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The preboiler phase The preboiler phase employs various mechanical and chemical methods to remove impurities from raw water entering the system. This is pre-treatment. The type of pre-treatment used depends on the requirements of the steam generating system and the quality of the water entering the system. For example, low pressure boilers can generally tolerate some degree of impurities, while high pressure boilers used to drive turbines require water that is virtually free of all impurities. Feedwater is provided in two ways: make-up water and condensate. Make-up is raw water used to "make-up" for water losses in the plant. Make-up water enters from the plant's water source and passes through pre-treatment equipment to remove impurities before it enters the boiler. Condensate, on the other hand, is water which is recycled from the boiler. After steam has been used in the plant, it cools and turns back to water called condensate. Condensate is collected and passed through condensate return lines back to the feedwater system. In this way, the water can be recycled over and over again. In addition to saving water, there are two other benefits to recycling condensate: • Since condensate is hotter than make up water, the amount of heat required to generate steam is reduced resulting in fuel savings. • Secondly, as condensate is generally high in purity, the amount of make-up requiring pre-treatment is also reduced. The preboiler section may also include equipment to recover heat energy from hot combustion gases. These gases contain a considerable amount of heat energy which can be used to heat the feedwater. Waste gases escaping up the stack cause the greatest loss of heat in a steam generating system. The heat is recovered in an economizer, a heat exchanger placed between the boiler and stack. The recovered heat is used to raise the temperature of feedwater before it enters the boiler. Using waste combustion gas to raise the temperature of feedwater increases boiler efficiency and reduces fuel consumption. Some plants employ deaerating feedwater heaters. In addition to heating the feedwater, deaerating heaters remove oxygen and other dissolved gases from the water.
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