What is Condenser ?
The primary functions of the Condensing plant are:
· To provide the lowest economic heat rejection temperature for the steam cycle
· To convert the exhaust steam to water for reuse in feed the cycle
· To collect the useful residual heat from the drains of the turbine feedheating plant, and other auxiliaries.
· In so doing, the circulating cooling water temperature being low enough, it creates a low back pressure (vacuum) for the turbine to exhaust to.
· This pressure is equal to the saturation pressure corresponds to the condensing temperature which is a function of the cooling water temperature.
· Since the enthalpy drop and hence turbine work, per unit pressure drop is much greater at low pressure than the high pressure end of the turbine.
· The condenser, by lowering the back pressure a little bit increases the work of the turbine, increases the plant efficiency, and reduce steam flow for a given output
· The lower the pressure, the greater the effects
Some other functions of Condenser: Additional Condenser Objectives
In addition to the condenser satisfying the primary functions, its design must also be capable of meeting the following objectives:
· To provide the turbine with the most economic back pressure consistent with the seasonal variations in CW temperature or the heat sink temperature of the CW system.
· To effectively prevent chemical contamination of the condensate either from CW leakage or from inadequate steam space gas removal and condensate de-aeration.
· The aim of the designs is to ensure that these objectives are met within the framework of the following practical considerations:
i. Economies of size, space and pumping power ii. Ease of maintenance and construction iii. An economical turbine back pressure is from 1.0 to 3.5 in Hg abs
Classification of Condensers:
• There are two broad classes of condensers:
Direct contact type condensers — the condensate and CW directly mix and come out as a single stream
Surface condensers — are shell-and-tube type Heat Exchanger where the two fluids do not come in direct contact and the heat released by the condensation of steam is transferred through the walls of the tubes into the cooling water continuously circulating inside them.
Direct Contact type condenser
When low investment is desired and condensate recovery is not a factor, direct-contact condensers are effective.
They are relatively simple to build and operate, are limited to sizes less than 250,000 lb. (114 tons) of steam per hour
Direct Contact type condensers can be of three types:
· Spray condenser
· Barometric condenser
· Jet condenser
Direct Contact Spray Type
· In a spray condenser the cooling water is sprayed into the steam which by mixing directly with cold water gets condensed.
· Part of the condensate, equal to the turbine exhaust flow, is sent back to the plant as feedwater.
· The remainder is cooled in a dry cooling tower to state 5 and is then sprayed on to the turbine exhaust thus, the cooling water continually circulates.
Direct Contact Barometer Type
· The cooling water is made to fall in a series of baffles to expose large surface area for the steam fed from below to come in direct contact.
· The condensed steam and the cooling water mixture falls in a tail pipe to the hot well below the tail pipe compresses the mixture to atmospheric pressure at the hot well by virtue of its static head
Direct Contact Jet Type
· In the jet-type Condenser the height of the tail pipe is reduced by replacing it with a diffuser
· The diffuser helps raising the pressure in a short distance than a tail pipe
· In all direct contact Condensers the non-condensable gases must be removed which is usually done with a steam-jet air ejector (SJAE)
Surface Type Condensers
· Surface Condensers shell and tube heat exchangers are mostly used in power plants
· For the convenience of cleaning and maintenance cooling water flows through the tubes and steam condenses outside the tubes
· Present-day condensers have heat transfer surface areas that exceed 1 million ft2 (93000 m2)
· Condensers are designed with one, two, or four water passes.
· The number of passes determines the size and effectiveness of a Condenser.
Single Pass Condenser
In an A single-pass condenser cooling water flows through all the Condenser tubes once, from one end to the other.
Two Pass Condenser
In Two-pass condenser water enters half the- tubes at one end of a divided inlet water box. And then passes through these tubes to an undivided water box at the other end. Then they reverse direction and passes through the other half of the tubes back to the other side of the divided water box.
Single Pass Condenser should be used or two pass?
A single-pass condenser with the same total number size of tubes, i.e., the same heat-transfer area, and same water velocity, requires twice as much water flows but results in half the water temperature rise and thus lower condenser pressure
Thus such a single—pass condenser is good for plant thermal efficiency and reduces thermal pollution, but requires more than twice the water and hence four times the pumping power
Water boxes are often divided beyond the divisions required by the number of passes
A divided water box single-pass condenser may have a partition in both the inlet and outlet water boxes at opposite ends of the condenser — allows half the condenser to operate while the other half is being cleaned or repaired.
