Surface Condenser in Thermal Power Plant
CLASSIFICATION OF SURFACE CONDENSERS
Surface Condensers are further classified as
(a) Down Flow (Two - pass) Surface Condensers
(b) Central Flow Surface Condensers
(c) Regenerative Surface Condensers
(d) Evaporative Condensers
(e) Inverted type Surface Condensers
WORKING PRINCIPLE OF SURFACE CONDENSERS
In these condensers steam and circulating water do not mix together. Water is allowed to flow through tubes and steam is passed over the tubes or vice versa. In either way there is an indirect contact of fluids and heat exchange takes place intersurface. Condensate can be recovered and is re-used as boiler feed water since it is free from impurities. Circulating water need not be very pure since it follows its own circuit. This marked advantage makes surface condensers suitable for marine engines. Absence of water treatment plant is an added economic advantage with these condensers. Following are the various types of surface condensers.
(a) Down Flow Surface Condenser:
The most commonly used Down Flow Surface Condenser is shown in figure.
In this condenser steam enters from the top and flows downwards over the nest of tubes in which cooling water is under circulation. Air extraction pump is fitted at the bottom of the condenser (not shown in Fig.). It sucks out any air from within the condenser and creates vacuum. This causes downward flow of steam.
Cooling water enters the condenser shell from the bottom. It first flows through the lower half of the tubes and then through the upper half as shown by arrows. A baffle plate separates these two halves of the nest of tubes. Circulating water which becomes hotter leaves the condenser from the top as shown.
Due to the fact that steam flows at right angles to direction of flow / of water, it is also called cross - surface condenser.
(b) Central Flow Surface Condenser :
Figure shows the transverse section of a Central Flow Surface Condenser. In this type, steam enters from the top and flows radially towards the nest of tubes as shown by arrows. Tubes carry circulating water.
The radial flow of steam can be caused by locating air extraction pump at the centre of tube nest. This type of condenser is more efficient than down flow type because steam has access to whole peripheral surface of the tubes.
(c) Regenerative Surface Condenser :
In this type of condensers condensate is heated by regenerative method. This is done by drawing the condensate out or the condenser and made to come in contact with fresh exhaust steam. As a result temperature of condensate increases which is now fed to the boiler. Feed water at higher temperatures possesses invariably higher liquid enthalpy (sensible heat). This renders quick evaporation in boiler and saving in fuel consumption.
(d) Evaporative Condenser :
Figure shows a simple sketch of an evaporative condenser. The exhaust steam is passed through a coiled and finned type pipe. The cooling water is sprayed down as shown. This water trickles over steam pipe and evaporates by absorbing latent heat of evaporation from the steam. Excess water falls down into the tank from where it is pumped to the header above. Evaporative condensers are more suitable to the small plants and are employed where cooling water is not available in large quantity.
(e) Inverted Type Surface Condenser :
In this type of condensers the air pump is fitted at the top of the condenser shell. Steam enters at the bottom and flows upwards over the nest of cold water tubes. Steam again reverses its path and flows towards the bottom. The condensate extraction pump is located at the bottom of the condenser.