Steam condensers: definition, introduction, types, factors affecting condenser selection - Mechanicaltalks

Steam condensers: definition, introduction, types, factors affecting condenser selection

STEAM CONDENSERS 

Definition


Steam condenser is a mechanical device which converts the low pressure exhaust steam from the turbine into water.In other words it is a device which is used to condense exhaust Steam of turbine into water.

Introduction and function


In order to get more work out of steam ,it has to be expanded to as low pressure as possible.But at the same time,if the steam be expanded too much below the atmospheric pressure,it becomes a problem to handle large quantity of high specific volume steam.
It can’t be discharged into atmosphere as atmospheric air being at higher pressure,
will get into it.Here we also take an eye of demineralised water that can’t be wasted and has to be reused.If it will compressed then we will have to spend more work than the work done by expansion,due to losses in pump.
One possibility to achieve this is that steam be first condensed and then compressed.
In this case for compressing low pressure water, work done will be less.
So, obviously the importance of condenser is realised in increasing the efficiency of steam turbine plant.
For condensing the steam,we require cooling water.Generally due to shortage of water,same water has to be used again and again.Therefore,the temperature rise of cooling water must be moderate.Usually for 1 kg of steam,1000 kg of water is needed.
Also colder the available water,more Will be steam condensation and hence lower back pressure.
Therefore,the temperature of the water sets limit for back pressure.Thats’s why in colder countries, lower vacuum pressure can be achieved.
The as we will summarise,the function of condenser are:
A.) To provide the lowest economy heat rejection temperature for the steam cycle.
B.) To convert the exhaust steam to condensate for the use in the cycle.
In addition,condenser also serves as a suitable point where the usable drains for feed heating and other auxillary plants are reintroduced in feedwater and make up water added.

Types of steam condensers


Condensers are mainly divided in two types.
1.) Mixing or contact type
2.) Surface type
1.) Mixing or contact type condenser:
In mixing type of condenser,water actually mixes with steam.Here heat transfer is by direct conduction.
Advantages-

* We require less quantity of cooling water.
* The effective area of heat transfer is reduced,thus this is small and economical.
Disadvantages-

* Here the cooling water used should either be of high purity or the condensate should be discarded,if the cooling water is not of high purity.
Mixing type condensers further classified as:
A.} Barometric type
B.} Low level jet type
A.} Barometric type condenser

In the barometric type of condenser,cold water is pumped on the top of condenser.
Water falls down the baffles and on its way comes across the uprising steam.Thus the steam is condensed and the condensate and cooling water move down.Since the condensate is at vacuum pressure,it can’t be discharged into atmosphere.In order that atmospheric air doesn’t get into condenser and condensate moves out itself,the pipe carrying the condensate is made 10.3 m long and always kept submerged into water.So the advantage here is that we require no extraction pump for pressing the condensate.
But as the condenser is to be located at a high level,it requires strong foundations.
B.} Low level jet type condenser

Since vacuum is there inside the condenser, water reaches in from pipe 2 and is sprayed from nozzle in form of fine jets and spray.
Here we require condensate extraction pump.
Some air always keeps on leaking into the condenser from the atmosphere and some air is also dissolved into cooling water which is given up while coming in contact with the hot steam.This air,if accumulated into condenser,will soon destroy the vacuum.So to maintain the vacuum,air must be continuously pumped out.But along the air some water vapour also moves out.This represents loss of condensate and more amount of work on the part of air extraction pump.
So we should try to have minimum quantity of water vapour and for this air extraction pump must be located at the coldest place i.e. near cooling water inlet.
     Left side barometric and right side 
    low level jet type condenser.

