In this article we will discuss about:- 1. Introduction- Necessity of Condensers 2. Classification of Steam Condensers 3. Sources of Air 4. Condenser Vacuum 5. Dalton’s Law of Partial Pressure 6. Vacuum Efficiency 7. Condenser Efficiency 8. Cooling Water Requirements 9. Capacity of Air Extraction Pumps 10. Function of Cooling Towers.
Contents:
- Introduction- Necessity of Condensers
- Classification of Steam Condensers
- Sources of Air in the Condenser
- Condenser Vacuum
- Dalton’s Law of Partial Pressure
- Vacuum Efficiency of Condenser
- Condenser Efficiency
- Cooling Water Requirements for Condenser
- Capacity of Air Extraction Pumps Used in Condenser
- Function of Cooling Towers in Condenser
1. Introduction- Necessity of Condensers:
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Steam will be generated in the boilers, after it is being superheated, it is expanded in the turbines for generating power. After generating the power, it is either exhausted into the atmosphere or it is exhausted into the condenser. In the condensers this steam will be condensed i.e., its phase will be changed from vapour to liquid by means of cooling water.
Figure 21.2 shows the theoretical indicator diagram for the steam turbine. Let the steam available in the turbine have the initial condition of pressure P1 and temperature T1. This steam is expanded in the turbines along 2 – 3 for generating work output and then it is exhausted into the atmosphere along 3 – 4 (slightly above atmospheric pressure). Let the final condition of steam be Pb and temperature T2. Then the work output is given by the area 1 – 2 – 3 – 4 – 1 on the indicator diagram.
According to Carnot’s maximum efficiency of any heat engine cycle is given by ƞcarnot = (T1 – T2)/T1.
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From this equation it is clear that the efficiency can be increased either by increasing T1 or by decreasing T2.
In the case of IC engines, efficiency is increased by increasing T1. (In this case T1 is temperature of products of combustion in the engine cylinder). When T1 is increased, then the metals which can withstand high temperature stresses are to be used in the construction of piston, cylinder, cylinder head etc., even though they are costly.
Whereas in case of external combustion engines, efficiency will be increased by decreasing T2. By the properties of steam to obtain lower temperature than T2, lower pressure has to be created. When the pressure of exhaust steam is lowered below Pb, (or atmospheric pressure) steam cannot be exhausted into the atmosphere. So, it has to be exhausted into a vessel which is at a lower pressure than Pb and this vessel is called as the condenser.
So, condenser is a device which condenses the exhaust steam and creates partial vacuum for the steam below atmospheric pressure. The phenomenon used in the working of the condensers is that, the volume of steam drops down heavily if it is condensed and creates a vacuum. For example- 1 kg of steam at 0.1 bar pressure occupies 14.93 m3 while its condensate occupies only 0.001076 m3.
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Thus, by installing the condenser it is possible to reduce the exhaust steam temperature from T2 to T’2 where T’2 is the saturation temperature of steam corresponding to condenser pressure P’b. Now as shown in P – V diagram, work done in a condensing plant is given by the area 1 – 2 – 6 – 5 – 1.
Thus, there is an increase in account of work by 3 – 4 – 5 – 6 by using a condenser. Consequently the thermal efficiency of the condensing unit will be higher than non- condensing unit. If the condensate is pure, then it can be used as feed water to the boiler. This recovery of feed water is also an incidental advantage.
2. Classification
of Steam Condensers:
Steam condensers are broadly classified as:
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I. Jet Condensers:
(a) Low Level Jet Condensers –
(i) Counter-Flow Type
(ii) Parallel-Flow Type
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(b) High Level Jet Condensers
(c) Ejector Condensers
II. Surface Condensers:
(a) Steam flowing through the tubes and water from outside
(b) Water flowing through the tubes and steam from outside
(c) Evaporative condensers
The main difference between the Jet condensers and Surface condensers is as given below:
In case of Jet condensers, the steam to be condensed directly mixes with the cooling water and the condensation will be effected with less amount of water.
Whereas in the case of surface condensers, the steam to be condensed and the cooling water are separated by means of a tube well and through which heat transfer takes place and the condensation will be effected. Here the amount of cooling water required will be more.
In case of Jet condensers, the temperature of condensate and cooling water are same, whereas, in case of surface condensers temperature of condensate will be more than the temperature of cooling water at the outlet.
In case of Jet condensers, the condensate collected will be impure. (Since the steam and cooling water mix with each other). So it has to be wasted. Whereas in case of surface condensers, it is pure and can be used as a feed water to the boiler.
The size, space required and cost of surface condenser is more, because it has to handle large quantity of water for the same capacity.
The most commonly used condensers are counter flow type Jet and Water tube type surface condensers.
