[Amps] Water cooled amp question

[David Kirkby]

Stan C. wrote:

> Hi David. Thank you for the input.

No problems.

> Yes I have the tube data sheet (and I am more then willing to get you 
> a copy) from Eimac. And yes they call for 7.7gpm at the full rated 
> plate dissipation of 5KW.

Seems a lot to me - see below.

> The only thing I can offer is that because it has a smaller surface 
> area compared to the 4CW40,000,that is why it needs higher water flow.

I don't know if that follows, as there is a well known relationship 
between energy, termpature rise and mass.

Energy = mass x specific heat capacity x T

where T is the temperature rise. The specific heat capacity of water is 
about 4.2 J/(g * deg C)

If one gallon of water weight 10 lbs (as I think it does), then

7.7 gallons/minute = 77 lbs/minute  = 1.28 pounds/second.

2.2lbs = 1 kg, so 1.28 pounds/s is 0.58 kg/s or 580g/s

Rearranging the above equation gives:

T = Energy/(mass * specific heat capacity)

5 kW = 5000J/s

therefore T = 5000/(5800*4.2)=20.5 deg C.

This suggests to me the temperature rise will be 20.5 deg C. Does the 
data sheet give anything to indicate that might be rightt? I would have 
thought the tube would have tollerated a lot more than 20 deg C rise.

But for the 40 kW tube, which needs about the same water flow, the 
temperature rise will be 160 deg C or so. That is only possible on a 
pressurised system, and whilst the data sheet gave a maximum pressure of 
10 bar, there was nothing to suggest it needed to be run pressurised.

I'm sure there are others here who know more about this than me, but a 
20 deg rise in temp seems pretty small to me.

My logic might well be flawed somewhere.

If you can email me some scans of the complete data sheet, I'll stick it 
on my web site.

-- 
Dr. David Kirkby, 
G8WRB

Please check out http://www.g8wrb.org/ 
of if you live in Essex http://www.southminster-branch-line.org.uk/