In this article we will discuss about the Condenser:- 1. Function of Condenser 2. Type of Condensers 3. Reasons for Inefficiency 4. Air Leakage 5. Measurement of Vacuum 6. Effect of Air 7. Methods for Obtaining Maximum Vacuum 8. Vacuum Efficiency 9. Coefficient of Performance or Efficiency.
- Function of Condenser
- Type of Condensers
- Reasons for Inefficiency in Surface Condenser
- Air Leakage in Condenser
- Measurement of Vacuum in Condenser
- Effect of Air in a Condenser
- Methods for Obtaining Maximum Vacuum in Condenser
- Vacuum Efficiency of Condenser
- Coefficient of Performance or Efficiency of Surface Condenser
1. Function of Condenser:
The function of condensers in steam power plants is to maintain a low temperature. This is necessary to obtain the maximum thermal efficiency from a given plant. The lowest temperature attained depends upon the temperature of the circulating water which condenses the steam.
When the steam expands to a pressure below the atmospheric pressure, it cannot be exhausted into atmosphere and therefore it exhausts in a vessel which is at a lower pressure than that of the exhaust steam. The exhaust steam is condensed in the vessel, which is known as condenser.
The temperature of condensed steam, commonly known as condensate, leaving the condenser is higher than that of the circulating water at inlet and the condensate is removed from the condenser by means of an extraction pump which discharges the condensate to the hot-well from where the boiler feed pump feeds it to the boiler.
Thus, we see that by installing condensers two aims are achieved:
(i) Greater amount of useful work is obtained from a given amount of steam than could be obtained without a condenser.
The gain in work by fitting a condenser is shown shaded on P- V diagram in fig. 12-1. By increasing the power developed per kg of steam, the steam consumption per kilowatt hour is reduced.
(ii) The recovery of condensate for the boiler feed water. The lowest practical exhaust pressure for most steam engines is 150 mm to 200 mm of mercury absolute. Steam turbines may expand steam upto 25 mm mercury absolute pressure or less.
Condensers are classified as jet condensers and surface condensers.
In jet condensers, steam to be condensed and the cooling water come in direct contact and the temperature of the condensate is the same as that of cooling water leaving the condensers. For jet condensers, the recovery of the condensate for re-use as boiler feed water is not possible.
In surface condenser steam to be condensed and circulating water do not come in contact; heat transfer between steam and circulating water is by conduction and convection.
Thus surface condensers provide both a low exhaust pressure and recovery of the condensate, whereas jet condensers provide only the low exhaust pressure; there will be more intimate heat exchange between cooling water and the steam with a practical disadvantage due to mixing of the cooling water the condensate, which disadvantage is obviated in surface condensers.
With jet condensers, exhaust steam mixes with cooling water, so this cooling water must be fresh and free from harmful impurities if the condensate is to be re-used for feeding into boiler.
With surface condensers, as the exhaust steam does not mix with cooling water, the cooling water need not be upto the high standard of purity demanded for modern boiler feed water. Salt water can be used; thus all marine steam installations are equipped with surface condensers.
In large steam turbine installations jet condensers are impractical because there is loss of condensate, the high power consumption of jet condenser pumps and the first cost of requisite air pumps. Therefore, for modern steam power plant surface condensers are used. However, with reciprocating steam engines and moderate size turbine units, the jet condensers are used especially where an abundant supply of good water is available.
(a) Low Level Jet Condenser:
Jet condensers in which cooling water, condensate and non-condensable gases are removed by a single pump are called low level low vacuum jet condenser because of limited air capacity of the pump.
In this condenser supply of cold water into the condenser chamber is drawn due to vacuum. Condensing or cooling water enters at the top and is distributed around the circumference from which the spiral nozzles discharge the water into the condensing cone.
Steam enters from the top and mixes with fine spray of cooling water in the condensing cone. The mixture of condensate and cooling water is removed from bottom by means of a pump. For higher vacuum air is removed by a separate pump fitted on the side of the condenser just at the same level as the condensing cone.
