In Denver, at 5000 ft, both the cfm AND the pressure both have to be increased by 20% vs sea level.
So here is an interesting question for you, Jim. If the CFM must increase by 20% due to the decreased air density (mass/volume) then why does Eimac say the static pressure increases by only 20% when earlier you said that it should increase by the square of the ratio of the CFM which would result in a 44% increase in this case?
The kicker is, blowers are all sea level rated @ 25 deg C ! You have to go through a series of math calcs to determine what sea level rated blower will do the job at higher elevations....and also higher ambient temps...if ambient temps are > 25 deg C.
Be careful here as you are double counting if you do that. If a blower manufacturer shows air and temperature derating curves, they are reworking the air density calculations from the supply side. If you have already done the calculations based on the load side (e.g. per the Eimac recommendation), you should not use their derated curves.
- Glenn W9IQ
Good questions Glenn. This is where Eimac's cooling notes in their 'care and feeding' don't jive with their published material for specific tube types....(anode diss, elevation and also intake air temps). The only blower de-rating curves I see are for 50 hz use...... on 60hz blowers. Typ they graph cfm vs backpressure for 60 hz, then again for 50 hz.
The example eimac uses in..'care and feeding' is a fictional tube that requires 100 cfm @ 1.0" pressure with an intake air temp of 25 C and located at sea level. Now the same fictional tube is required to be operated at 8000' and 40 C intake air temps. Works out to 142 cfm @ 1.42". After the maths, the blower selected from the catalog must be rated for 142 cfm @ 2.02" pressure and 25 deg C at sea level....to deliver 142 cfm @ 1.42" at 8000 feet + a 40 deg C intake temp.
