Ian,
Correct, The speed of the air over the tube fins (or body of a glass tube) is
what cools as it takes the heat away. The faster you do this, the cooler the
object is. You have to have a certain amount of pressure though to force the
air through the socket and tube fins. That's why most use a blower if they're
going to feed it into the under-chassis space and out through the tube. In
other words, the blower has to create enough pressure in the chassis so it will
seek the lower pressure easily. A regular fan wont do this and can't be used in
this respect as it cant create the pressure needed to do it. However, some have
used a large fan mounted directly under the tube and socket to blow air through
it. Of course they're not trying to pressurize a whole chassis and have the
opening at a 90 degree angle to the outlet either. I'm still a little skeptical
about this way myself though and don't trust it enough to cool an expensive
tube. What I've seen though is blowers that are rated
at around 2800 RPM to 3600 RPM used for this application and he's got one
slower which I know won't cool the tubes as good. Without Alpha telling him the
motor size, I'm not for sure how he can match it to what they had. About the
only way is do these tests and see if they match published specs for flow and
pressure for the tubes he has. The bag test I had forgot about, but will work.
Best,
Will
*********** REPLY SEPARATOR ***********
On 5/24/06 at 7:25 AM Ian White GM3SEK wrote:
>Will Matney wrote:
>>
>>The pressure on the outside is the low pressure and the pressure in the
>>chassis is the high. With the blower off, that's the low pressure or
>>room pressure where the water is settled. The difference between low
>>and high shows the back pressure. The low side though, I don't
>>understand what your saying? I always checked them the same as the set
>>ups shown in the handbooks showing the same setup using the tubing. Are
>>you counting the pressure above the chassis as the low pressure? It
>>should be the same as the outside pressure unless the cabinet is not
>>ventilated well enough. On most Alpha photos I've seen the tubes
>>chimney runs right to the top and out the vents. You could check it
>>that way but in reality it should be the same, especially if it set up
>>that way. I doubt that perf metal will hold back the outgoing air flow
>>very much. Now if above the chassis wasn't ventillated very good to the
>>outside where it caused a restriction to flow, then yes, the above
>>chassis would be used I
>> would think.
>>
>Pressure can be tricky to measure at low levels. It is like trying to
>measure voltage with an insensitive meter, and no clear idea where you
>should apply the two probes. You can measure "something"... but you
>don't really know what it means.
>
>As Will points out, the pressure that you measure depends on far too
>many different variables: the shape of the enclosure, exactly where you
>locate the probe, how the end of the tube is oriented relative to the
>air flow, and what you count as "the low side".
>
>But it doesn't matter anyway. What actually provides the cooling is the
>flow rate of air. A pressure measurement is only an indirect way of
>measuring the volume flow rate through a particular tube and socket - so
>why not go straight to what you really need to know?
>
>A volume flow measurement is SO EASY!
>
>Ten years ago, K6GT described a very simple way of measuring volume flow
>rate, by timing how long it takes to inflate a thin plastic garbage bag.
>It is WAY easier and more meaningful than trying to measure pressure.
>
>Original reference: AB6YL (now K6GT), QST 5/96.
>
>A quick search of the AMPS archives for "plastic bag" yields:
>/archives//html/Amps/1998-03/msg00171.html
>/archives//html/Amps/1998-03/msg00172.html
>/archives//html/Amps/1998-03/msg00430.html
>/archives//html/Amps/2001-10/msg00117.html
>
>To see these messages, click the link to the AMPS home page (at the
>bottom of every posted message) and then click "Searchable archives".
>
>
>>What most have been doing with the high speed blowers is cutting the
>>speed back with a rheostat. Then if the temperature gets so hot, a
>>thermostat shorts out the rheostat and full voltage is applied to the
>>motor. In his case, my guess he has a fan already slow enough, and I'm
>>afraid may be too slow.
>>
>Cutting back the rpm from 3000 to 1000 would be pretty disastrous. Both
>the pressure and the volume flow rate would reduce by a large factor.
>Are you sure they have sent you the correct motor?
>
>
>
>
>--
>73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
>http://www.ifwtech.co.uk/g3sek
>
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