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For this discussion I will describe how a plain bell whistle works.
In order to visualize the parts of a steam whistle I took an actual 3” Lunkenheimer plain bell whistle and cut it in half with a band saw. Then I took a digital picture of it and edited the image and labeled the parts.
Whistles are designated as to whether or not the have a built-in or integral valve. Whistles with integral valves are designated wiv and whistles without valves are designated as wov. (Whistles without valves will require a valve downstream to control steam flow to the whistle.)
 Whether the whistle is wiv or wov, steam enters the whistle through the inlet and pressurizes the bowl.
Steam then exits from the bowl through the steam slot, which is a circular or annular opening between the bowl of the whistle, and the languid plate or spreader plate.
Steam exiting the steam slot hits the lower edge of the bell of the whistle. Some passes into the bell and some passes outside the bell.
Because of differing pressures inside and outside the bell a Venturi phenomena sets up a standing wave of vibrating air molecules and the whistle resonates at a frequency determined by the physical dimensions of the bell, principally the length but also the diameter.
Most plain bell whistles are adjustable i.e. the bell can be screwed up or down on the center post or stud. This adjustment does not change the pitch of the whistle as that is fixed by the dimensions of the bell. However, by adjusting the bell up and down the whistle can be set to achieve maximum resonance at a given steam pressure.
For higher steam pressure the bell needs to be adjusted up and for lower pressures adjusted down. If the bell is adjusted too far up it will not be able to resonate and will produce a poor quality sound and if adjusted too far down the whistle may become unstable and instead of resonating at the appropriate frequency may overblow and resonate at the third harmonic.
Single bell chime whistles are not generally adjustable, as are the plain bells. However to compensate for that the steam exits the steam slot and splits on the lip of the bell which is usually an arch. In most instances the arch comes down all the way to the steam slot. In effect, the height of the bell varies from zero to the maximum height of the arch above the steam slot. This would seem to be a compromise in efficiency but at least two points on the arch will be at maximum efficiency to produce resonance and everything above or below that will be operating at less efficiency. In practice this seems to work very well and is able to compensate for a wide range of pressures.
What are the various parts of a steam whistle? Just click here!
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