Steam Condensers

STEAM CONDENSERS:

A closed vessel in which steam is condensed and in which vacuum is maintained is called a condenser. Condensation takes place when heat is abstracted from the steam. For this purpose a cooling medium such as cold water is used. A condenser is placed at the exhaust end of a steam engine or steam turbine. Steam leaving the prime-mover contains considerably large amount of heat. Exhaust steam is, therefore, passed into the condenser, where heat exchange between steam and cold circulating water takes place. Condensed steam is called condensate and it is in liquid form. This is re-fed to the boiler via a hot well.

If 1 kg of dry steam at 1.013 bar and with a volume of 1.673 m3 contained in a closed vessel is condensed into water at a temperature of 100°C, the liquid (condensate) would occupy only 0.001044 m3 space and pressure would fall to 0.2 bar. This enormous shrinkage creates a region of emptiness called vacuum.

We have Efficiency = 1— T2/T1 for a steam plant. Efficiency will be maximum when T1 is highest and T2 is lowest. Low exhaust temperature means low exhaust pressure. Lowest pressure is possible only when steam is condensed in a closed vessel. With- the vessel open to atmosphere the condensation, of course will take place, but pressure will not fall below the atmosphere. Thus condenser serves the purpose of creating vacuum and low pressure.

ADVANTAGES OF CONDENSATION AND CONDENSERS :

1. Steam expands to lower back pressures, thus providing larger enthalpy drop, consequently more work is done and plant efficiency increases.

2. Reduction in steam consumption per kW hour. For instance increase in vacuum from 710 to 735 mm of mercury gives about 45% reduction in steam consumption.

3. The effect of partial vacuum created in a condenser gives the prime mover 0.75 to 1.0 bar of more working pressure without any increase in boiler pressure or increase in fuel consumed. Greater working pressures result in greater power output.

4. The condensate can be pumped back to the boiler as feed water without any chemical treatment. Temperature of this condensate being higher than fresh water, time of evaporation is reduced. This results in fuel economy.

5. As no water treatment or softening is necessary capital running cost of the plant reduces.

CLASSIFICATION OF CONDENSERS:

Condensers are broadly classified into two types: Jet Condensers and Surface Condensers.

(1) Jet Condensers: These are also called Mixing Condensers and are further classified as

(a) Low Level Parallel Jet Condensers

(b) Low Level Counter flow Jet Condensers

(d) Ejector Condensers

(2) Surface Condensers:

These are further classified as

(a) Down Flow (Two - pass) Surface Condensers

(b) Central Flow Surface Condensers

(d) Evaporative Condensers

(e) Inverted type Surface Condensers