Divided water boxes have duplicate inlet and outlet connections, each with its own circulating water circuit.
Many large modern-day power plants usually have two or more low-pressure turbine sections in tandem.
The condenser may be divided into Corresponding sections or shells.
Single Pressure Condenser
When the turbine exhaust pressure in all sections is same, i.e. when the exhaust ducts are not isolated from each other, it is known as a single-pressure condenser.
If the exhaust ducts are isolated from each other, these individual condenser shell pressures will increase because the circulating water temperature will increase as it flow from shell to shell — a multi-pressure condenser
A multi-pressure condenser results in efficiency improvement because the average turbine back pressure is less compared with that of a single-pressure condenser (Which is determined by the highest circulating water temperature)
In essence, condensers are almost custom designed to suit individual requirements of steam flow available cooling water flow and temperature, available space and other variables.
Surface Type Condenser Design Considerations
A condenser design can be established for a given performance rating based on eight principal variable which are
· Total heat transferred, which is a function of
Ø Weight of steam to be considered.
Ø Enthalpy of steam less enthalpy of condensate.
Ø Enthalpy loss or gains of drains and make up.
· Absolute static steam pressure.
· Cooling water flow rate
· Cooling water inlet temperature.
· Cooling water outlet temperature.
· Cooling water velocity through tubes.
· Effective heat transfer surface, which is a function of:
Ø Number of tubes Tube length Tube diameter Tube thickness Tube material Number of cooling water passes
Ø Service conditions:
o Tube cleanliness
o Air in-leakage
The tube material can be:
- Cupronickel (70% Cu, 30% Ni)
- Aluminum brass (76% Cu, 22% Zn. 2% AI
- Aluminum bronze (95% Cu, 5% AI)
- Muntz metal (60% Cu, 40% Zn)
- Admiralty alloy (71 % Cu, 28% Zn, 1 % Sn)
- Stainless steel
De-aeration or Air Removal of condenser
Air may leak into the condenser shell through flanges or sometimes comes along with steam which has leaked into the exhaust end of the turbine along the shaft
This air affects the condenser performance badly because of the following reasons:
· It reduces the heat transfer considerably
· It reduces the condenser vacuum and increases the turbine exhaust pressure thus reducing the turbine output
Good de-aeration within a condenser requires time, turbulence and good venting equipment
· The cold condensate falling from the lower tubes must have sufficient falling height and scrubbing steam for reheat and de-aeration
· The scrubbing steam is provided by allowing some of the incoming steam to pass through an open flow area directly to the bottom tubes to reheat the condensate — non condensable are more easily released from a hotter than a colder liquid.
· Once the non-condensable are released, they are cooled to reduce their volume before being pumped out of the condenser
· For this a number of water tubes, about 6 to 8% in the tube bundle, are set aside for this function.
· This, called an air-cooler section, is baffled to separate the non condensables from the main steam flow
· Most of the condensation takes place on the main bank of tubes and the air is drawn over another smaller bank which is shielded from the main bank by a baffle and is called the a aircooler.
· Here, further condensation takes place at a lower temperature and thus, there is saving in feedwater as well as in air ejection load.
· Jet pumps are used to pump out the non-condesables — known as steam jet air ejectors (SJAE)
Ø In a two-stage ejector, main steam is used at a reduced pressure that enters a driving nozzle in the first stage ejector
Ø It exits with a high velocity and momentum and reduced pressure
Ø This reduced pressure draws in the non-condensables from the condenser
Ø By a process of momentum exchange, the gases are entrained by the steam jet
· The combined flow of steam and gas is now compressed in the diffuser of the first-stage ejector and discharged into a small inter-condenser, where the steam is condensed by passing across cooling pipes in much the same manner as the main condenser.
· Cooling here, however, is accomplished by the main condenser condensate and is part of the feedwater heating system, resulting in improvement in efficiency of the plant.
· The non-condensables and any remaining steam are then passed to the second stage ejector, where they are compressed further and passed to an after-condenser
Vacuum efficiency of a Condenser
Sometimes a term called ‘Vacuum efficiency’ is often used a regard to a condenser
It is defined as:
Vacuum efficiency = (Vacuum produced by steam condenser inlet/Barometric pressure - Saturation pressure at exhaust steam pressure)
Another term called ‘condenser efficiency’ is also used sometimes.
Which is defined as:
Condenser efficiency = (Actual temperature rise of cooling water/Maximum temperature rise of cooling water)