Misaloli na Ƙaifiyoyi na Tsarin Rankine

Masugabinsa mai tsarki suna daidai na kula da yawan gina kashi a yammacin Asia. Saboda haka, wanda ya yi amfani da yawan kimi da take da shawarar da zama da tsafta a kan gina kashi ya haɗa da tsafta a kan ingancin ruwa da kuma sauya da kudin gaso na kuliya.

Saboda haka, ba za a iya gano wata fatar da ke nuna cewa an samun hanyar da za su iya sauya da tsafta a kan masugabinsa mai tsarki.

Dalilin wata ilimin da kuma gajerar da ke nuna shine da za su iya sauya da tsafta mai tsarki na IEE-Business. Saboda haka, wasu hanyoyi da za su iya sauya da tsafta mai tsarki sun hada da:

• Ta haka da kan tsayawa da ake bayar da shi a kan mutanen da ake amfani da su (tsarki) a kan maimaita da ake amfani da su (condenser). (Tsayawa da ake bayar da shi a kan condenser)

• Ta haka da kan tsayawa da ake bayar da shi a kan tsarki da ake amfani da su a kan turbine

Tsayawa da ake Bayar da Shi a Kan Condenser

Tsarki ta fito daga turbine da ta zama a kan maimaita da ake amfani da shi (condenser) a kan abu mai tsarki da ake amfani da shi. Tsayawa da ake bayar da shi a kan condenser yana taimakawa da ake bayar da shi a kan turbine saboda ana iya zama da shi a kan turbine.

A nan T-s diagram, ana iya gano da fahimta da ake bayar da shi a kan tsayawa da ake bayar da shi a kan condenser.

Muhimmanci na Tsayawa da ake Bayar da Shi a Kan Condenser

Don in iya samun tsafta, Rankine Cycle yana iya amfani da tsayawa da ake bayar da shi a kan condenser da ke da ita da ita. Amma tsayawa da ake bayar da shi a kan condenser yana ba da dalili da ake bayar da shi a kan tsayawa da ake bayar da shi a kan area.

A nan T-s diagram, ana iya gano cewa abin da ake karshe a kan net work output ita ce da ake bayar da shi a kan tsayawa da ake bayar da shi a kan condenser daga P4 zuwa P4’.

Lafiya na Tsayawa da ake Bayar da Shi a Kan Condenser

Muhimmancin da ake bayar da shi a kan tsayawa da ake bayar da shi a kan condenser ba a tabbas da lafiya ba. Saboda haka, wasu abubuwa masu lafiya na tsayawa da ake bayar da shi a kan condenser sun hada da:

• Yadda ake amfani da heat input a kan boiler da ke da ita da ita (dalilin tsayawa da ake bayar da shi a kan condenser)

• A nan tsayawa da ake bayar da shi a kan condenser, yawan da ake amfani da moisture content a kan tsarki a kan stage na biyu na turbine yana ƙara. Yadda ake amfani da dryness fraction a kan tsarki a kan stage na biyu na turbine ba a tabbas da lafiya ba saboda hakan yana ƙara da tsafta da kuma lafiya na turbine blades.

Abubuwan da Ake Bayar da Shi a Kan Tsayawa da Ake Bayar da Shi a Kan Condenser

Gaba-gaban abubuwan da ake bayar da shi a kan tsayawa da ake bayar da shi a kan condenser yana haɗa da muhimmanci, saboda yadda ake amfani da heat input a kan boiler yana da ƙarin, amma yadda ake amfani da net work output yana da ƙara a kan tsayawa da ake bayar da shi a kan condenser. Kuma dryness fraction a kan tsarki a kan stage na biyu na turbine ba a bincike da 10-12% ba.

Super Heating The Steam to Higher Temperature

Superheating of steam is the phenomenon in which heat is transferred to the steam to super heat the steam to higher temperature by maintaining the constant pressure in the boiler.

The shaded area in the above T-s diagram clearly showing the increase in net work (3-3’-4’-4) on account of increase in superheat temperature of steam.

Additional heat input in the form of energy, leaves the cycle as work i.e increase in work output surpass the additional heat input and heat rejection. Thermal efficiency of the rankine cycle increases on account of increase in steam temperature.

Positive Effects of Increasing the Steam Temperature

One desirable effect of increasing the increasing the steam temperature is that it doesn’t allow to the last stage mositure % of steam to increase. This effect can be easily seen on the T-s diagram (Fig:2) above.

Negative Effects of Increasing the Steam Temperature

Increasing the steam temperature results in small increase in heat input. There is a limit to which the steam can be superheated and used in the power cycle. These limiting factors are related to metallurgical proveness at high temperature and economical viability.

Presently in supercritical power generating units, steam temperature at turbine inlet is around 620oC. Decision of any further increase in steam temperature can be judiciously taken only after doing the metallugical due diligence and evaluation of the cost-implications.

Net Effects of Increasing the Steam Temperature

From the T-s diagram (Fig:2) the net effect of temperature increase is more towards positive side, because the gain from the network output surpasses the increase in heat input and slight increase in heat rejection. So it is always beneficial to increase the steam temperature after accessing the reliability and economic viability.

Increasing Boiler Pressure With Sub Critical Parameters

Alternative way of increasing the Rankine cycle efficiency is by increasing the boiler operating pressure and thus in a way related with the temperature at which boiling is taking place in the boiler. Thus the thermal efficiency of the cycle increases.
By the help of T-s diagram the effect of Increase in boiler pressure on the performance of the cycle can be clearly seen and understood.

Because of increase in boiler pressure, Rankine cycle shifts slightly towards left as shown in the Fig:3 on T-s diagram and thus following can be concluded from it:

• Substaintial increase in net-work, as shown in the pink colour shaded area of the above figure.

• As the cycle shift slightly towards left, so their is decrease in net work during the expansion of steam in the turbine. ( as shown in above fig:3 sheded in grey colour.

• Reduction in the heat-rejection to the cooling water in the condenser.

Thus net-effect is marked increases in the thermal efficiency of the cycle on account of these measures.

Increasing the Boiler Pressure with Super Critical Parameters

In order to increase the thermal efficiency of the Rankine cycle, super-critical pressure is used in steam-generators used in the present time. When the steam generators operates above 22.06Mpa then the steam generators are called super-critical steam-generators and the plant is called super-critical power generation plant. Because of the higher operating pressures these plants are know for giving higher efficiencies.

Re-Heat Rankine Cycle

Re-heat Rankine cycle is for taking the advantage of increased cycle efficiency at higher boiler pressure without compromising on moisture content of the steam in the last stages of the turbine.

Higher cycle efficiency is possible with re-heating cycle that too without compromising on dryness fraction this is possible by expanding the steam in turbine in two stages by re-heating it in between. Re-heating is practically acceptable way of tackling the problem of excessive moisture in the last stages of the turbine.

Theoretical Way of Reducing the Last Stage Moisture

Theoretically one way is to super heat the steam to a higher temperature before steam enters the turbine but there is a limit above which metallurgical limitations of handling high steam temperature prevents it from further increase beyond 620oC. Super critical power plants are running