[time-nuts] Fan for old HP synthesizer

[Dr. David Kirkby]

Magnus Danielson wrote:
> From: "Poul-Henning Kamp" <phk at phk.freebsd.dk>
> Subject: Re: [time-nuts] Fan for old HP synthesizer (was RE: time-nuts Digest, Vol 21, Issue 20)
> Date: Tue, 18 Apr 2006 21:43:18 +0200
> Message-ID: <52293.1145389398 at critter.freebsd.dk>
> 
>>In message <001d01c6631e$8f49b960$8301000a at fakie>, "Christopher Hoover" writes:
>>
>>
>>>As someone else mentioned, the form factor is often quasi-standard.  Poring
>>>over the Digi-Key catalog may be your best bet.
>>>
>>> Mtg Hole Ctr Sp - English : 3.25 in
>>> Thkns - English           : 1.5 in
>>
>>This is what's called a 92mm x 92mm x 38mm fan these days.
>>

>>There are many 92x92x25 mm fans available, you may be able to
>>substitute one of these if 92x92x38 is impossible to find.
> 
> 
> When checking the EBM-Papst site (http://www.ebmpapst.us) the 92x92x25 mm fans did not
> reach the same CFM, but those that I found (3212JN and 3214JN) was not doing 117 VAC
> (they where doing 12 VDC and 24 VDC respectively). While Papst isn't all there is to fans
> they are usually a good benchmark on what is available.

You have to be careful with cfm ratings on fans. For small fans like 
this, the cfm rating will be in free air - i.e. no restrictions near by. 
For some larger fans, which must not be operating in free air, then this 
is obviously not so.

As soon as you restrict the flow by putting obstructions in the path of 
the airflow, so the flow rate drops. At a particular back pressure, the 
flow drop becomes zero. The actual shape of the flow vs pressure curve, 
which you see in fan data sheets, can be changed by the design of the 
fan blades.

I had an email from one of the fan manufacturers some time back when I 
was trying to replace a really odd sized fan in my Sun Ultra 80. (Sun's 
answer is to buy a new DC-DC converter with the fan on it, which was not 
too attractive.) Anyway, the fan manufacturer said that if a fan is 
designed for a particular customer, then it is quite common to optimise 
the fan blade design for there particular application.

If the old fan is still working, and you can measure the air inlet and 
outlet temperatures, then find the difference between them. If the new 
fan results in a smaller difference between air input and air output 
temperatures, then the flow is greater so all is well.

Obviously if this is in a cabinet with multiple fans, it is best to 
measure the difference only over the region being cooled by this fan.

HP kit is usually well over-engineered, but you need to be careful as 
you are at high altitude, where the air is thinner and so less effective 
at cooling. Your safety margin will be smaller than for someone at sea 
level.


dave