Cement, Energy and Environment

.. -- I: f: t : £ .. I: .... ·'&rc.~r..h. :, :t .......-....... - • 1t"'C .....__.,_ ....... •w.4"'C .,~----------------~. : f I : :I f : • I : £ : I = • • :.-.. ...... : • l.rC - ........... • I'C . ..... , ... " ... _____ ..,.... Figure 4. Water cooling in a wet cooling tower. left: dry ambient air conditions; right: humid ambient air. The back pressure after the turbine depends on the outside air temperature (OAT): the warmer the weather, the higher the back pressure and thus the lower the produced electrical power. After the turbine, the organic vapour flows through a heat recovery heat exchanger and is cooled. Then the vapour enters to condenser, where it is liquefied and slightly undercooled. Finally, the liquid is again put under high pressure in the fluid pump and conveyed via the heat recovery heat exchanger (heat recovery from the vapour) to the evaporator and the cycle is closed. 10.7 11).6 10..S ~ . • 10.4 I uu 1 ' 10.2 f 10.1 ~ 10.0 u ..... 25 25 zs 25 rt1 ,...._ zo 40 110 1'1'1 Heat dissipation Wet cooling tower As with every thermal power plant, the vapour has to be liquefied. Therefore the condensing heat has to be discharged to the environment. For this process step a condenser is necessary. Condensers can conventional wet cooling towers or dry air condensers. Wet cooling towers are basically capable of reaching temperatures below ambient (theoretically wet bulb temperature) through the evaporation effect of water in the air. zs 30 100 zo 40 110 110 100 Figure 5. Influence of air humidity on heat dissipation in a dry air condenser. Blue curve: cool ambient air at various r.h.; red curve: warm air. Figure 4 shows that with warm and very dry air (e.g. 30' C/20% r.h.) the unde~~ooling effect might (theoretically) reach around 14'C; in reality (tower efficiency), the undercooling will be roughly 10' C. When the ambient air is warm and humid (e.g. 30'C/80% r.h.), this additional cooling effect is reduced to approximately 2'C. If, in addition, an intermediate cooling water cycle is installed between condenser and cooling tower, the undercooling effect on the condensing temperature is affected even more negatively. Water for cooling towers has to be treated, which requires considerable investment and operating costs. Dry air condenser On the other hand, dry condensers profit from humid air. The enthalpy of ambient air increases with rising humidity and consequently more heat power can be dissipated per kg of air; i.e. the same condenser is more efficient when the air is more humid. Figure 5 shows that a dry air condenser can dissipate with dry air (30' C/20% r.h.) roughly 10.2 kW of heat power per kg/s of cooling air. When the humidity rises to e.g. 80% r.h. 30 -·~ >-