Cooling flow and pressure are like voltage, amps, and resistance
the figures as given in the data sheets tell two things. That the tube
needs a given flow in CFM for a given power and that the "fan" must be
capable of delivering so much pressure at the socket.
With that given, if the tube (3CX-800) requires 19 CFM you *WILL* have
0.50" of pressure at the socket. Conversely IF you have 0.50 inches at
the socket you will have 19 CFM. However if there is a down stream
restriction the pressure will rise and the flow will drop which would be
quite unusual. I'd not think this would be a normal situation, but when
designing an amp it is possible to not leave enough venting area for the
cooling to maintain proper flow.
Possibly one of the reasons for the difference between the data sheets
would be a redesign of the anode cooler fins. Another of course could be
a change to a more conservative CFM per watt
One point is the lower the pressure and/or flow the less efficient the
heat transfer and it is not linear as far as I can see.
73
Roger (K8RI)
Joe Subich, W4TV wrote:
> The current Eimac data sheet gives the cooling requirements
> at sea level as:
>
> Anode CFM Pressure
> Diss In. Water
> ---------------------------------
> 400 6 0.09
> 600 11 0.20
> 800 19 0.50
>
>
> However, the original (1983) Eimac data sheet for the 3CX800
> gave the following:
>
> Anode CFM Pressure
> Diss In. Water
> ---------------------------------
> 800 19 0.35
>
>
>
>> The cooling requirements are the same per watt of dissipation
>> for the 8877 as they are for the 3CX800.
>>
>
> Not according to the Eimac data sheet (1971) for the 8877
>
> Anode CFM Pressure
> Diss In. Water
> ---------------------------------
> 500 7.7 0.10
> 1000 20.3 0.23
>
>
> Each of the points has a different cfm/W value:
> 6/400 <> 19/800 <> 7.7/500 <> 20.3/1000
>
> 73,
>
> ... Joe, W4TV
>
>
>
>> -----Original Message-----
>> From: amps-bounces@contesting.com
>> [mailto:amps-bounces@contesting.com] On Behalf Of Paul Decker
>> Sent: Sunday, December 06, 2009 12:11 AM
>> To: garyschafer@comcast.net
>> Cc: amps@contesting.com
>> Subject: Re: [Amps] Advice
>>
>>
>>
>>
>> Hi Gary,
>>
>>
>>
>> I don't know if it's a linear relation. Perhaps a typo, but
>> on page 13.19 of the 1995 handbook, it says 12 CFM at 0.09"
>> back pressure for two 3cx800's operating at 400 Watts
>> dissipation each.
>>
>>
>>
>> Paul (KG7HF)
>>
>>
>> ----- Original Message -----
>> From: "Gary Schafer" <garyschafer@comcast.net>
>> To: "Paul Decker" <kg7hf@comcast.net>
>> Cc: amps@contesting.com
>> Sent: Saturday, December 5, 2009 11:57:34 PM GMT -05:00
>> US/Canada Eastern
>> Subject: RE: [Amps] Advice
>>
>>
>>
>>
>> Hi Paul,
>>
>> I didn’t mean that I didn’t understand the 2 nd paragraph but
>> that I didn’t read it. I just overlooked it when I read your
>> first post with it.
>>
>>
>>
>> I do think that you have the cooling requirements wrong for
>> the 3CX800 though. I got the info from the 2000 ARRL handbook
>> section on amplifiers. There is a table there that list the
>> cooling requirements.
>>
>> The cooling requirements are the same per watt of dissipation
>> for the 8877 as they are for the 3CX800.
>>
>>
>>
>> By the way I see that I made a type in one place where I show
>> .03333 cu ft/watt. It should be .02333 cu ft/watt.
>>
>>
>>
>> At 400 watts dissipation each they should require around
>> (.02333 x 400) 9.3 cu ft of air each or 18.6 for a pair for
>> 800 watts dissipation.
>>
>>
>>
>> A single 3CX800 operated at 800 watts dissipation would
>> require 19 cu ft of air per the chart at .5 back pressure.
>>
>>
>>
>> 73
>>
>> Gary K4FMX
>>
>>
>>
>>
>>
>>
>>
>> From: Paul Decker [mailto:kg7hf@comcast.net]
>> Sent: Saturday, December 05, 2009 6:26 PM
>> To: garyschafer@comcast.net
>> Cc: amps@contesting.com
>> Subject: Re: [Amps] Advice
>>
>>
>>
>>
>> Hi Gary,
>>
>> The point of the second paragraph was that with a pair tubes
>> operating under their specificed dissipation rating require
>> less cooling then a single tube operating at or above its
>> dissipation range.
