Convergent-divergent nozzlesSteam leaving the boiler has high heat energy, low kinetic energy. The amount of heat energy or enthalpy is dependent on the pressure and condition of the steam ( dryness fraction, degree of superheat )
If the pressure is then dropped, then some heat energy must then be released. This heat may be used to perform work or be allowed to manifest itself as an increase in velocity.
Assuming the mass of steam must pass a point at any time, then;
C.S.A is proportional to specific volume of steam/ velocityAt inlet to nozzle the specific volume of the steam is relatively low, and rate of increase is low
Velocity increases at a greater rate
C.S.A is proportional to specific volume/ velocity
Therefore, area required for flow contracts As expansion proceeds, rate of change of specific volume increases to a point where it overtakes the rate of change of velocity and an increase in C.S.A is requiredThe point immediately prior to this is the min C.S.A and is called the throat.
If the remainder of the path is then kept constant then this nozzle is called convergent and the steam will leave the nozzle with no discontinuity of flow
The amount of steam discharged will depend upon inlet/exhaust pressure ratio.
Exhaust pressure = 0.55 inlet pressure ( suphtd )
Exhaust pressure = 0.58 inlet pressure ( sat )This is called the critical pressure as no drop in exhaust pressure will increase the flow.
If the steam flow enters a pressure less then the critical then the expansion becomes uncontrollable and there is a rapid dissipation of energy, scattering the stem and causing turbulence in the steady flow. If a divergent section is attached then expansion is controlled by gradually increasing the area making the discharge pressure equal to the back pressure.
Steam leaves the nozzle without discontinuity of flow.
Divergent section has an angle of divergence of 8 to 10o to centre lineConverging section made as short a possible as rapid contraction to damp turblence and help stream line for laminar flow.
Expansion theoretically adiabatic.Wear, erosion, deposits create turbulence and reconvert some k.e. back to heat energy.
Nozzle plate and BoxesThe nozzles may be formed by machining of the nozzle plate, or by casting in steel partition plates. Alternately, nozzles may be fabricated of vanadium-molybdenum steel and welded into segments. These may be fitted into the nozzle box which is welded to the turbine casing.
Different nozzle designs
The nozzle box is made of a similar material to the nozzle ring and is welded into the casing, followed by stress relieving
Copper end seals let into radial recesses in the T-Slot cut down circumferential leakage
Continuous 360' nozzle plate minimises tip leakage over the blades.