| Term | Definition |
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| Waste Heat | Sensible heat in non-combustible gases discharged to the environment.
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| Water | A liquid composed of two parts of hydrogen and sixteen parts oxygen by weight.
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| Water Column | A vertical tubular member connected at its top and bottom to the steam and water space respectively of a boiler, to which the water gauge, water level controls, and fuel cutoff may be connected.
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| Water Gauge | The gauge glass and its fittings for attachment.
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| Water Hammer | A sudden increase in pressure of water due to an instantaneous conversion of momentum to pressure.
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| Water Level | The elevation of the surface of the water in a boiler.
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| Water Softener | Removes hardness (CaCO3) from water through an ion exchange of sodium with calcium and magnesium.
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| Water Tube | A tube in a boiler having the water and steam on the inside and heat applied to the outside. |
| Water Vapor | A synonym for steam, usually used to denote steam of low absolute pressure.
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| Weep | A term usually applied to a minute leak in a boiler joint which forms droplets (or tears) of water very slowly.
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| Wet Steam | Steam containing moisture.
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| Wet-Bulb Temperature | The lowest temperature which a water wetted body will attain when exposed to an air current. This is the temperature of adiabatic saturation, and can be used to measure humidity.
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| Wetback Boiler | Firetube boiler design wherein the back portion of the boiler has a water jacket.
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| Wetness | A term used to designate the percentage of water in steam. Also used to describe the presence of a water film on heating surface interiors.
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| Windbox | A chamber below the grate or surrounding a burner, through which air under pressure is supplied for combustion of the fuel.
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| Windbox Pressure | The static pressure in the windbox of a burner or stoker.
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