Re: L Bladeless Fan Technology

[hhoyt]

The Dyson ad is misleading: it merely has no EXPOSED blades. What it is
is a venturi pressurized by a bladed fan in the base. In this way it is
trading high-pressure low-velocity for low-pressure high-velocity which
is appropriate for a fan in free-air to cool human bodies.  Fans
designed to push air through heatsink fins are designed for moderate to
high back-pressure depending on fin density and height.

When we modeled our convective heatsinks for the KX2 & KX3, we also did
designs with fans (and brought some to Dayton).  After iterating a
heatsink design in ThermalCAD for hours to get the last percent more
convective cooling, it is sobering and amazing how much heat you can
remove by forcing air to remove the stagnant boundary layer of air from
the heatsink.  It makes you realize natural convection is a very weak
motor for air movement.

A tube amp can potentially be quieter than a solid state unit due to the
fact that the total heat removal in BTUs is directly proportional to the
temperature differential between the air and the object being cooled
(Newton's Law of Cooling).

Analyzing the thermal path in each situation; the anode fins of a common
3CX or 4CX series tube are directly cooled by the air and can run at
250°C, whereas the transistor has some additional series thermal
resistances to overcome before meeting the cooling air which can reduce
the effective heatsink temperature at FET max junction temperature and
heat flux to 125°C.  Assuming 25°C air, there can be a 225°C
differential between air and tube anode, and a 100°C differential
between the FET heatsink and air.  This means for any given volume of
air passing through the two devices, there can be 2.25 times as many
BTUs removed from a tube.  The other issue is the fact that the ceramic
tube fins are designed for high pressure low-volume cooling, and most
non-ducted FET heatsinks are designed for lower pressure, higher volume
air flow. High-velocity fans typically create more noise than do higher
pressure fans due to the relatively slower air movement and lower
turbulence past the fins.  This being said I have used some really noisy
tube amps.

I opt for an Elecraft KPA amp for many reasons; the fantastic K-Line
integration, included ATU (in the 1500) and protection circuitry they
design in.  I use a KPA500 which also can get loud, but I appreciate
what it takes to remove 500 W of heat to keep a 500 W amp cool (assuming
~50% efficiency).  Based on the reliability of the KPA500, I also do not
think there is a tube amp on the market which protects it's devices as
well as Elecraft does their FETs.  In order to address the noise issue,
perhaps a remote control like the Acom 2000A offers is a good option for
the KPA1500 at this point?

I was not paid to make this endorsement, just a happy customer.

Cheers & 73,

Howie - WA4PSC
www.proaudioeng.com


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[Edward R Cole]

Howard,

Good points on cooling.

Those of olden tymes, recall the fanless 6146, 813, etc.

The 3CX and 4CX ceramic tubes need high pressure for fast air
movement to keep them alive.  Note; cooling on these tubes is applied
with filament voltage and run several minutes after removing voltage
for cool-down.  My 2m-8877 is run this way.  I use a Lunar Link
blower and seven-inch exhaust fan (originally for cooling 19-inch
cabinets).  Fan causes partial vacuum for blower to work into.

Solid state amps occupy smaller "real estate" for similar power
levels so big issue is heat conduction from the transistor to a air
interface (cooling fins).  I see most of the new LDMOS kilowatt level
devices are mounted on thick copper bases to provide the lowest heat
resistance and then mounted to aluminum heat sink fins.  I would
never think of running with out some air movement assistance (fans).

Personally all my VHF+ high power LDMOS amps are being installed outside;
1.  Primarily to lower coax run losses
2.  Remove audio noise from the shack
3.  Benefit from cool ambient air  (here in Alaska summer air temp
runs 15 to18c; winter its -5 to -10c on average).  This might not be
helpful if you live in hot climates and using climate-controlled
interior air is better.

My 1000w 6m amp uses five fans in a semi venturi manifold so air is
also drawn thru cooling holes to interior ckt board.  My 1296 600w
LDMOS has 2-1/2 inch tall heats ink fins which I capped with a short
manifold and two fans covering entire heat sink top to exhaust hot air.
Only my 30w driver amp which is run 7.5w output is not
fan-cooled.  Have many days running outside this way.  Outdoor
enclosure has both 600w PA and 7.5w driver inside with air flow
assisted by the two 120mm fans.

73, Ed - KL7UW


-----------------------------------------
From: Howard Hoyt <[hidden email]>
To: [hidden email]
Subject: Re: [Elecraft] L  Bladeless Fan Technology
Message-ID: <[hidden email]>
Content-Type: text/plain; charset=utf-8; format=flowed

The Dyson ad is misleading: it merely has no EXPOSED blades. What it is
is a venturi pressurized by a bladed fan in the base. In this way it is
trading high-pressure low-velocity for low-pressure high-velocity which
is appropriate for a fan in free-air to cool human bodies.? Fans
designed to push air through heatsink fins are designed for moderate to
high back-pressure depending on fin density and height.

When we modeled our convective heatsinks for the KX2 & KX3, we also did
designs with fans (and brought some to Dayton).? After iterating a
heatsink design in ThermalCAD for hours to get the last percent more
convective cooling, it is sobering and amazing how much heat you can
remove by forcing air to remove the stagnant boundary layer of air from
the heatsink.? It makes you realize natural convection is a very weak
motor for air movement.

A tube amp can potentially be quieter than a solid state unit due to the
fact that the total heat removal in BTUs is directly proportional to the
temperature differential between the air and the object being cooled
(Newton's Law of Cooling).

Analyzing the thermal path in each situation; the anode fins of a common
3CX or 4CX series tube are directly cooled by the air and can run at
250?C, whereas the transistor has some additional series thermal
resistances to overcome before meeting the cooling air which can reduce
the effective heatsink temperature at FET max junction temperature and
heat flux to 125?C.? Assuming 25?C air, there can be a 225?C
differential between air and tube anode, and a 100?C differential
between the FET heatsink and air.? This means for any given volume of
air passing through the two devices, there can be 2.25 times as many
BTUs removed from a tube.? The other issue is the fact that the ceramic
tube fins are designed for high pressure low-volume cooling, and most
non-ducted FET heatsinks are designed for lower pressure, higher volume
air flow. High-velocity fans typically create more noise than do higher
pressure fans due to the relatively slower air movement and lower
turbulence past the fins.? This being said I have used some really noisy
tube amps.


73, Ed - KL7UW
   http://www.kl7uw.com
Dubus-NA Business mail:
   [hidden email]

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