This routine helps with blower (fan) selection for power tubes.
It determines the correct amount of cooling air and at the required
back-pressure for cooling of power tubes. The 'formulae' used were
taken from various sources, including the Burle application note
'Cooling of BURLE Power Tubes.` Since many tubes are manufactured in America,
which still has not accepted the metric system, most tube data is in imperial
units. Pressure is in units of inches of water, flow rate in cubic feet per
minute, altitudes in feet etc.
If you then measure the back-pressure of the fan at STP (20øC/1 atm) with the
tube in, the program estimates the amount of cooling air flowing at STP.

You may pick data on the following tubes:
 0) 2C39A          1) 4CX250B.   2) 4CX250K          3) 4CX350A or F
 4) 4CX1000A       5) 4CX1500B   6) 3CX1500A7/8877   7) 3CX5000A7
 8) 7213/7214      9) 8794      10) Y831            11) Other type