In such case the condenser is known as a low level high vacuum jet condenser. See fig. 12-2.(b). A vacuum breaker which is a float control valve is provided just below the condensing cone. It prevents water from rising to too high a level in the condenser by allowing air to break the vacuum.
(b) High Level Jet Condenser or Barometric Condenser [Fig. 12 2(a)]:
In this condenser, the condensing chamber is elevated about 11 metre above the discharge of condensate and cooling water, which is discharged through a tail pipe. This condenser does not need a pump to remove the mixture from the condenser.
This function is carried out by height provided. Water stands in the tail pipe to a height according to vacuum. Normal height is about 9.5 metre for the most efficient condenser. Water is allowed to enter from the top and is broken into fine streams by suitably arranged baffles.
Steam enters from the bottom of the condensing chamber. Air released from steam and water rises to the top thereby coming in contact with cold water and so it gets cooled and devapourized. It is removed by means of a pump fitted at the top.
(c) Ejector Condenser:
The main principle of this condenser is that the momentum of flowing water removes mixture of condensate and cooling water against atmospheric pressure. Exhaust steam enters at the top and surrounds the cones through which cooling water flows at high velocity.
Steam is drawn into it due to vacuum produced by this flow of water, thereby condensing steam. Mass of condensate, cooling water and air is discharged out of the condenser, due to this velocity, against the atmospheric pressure.
(a) Standard Type:
In standard surface condensers, the steam to be condensed is made to flow over the outside of the tubes, fig. 12-3, while cooling or circulating water is made to flow inside the tubes. This flow pattern is used because the clean steam does not contaminate the outside of the tubes, which would be difficult to clean.
(b) Water Works Type:
Circulating water is frequently dirty and leaves deposits on the inner surface of the tubes. When condenser is used in conjunction with a pumping engine, the reverse arrangement is generally provided i.e. steam is allowed to pass through the tubes and water passes over the outside of the tubes. Such surface condensers are known as water works condenser.
(c) Evaporative Condensers:
In evaporative condenser, which is another form of surface condenser, the shell is omitted. The steam is fed through the condenser tubes and over these tubes, water is sprayed. The cooling is effected mainly by evaporation of water form atmosphere.
Surface condensers may be single pass or two pass. In single pass condensers water flows in one direction only through all the tubes, while in two pass condensers water flows in one direction through half the tubes and return through the remainder. Some condensers may have three or more passes for cooling water.
Fig. 12-3 shows one form of standard type of surface condenser having two passes. Steam enters at the top and passing downwards over the tubes, through which cooling water is flowing, is condensed and condensate is removed at the bottom, by means of an extraction pump. Cooling water enters at one end of the tubes situated in the bottom half of the condenser and after flowing to the other end returns through the tubes situated in the top half of the condenser.
Condenser tubes are usually made of red brass or metal for pure water and admiralty brass for salt and impure river water. The outside diameters of tubes vary from 2 cm to 3 cm, using No. 18 B.W.G. The condenser may have one to eleven thousand tubes.
Surface condensers may operate on wet vacuum system or on dry vacuum system. For operation on wet vacuum system, two pumps are required, a circulating pump and a wet air pump. The wet air pump removes both air and condensate from the condenser.
For operation on dry vacuum system, three pumps are required, a circulating or cooling water pump, a condensate pump for removing the condensed steam and a dry air pump which removes air from the condenser.
Now-a-days circulating water pumps and condensate pumps are of the centrifugal types and dry air pump is of the steam jet type single-stage or multi-stage.
The salient features of the modern trend in the design of the condenser is to shield the air exit from the downstream of condensate by means of a baffle and thus air is extracted with only a comparatively small amount of water vapour.
A section of tubes is screened by the baffle to form an air cooling section as shown in fig. 12-4. The air cooling section reduces the required capacity of the air pump and the mass of the steam removed by the air pump.
The principle of evaporative condenser may be explained as shown in fig. 12-6. Steam to be condensed enters at the top of a series of grilled tubes around which a film of cold water is falling. At the same time a current of air circulates over the water film.
