Aerodynamics of a vent?

[N_Cook]

Probably equivalent to coalfire chimney design.
Recently put in a cab top vent for a valve amp in desperate need of one.
The convection air coming out was 40 degree C higher than ambient so metal
of grill got fairly warm, better out than in. I was thinking next time
instead of 2 x 3 inches would be double that and reduce the grill
temperature. But would it, and would the convective effect be better, ie
the output valves running cooler with a larger vent.

Restricting the aperature in a fireplace increases the draught, to make a
lack lustre fire come to life, me was thinking.
No actual chimney , here, just a vent directly over the output bottles,
introduced milled holes in the pcb around the sockets and existing anular
gap around the bottles, through the steel chassis.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

 

[DaveM]

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:gea66r$bj7$1@registered.motzarella.org...

Probably equivalent to coalfire chimney design.
Recently put in a cab top vent for a valve amp in desperate need of one.
The convection air coming out was 40 degree C higher than ambient so metal
of grill got fairly warm, better out than in. I was thinking next time
instead of 2 x 3 inches would be double that and reduce the grill
temperature. But would it, and would the convective effect be better, ie
the output valves running cooler with a larger vent.

Restricting the aperature in a fireplace increases the draught, to make a
lack lustre fire come to life, me was thinking.
No actual chimney , here, just a vent directly over the output bottles,
introduced milled holes in the pcb around the sockets and existing anular
gap around the bottles, through the steel chassis.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

Generally speaking, the ratio of outlet area to intake area should be at least
unity. Greater than unity is better. To reduce the temperature of an air mass,
increase its volume; either with a blower or by enlarging the outlet area.

The relationship between temperatures, power and air volume can be shown by

Qa = Pd/((To/Ti)-1)

where Qa = Airflow in CFM
To = outlet temperature in degrees Kelvin
Ti = inlet temperature in degrees Kelvin
Pd = Power dissipated by active device(s)in watts

You can get an idea of the effectiveness of your cooling design by placing
thermometers at the inlet and outlet of your cabinet, and using the calculated
Pd of your amp, calculate the airflow volume with the formula above. Then,
restrict some of the vent holes, remeasure the temperatures and recalculate.
Those measurements should show that the more air flow, the lower the outlet
temperature.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Life is like a roll of toilet paper; the closer it gets to the end, the faster
it goes.

 

[Engineer]

On Oct 29, 7:29 pm, "DaveM" <masondg4...@comcast99.net> wrote:

"N_Cook" <dive...@tcp.co.uk> wrote in message

news:gea66r$bj7$1@registered.motzarella.org...

(snip)


Not addressing the question on vents; I'll just add that my tube
amplifiers are NOT in cabinets or cases (actually, one is, a Stromberg-
Carlson, but I don't use it - just a "collector"!) My reasons are: 1)
glass vacuum tubes (valves) look really good without cabinets -
particularly in a dark room (you can always tart-up the chassis with
polished wood sides, black-painted transformers, gold trim, etc.), and
2) they run cooler.
Cheers,
Roger
Thornhill, Ontario, Canada.

 

[N_Cook]

Engineer <junk2007@rogers.com> wrote in message
news:e5011776-2ac5-41d9-8a15-c694c00c6e15@v13g2000pro.googlegroups.com...
On Oct 29, 7:29 pm, "DaveM" <masondg4...@comcast99.net> wrote:

"N_Cook" <dive...@tcp.co.uk> wrote in message

news:gea66r$bj7$1@registered.motzarella.org...

(snip)


Not addressing the question on vents; I'll just add that my tube
amplifiers are NOT in cabinets or cases (actually, one is, a Stromberg-
Carlson, but I don't use it - just a "collector"!) My reasons are: 1)
glass vacuum tubes (valves) look really good without cabinets -
particularly in a dark room (you can always tart-up the chassis with
polished wood sides, black-painted transformers, gold trim, etc.), and
2) they run cooler.
Cheers,
Roger
Thornhill, Ontario, Canada.



You don't get windmilling guitarists in a domestic environment, let alone
clumsy grunts throwing kit around.

Otherwise amp controls at the top and 12 inch speakers mean that the valves
tend to be usually mounted upside down otherwise it means an extra six
inches of cab height.


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/