In this article we will discuss about:- 1. Introduction to Steam Condensers 2. Components for Condenser Installation 3. Classification.
Introduction to Steam Condensers:
A steam condenser is a closed vessel which is maintained at low pressure, lesser than atmospheric pressure. The condenser acts as a heat exchanger where exchange of heat occurs between steam and cold circulating water. In this system, heat rejection at atmospheric temperature takes place by condensing exhaust steam from prime mover. The condensate can be reused as feed water to the boiler.
The condensate can be supplied to the boiler by means of a high-capacity feed pump. The hot water which comes out from condenser is pumped to the cooling tower in order to become cooled and to supply this to condenser. The process is repeated again and again. The thermal efficiency of the plant can be improved by the use of condenser. Steam condenser is one of the main constituents of a steam power plant as shown in Fig. 4.15.
Condenser is a device which is used in a steam power cycle in order to:
(a) Increase the thermal efficiency of the plant reducing the exhaust pressure and thereby reducing the exhaust temperature. The pressure range becomes higher and the capability of the prime mover to work is increased. Thus, more work can be obtained by increasing the pressure range.
By the use of condenser, the exhaust pressure can be maintained (nearly) at saturation pressure corresponding to the atmospheric water temperature. For example, at 20°C, the pressure can be as low as 0.02 bar. Thus, the thermal efficiency as well as the work output increases substantially.
(b) Save the water cost by condensing the exhaust steam from a prime mover into pure water form which can be reused as hot feed water. In large power plants, about 105 kg/hr or so feed water may be used. Thus, there is a saving in water cost. The condensate which is obtained after condensation is of the purest form.
If there is no loss of water, the same volume can be supplied over and over again. For a good thermal power plant, only 5% loss of feed water is permitted which can be compensated by preparing purest form of feed water by water purification system.
The operation of the plant is well affected by the condenser. The power plant performance will deteriorate if condenser performance becomes poor. The advantages of introducing condenser are quite apparent which can be shown in Fig. 4.16.
Figure 4.16 represents the expansion of steam on p-V and h-S plot in case of steam prime mover. The shaded area shown in Fig. 4.16 represents the extra work available by exhausting the steam into a condenser instead of exhausting it into atmosphere.
Installation of Steam Condenser:
Figure 4.17 shows the complete installation of a condenser in a steam power plant. In this case, the cold circulating water used in condenser is not re-circulated again and again but is discharged into the downward stream of river whereas at a place where no river is available, cooling water is re-circulated again and again by passing it through a cooling tower or cooling pond or spray pond.
Following are the components required for condenser installation:
(b) Cold circulating water pump
(c) Condensate extraction pump
(d) Hot well
(e) Boiler feed pump
(f) Air extraction pump
(g) Cooling tower
It is a closed vessel which is maintained at low pressure, lesser than atmospheric pressure. The exhaust steam from prime mover is exhausted in the vessel in order to have condensation by using cold water at atmospheric temperature. Since the quantity of steam is quite high, the amount of cooling water required to condense the steam in condenser is extremely large (about 50-100 kg of water/kg of steam). This acts as a heat exchanger.
The heat of steam is absorbed by the cold circulating water. The water which comes out from the condenser is hot enough which cannot be reused as cold circulating water. It is, therefore, essential that a source of cold circulating water must be available throughout the operation. If the source is not available, such as cooling pond, river, lake, etc., then the cold circulating water is to be prepared by spray pond (for a small unit) or cooling tower (for large thermal power plant).
(b) Cold Circulating Water Pump:
This is used to circulate water through condenser.
(c) Condensate Extraction Pump:
The condensate available at low pressure cannot flow from low pressure to atmospheric pressure. The function of the condensate extraction pump is to extract the condensate from condenser and supply this to hot well at atmospheric pressure.
(d) Hot Well:
Hot well is a tank between the condenser and the boiler wherein the condensate coming from the condenser is stored.