A natural or forced air current causes rapid evaporation of the water film with the result that the steam flowing through the tube .gets condensed. The vapour of the cooling water passes off with heated air. The remainder of the cooling water at increased temperature is connected and used again after its temperature is resorted to the original value by the addition of the requisite quantity of cold make up water.
This type of the condenser is restricted to small powers on account of nuisance which would result from production of cloud in a populated area. It can take overload for a short period without seriously affecting the vacuum.
In this case, the mass of water required for condensing the steam can be reduced by evaporating the water under a small partial pressure.
The following are the requirements of an ideal surface condenser plant for a steam turbine or a reciprocating steam engine:
(i) The pressure drop of steam while passing through the condenser should be reduced to minimum.
(ii) Steam should enter the condenser with the least possible resistance.
(iii) As the air is a poor conductor of heat, air should be removed from the heat transmitting surfaces rapidly.
(v) Air to be extracted from the surface condenser should be cooled to a low temperature and should be free from entrained water vapour.
(vi) Condensate should be removed as quickly as possible from the heat transmitting surface at the maximum practicable temperature so as to have higher thermal efficiency.
(vii) There should be least resistance to flow of circulating water through the tubes and the heat transmission rate through the walls of the tubes should be maximum. The regenerative type of surface condenser has been designed by Metropolitan Vickers Ltd. in which the condensate is reheated to a temperature more nearly that of the exhaust steam. The air is drawn from the centre of the condenser, but in order to effect the maximum cooling of air, it is passed through a nest of cooler tubes before final extraction.
The surface condenser becomes inefficient and the performance of the condenser is drastically reduced.
There are various reasons for inefficiency in the surface condenser as below:
(1) The pressure inside the condenser is less than atmospheric and it has vacuum. It is necessary that the pressures should be as low as possible to get maximum effect for unit mass of steam. The pressure in the condenser depends on the amount of air. The prevention of leakage of air in the surface condenser is difficult because of the presence of the high vacuum in the condenser.
(2) The air leakage results in lowering the partial pressure of the steam and temperature. This naturally results into increase in latent heat and hence more cooling water is required.
(3) The under-cooling of the condensate provides effect of low overall efficiency.
(4) There is pressure drop in the tubes when the steam flows over the tubes. This pressure drop decreases the vacuum which reduces the efficiency.
(5) The condenser uses brass tubes and the conduction of heat is not perfect, this reduces the efficiency.
(6) The circulating water passes through the condenser with high friction and reduction in velocity causes the low efficiency.
(1) Sources of Air Leakage in the Condenser:
The condenser has high vacuum, therefore air leakage occurs in the condenser.
The main sources of air leakage in the condenser are:
(i) Leakage of air from atmosphere at the joints of the parts which are at pressure less than atmospheric pressure.
(ii) Air is coming from the boiler along with steam. This air enters in the dissolved form in the feed water.
(iii) In case of jet condenser, quantity of air accompanies the injection of water.
(2) Prevention of Air Leakage in the Condenser:
In order to prevent the sources of leakage in the condenser the following procedure is adopted:
(i) The design of condenser and use of vacuum joints.
(ii) By giving treatment of feed water to the boiler.
(iii) Keeping all the joint tight and sealed.
(3) Detection of Air Leakage in the Condenser:
The air leakage can be detected by:
(i) Keep condenser under pressure and apply soap water at the joints where the infiltration is possible.
(ii) Use Pepprament oil on the suspected joints during condenser operation and check on the odour in the discharge of the air ejector.
(iii) The large leakages can be detected by moving candle flame over the possible opening.
5. Measurement of Vacuum in Condenser:
The pressure inside the condenser is less than atmospheric, and in order to obtain the maximum work from unit mass of steam, the pressure should be as low as possible. The minimum pressure that can be attained depends on the temperature of the condensate and on the air present in the condenser.
It is found that in central power stations 5 per cent reduction in steam consumption is achieved by increasing the vacuum from 70.0 to 73.0 cm of mercury. Thus, we see that high vacuum should be maintained in the condenser.
The vacuum is measured in either mm or cm of mercury. It depends upon the barometric pressure as well as the absolute pressure in the condenser. The absolute pressure in the condenser is the sum of the pressures of the steam and air present in the condenser.
