The _pressure_ producer contains the following essential parts:--
The _generator_, a vertical furnace fed from the top through an air-tight trap, and shut off below from the outside atmosphere by having its foot immersed in water. Any fuel or ashes which fall through the bars into the water can be abstracted without spoiling the draught. Air and steam are forced into the generator, and pass up through the fuel with the chemical results already described. The gases then flow into a _cooler_, enclosed in a water-jacket, through which water circulates, and on into a _scrubber_, where they must find their way upwards through coke kept dripping with water from overhead jets. The water collects impurities of all sorts, and the gas is then ready for storage in the gas-holders or for immediate use in the engines.
A pound of anthracite coal thus burnt will yield enough gas to develop 1 h.p. for one hour.
_Suction Gas Plants._--With these gas is not stored in larger quantities than are needed for the immediate work of the engine. In fact, the engine itself during its suction strokes _draws_ air and steam through a very small furnace, coolers, and scrubbers direct into the cylinder. The furnace is therefore fed with air and water, not by pressure from outside, but by suction from inside, hence the name "suction producer." At the present time suction gas engines are being built for use on ships, since a pound of fuel thus consumed will drive a vessel further than if burnt under a steam boiler. Very possibly the big ocean liners of twenty years hence may be fitted with such engines in the place of the triple and quadruple expansion steam machinery now doing the work.
Every iron blast-furnace is very similar in construction and action to the generator of a producer-gas plant. Into it are fed through a hopper, situated in the top, layers of ore, coal or coke, and limestone. At the bottom enters a blast of air heated by passing through a stove of firebrick raised to a high temperature by the carbon monoxide gas coming off from the furnace. When the stove has been well heated the gas supply is shut off from it and switched to the engine-house to create power for driving the huge blowers.