What is Steam Condenser?
A surface condenser or steam condenser is a water-cooled shell and tube heat exchanger used to condensate the exhaust steam from the steam turbine in thermal power stations. The steam is converted from a gaseous to a liquid state at a pressure level below atmospheric pressure.
A steam condenser is a closed vessel-type heat exchanger applied to convert low-pressure exhaust steam from the turbine to water. A steam condenser is a mechanical device employed to condense the exhaust steam of the turbine into the water. It conducts this process with the help of cooling water that circulates inside it from the cooling tower.
The pressure within a steam condenser is held below the atmospheric pressure to enhance efficiency. It is commonly used to lower the backpressure of the exhaust at the turbine end.
Where cooling water is in short supply, an air-cooled condenser is often used. An air-cooled condenser is, however, significantly more expensive and cannot achieve as low a steam turbine exhaust pressure (and temperature) as a water-cooled surface condenser.
Surface condensers are also used in applications and industries other than the condensing of steam turbine exhaust in power plants.
Why it is required?
In thermal power plants, the purpose of a surface condenser is to condense the exhaust steam from a steam turbine to obtain maximum efficiency, and also to convert the turbine exhaust steam into pure water (referred to as steam condensate) so that it may be reused in the steam generator or boiler as boiler feed water.
The steam turbine itself is a device to convert the heat in steam to mechanical power. The difference between the heat of steam per unit mass at the inlet to the turbine and the heat of steam per unit mass at the outlet from the turbine represents the heat that is converted to mechanical power.
Therefore, the more the conversion of heat per pound or kilogram of steam to mechanical power in the turbine, the better is its efficiency.
By condensing the exhaust steam of a turbine at a pressure below atmospheric pressure, the steam pressure drop between the inlet and exhaust of the turbine is increased, which increases the amount of heat available for conversion to mechanical power.
Most of the heat liberated due to condensation of the exhaust steam is carried away by the cooling medium (water or air) used by the surface condenser.
Functions of the Steam Condenser
The purpose of using a steam condenser is:
- Helping keep the pressure low (below atmospheric pressure) at the steam turbine end to get maximum possible energy and reduce the specific steam consumption of a power plant.
- Supplying pure feed water to and from the hot well. The water is again pumped to the boiler using a boiler feed pump.
- Raising the heat transfer rate by removing other non-condensable gases from the exiting steam.
Steam Condenser Parts
The components required to operate a steam condenser are as follows.
- Condenser: It is used to condense the steam. The steam with low pressure delivers its heat to the coolant (here water from the cooling tower), and it converts into the water through the condensation process.
- Condensate Extraction Pump: A Condensate Extraction Pump (CEP) is a pump installed between the condenser and the hot well to transfer the condensate from the condenser to the hot well.
- Hot Well: It is a pit between the condenser and the boiler. It takes the condensate-by-condensate pump from the condenser. The feed water is carried from the hot well to the boiler.
- Boiler Feed Pump: It is a pump located between the hot well and boiler to deliver the feed water from the hot well to the steam boiler. This is accomplished by increasing the condensate pressure above boiler pressure.
- Air Extraction Pump: It is a pump utilized to extract or remove the air from the steam condenser.
- Cooling Tower: It is a tower containing cold water, and this water is provided to circulate inside the condenser to cool the steam.
- Cooling Water Pump: It is a pump installed between the condenser and the cooling tower. It circulates the coolant within the condenser.
How does a Steam Condenser Work?
A flow of cooling water is continuously circulating from the condenser inside a steam condenser to the cooling tower and the cooling tower to the condenser. When low-pressure exhaust steam leaves the turbine and passes through the condenser, it loses the heat and condenses into water. The circulating cooling water performs the extraction of the heat from the steam.
Two types of devices are installed on the condenser: the condensate extraction pump and the air extraction pump. Therefore, when the steam condenses into water, it re-circulates again to the steam generator with the help of a condensate extraction pump.
Using the air extraction pump, a vacuum (the pressure below the atmospheric pressure) is created inside the condenser so that cooling water can circulate easily and also the condensate flow can be stabilized.
Due to the low pressure inside the condenser, the air enters the system so that the condenser is filled with a mixture of water, air, and steam.
Types of Steam Condensers
The steam condensers are broadly classified into two types:
- Surface condensers (or non-mixing type condensers). In surface condensers, there is no direct contact between the exhaust steam and the cooling water.
- Jet condensers (or mixing type condensers). In jet condensers there is direct contact between the exhaust steam and cooling water.
1. Surface Condenser
The surface condenser is designed to condense and deaerate the exhaust steam from the main turbine and provide a heat sink for the turbine bypass system. In surface condensers, there is no direct contact between the exhaust stream and the cooling water.
The exhausted steam from the LP turbines is condensed by passing over tubes containing water from the cooling system. The steam condenses when it comes in contact with the cold surface of the tubes and due to the heat transfer to cooling water by conduction and convection.
These tubes are usually made of stainless steel, copper alloys, or titanium depending on several selection criteria (such as thermal conductivity or corrosion resistance). Titanium condenser tubes are usually the best technical choice; however, titanium is a very expensive material and the use of titanium condenser tubes is associated with very high initial costs. In general, there are two types of surface condensers:
- Water-cooled surface condenser
- Air-cooled surface condenser
In thermal power plants, where cooling water is in short supply, an air-cooled condenser can be used. An air-cooled condenser is, however, significantly more expensive and cannot achieve as low a steam turbine exhaust pressure (and temperature) as a water-cooled surface condenser.
The water gets warmed in the condenser is discharged into the cooling system (i.e., cooling tower, river, sea, or cooling pond). The condensate collected from these condensers is reused as feedwater in the boiler.
Since the cooling water and steam do not mix, the condensate is recovered and any kind of cooling water can be used. In comparison to jet condensers, in surface condensers a high vacuum can be maintained, therefore greater thermal efficiency can be achieved.
On the other hand, surface condensers are bulky, require a large area,s and have high capital costs. But these capital costs can be recovered by the improved thermal efficiency (i.e., higher) saving in running costs.
Thus, these condensers are most suitable for modern thermal power plants. These are generally used where a large quantity of inferior water is available and better quality of feedwater is to be supplied to the boiler.
2. Jet Condenser
In jet condensers, the cooling water is sprayed on the exhaust steam and there is direct contact between the exhaust steam and cooling water. The process of condensation is very fast and efficient, but here cooling water and condensed steam are mixed up.
The condensate then cannot be reused as feedwater to the boilers. The temperature of the condensate is the same as that of the cooling water leaving the condenser. Due to the more intimate mixing of steam and cooling water jet condenser requires less quantity of cooling water for the condensation of steam.
In general, jet condensers require less building space and they are simpler in construction and lower in capital cost. Despite these advantages, jet condensers are not usual in thermal power plants especially due to the loss of condensate.
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.
