PRINCIPLES OF NAVAL ENGINEERING 



and for any specified temperature there is a 

 corresponding saturation pressure. 



A liquid \which is under any specified pres- 

 sure and at the saturation temperature for that 

 particular pressure is called a saturated liquid. 

 A liquid which is at any temperature below its 

 saturation temperature is said to be subcooled 

 liquid . For example, the saturation temperature 

 which corresponds to atmospheric pressure 

 (14.7 psia) is 212°F for water. Therefore, water 

 at 212°F and under atmospheric pressure is said 

 to be a saturated liquid. Water flowing in a river 

 or standing in a pond is also under atmospheric 

 pressure, but it is at a much lower temperature; 

 hence, this water is said to be subcooled. 



A vapor which is under any specified pres- 

 sure and at the saturation temperature corre- 

 sponding to that pressure is said to be a saturated 

 vapor . Thus, water at 14.7 psia and 212°F pro- 

 duces a vapor known as saturated steam . As 

 previously noted, it is impossible to raise the 

 temperature of a vapor above the temperature 

 of its liquid as long as the two are in contact. 

 If the vapor is drawn off into a separate con- 

 tainer, however, and additional heat is supplied 

 to the vapor, the temperature of the vapor is 

 raised. A vapor which has been raised to a tem- 

 perature that is above its saturation tempera- 

 ture is called a superheated vapor , and the 

 vessel or container in which the saturated steam 

 is superheated is Called a superheater . The ele- 

 mentary boiler and superheater illustrated in 

 figure 8-10 show the general principle of gen- 

 erating and superheating steam. Practically all 

 naval propulsion boilers have superheaters for 

 superheating the saturated steam generated in 

 the generating sections of the boiler; the steam 

 is then called superheated steam. The amount 

 by which the temperature of a superheated vapor 



exceeds the temperature of a saturated vapor 

 at the same pressure is known as the degree of 

 superheat . For example, if saturated steam at a 

 pressure of 600 psia and a corresponding satu- 

 ration temperature of 486° F is superheated to 

 786° F, the degree of superheat is 300° F. 



For any substance there is a critical point 

 at which the properties of the saturated liquid 

 are exactly the same as the properties of the 

 saturated vapor. For water, the critical point 

 is reached at 3206.2 psia (critical pressure) 

 and 704.40°F (critical temperature). At the 

 critical point, the vapor and the liquid are 

 indistinguishable. No change of physical state 

 occurs when the pressure is increased or when 

 additional heat is supplied; the vapor cannot be 

 made to liquefy and the liquid cannot be made 

 to vaporize as long as the substance is at or 

 above its critical pressure and critical tem- 

 perature. At this point, we could no longer refer 

 to "water" and "steam", since we cannot tell 

 the water and the steam apart; instead, the 

 substance is now merely called a "fluid" or 

 a "working fluid." Boilers designed to operate 

 above the critical point are called supercritical 

 boilers. Supercritical boilers are not used at 

 present in the propulsion plants of naval ships; 

 however, some boilers of this type are used in 

 stationary steam power plants. 



Representation of Properties 



The condition of a working fluid at any point 

 within a thermodynamic cycle or system is 

 established by the properties of the substance 

 at that point. The properties that are of special 

 interest in engineering thermodynamics include 

 pressure, temperature, volume, enthalpy, en- 

 tropy, and internal energy. These properties 



FEED WATER 



^ SATUF 



SATURATED STEAM 



GENERATING 

 FURNACE 



SUPERHEATED STEAM 



38.3 



Figure 8-10.— Elementary boiler and superheater. 



184 



