Steam Condenser | Types , Function , Diagram , Advantages
Steam condensers are devices in which the exhaust steam from the steam turbine is condensed by means of cooling water. Condensation can be done by removing heat from exhaust steam using circulating cooling water. During condensation, the working substance (steam) changes its phase from vapour to liquid and rejects latent heat as shown in Figure 1.
The primary object of a condenser is to maintain a low pressure on the exhaust side of the rotor of steam turbine. This enables the steam to expand to a greater extent which results in an increase in available energy for conversation into mechanical work. The secondary object of condenser is to supply to the boiler pure and hot feed water, as the condensed steam which is discharged from the condenser and collect in a hot well can be used over again as feed water for the boiler. The use of a condenser in a power plant is to improve the efficiency of the power plant by decreasing the exhaust pressure of the steam below atmospheric pressure. Another advantage of the condenser is that the steam condensed may be recovered to provide a source of pure feed water to the boiler and reduce the water softening capacity to a considerable extent.
Advantages of a condenser in a steam power plant
The main advantages of incorporating a steam condenser in a steam power plant are as follows:
• It increases the efficiency of the power plant due to increased enthalpy drop.
• It reduces back pressure of the steam which results in more work output.
• It reduces temperature of the exhaust steam which also results in more work output.
• The condensed steam can be reused as feed water for boiler which reduces the cost of power generation.
• The temperature of the condensate is higher than that of the fresh water which reduces the heat supplied per Kg of steam produced.
Function of condenser
The main function of condenser is to convert gaseous form of exhaust steam into liquid form at a pressure of below atmosphere. Cooling medium is used water to convert steam into water.
Others important functions of condensers:
• Function of the condenser is to create a vacuum by condensing steam
• Remove dissolved non – condensable gases from the condensate.
• Providing a leak tight barrier between the high grade condensate contained within the shell and the untreated cooling water.
• Providing leak tight barrier against air ingress, preventing excess back pressure on the turbine.
Elements of a steam condensing plant
The main elements of a steam condensing plants are:
• A condenser in which the exhaust steam is condensed
• Supply of cooling water for condensing exhaust steam
• A pump to circulate the cooling water in case of a surface condenser
• A pump called the wet air pump to remove the condensed steam (condensate) the air, and uncondensed water vapour and gases from the condenser (separate pump may be used to remove air and condensed steam)
• A hot well where the condensed steam can be discharged and from which the boiler feed water is taken
• An arrangement (cooling pond or cooling tower) for cooling the circulation water when a surface condenser is used and the supply of water is limited
Types of condensers
The steam condensers are classified as follows:
1. Jet condensers (mixing type condensers)
a. Parallel flow jet condenser
b. Counter flow jet condenser (low level)
c. Barometric or high level jet condenser
d. Ejector condenser
2. Surface condensers (non mixing type condensers)
a. Down flow surface condenser
b. Central flow surface condenser
c. Regenerative surface condenser
d. Evaporative condenser
Parallel flow jet condenser
In parallel flow jet condenser both the steam and the water enters from the top and flows in the same direction as shown in Figure 1. The exhaust steam is condensed when it mixes up with water. The condensate and the cooling water are delivered to the hot well from where surplus water flows to the cooling pond through an overflow pipe. Sometimes a single pump know as wet air pump is used to remove both air and the condensate but generally separate air pump is used to remove air as it gives a great vacuum.
Counter flow or low level jet condenser
In counter flow or low level jet condenser, the exhaust steam enters from bottom and mixes with the down coming cooling water as shown in Figure 2. The air pump mounted at the top of the condenser shell creates vacuum as it suck air. This draws the supply of cooling water which falls from a large number of jets through perforated conical plate.
The water then falls in the trays and flows through second series of jets and mixes with the exhaust steam entering at the bottom. This cause rapid condensation after which the condensate and the cooling water are delivered to the hot well by the condensate extraction pump.
Barometric or high level jet condenser
This type of condenser is provided at a high level as shown in Figure 3 having a long tail pipe. The exhaust steam enters from the bottom and flows upwards. This steam then mixes with cooling water which falls from the top through various baffles. The vacuum is created by the air pump placed at the top of the condenser shell. The condensate and the cooling water flows downwards through a vertical tail pipe to the hot well without the aid of any pump. The surplus water from the hot well flows to the cooling pond through an overflow pipe.
In ejector condenser, the steam and water mix-up while passing through a series of metal cones as shown in Figure 4. Water enters from the top through a number of guide cones. The exhaust steam enters the condenser through a non return valve. The steam and air then pass through the hollow truncated cones. After that it passes through the diverging cone where its kinetic energy is partly transformed into pressure energy. The condensate and the cooling water are then discharged to the hot well. The high exit pressure in the diverging cone allow discharged of water automatically into the hot well at atmospheric pressure
In surface condenser, the condensate does not mix up with the cooling water. So the whole condensate can be reused in the boiler. This type of condenser is used where is only limited quantity of fresh water is available like ships. A sectional view of a two pass surface condenser is shown in Figure 5. It consists of a horizontal cylindrical vessel made of cast iron packed with tubes for cooling water. The cooling water flows in one direction through the lower half of the tubes and in opposite direction through the upper half. The water tubes are fixed into vertical perforated type plates at the ends so that leakage of water should not occur into the central condensing space. The steam enters from top end. The extraction pump at the bottom sucks the condensate resulting in the downwards flow of steam over the water tubes.
Down flow surface condenser
In down flow surface condenser, the steam enters from the top as shown in Figure 6. The exhaust steam is forced to flow downwards over the water tubes due to suction of the extraction pump at the bottom. The suction pipe of the dry air pump is provided near the bottom and is covered by a baffle so that the condensed steam does not enter into it. As the steam flow perpendicular to the direction of flow of cooling water, it is also called cross flow surface condenser.
Central flow surface condenser
In this type of surface condenser the suction pipe of the air extraction pump is placed in the center of the tube nest as shown in Figure 7. The exhaust steam from turbine enters from the top and flows radially inwards over the tubes. The condensate is collected at the bottom. The advantage of central flow type surface condenser over the down flow type is that the steam flows over the whole periphery of the water tubes as the steam flows radially inwards.
In evaporative condenser the steam flows enters the gilled pipes and flows backwards and forwards in a vertical plane as shown in Figure 8. The water pump sprays water on the pipes which condenses the steam. The main advantage of this type of condenser is that the quantity of cooling water needed to condense the steam can be reduced by causing the circulating water to evaporate which decrease the temperature. The remaining water is collected in the cooling pond.
Regenerative surface condenser
In this type of condenser the condensate after passing the tube nest is heated using regenerator which is located inside the exhaust steam which raises the temperature of the condensate. This increases the efficiency of the plant as water in the boiler requires less heat input.
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