Thermal Power Plant is based on the Rankine cycle and in Ideal Rankine cycle, there are 4 processes and those are Heat Addition at Constant Pressure in Boiler, Isentropic Expansion in Turbine, Constant Pressure Heat Rejection in Condenser and Isentropic Compression in Pumps. So now we will discuss about the condenser, which plays a very vital role in the cycle.
So what is the purpose of condenser? Condenser condenses the steam which comes from the turbine into water at a very low pressure. But here the question arises why we need to condense the steam into water.it will be better if we directly send the steam back to the boiler without converting.
The reason is that if we want to dump the exit steam from the turbine to the boiler then we have to use a very large capacity compressor, which may take very high auxiliary power or you can say more than the power turbine is generating and that is not economical. As the total auxiliary consumption has to be as low as 9% of the total power generated, then only you will be able to supply more power to the customers. Also In mechanical term, we call it as a PMM2 (Perpetual Motion Machine),which takes heat from one source and gives work without rejecting any heat to the sink, But we know that we may not able to convert the total amount of heat into work. So when we use the condenser then high specific volume steam converts into low specific volume water and it’s easy to pump less quantity of water in comparison of high quantity steam. So for power plants condenser plays a very vital role and condensers are broadly classified into two types;
1) Direct Contact Type Condenser
2) Surface Type Condenser
Direct contact type condenser is not used commonly because in this type of condenser, we directly sprinkle the cooling water into the steam and to do that we require cooling water of the same quality as that of steam which is very costly and not economical. So the commonly used condenser is of Surface Type Heat Exchanger, in which the cooling water is passed through the tubes and the steam flows outside the tube. This water takes the heat of the water and circulates continuously for converting steam into water.
In the Rankine cycle, if the area of dome increases then the efficiency also increase. So our approach is to go for lesser pressure in the condenser, then only the cycle efficiency will increase by increasing the area of the dome of the Rankine cycle, but because of not achieving Absolute Vacuum in the condenser, we try to maintain low pressure or high vacuum in the condenser. This is done by the help of Ejectors or Vacuum pumps.
Multi-pass Double Flow Type Condenser
For a 135MW unit, the Multi-pass Double Flow Type Condenser consists of around 17,000 tubes through which the cooling water flows and the steam flows on the outer surface of the tubes.Below is the internal diagram of the condenser.
In the operation of the condenser, first of all we pull the vacuum in the condenser then we start the cooling water (CW) pump and then we charge condenser from the steam side. But the main reason for the creation of vacuum in the condenser is that when high volume steam converts into the low volume water then the vacuum is generated in the condenser. For maintaining vacuum in the condenser we use 2 types of equipment;
1) Air Ejector
2) Vacuum Pump
In the air ejectors, we also use steam for its operation. Its shape is like a nozzle-diffuser section. When the steam passes through the nozzle type of section then its pressure energy is converted into the kinetic energy and in the inlet section of nozzle pressure is dropped and it sucks the air from the condenser. In ejector, there are no moving parts so its maintenance is very easy.
(Nozzle-Diffuser in Ejector)
The other type of air ejecting equipment is Vacuum pump. In the vacuum pump no need of steam to operate it instead of that we have to use water in that. When the impeller of the vacuum pump starts rotating in the eye of impeller low pressure area is made and by the help of that vacuum is maintained in the condenser. This vacuum pump is effective in the starting of the condenser to pull the vacuum and maintain it throughout the operation.
In condenser after condensing the steam, the steam converts into water by the help of cooling water. That cooling water (CW) after heat exchanging goes to the cooling tower. Where the heat collected from the steam in the condenser is removed by the help of direct contact with air. The purpose of the cooling tower is to cool the hot water coming out from the condenser. Also cooling tower is of so many types like forced draught cooling tower and Natural Draught cooling tower. In forced draught cooling tower we use fans but mostly in larger units we use Natural Draught Cooling Tower. When the water comes from the condenser after exchanging heat from the steam, it goes up in the cooling tower by the help of riser tubes and then it falls through the nozzle. When it sprinkles, the heat in the water is carried by the air which comes from the downside and carries heat with it. After cool down the water by the help of cooling water (CW) pump is again sent to the condenser.
After the condenser, the condensed water or condensate goes to the hot well. The Hot well serves 2 purposes. It gives head and flow to the condensate extraction pump (CEP) for minimum circulation at starting and on the other hand it also stores the water which is condensed in the condenser. After the hot well, this water goes to the gland steam cooler and LP heater and then to the Deaerator.
Factors affecting Condenser performance –
- Cooling Water Temperature – if the cooling water temperature is high then its heat exchanging capacity is low and it decrease the performance of the condenser.
- Cooling Water Quantity – if the cooling water quantity is less than the required then proper heat exchange in condenser will not be there, resulting a decrease in performance.
- Fouled And Dirty Tubes Of Condenser – this also restricts the heat exchange process and it degrades the water quality.
- Malfunctioning In Ejector And Vacuum Pump – because of this there is vacuum drop in condenser which affecting directly plant efficiency and condenser performance. It also leads to the LP Hood heating due to the back pressure developed in the Condenser.
As condenser is very crucial so there are certain measures we have to take and some factors which are important with respect to plant running and tripping (Stop Conditions) –
- Hot Well Level High ( Because of this there are chances of drop in vacuum)
- Hot Well Level Low (Because of this chances of CEP trip)
- CEP Trip (if in case running as well as standby CEP trip then plant trip)
- Cooling Water Pump Trip (if in case all the CW pump trip then plant trip)
- Condenser Vacuum Drop (If condenser Vacuum drop continuously then plant trip)
With this we see that condenser Vacuum plays an important role in the plant efficiency. The falling of vacuum has many reasons like malfunctioning of ejector, CW pump trip, and Vacuum breaker valve not close properly, leakage of air in Vacuum system or air ingress through drain which directly open to atmosphere. Because this Vacuum drop there is increase in the exhaust hood temperature which further deteriorate the vacuum.