Principles of naval engineering - Historic Naval Ships Association

PRINCIPL5S OF NAVAL ENGINEERINGdecreases (in relation to the inlet pressure).However, no further increase in steam velocitywill occur when the outlet pressureis reduced below 55 percent of the inlet pressure.When the pressure at the outlet area ofa nozzle is designed to be higher than thecritical pressure, a simple convergent (parallel-wall)nozzle may be used. In this type ofnozzle, shown in figure 12-1, the cross-sectionalarea at the outlet is the same as the crosssectionalarea at the throat. This type of nozzleis often referred to as a nonexpanding nozzlebecause no expansion of steam takes placebeyond the throat of the nozzle.INLETREGIONTHROATOUTLETREGION147.90Figure 12-1.— Simple convergent nozzle.When the pressure at the outlet area of anozzle is designed to be lower than the criticalpressure, a convergent-divergent nozzle is usedto control the turbulence that occurs whensteam expansion takes place below the criticalpressure ratio. In this type of nozzle, shownin figure 12-2, the cross-sectional area of thenozzle gradually increases from throat to outlet.The critical pressure is reached in thethroat of the nozzle, but the gradual expansionfrom throat to outlet allows the steam to emergefinally in a steady stream or jet. Becauseexpansion takes place from the throat to theoutlet, this type of nozzle is often called anexpanding nozzle.The decrease in thermal energy of thesteam passing through a nozzle must equalthe increase in kinetic energy (disregardingirreversible losses). The decrease in thermalenergy may be expressed in terms of enthalpyaswhere- h.hi enthalpy of the entering steam, in BTUper poundh2 = enthalpy of the steam leaving the nozzle,in BTU per poundINLETREGIONFigureTHROATOUTLETREGION147.9112-2.— Convergent-divergent nozzle.The kinetic energy of the steam jet leavingthe nozzle may be determined by using theequation for mechanical kinetic energy:whereKEWV^2gKE = mechanical kinetic energy, in footpoundsW = weight of the flowing substance, mpounds per secondV = velocity, in feet per secondg = acceleration due to gravity (32.2 feetper second)Since we have taken the enthalpy per poundof the entering and departing steam, let us assume1 pound of steam per second flowing fromthe nozzle. The kinetic energy of this pound ofsteam will then be expressed by320KEA2gwhere V2 is the velocity, in feet per second,of the steam leaving the nozzle. We may nowequate the expression for the decrease in thermalenergy and the expression for the increase inkinetic energy, Thus,64.4(hj - hg) (778) ft-lbSince 1 BTU is equal to 778 foot-pounds,we have multiplied the expression for the decreasein thermal energy by 778. This putsboth sides of the equation in terms of footpounds.The kinetic energy of the steam leaving thenozzle is directly proportional to the square of