 
2.) Surface type condenser:

In surface condenser water is not at all mixed with the steam.
Usually in a round shell there are large number of tubes, through which cold water flows.Hot steam is at outside of these tubes and due to heat transfer,the steam is condensed and the condensate collected at the bottom of condenser.
The condenser shell of a modern units has a large construction and is fabricated in several sections to assembled at site.
The condenser shell to turbine exhaust is made by butt welding or bolting.
A baffle is provided near the air outlet,so that air cooling zone is provided near the baffle and thus the air extraction pump sucks out less amount of water vapour.
Here the tubes should be arranged properly so that steam always in contact with some tube.Thus the effective cooling area is increased.
                   Surface type condenser

Factors affecting selection of condensers


A condenser should be able to create the lowest possible vacuum and condensate the steam at the best possible point of efficiency
consistent with the cost.The design factors for condenser are: steam load,back pressure and cooling water temperature.
The following eight variables deserve special attention as these influence the initial cost, operating cost and physical size of condenser.
1.) Influence of cooling water temperature:

It is important to understand that the actual back pressure in a condenser will be quite consistent with the actual cooling water temperature available.
It is interesting to note that in the case of typical 200 MW power plant using single pass condenser having 22 mm diameter,9 m
long tubes with 2.1 m/s water velocity.In order to produce 50 mm back pressure condensing surface following different cooling water temperature would be required-
               9600 m^2 with 27°C
               7800 m^2 with 24°C
               12500 m^2 with 30°C
It is cleared by above data that there would be a great change in surface requirement of condenser and hence it’s cost with change in cooling water temperature to obtain same back pressure.
2.) Number of passes in condenser:

The choice between single and two pass condenser is dependent principally upon the quantity of cooling water available and the pumping head external to the condenser.
It is generally noted that a single pass condenser is somewhat smaller in size and therefore cheaper in Cost.
However the two pass condenser would need less quantity of cooling water but more pumping head to pump water through the condenser.
If the head, external to the condenser is more,then the total pumping capacity for two pass condenser will be low because of less requirement of cooling water.
For this reason,two pass condensers are usually used with cooling towers.
Single pass condensers would be best power station located adjacent to large river or lake.
3.) Tube size

It has been found that the condensing surface requirement may be 6% less for 25.5 mm outside diameter tubes and 6% more for 19.5 mm tubes as compared to 22.5 mm tubes.It is generally noted that the size of the tube increases with the size of power plant.
The cooling water requirement and pumping power increases with increase in size of tubes.
However, condenser first cost is inversely proportional to the size of the tubes and hence the decision for size of tube can taken between first condensing cost and cooling water pumping cost.
Another important factor to favour larger diameter tubes is that it has less tendancy to plug or foul.
4.) Tube length

Tubes length usually varies between 7.2 m to 9 m.The cooling water requirement,B.H.P. and relative cost decreases with increase in length.Only there a slight increase in surface requirement with increase in length.
Longer tubes length are generally preferred,due to the consideration being given to the space required to pull the tubes
and the means to obtain proper steam distribution to the entire tube length and in order to get good performance.
5.) Tube material

Admiralty metal ( 70% copper,29% zinc,1% tin) is the most commonly used material for condenser tubes, because of its high heat transfer.It is interesting to note that Aluminium would require minimum surface area,but isn’t used because of it’s high initial cost.
Other materials used for the condenser tubes are arsenic copper,muntz, aluminium brass, aluminium bronze etc.
6.) Cooling water velocity

The normal value of cooling water velocity ranges from 1.8 to 2.4 m/s.As the cooling water velocity increases heat transfer is improved, friction losses and B.H.P. requirement increase,the cooling water requirement increase and the condenser surface requirement decreases for the same
job.
The tube life may decrease with higher velocity due to possible increment in erosion and corrosion.
7.) Cleanliness factor

Usually cleanliness factor of 85% is assumed.It depends on the contamination of cooling water available.It should be determined very carefully as the size and cost of condenser increase with decrease in cleanliness factor.
8.) Size and shape

The size and shape of the condenser must be determined accurately to do the job best.
Smaller sizes (1 to 3.6 m) are usually made round.Oval condenser limit overall height in small areas.Rectangular shapes are advantageous in limited spaces.
Condenser shape is an important economic consideration and it’s shape should be Such that steam distribution is adequate and sufficient sealing head is provided for condensate pumps.
The final decision about the construction of a condenser can be taken only after careful evaluation of all above factors,the turbine heat rate improvement, cost of pumping cooling water and capitalisation of reduced station capacity.
That was all about Steam condenser introduction, types and different factors affecting condenser.Hope you all like this article.
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