Monday, December 29, 2008
In order not to reduce the efficiency of a steam turbine plant, any am mount of steam that has been utilized to drive the turbine need to be restored back to water form, condensate.As steam being expanded across the steam nozzles, diaphragm plates, blading, it reduce its steam profile and becomes heavier and denser, becomes larger droplets, wet steam. In order to convert to water at the same temperature as exhaust steam at same pressure, both steam and water must coexist.In order to achieve that the condenser must not over cool the steam, only extract the latent heat.
To have these conditions, the condenser must be below atmospheric pressure, vacuum.
The cooling medium is either sea water or treated water(including river water),passes through condenser tubes. At the outside of the tubes is steam. Heat transfer occur here.
There are various designs of condensers
One of them is wet bottom condenser.
From the diagram it shows that the turbines sits on top of the condenser.These arrangement is called under slung type. The hot well , which is the condenser bottom, is always filled with water either from make up arrangement or condensation . The condensate is extracted outwards to next heaters before reaching the boiler feed water tank, maybe condensate preheater or Low Pressure heater, then High pressure heater and finally feed water. Non condensible vapour and air and a bit of wet steam is drawn into air ejector or liquid ring primer pump or vacuum pump, re cooled and re condensed again , finally discharges to atmosphere. Because the hot well level is maintained sufficiently the extraction pump receives its requiered suction head, preventing cavitation.In the event of loss of water level in the hotwell protection is installed, makeup demin water will be injected, or in severe loss case eventually may even trip the turbines.We will discuss later about this.
The position of the condenser plane whether is is the same level as the turbine or under the turbine nominates, whether it is axial single plane or underslung. Normally the condenser is vacuum. However there is what is known as atmospheric condenser.
The source of steam is coming from heating steams fuel traceheating lines,drains systems which normally contaminated. The steam is normally lower in pressure and temperature hence quality.But to discharge it overboard is not economic. Did you know how much of heat energy required to increase 1 deg C of water temperature?
To turn a liter of ice to water requires 80000 calories, and to turn water to saturated steam requires 540000 per liter.You have so many calories to turn them into steam whatever the condition, surely you will not discard it. Why not captured back the H2O and reused back?What will you used as cooling medium? Yes, the freshly exracted condensate from the condenser. You will gain a few degress of heat as it absorb the heat from the steam and drains.Thus reduce the ammount heat energy required to bring to next heating stage, finally the boiler.
So the bad quality steam is retrieved back in maybe wet steam or condensate. What are you going to do with it. You cannot send it to boiler. Yes you can treat it . OK that will be discussed later.In the meantime we store the product in atmospheric drains tank. Sometimes know as flash tank
Notice the top part is for venting the vapour. The bottom is for water and condensate retrieval. The inlet is at the center, where the steam and water will be twirling like a cyclone in order to bring down the pressure and temperature of the fluid
Posted by aza ni at 9:11 AM