| Term | Definition |
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| Radiation Loss | A comprehensive term used in a boiler-unit heat balance to account for the conduction, radiation, and convection heat losses from the boiler to the ambient air.
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| Rate of Blowdown | A rate normally expressed as a percentage of the water fed.
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| Rated Capacity | The manufacturer's stated capacity rating for mechanical equipment; for instance, the maximum continuous capacity in pounds of steam per hour for which a boiler is designed.
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| Rating | See "Load."
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| Raw Water | Water supplied to the plant before any treatment.
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| Reaction | A chemical transformation or change brought about by the interaction of two substances.
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| Reassociation | The recombination of the products of dissociation.
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| Recirculation | The reintroduction of part of the flowing fluid to repeat the cycle of circulation.
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| Reducing Atmosphere | An atmosphere which tends to 1) promote the removal of oxygen from a chemical compound; 2) promote the reduction of immersed materials.
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| Reduction | Removal of oxygen from a chemical compound.
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| Refractory | Brickwork or castable used in boilers to protect metal surfaces and for boiler baffles.
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| Relative Humidity | The ratio of the mass of water vapor present in a unit volume of gas to the maximum possible mass of water vapor in unit volume of the same gas at the same temperature and pressure.
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| Relief Valve | (a.k.a. Safety Relief Valve) An automatic pressure relieving device actuated by the pressure upstream of the valve and characterized by opening pop action with further increase in lift with an increase in pressure over popping pressure.
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| Residual Fuels | Products remaining from crude petroleum by removal of some of the water and an appreciable percentage of the more volatile hydrocarbons.
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| Resin | A bead-like material used in chemical exchange for softeners and dealkalizers.
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| Resistance | Impediment to gas flow, such as pressure drop or draft loss through a dust collector. Usually measured in inches water column (or, "wc").
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| Return Flow Oil Burner | A mechanical atomizing oil burner in which part of the oil supplied to the atomizer is withdrawn and returned to storage or to the oil line supplying the atomizer.
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| Ringelmann Chart | A series of four rectangular grids of black lines of varying widths printed on a white background, and used as a criterion of blackness for determining smoke density in stack gas streams.
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| Rotary Oil Burner | A burner in which atomization is accomplished by feeding oil to the inside of a rapidly rotating cup.
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