STEAM CONDENSERS

Vapor is converted into liquid by

losing latent heat at the saturation

temperature corresponding to a

certain pressure. Condensers are

used in steam engines, steam turbines

of a thermal power station. Condensers

are used in refrigeration and air

conditioning plants too. Mostly shell

and tube type condensers are used.

In these condensers, water circulates

through the tubes and steam in the

shell.

Steam condenses on the outer surface of the tubes. Pressure in steam condensers is far below atmospheric pressure. Condensers used in Refrigeration and Air conditioning have pressure far above atmospheric pressure. These are not being discussed here. However condensers used in steam engines and steam turbines have pressure much less than atmospheric pressure (i.e. vacuum pressure). Steam condensers have the following objectives.

1. 1. Large pressure drop in steam turbine gives more power and thus higher efficiency.

2. Steam condensed gives distilled water which can be safely used as feed water in the boiler.

3. The temperature of condensate is much higher than the temperature of fresh water. Thus heat supplied in the boiler will be reduced.

4.  The reuse of condensate reduces the cost of power generation.

5.  Pressure in the condenser far below atmospheric which increases the expansion ratio and hence the work obtained increases.

TYPES OF STEAM CONDENSERS

Fig. Surface Condenser

Fig. Jet Condenser

1. Surface condenser
2. Jet condenser

Surface condenser works under high vacuum. Vacuum is maintained with a vacuum pump. Exhaust steam and cooling water do not mix. Steam is condensed by the surface being cooled by the water. The condensed steam gives distilled water. Hence pure water from condensed steam can be used as feed water to the boiler. Jet condensers also work under vacuum. Exhaust steam and water come in direct contact. This condensate cannot be used as feed water to the boiler since it will contain impurities.

COMPARISON BETWEEN SURFACE AND JET STEAM CONDENSERS

Sr. No.	Surface condenser	Jet condenser
1.	Exhaust steam and water do not come in direct contact	Exhaust steam and water come in direct contact
2.	Steam condensed give pure water	Condensate is not pure water.
3.	Condensate is used as boiler feed water	Condensate cannot be used as boiler feed water
4.	Commonly used condensers	Not commonly used condensers
5.	High vacuum (around 730 mm of Hg) is possible. Gives more power	Relatively less vacuum is possible. Gives less power
6.	Higher efficiency	Lower efficiency
7.	Any water available can be used.	Pure water is to be available in large quantity otherwise chemical treatment will be necessary for the boiler feed water
8.	Bulky in size and needs more space	Compact and needs less space
9.	Capital cost is higher	Capital cost is lower
10.	Running cost is higher	Running cost is lower
11.	Maintenance cost is higher	Maintenance cost is lower
12.	Vacuum is maintained with a centrifugal air pump which removes air and other non- condensing gases	Vacuum is maintained with a centrifugal  air pump which removes air and other non- condensing gases
13.	Complex design	Simple in design

 

Efficiencies of a Condenser
1. Condenser Vacuum efficiency
2.  Condenser Overall Efficiency

CONDENSER VACUUM EFFICIENCY

η_vac = Actual vacuum/ maximum vacuum possible
EXAMPLE
Steam enters a condenser at 30⁰ C and the barometer reads 760 mm of Hg. The vacuum available in the condenser is 710 mm of Hg. What is the vacuum efficiency?
Saturated pressure of steam at 30⁰ C from steam tables = 31.8 mm of Hg
Maximum vacuum possible = 760 – 31.8 = 728.8 mm of Hg
η_vac = 710/728.8 = 97.4 %

OVERALL CONDENSER EFFICIENCY

η_con = Rise in cooling water temperature / (Sat temp of steam –Cooling water inlet temp)

EXAMPLE

Steam enters a condenser at 30⁰ C and the barometer reads 760 mm of Hg. The vacuum available in the condenser is 710 mm of Hg. The inlet and outlet temperatures of cooling water are 20 and 27.5 ⁰ C. Find the condenser efficiency.

η_con = (27.5 – 20)/(30 –20) = 7.5/10= 75 %

 

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