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## [Amps] FW: Ceramic valve exhaust air temperature

 To: [Amps] FW: Ceramic valve exhaust air temperature "Matt" Fri, 27 Oct 2017 01:24:31 -0500
 ```Hi Alan, 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 non-compressed airflow. 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 here... :) 73 Matt KM5VI -----Original Message----- From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Alan Ibbetson Sent: Thursday, October 26, 2017 5:24 PM To: amps@contesting.com 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 _______________________________________________ Amps mailing list Amps@contesting.com http://lists.contesting.com/mailman/listinfo/amps --- This email has been checked for viruses by AVG. http://www.avg.com _______________________________________________ Amps mailing list Amps@contesting.com http://lists.contesting.com/mailman/listinfo/amps ```
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