Wakati mvua ya kutosha inayotokana na jiko la mvua hutumika tena kupitia katika muktadha ya kutumia joto, hii itaanza kujitokeza juu ya evaporation au saturation.
Mvua inatafsiriwa kama mvua ya juu, ikiwa ni juu ya temperature yake ya saturation. Daraja ya mvua ya juu ni mara moja na temperature ya mvua ya juu.
Mvua ya juu inaweza kutumika tu kwenye mvua ya kutosha na si kwenye mvua yenye moisture. Kupata mvua ya juu, mvua ya kutosha inapaswa kupita kwenye heat exchanger nyingine. Hii heat exchanger inatafsiriwa kama secondary heat exchanger ndani ya jiko. Mvua ya chafu yanayotoka kutoka jiko ni njia bora ya kutumia joto kwenye mvua ya kutosha.
Mvua ya juu inatumika kwenye viwanja vya umeme wa mvua kwa kutengeneza umeme. Katika turbine za mvua, mvua ya juu inajitokeza upande mmoja na kuondoka upande mwingine kwenye condenser (inaweza kuwa ya maji au air cooled type). Tofauti ya mvua ya juu energy kati ya turbine inlet na outlet huonyesha rotor ya turbine. Kuna uzalishaji wa mvua ya energy wakati inapita kwenye rotor ya turbine.
Kwa hivyo ni muhimu kuwa na mvua ya juu kwa safi kwenye turbine inlet, ili kukosa condensation ya wet steam kwenye sehemu ya mwisho ya rotor ya turbine.
Katika msingi rotor wa turbine una stages nyingi na mvua inapaswa kupita kwenye kila stage kabla ya kufikia condenser. Kwa hivyo ikiwa mvua ya juu haijawekwa kwa safi kwenye turbine inlet, mvua inaweza kusafa kwenye stages zilizochachezi zaidi za rotor na baada ya kila stage.
Mvua ya chafu kwenye sehemu ya mwisho ya rotor ni ngumu kwa sababu inaweza kuwa na Water Hammer na ukosefu mkubwa wa blades za turbine. Kukabiliana na tatizo hili ni vizuri kutengeneza parameters za mvua kwenye turbine inlet kwa njia inayoweza kubofya mvua ya juu kwenye turbine inlet na exhaust za turbine zinatumika kwenye steam parameters karibu na saturation.
Moja ya sababu muhimu za kutumia mvua ya juu kwenye turbine za mvua ni kuboresha thermal efficiency ya cycle.
Uwezo wa mesin ya joto unaweza kupatikana kwa kutumia:
Uwezo wa Carnot Cycle: Ratio ya tofauti ya temperature kati ya inlet na outlet kwa temperature ya inlet.
Uwezo wa Rankine cycle: Ratio ya heat energy kwenye turbine inlet na outlet kwa total heat energy imetumika kutoka steam.
2. Mfano wa kutathmini uwezo wa Carnot Cycle na Rankine Cycle.
Explanation kwa mfano:
Turbine inatumika na mvua ya juu kwenye 96 bar na 490oC. Exhaust ni kwenye 0.09 bar na 12 % wetness.
Temperature ya saturated steam ni : 43.7oC
Determine na Compare uwezo wa Carnot Cycle na Rankine cycle.
Njia ya kutathmini uwezo wa Carnot cycle :
Njia ya kutathmini uwezo wa Rankine cycle :
Where,
Sensible heat in condensate corresponding to exhaust pressure of 0.09 bar in KJ/Kg = 183.3
3.
Steam-Phase diagram ni grafu ya data iliyotolewa kwenye steam table. Steam-Phase diagram hutumika kwa kuhusu enthalpy, temperature kulingana na pressures mbalimbali. Liquid Enthalpy hf. Hii inawakilishwa kwa line A-B kwenye phase-diagram. Wakati maji anastart kutoa heat kutoka 0o C, basi itareceive liquid enthalpy yote kwenye saturated water line A-B kwenye phase diagram
Enthalpy of Saturated Steam (hfg): Heat addition results in change in phase to saturated steam and is represented by (hfg) on phase diagram i.e B-C.
Dryness Fraction (x): When heat is applied then the liquid start changing its phase from liquid to vapour and then the dryness fraction of the mixture starts increasing i.e moving towards unity. In the phase diagram dryness fraction of the mixture is 0.5 at exactly mid of the line BC. Similarly at point c on the phase diagram dryness fraction value is 1.
Line C-D Point c is in the saturated vapour line, any further heat addition results in increasing the steam temperature i.e beginning of steam superheating represented by line C-D.
Liquid Zone → Region towards left side of the saturated liquid line
Super heat zone → Region towards right side of the saturated vapour line
Two phase Zone → Area between the saturated liquid and saturated vapour line is mixture liquid and vapour. Mixture with varied dryness fractions.
Critical Point → It is the Apex point where saturated liquid and saturated vapour lines meet. Enthalpy of evaporation diminishes to zero at critical point, it means that water changes directly to steam at critical point and thereafter.
Maximum temperature which liquid can attain or exist is equivalent to critical point.
Critical point Parameters → Temperature 374.15oC
Pressure → 221.2 bar
Values above this are super-critical values and are useful in increasing the efficiency of the rankine cycle.
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