I. Jet Condensers:
(a) Low Level Jet Condensers:
(i) Counter-Flow Type Jet Condensers:
In this type, the direction of flow of steam and cooling water are opposite.
In this condenser, the cold water is drawn up in the condenser from the cooling pond, due to the vacuum head created in the condenser shell. This condenser consists of a cylindrical shell arranged with 2-3 water trays with perforations to break up water into small jets. The exhaust steam and any mixed air enters the condenser at lower portion and tries to ascend up through the falling sprays. Thus, when the steam comes in contact with the cold water particles gets condensed and the air gets cooled.
This cooled air will be removed through the air pump at the top. The mixture of condensate and cooling water descends down through the vertical pipe to the centrifugal extraction pump and is pumped to the hot well. If the condensate in the hot well is pure then it can be pumped to the boiler as a feed water otherwise it has to be wasted.
The capacity of air pump required is small, since it has to handle cold air and water vapour. These condensers can be directly installed below the turbines. Such condensers have a disadvantage of flooding the steam turbine, if the condensate extraction pump fails.
(ii) Parallel Flow Jet Condenser:
In this case, the direction of flow of exhaust steam and cooling water are parallel. In this case, the steam enters from top of the condenser and the cooling water just below it from sides as shown in Fig. 21.4 and rest all the arrangements are similar to counter flow Jet condenser. A wet air pump is provided at the bottom which removes air, condensate and cooling water.
(b) High Level Jet Condenser:
High level Jet condenser is also called as barometric Jet condenser since it placed above the atmospheric pressure equivalent to 10.33 m of water pressure. In this case, a long tail pipe of length more than 10.33 m is provided to the bottom of the condenser as shown. This facilitates the condensate and coolant to be discharged from the condenser under gravity hence condensate extraction pump need not be provided. Another advantage of such arrangement is that the water from the hot well will not be able to rise into the condenser and flood the turbine due to the vacuum head in the condenser. The other details and working are similar to low level counter-flow jet condenser.
(c) Ejector Condenser:
Ejector condenser consists of a central vertical tube in which a number of cones or converging nozzles are fitted. The exhaust steam enters at the left and surrounds the central tubes with ports. The cooling water enters at the top under a head of 5-6 m and attains a high velocity while passing through the condenser.
In flowing port the steam ports, the water produces vacuum. The vacuum causes the exhaust steam and air to flow through the ports in the tube and mix with the cooling water. The exhaust steam gets condensed, as a result of which the vacuum is further increased. The condensate and air after passing through converging cones pass through the diverging cones and in doing so water gains the momentum which forces out condensate and air against the atmospheric pressure. So in this case, no air pump is required.
This type of condenser is usually fitted with a non-return value to prevent a sudden backward flow of water into the engine exhaust pipe, in case of sudden failure of water supply to the condenser.
(II) Surface Condensers:
(Cooling water flowing through tubes and steam from outside)
A surface condenser essentially consists of a Mild Steel rolled shell, closed at both the ends by dished ends (also called as water boxes) as shown. Ends of the shell and dishes are fitted with flanges. A tube sheet is provided in between the shell flanges and dish end flange on either side. A number of water tubes are fitted into these tube sheets. The ends of the tubes are either expanded or welded in order to have leak-proof joint.
The exhaust steam from the engine enters at the top of the condenser and gets condensed when it comes in contact with the cold surfaces of water tubes, through which cooling water is being circulated.
The cooling water enters at one end of the tubes (as shown in Fig. 21.6) situated at the lower half of the condenser and after flowing to the other end it returns in the opposite direction through the tubes situated in the upper half of the condenser. This is obtained by providing pass partition plates in the dished ends. In this type since the water traverses 2 times, it is called as 2-flow surface condenser. If the water traverses more than two times then it is called Multi-flow surface condenser.
This condenser requires two pumps viz., wet air pump to remove air and condensate and secondly water circulating pump.
Another way of classifying the surface condensers is according to direction of flow of exhaust steam viz. –
(i) Central flow surface condenser
(ii) Down flow surface condenser
(iii) Evaporative condenser
(i) Central Flow Surface Condenser:
Figure 21.7 shows the end view of central flow surface condenser. In this case, air extraction pump is provided at the centre. So, negative pressure will be created at the centre. In this case, the exhaust steam and any air enters from top and will be compelled to flow towards the centre as shown. During its flow, it comes in contact with the cold surface of the water tubes and gets condensed, and the condensate will be removed by means of condensate extraction pump.
(ii) Down Flow Surface Condenser:
Figure 21.8 shows the end view of the down flow surface condenser. In this case a baffles is provided throughout the length of the condenser. It consists of two pumps, one is for removing the condensate and other is the air pump. In this case exhaust steam mixed with any air enters from top and descends down as shown by arrows.
