watts total dissipation limit @ 11.3
> cfm...
>
> 446w - 50w (heater) = 394w net dissipation @ 11.3 cfm
>
> 394w / .6 efficiency = ~657w output or ~1050w input...
>
> It would seem that there could perhaps be a shortfall when holding a long
> dead key tune up or running a RTTY QSO.
>
> Matt
> KM5VI
>
>
>
>
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Gary Smith
> Sent: Monday, February 03, 2014 2:10 AM
> To: amps@contesting.com
> Subject: [Amps] Air temps leaving the amp.
>
> Before working the 160M contest last week I put the digital temperature
> probe I use for cooking meat in the oven into the air flow from my amp
> just
> to see what temps I generate when running a frequency.
>
> The room temp is 68F and at idle the temp sits at 82 as it exits the amp.
> Running pretty steady CQ and replying, the air temp gets up to
> 142 when I'm hot (pun intended) & heavy into the contest. According to the
> specs n the Eimac sheet it gives the amount of air that needs to be moved
> to
> handle 1500W anode dissipation as 38CFM to hold the temperature below 225C
> (437F) at 50C (122F) ambient temp.
>
> http://www.ok1rr.com/tubes/eimac/3CX1500A7.pdf
>
> Since they don't give air exit temps in the data I don't know if my 142F
> means I need to speed up the blower or if all is well as is. I'd rather
> keep
> the noise level low but would rather have more fan noise than a burnt
> tube.
>
> Idears?
>
> 73,
>
> Gary, KA1J
>
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