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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