The steam when comes in contact with the cold surfaces of tubes it gets condensed and the air gets cooled. Air is further cooled when it flows from below the baffle. So the air is cooled to the minimum temperature before it is extracted. Due to this cooling of air, its specific volume will be reduced, so it reduces the pump capacity. So, it reduces the energy consumption for running the air pump.
(iii) Evaporative Condenser:
When the supply of cold water is extremely limited, then the evaporative condenser is used since it uses very small amount of circulating and make up water (since the coolant is used again and again). However these condensers are used with small power plants.
In this case the exhaust steam from the steam engine enters into a coil of gilled pipes or series of tubes, the outlet of which is connected to the wet air pump.
The water which is pumped up by means of a pump is sprayed from the top and descends down. The water falling downwards forms a thin film over the pipes as it falls from one level to another and remainder water will be collected in the water tank.
A natural or forced air current causes rapid evaporation of this film of water. The rapid evaporation of film of water results into condensation of steam. The evaporated vapour will be removed by means of air.
3. Sources of Air in the Condenser
:
The sources of air in the condenser are as follows:
1. Leakage through packing glands and microscopic holes in shell joints since the pressure inside the condenser is less than the atmospheric pressure.
2. Leakage through relief valves and other accessories.
3. Feed water contains dissolved air, which is liberated in the boiler during steam formation the exhaust steam from the steam engine carries air.
4. In case of jet condenser some air comes in with the injection/cooling water (in which it is dissolved).
Following are the effects of air, leakage in condenser on its performance:
1. Back pressure in the steam power plant increases and corresponding work output decreases and the very purpose of using the condenser is defeated.
2. Due to poor thermal conductivity of air, rate of heat transfer is low therefore more cooling water is required to be supplied.
3. For maintaining the required vacuum, the air pump is to be provided.
4. Condenser Vacuum:
The vacuum in the condenser is usually expressed in mm of Hg. The absolute pressure in the condenser is equal to the difference of barometric pressure and the vacuum pressure as shown in Fig. 21.10.
5. Dalton’s Law of Partial Pressure
:
This law states that “Total pressure exerted by mixture of gases or mixture of gas (air) and vapour (steam) is equal to the sum of partial pressure of constituents, if they would occupy the same volume and are at the same temperature”.
∴ Pressure of mixture in condenser = Partial pressure of steam + Partial pressure of air.
Pc = Ps + Pa
If the condenser temperature is known, Ps is found from steam tables and Pc can be read from the vacuum gauge.
∴ We can write, Pa = Pc – Ps
Knowing the partial pressure of air Pa its mass ma can now be calculated with the help of ideal gas equation PaVc = maRTa where Vc = volume of condenser; Ta = temperature; R = characteristic gas constant of air.
Also note that – Each constituent is considered to occupy condenser volume Vc.
6. Vacuum Efficiency
of Condenser:
It may be defined as the ratio of actual vacuum as recorded by the vacuum gauge to the ideal vacuum.
7. Condenser Efficiency
:
There is no standard method of determining the condenser efficiency but a method adopted by the well-known makers of steam turbine M/s Parson & Co. has been widely used in engineering practice.
8. Cooling Water Requirements
for Condenser:
In a condenser, cooling water absorbs heat from steam to be condensed and the temperature of cooling water increases. In jet condensers steam to be condensed directly mixes with cooling water and hence temperature of condensate and cooling water are same.
9. Capacity of Air Extraction Pumps Used in Condenser:
We know that, absolute pressure in the condenser –
10. Function of
Cooling Towers in Condenser:
The function of the cooling tower is to cool the cooling water of the condenser, by the current of air flowing in the opposite direction.
A large amount of cooling water is required for condensation in large capacity power plants. If the water is freely available either from the river or lake, then the water can be directly pumped from the river to the condenser as shown in Fig. 21.13 (a).
If the water is not freely available, then a cooling tower has to be used for cooling the hot water of condenser. The cooled water can be used again as cooling water for the condenser.
Figure 21.13 (b) shows the hyperbolic cooling tower. It is usually made of steel Reinforced cement concrete. The hot water from the condenser is supplied to the ring troughs which are placed at 8-10 m above the ground level. The nozzles are provided on the bottom side of troughs to break up water into sprays.
The air rises up from the pond in the opposite direction of water flow and absorbs heat from the falling water spray. The cooled water is collected into a pond built below the tower. This type of cooling tower is generally used since it is very efficient, however, it needs about 3-5 % of makeup water for compensating the evaporation losses.
The other types of cooling towers are:
(i) Natural Draught Towers:
In this type the circulation of air is obtained by virtue of pressure difference of the air inside and outside the tower. Here no fan is required.
(ii) Forced Draught Tower:
In this case, the circulation of air is obtained by means of fans provided at the bottom of the tower.
(iii) Induced Draught Tower:
In this case, the circulation of air is obtained by providing a fan at the top of the tower.