(e) Boiler Feed Pump:
The function of a boiler feed pump is to supply the condensate from the hot well into the boiler by increasing the pressure of condensate above the boiler pressure (about 6% more than the boiler pressure).
(f) Air Extraction Pump:
The function of an air extraction pump is to extract the air which has entered into the condenser through mechanical joints of condenser owing to the large pressure difference between atmospheric pressure and the vacuum inside the condenser. This is necessary to maintain vacuum.
(e) Cooling Tower:
In most of the plants, cold circulating water remains unavailable throughout the year in the river. Under such circumstances, the hot water coming out from condenser is to be cooled by sending this water to the cooling tower and is allowed to fall in the form of spray. The hot water is cooled by evaporative cooling, rejecting heat to the atmospheric air. The cold water is again reused in the condenser. During this process of evaporation, there is a loss of water which is compensated from outside taking water from river or other sources. The typical cooling towers are shown in Figs. 4.18(a) and 4.18(b).
Classification of Steam Condensers:
Steam condensers are of two types:
(a) Jet condenser/mixing-type condenser
(b) Surface condenser/non-mixing-type condenser
(a) Jet Condenser:
In a jet condenser, the exhaust steam from steam prime mover and cold circulating water come in direct contact with each other and condensation takes place by the proper mixing of exhaust steam with cold water, by spraying water into steam. The condensate obtained cannot be reused as feed water to the boiler unless the cold circulating water is pure enough. This type of condenser is very effective. Two types of jet condensers have been shown in Figs. 4.19(a) and 4.19(b).
(b) Surface Condenser:
In a surface condenser, the exhaust steam and the cold circulating water do not come in direct contact with each other. The steam passes over the outer surface of the tube through which cooling water is circulated. There may be single pass or double pass or multi-pass condensers, which depends on the size of the condenser.
In single pass condenser, the water flows in one direction only whereas in double pass, the water flows in one direction through the tube and returns through the remainder tubes, as shown in Fig. 4.20. The condenser box is divided into two parts by baffle plate.
i. Down Flow Type Surface Condenser:
Figure 4.20 shows a down flow type of surface condenser. It consists of a shell which is generally of cylindrical type. It has two cover plates at both the ends and the end of the tubes are connected with these two plates. A baffle plate divides the water box in two parts in case of double pass condenser.
The cooling water enters in one side of the shell at the lower half and after passing into the tubes it comes out through the upper half section. The exhaust steam entering the shell from the top flows down over the tube surface and gets condensed. Finally, the condensate is removed by an extraction pump. In this case, the steam flows in a direction right angle to the direction of the flow of water and so it is called cross-flow surface condenser.
ii. Central Flow Type Surface Condenser:
This has been shown in Fig. 4.21. In this type of condenser, the suction pipe end of the air extraction pump is placed in the center of the tubes which results in the radial flow of the steam. It ensures better contact between the outer surface of the tubes and steam.
iii. Evaporative Condenser:
Figure 4.22 shows the schematic diagram of an evaporative condenser. These condensers also come under the category of surface condenser. They are very much suitable for the places where dry air and limited quantity of water is available.
The exhaust steam from steam prime mover is passed through large number of tubes and cold circulating water is allowed to fall in the form of spray on the outer surface of the tube carrying steam.
The cool and dry air is drawn over the surface of the tube with the help of induced fan. This causes the withdrawal of heat from steam by the evaporation of water spray. The evaporation of water takes place by utilizing the latent heat. The air passing over the tube carries water in the form of vapour. These condensers are found to be more effective because of the effective heat transfer.
Comparison between Jet Condenser and Surface Condenser:
i. This is used in case of small plant.
ii. This is used where clean fresh water is available in plenty. There are several mechanical arrangements in this type of condenser. The mixing of cooling water and exhaust as parallel flow and counter flow steam can be arranged.
i. This is used in case of large plants
ii. Non-mixing type of steam condensers are of shell and tube type with steam flowing over water tubes. The steam flow may be directed downwards or towards the center.