----- Original Message -----
From: "Gary Schafer" <gschafer@mediaone.net>
To: "Ian White, G3SEK" <g3sek@ifwtech.com>
Cc: <amps@contesting.com>
Sent: Tuesday, March 05, 2002 6:33 PM
Subject: Re: [Amps] Fans for 4cx1000/1500 question
>
>
> "Ian White, G3SEK" wrote:
>
> > Richard wrote:
> > >>I need to know how to determine if the following fans will be adequate
> > >>for an 4cx1000/1500 amp. While I can find the cfm specs, I cannot
> > >>find the back pressure specs.
> > >
> > >Ordinary fans do not create enough air pressure to cool external anode
> > >tubes. A centrifugal blower makes more pressure. I like to mount the
> > >tube/socket in a cardboard box and measure air pressure with a
> > >manometer.
> > >
> > >
> > Agreed about the cardboard box mock-up, just to see if the blower is big
> > enough.
> >
> > Broadening the answer, it is better to measure the cfm directly by
> > letting the exhaust air blow up a thin plastic bag, and timing how long
> > it takes (as recommended here by K6GT). I use a 55 litre swing-bin bag
> > which typically inflates from flat to full in a few seconds... don't
> > know what size they are in the USA.
> >
> > Measuring cfm also allows you to use other cooling methods, which may be
> > better than the manufacturer's standard system of blowing in through the
> > input compartment, through the socket and out through the anode (aka the
> > missionary system).
> >
> > Blowing into a sealed output compartment and letting the hot air out
> > through a chimney on the *top* of the tube has many advantages.
> > Typically 25-30% of the airflow is bled down through the socket and out
> > through an exhaust port in the input compartment.
> >
> > This method has been very widely used for the last 30-40 years in VHF
> > amps, where the necessary sealed output compartment is standard
> > practice. Advantages are:
> >
> > 1. It circulates *cold* air all around the tank components, unlike the
> > normal system which circulates hot air.
> >
> > 2. The pressure drops upwards through the anode cooler and downwards
> > through the socket are now in parallel, not in series as in the normal
> > method, so the blower has to provide a lower pressure. That generally
> > means that the same blower will provide *much* more airflow through the
> > anode in this configuration, as well as cooling the base of the tube
> > very adequately.
> >
> > An hour or so spent with cardboard boxes, duct tape and a plastic bag
> > will show very clearly which method is superior, and whether your blower
> > is big enough.
> >
> > I wouldn't recommend every sucking hot air into the amplifier, or
> > through the blower.
> >
> > --
> > 73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
> > 'In Practice' columnist for RadCom (RSGB)
> > http://www.ifwtech.com/g3sek
> >
>
> Interesting thoughts Ian. Would you think that pressurizing the output
> compartment on an hf amplifier with a tube like a 4CX250 or a 4CX1000
would
> be practical?
> It sure would help on the 4CX250 if some back pressure relief could be
had.
> Looking at that tube though it seems that most of the air would go out the
> bottom rather than the dense plate heat sink.
>
> 73
> Gary K4FMX
>
>
There are some more points that may be of interest:
1. Airflow are also in the USA and can be found at www.airflow.com
2. On my 6m 8877 amp I use only a 40BTFL blower which is rated at
48.7 litre/min. This blower is about 20% smaller than the 45CTL I used
on the 4CX1000As on 2m. Both have a hefty motor and really deliver
the air.
3. No-one's mentioned the use of Plenum Chambers yet - they can be
a useful method of turning a good blower (with inappropriate
characteristics)
into a useful cooling device for an amplifier. The resistance
characteristics
of, for example a grid cooled amplifier, may not suit the optimum static
pressure versus airflow ratio (noted elsewhere) for a particluar blower, a
Plenum Chamber (aka "buffer") may be used to aide in some cases.
4. Other tests or metrics that can be considered (although admittedly after
you've built the amp) are (a) the ratio of output air temprature to input
(ambient) air temprature - since this is directly related to rate of flow
and
how many BTU (watts) you're removing and (b) thermal recovery time -
after a good period of TX (long contest CQ) how long (or how quickly)
does the air temprature from the anode outlet return to near ambient?
On a summer's day (in contest mode) with the ambient at say 25C the
outlet temprature on my 8877 6m amp running full-tilt doesn't rise
above 50C and is back within 3-4C of ambient in under 10 seconds.
Some amps frighten me - my Commander-II 3CX800 amp used in a
contest on 144MHz never recovers thermally between calls/QSOs
and even with a 10W 80mm PAPST fan helping suck the air out of
the anode it runs too hot...
Mike G8TIC/M0VHF
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