Specific sub-systems - The Winding System
The most important criterion for the organ blower is that it should be able to supply at the very least as much air as the organ might ever consume, and also that the pressure of the air in the wind distribution system will at all times be slightly higher than the highest pressure used in the organ. Nevertheless, the typical operating characteristic of centrifugal blowers is that the output pressure varies inversely with the flow.
The air demands of a pipe organ are extremely variable, because unlike the demands of air that you might find in, for example a factory or other large consumer of compressed air, there is no predictability to air demand in a pipe organ. The demand depends only on the needs of a particular piece of music and the artistic whims of the musician. At one instant, the musician might only be playing a couple of notes on the smallest pipes of the instrument, and then a fraction of a second later there might be a huge chord with the sforzando control in operation, and a second after that, there might be a moment of silence. Thus, very quickly the air demand can go from very small to many thousands of CFM and then drop to zero.
Because centrifugal blower output pressure varies inversely with the flow, this implies that the pressure of the air in a pipe organ wind distribution system varies considerably and in a random manner when the instrument is being played. However, the requirements of organ pipes are such that they must be supplied at all times with a pressure that is as steady as possible, with one exception which we'll look at later. And even in that one exception, the pressure must still vary in a very precise and controlled manner and within fairly tight limits.
Therefore, we encounter the second major pieces of equipment in an organ winding system after the blower, and these are the instrument's pressure regulators. The pressure regulators serve several important functions among which are 1, to supply absolutely steady pressure to the organ pipes regardless of demand, and 2, concurrently to filter out transient pressure excursions in the winding system, and 3, to isolate each division of the organ from pressure variations in the supply that are created by instantaneous demands in other divisions.
Although in small instruments one regulator might be sufficient, in most large instruments there are many different pressure regulators, each responsible for only a certain section. In some cases, there might even be a specific pressure regulator for just one rank of pipes. It is also common practice to put the bottom one or two octaves of very large, low-pitched ranks on their own regulators, entirely separate from anything else. Pressure regulation in pipe organs has always been a problem, but in the modern instrument with multiple regulators that are supplied by centrifugal blowers, it is possible to maintain extremely steady pressure at the organ pipes regardless of what the instantaneous demand might be. And this is very important, because the pitch and also the sound level or volume of organ pipes are in all cases at least somewhat influenced by the instantaneous pressure of the air being supplied, and different types of organ pipes will change pitch differently with supply pressure changes. As an example, if we have a Middle C concert flute pipe and sound with it a Middle C Trumpet pipe and both are designed for operation on 7" wind pressure, they will not be exactly in tune with each other if the pressure is lowered to 6" or raised to 8" although they would most likely both still work.
In early pipe organs, especially those that were supplied with air from large bellows, pressure regulation could be a problem, and sometimes in listening to one of these instruments, you can notice audible "hiccups" in the tone of some pipes when some notes are being held on and then others are played in addition. In good pipe organ design, there should be no significant unsteadiness in wind, no hiccupping in pipe tone, no sagging of pressure when the musical demands require a large number of pipes to sound at once, and likewise, no rebound or temporary over-pressure when you let go of a big chord; all of which defects you will frequently find in many early pipe organs.
It is curious to note that in some rather "learned" dissertations extolling the virtues of these early mechanical action pipe organs that you can find whole paragraphs devoted to reasons why unsteady wind in an instrument is a good thing and there is even a euphemism for it: "flexible wind." It is my opinion of course, but I am amazed at how sometimes in some published information about pipe organs, defects which the earlier systems had are described as ideals to strive for in good organ design. In my opinion, unsteady wind and the resulting hiccuping in pipe tone, and sagging or wildly fluctuating wind pressures are nothing more than defects. Calling it flexible wind still doesn't prevent it from being a defect. The only reason why I could think that such defects might possibly be acceptable in a new pipe organ would be if the instrument is to be an accurate replica of an instrument from an earlier time prior to the use of centrifugal blowers and modern pressure regulating systems.
In spite of all of these seemingly stringent conditions, modern pipe organ pressure regulators are very simple devices, both in their construction and also in their method of operation. On the next page we have pictures of a pipe organ pressure regulator, and also a cross-sectional view of one type which shows what a typical one looks like, and from which, along with the text, you will easily understand how it works. In good design, these regulators will hold air pressure to within ± 2% or even closer.