Considering airflow at std. room conditions, temperature rise can be
approximated as follows:
Temp Rise = watts x 3.16 / cfm
For the derivation...
Q = Cp x mass flow rate x delta T
Mass flow rate = volumetric flow rate x density
For air at standard conditions Cp = ~0.24 btu/lb*F & density = ~.075 lb/cf
Therefore; Btuh = Btu/hr = (0.24 btu/lb*F) x (cf/min) x (60 min/hr) x
(.075 lb/cf) x (delta T in F) = 1.08 x cfm x delta T. This is a common
engineering formula in the states for estimating the sensible heat gain of a
By algebraic rearrangement, Delta T = Btuh / (1.08 x cfm)
However, 1 watt = ~3.41 Btuh so...
Delta T = Btuh x (watts/3.41 Btuh) / (1.08 x cfm) = ~3.16 x watts / cfm
To apply this to your specific Eimac temperature limit question:
At 575w dissipation, Detla T = (3.16)x(575) / (7.8) = 233 F
50C = ~122 F, therefore exhaust temp =~ 233+122 = 355 F (wow!)
Bear in mind that the anode dissipates input power - output power + heater
input. So for a 60% plate efficiency at 1500w out, the anode dissipation
would be something like 2500 - 1500 + 75 = 1075 watts
If the blower on an 8877 blower moved 38 cfm, 1075w dissipation would
produce a temperature rise of about 89F. Considering ambient room
temperature of 71F this would be about 160F discharge temperature.
I have not ever tried to measure the discharge air temperature off my own
8877 but I can tell you that the discharge air gets mighty warm when the amp
is keyed up at 1500w out. I have no idea what the airflow is.
Hope this helps you and that I got my numbers right - it's pretty late
From: Amps [mailto:email@example.com] On Behalf Of Alan Ibbetson
Sent: Thursday, October 26, 2017 5:24 PM
Subject: [Amps] Ceramic valve exhaust air temperature
I'm looking for a sanity check on some calculated ceramic tube exhaust air
temperatures predicted by the data sheets.
Given the blower inlet temperature, the Specific Heat of air (Cp) and the
rate of air delivery, simple arithmetic predicts the exhaust temperature at
a given tube dissipation: temp rise = watts/air mass per sec/Cp. Or have I
got this wrong?
The reason I ask is that the Eimac data sheet for the 8877 says it needs
7.8 cfm air at 50C ambient to keep the seals at a safe temperature when the
tube is dissipating 500W (plus 75W for grid/heater). If I have the "simple"
calculation right this results in an exhaust air temperature of 134C above
ambient, so around 160C (320F) in an averagely warm shack. Is it really
going to be this hot? It seems to me that the cabinet paintwork could be
damaged as well as people/pets.
The data sheet air volumes at higher dissipation rise in much more than
simple linear progression, presumably due to the kinetics of the airflow
over the tube and socket. Hence the predicted exhaust temperature for an
8877 at 1500W anode dissipation with the recommended 38 cfm airflow is 74C
above ambient (so maybe 100C). This will still burn you but at least the
paint on the amplifier cabinet probably won't melt.
Has anyone actually used an 8877 at 500W Pd (a fairly typical UK-legal
figure) on, say, RTTY with a puny 7.8 cfm blower? Did the cabinet feel like
it was about to catch fire? Or are my numbers all messed up?
The reason I ask is that there is no warning about the possible dangers of
these exhaust temperatures in the Eimac data sheet, which might lead novice
builders astray. I would feel happier with a target of 30C temperature rise,
which requires 35 cfm for 500W anode dissipation, 65 cfm for 1KW and 100 cfm
for 1.5KW, all at 100% duty. I suspect that the backpressure will defeat
most practical-sized blowers even when asked to deliver 65 cfm, let alone
100 cfm, especially in a conventional 'through the SK2210 socket'
configuration. Maybe you could get closer with the K2RIW approach of
pressurising the anode compartment?
73, Alan G3XAQ
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