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In French, tuyau means pipe or conduit. Literally then, a tuyau is almost any type of rigid or flexible pipe. More commonly, however, a tuyau references a specific hydrostatic pressure mechanism involving underwater pipes and a plenum chamber to supply compressed air to operate machinery. The mechanism was used as an early forge device by Romans and greatly changed how iron was processed in ancient times. Compression from a tuyau functioned much like bellows in more modern forges.
Roman iron-working forges further developed the device over the centuries, with some forges featuring tuyaux with water-powered hammers to work molten iron ore. Since the device had no real moving parts or wearable seals, it was easy to set up near streams and other bodies of water. Deep water allowed the tuyau to produce higher compression and therefore, more power. Compressed air from a pipe-only based water system is cool and surprisingly dry, in spite of being derived from water.
As for specific construction, both ancient and modern tuyaux feature a pipe with internal nozzles, placed with one end facing upstream. Water flows into the pipe while the nozzles help to create more air bubbles in the incoming water. Increasing the water inside the pipe results in the pipe sinking deeper into the water, thus increasing hydrostatic pressure. Gravity, water depth, and water temperature all factored into how much pressure is produced during this process.
At the opposite end of the pipe, a small plenum chamber attracts the air bubbles and forces water flow sideways. Higher hydrostatic pressure increases the number of air bubbles pushed into the plenum chamber. Air bubbles inside the plenum chamber pop as a result of changing pressures, supplying compressed air that can be fed through another pipe to power forges, water hammers, and other mechanisms. Once air bubbles pop, the hydrostatic pressure in the tuyau decreases, the pipe rises, and the process repeats.
Decreasing hydrostatic pressure combined with the release of compressed air powers the necessary water discharge during the final processing phase of a tuyau system. Pipes rise out of the water as pressure drops, in order to pump water back into the stream. Each step of the process repeats and overlaps, providing a consistent supply of compressed air. While consistent in terms of supply, the force or pressure of compressed air delivery is not always as consistent.
Use of a tuyau system helped to change the placement of iron-working operations in Roman times. Rather than focusing on supplies for fires and thus locating forges near forests or coal supplies, operations could instead focus on power generation. Understandably, forges were thus moved closer to streams and other bodies of water with a good current in order to take advantage of falling water systems like a tuyau. Many improvements over the centuries came from modifying water-power systems used in mills and other industries.