Following on from Tony Decap's useful description of organ blowers and
also Don Shenbarger's information, I wanted to correct an earlier piece
of information where I believe the writer is assuming that you need a
spill valve with a blower.  This is a commonly held misconception.  If
you are using a centrifugal type blower (the correct type for an organ)
as opposed to a vortex type, then a spill, or safety valve is not
required.  If no air is being used by the organ, then all that happens
to the air from the blower is that it is compressed and heated up in
the ducting.  It won't cause the regulator to explode!  That is because
the fan operates on the centrifugal principle.  It will only deliver
what is being called for.

In fact if you do have a spill valve then the blower will be working
harder, since it is blowing air to the outside, almost infinitely, as
Don Shenbarger's information so clearly describes.

My advice to anyone wanting an organ blower is to get a professional
to design and build it for you.  There are complicated calculations
required in designing the right machine, for it to be successful and
safe!  It may be an expensive item but some of them are doing 2,500
rpm with a fan diameter of twenty inches perhaps.  That is a
potentially lethal machine, if not built and balanced correctly.

Andrew Leach,
UK