>>
>>
>>
>> Two 3cx800's operating at legal limit only require 12 CFM at
>> about 0.09 - 0.1" of back pressure, this is because the tubes
>> are only operating at 400W dissipation each.
>>
>>
>>
>>
>>
>> Paul Decker (KG7HF)
>>
>>
>> ----- Original Message -----
>> From: "Gary Schafer" <garyschafer@comcast.net>
>> To: garyschafer@comcast.net, "Paul Decker"
>> <kg7hf@comcast.net>, amps@contesting.com
>> Sent: Saturday, December 5, 2009 5:11:10 PM GMT -05:00
>> US/Canada Eastern
>> Subject: RE: [Amps] Advice
>>
>> Paul, sorry I missed your second paragraph.
>> Air flow needed depends on how much power is being
>> dissipated. If you divide
>> the airflow by the dissipation rating of the 8877 as an
>> example 35/1500 =
>> .02333 cu ft/watt. multiply that by 1600 (for a pair of
>> 3CX800s) and you get
>> 37.3 cu ft of air flow. The cooling efficiency of both are
>> about the same
>> .02333 cu ft/watt.
>>
>> So if you ran the pair of 3CX800s at the same power level as
>> the 3CX1500 it
>> would require just about the same amount of air flow. .03333
>> x 1500 on the
>> pair of 3CX800s = 35 cu ft.
>>
>> I don't know what the back pressure would look like on the
>> pair of 3CX800s
>> at reduced air flow but it would probably be pretty close to
>> the 3CX1500.
>>
>> 73
>> Gary K4FMX
>>
>>
>>> -----Original Message-----
>>> From: amps-bounces@contesting.com
>>>
>> [mailto:amps-bounces@contesting.com]
>>
>>> On Behalf Of Gary Schafer
>>> Sent: Saturday, December 05, 2009 4:40 PM
>>> To: 'Paul Decker'; amps@contesting.com
>>> Subject: Re: [Amps] Advice
>>>
>>> The chart that I am looking at says: 8877 requires 35 cu ft at .41
>>> back
>>> pressure.
>>>
>>> A 3CX800A7 requires 19 cu ft at .5 back pressure. Two of those would
>>> require
>>> 38 cu ft at .5 back pressure.
>>>
>>> 73
>>> Gary K4FMX
>>>
>>>
>>>
>>>> -----Original Message-----
>>>> From: amps-bounces@contesting.com
>>>>
>> [mailto:amps-bounces@contesting.com]
>>
>>>> On Behalf Of Paul Decker
>>>> Sent: Saturday, December 05, 2009 4:26 PM
>>>> To: amps@contesting.com
>>>> Subject: Re: [Amps] Advice
>>>>
>>>>
>>>>
>>>> When I was writing the original reply I was thinking more
>>>>
>> along the
>>
>>>> lines of different tubes for example a single 8877 requires more
>>>>
>>> airflow
>>>
>>>> and has more back pressure than two 3cx800's.
>>>>
>>>>
>>>>
>>>> Agreed, two tubes of the same type would require more
>>>>
>> airflow than a
>>
>>>> single tube. However, I think we could assume that two
>>>>
>> tubes of the
>>
>>>> same type would produce 2x the power of the single tube
>>>>
>> amp. If the
>>
>>>> twin tube amp were run at the same output as the single holer, it
>>>>
>>> would
>>>
>>>> also follow to reason the airflow requirements could for the two
>>>> holer
>>>> could be reduced because each tube is only being driven
>>>>
>> to a reduced
>>
>>>> output.
>>>>
>>>>
>>>>
>>>> Paul
>>>>
>>>>
>>>>
>>>>
>>>> ----- Original Message -----
>>>> From: "Gary Schafer" < garyschafer@comcast.net >
>>>> To: < dezrat1242@yahoo.com >; < amps@contesting.com >
>>>> Sent: Saturday, December 05, 2009 11:04 AM
>>>> Subject: Re: [Amps] Advice
>>>>
>>>>
>>>> There is no free lunch. While it may seem like you could
>>>>
>> get by with
>>
>>>> less
>>>> air flow it doesn't follow.
>>>> A pair of tubes will require twice the air flow at the same back
>>>> pressure as
>>>> a single tube. Tubes being the same of course.
>>>>
>>>> 73
>>>> Gary K4FMX
>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> Amps mailing list
>>>> Amps@contesting.com
>>>> http://lists.contesting.com/mailman/listinfo/amps
>>>>
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>>
>
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