If we want to know the absolute pressure in the condenser, we should know the barometric reading as well as the vacuum gauge reading. The vacuum is usually referred to a standard 76 cm barometer.
Corrected vacuum = 76 – (barometric height – vacuum) cm.
Absolute pressure in condenser in cm of mercury
= barometric height – vacuum reading.
Pressure equivalent of 1 cm of mercury is 1/750 = 0.001333 bar.
Absolute pressure in a condenser depends upon the amount of air present in it and temperature of the condensate. Mixture of air and steam in a condenser may be regarded as an atmosphere which is always saturated. Each is responsible for part of the pressure.
Dalton’s law of partial pressures states that the total pressure within the condenser is equal to the sum of the pressures which each constituent would exert separately if it alone occupies space of the condenser.
Assuming there is a small amount of water in the condenser so that the steam is in contact with the water, the pressure Ps which it exerts on the wall of the condenser depends upon its saturation temperature. If we know the saturation temperature we can determine the value of Ps.
The pressure of air present in the condenser depends upon the mass of air present and the temperature of air vapour mixture and is given by:
The effects of air leakage in the condenser are:
(a) It increases the pressure in the condenser which limits the amount of work done by unit mass of steam in the engine or turbine.
(b) It lowers the partial pressure of steam and temperature. This means that the latent heat increases and therefore more cooling water is required and the undercooling of the condensate is likely to be more severe with a resulting lower overall efficiency.
(c) It reduces the rate of condensation of steam, since the abstraction of heat by the water circulating through the tubes is then partly from the steam and partly from the air.
Steam jet air ejectors are used to remove air from condensers and a de-aerating plant is used to remove dissolved air from feed water before pumping it through the economiser into the boiler.
To check whether there is air leakage in the condenser or not, the plant is run until conditions are steady. Afterwards the steam supply from the engine is shut-off; simultaneously the air and condensate extraction pumps are closed down so that the condenser is isolated. The readings of the vacuum gauge and the thermometer are noted. If there is a leakage after some time the temperature and vacuum will fall.
There are various methods used for obtaining maximum possible vacuum in the condenser.
(1) Air Pump:
The air pumps are used to maintain the desired vacuum in the condenser by extracting the air and other non-condensable gases. The air pumps can remove mixture of condensate and non-condensable gases. Some pumps removes dry air only.
These are classified as:
(i) Wet air pump
(ii) Dry air pump.
(2) Steam Air Ejector:
The wet air pump uses the steam air ejectors to remove the air from the mixture. The operation of the ejector is to utilize the viscous drag of a high velocity steam jet for the ejection of air and other non-condensable gases from a chamber
(3) De-Aerated Feed Water:
The de-aeration of feed water helps in maintaining better vacuum in the condenser and controlling corrosion.
(4) Air Tight Joints:
The joints of the steam power plant can be made air tight by use of suitable packaging material and piping system. The inspection should be conducted regularly.
The lowest pressure which can exist in a given condenser is that of the vapour corresponding to the condensate temperature. This would be the pressure in a perfect condensing plant. In fact there will always be some air present in the condenser due to leakage and due to dissolved gases in the steam which enter the condenser. The air exerts its own partial pressure and therefore lowers the partial pressure of steam during condensation.
The efficiency of and engine or any machine is defined as the ratio output by input. For condensers the meaning of output and input is obscure. To formulate some standard of reference we borrow the term Coefficient of Performance (COP) which is used with refrigerating machines.
An ideal condenser should only remove the latent heat of steam. There should be no undercooling of the condensate. The condensation should be achieved by using minimum quantity of circulating water. To satisfy this condition the outlet temperature t2 of the circulating water should theoretically equal to ts the saturation temperature of steam corresponding to the vacuum required.
In an actual condenser a certain amount of under cooling is necessary to maintain the vacuum. If, therefore, the temperature of the condensate is tc and the temperature of cooling water inlet is t1, the coefficient of performance of a condenser is defined as under.
The usual value of coefficient of performance is in the region of 1 to 1.2.