ELEMENTS OF A CONDENSING PLANT
(i) a closed vessel called condenser
(ii) a condensate extraction pump
(iii) dry air pump to remove air and non-condensable gases
(iv) a feed water pump
(v) a cooling water pump
(vi) cooling tower. Cooling ponds are used in place of cooling towers in some plants.
SOURCES OF AIR LEAKAGE IN CONDENSER:
Following are the means through which air is present in a condenser,
(i) Leakage through packing glands and very small holes in the shell.
(ii) Leakage through joints and vents from atmospheric relief valves and other accessories.
(iii) Dissolved gases or air in boiler feed water, which ultimately enters the condenser with exhaust steam.
EFFECT OF LEAKAGE AIR ON CONDENSER PERFORMANCE:
The presence of air in the condenser has the following effects on condenser operation
Back pressure (or exhaust pressure) in the steam engine or turbine increases and consequently workdone reduces. Thermal efficiency of the power plant reduces.
For the same absolute pressure (partial pre., of steam + partial pre., of air), the partial pressure of steam decreases with air infiltration. Saturation temperature decreases; latent heat (enthalpy of evaporation) increases. So, condenser requires larger quantity of cooling water.
Air being a poor conductor of heat, its presence in the condenser affects rate of heat transfer. Thus to fulfil the requirement, surface area of tubes is to be increased. The Size of condenser increases. Rate of steam condensation is reduced.
AIR PUMP :
Presence of air in condenser has bad effects on the condenser operation. Air must be completely extracted out using extraction pumps. The pump may be of dry type which removes only air, or wet type which removes air, condensate and non condensable gases.
Fig shows the working of Edward's reciprocating wet type air pump (or air extractor). It has no suction valve as it is there in the ordinary pumps. It is connected to the condenser.
When the piston is at the top of the barrel, the condensate and air from the condenser are collected in the conical portion of the barrel. On the downward stroke of the piston, partial vacuum is created above the piston since delivery valve is kept closed. When the piston uncovers the ports during its further downward movement, the air and water vapour entrapped in the conical portion, rush into the barrel space above the piston through these ports.
During return journey (upward movement) of the piston, the above mixture is compressed and pressure rises. When pressure exceeds the atmospheric pressure, the delivery valve opens. The mixture of air and water vapour escapes out to the hot well, over a weir. The weir maintains sufficient head of water above the delivery valve to prevent air leakage into the pump barrel.
The watersealed relief valve operates when the pressure inside the barrel exceeds atmospheric pressure.
CONDENSER VACUUM :
The vacuum obtainable in the condenser depends on tightness of valve joints, amount of air infiltration and temperature of steam after condensation. Vacuum will not be uniform throughout. It is least at the air pump section, high in.the body of the condenser and still higher at exhaust valve. The degree of vacuum measured by the vacuum gauge mounted on condenser can be expressed in the following ways.
(i) By excess of atmospheric pressure over the observed vacuum:
For example 650 mm vacuum means, atmospheric pressure is 650 mm of Hg above the condenser pressure.
(ii) By the percentage vacuum : It is the ratio of observed vacuum to atmospheric pressure. For example, if the gauge reads 650 mm Hg, with the barometer reading of 750 mm Hg, the percentage vacuum
650/750 x 100 = 86.67%.
(iii) By the absolute pressure : It is the difference between barometric reading and vacuum gauge reading.