Section IV CONDENSATE SYSTEM 1. GENERAL DISCUSSION (a) Description.-The condensate system is that part of the steam cycle in which water flows from the main condenser toward the boilers while it is being prepared for use as feed water. The several elements of this system are: the main condenser in which steam is condensed and collected as condensate; the main condensate pump which removes condensate from the main condenser; the main air ejector, through which condensate is passed to serve as cooling water, and which has the function of removing air from the main condenser; and the deaerating tank. which finally prepares condensate for use as feed water. Piping connects all these elements. thereby requiring that each element operate in perfect coordination with each other element in order that proper operation of the system, as a whole. may be maintained. As in all other systems, hereinafter discussed, we will first describe the action within each element and thereafter show by description of the piping how all these elements are connected together into a single system. (b) Notes on Vacuum.-In all ocean-going marine power plants a condenser is employed to convert the steam exhausted from the main engines back into water so that it can be returned to the boiler as feed water. In these condensers a vacuum is maintained at as high a level as possible in order to obtain maximum economy and maximum power. In a strict sense, the vacuum in the condenser is not a vacuum. All gages indicate pressures which start with zero at atmospheric pressure. Therefore, we use the term "vacuum" to indicate that the pressure is less than atmospheric. The average atmospheric pressure at sea level is 14.7 p.s.i. If we have a pressure which is 4.7 p.s.i. less than atmospheric the gage will indicate a vacuum, but there will be 10 p.s.i. of pressure remaining. This pressure is referred to as "absolute" pressure to differentiate it from "gage" pressure (above atmospheric). These absolute pressures are generally referred to as "vacuum" in inches of mercury. A column of mercury with 0 p.s.i. absolute pressure above it will he held up by the atmospheric pressure (14.7 p.s.i.) to a height of 30 inches, where the weight of the column of mercury becomes equal to the 14.7 p.s.i. of atmospheric pressure. If an absolute pressure greater than 0 p.s.i. exists above the mercury, the height of the column will be reduced by approximately 2 inches for each 1 p.s.i. of pressure. Thus, when we say there is 28 inches of vacuum, there is actually 1 p.s.i. of absolute pressure. Atmospheric pressure is, however, variable. With a constant absolute pressure in the condenser, variations in atmospheric pressure will create corresponding variations in the amount of vacuum shown on a gage. Therefore, a change in vacuum gage reading may not necessarily indicate a change in condition of the condenser but may merely reflect a change in atmospheric pressure. Checking the absolute pressure gage or the barometric height will indicate whether such is the case. (c) Condenser Temperatures.-Under normal operation the condenser will he filled with steam under the absolute pressure existing in the condenser. A definite relation exists between the pressure of saturated steam and its temperature. An absolute pressure of 1 p.s.i. will give a corresponding gage reading in vacuum of approximately 28" of mercury. The temperature of saturated steam at 1 p.s.i. absolute pressure will be 101.2 degrees F. If some external condition causes the condenser temperature to remain hotter than this, or if the cooling water temperature is not held below this, the steam will not be condensed and the absolute pressure in the condenser will rise, reducing the vacuum shown on the gage. It is apparent that the greater the vacuum obtained, the lower will be the temperature of the steam. The lower the temperature of the steam in the condenser, the more heat we have available for conversion into mechanical energy in the engines. For maximum power, then, the vacuum should be carried at as high a level as possible. In doing this, care should be exercised to see that. the temperature of the condensate and of the cooling water discharged is not reduced 15