[ Michael J Taylor asked about his organ blower: ]

> At what speed should this type of fan rotate to deliver around
> 7 to 8 inches?

Pressure is related to the square of the speed, so you need to double
the speed to increase the pressure 4 times.  That will provide you with
8" water gauge pressure if the present pressure is 2".

Some other things to consider --

1.  The velocity is related linearly to the speed of the blower speed,
so doubling the speed will increase the volume of air sent through the
system by 2.

2.  The horsepower is related to the cube of the blower speed, so
doubling the speed will increase the horsepower required by 8.  If you
are pulling a full load on your 1/4 HP motor, you will need a 2 HP
motor to handle the change.

3.  The impeller may not be rated for 3000 RPM.  Failure to observe
the safe operating speed of a fan often results in turning the fan into
shrapnel -- suddenly, catastrophically and without warning.

4.  The rules above apply to an otherwise unchanged system.  If you
change anything that has an influence on the number or size of openings
out of the system, the pressure will change.  In other words, if you use
more pipes at one time, the pressure will be lower.

5.  Running a fan in an unpressurized situation creates a greatly
increased horsepower load, so a dry run of the new drive arrangement
with the blower sitting open to the room will have a high power
requirement and may burn up a motor.  In other words, once you have
designed a system to run using a pressure blower, you need to run it
under conditions of designed pressure or the motor will overload.  At a
minimum, you can block off the blower inlet to prevent motor overload
until you have a grip on how it will perform.  (Pressure on the outlet
is equivalent to a vacuum on the inlet in terms of blower performance.
All that matters is pressure across the impeller.  You can usually draw
8" of vacuum safer than you cap off 8" of pressure.)

6.  Your ducting needs to be able to withstand the pressure.  A pressure
of 8" water gauge is nominally 1/3 PSI for reference.  If you have a
wood box in your system that is made of 1" boards and is 8" square
(outside) X 6 ft long, the force trying to blow your box apart is about
130 pounds on each board.

You really need to balance the volume of air you require with the
pressure you require to obtain a correct design.  It will be helpful
if you can provide the blower's nameplate data, manufacturer's name and
model number.  If the blower has no nameplate, it is probably not
intended for use as a pressure blower and may be dangerous in such
service.

If you can determine the specific details, I may be able to help in
blower selection.  Otherwise you may find better advice from people who
work with organs.  My experience in blower applications at the pressure
you desire is extensive but does not include organs or musical
instruments.

Don Shenbarger