A condenser is a device or unit used to condense a gaseous substance into a liquid state through cooling. In so doing, the latent heat is released by the substance and transferred to the surrounding environment. Now you may wonder what the point of maintaining vacuum in condensers is. In this article, Linquip has provided thorough information about the reason why vacuum is maintained in condensers.
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Vacuum Importance in Condensing System
Not all condensers maintain vacuum but definitely those working in boiler system do. Water is used to condense steam back to liquid water in any form of boiler system, whether on board ship or in power plants. As water and steam has a volume ratio of 1:1600; that means steam is condensed to the size of 1/1600 times than before. In other words, the specific volume of water at a given pressure is many times lower than that of steam. This enormous reduction in volume creates the vacuum inside the condenser. This partial vacuum will draw in more gas to be cooled. Now to ensure proper and continuous flow of steam to condensers at all times, vacuum must be maintained externally. In other words, it is impossible to prevent the entrance of air and other non-condensable gases into the condenser. A vacuum is maintained in the condenser so that steam can easily flow and more work can be extracted from the steam in the turbine; this is the reason why vacuum is maintained in condensers. Therefore, some methods must exist to initially cause a vacuum to exist in the condenser. This necessitates the use of vacuum-producing devices to establish and help maintain condenser vacuum.
Vacuum-producing Device Types
The three most common vacuum-producing devices are steam-jet air ejectors (SJAE), liquid ring vacuum pumps (LRVP), and a hybrid of ejector and pump.
- Ejectors can be operated with many different motive fluids: steam, air, organic vapor and other gases. They can handle corrosive and slugging liquids, solid and abrasive suction fluids without damage. They also have explosion-proof construction and can be installed indoors or outdoors with versatile mounting design. Furthermore, low initial cost, low maintenance cost and long life are other advantages of ejectors.
- Liquid Ring Vacuum Pumps are the most used vacuum-producing devices besides ejectors in industry. They are reliable with simple design involving only one rotating part, which is not subject to wear. In addition, they can handle condensible vapors or even slugs of liquid entrained in the gas stream without damage to the pump or affecting performance. They are also resistant to contaminants entering with the gas stream and include wide choice of materials available for handling most gases and seal liquids.
- Hybrid of ejector and pump is often used to reduce energy costs and the size of the vacuum pump. Its other advantages include operating simplicity, ability to handle corrosive vapors, available space, and the cost and availability of spare parts.
Vacuum System Application
The primary application of vacuum systems in the power generation industry is evacuating the air and other non-condensable gases from the shell side and the waterbox side of a steam surface condenser (shell and tube condenser). On the shell side, the vacuum equipment is used for two purposes: rapid evacuation of air to reduce condenser pressure before steam turbine starts, the “hogging mode,” or for continuous removal of air and associated water vapor from the main condenser when the steam turbine is in operation, the “holding mode.” The elimination of air and other non-condensable gases from the condenser shell side is necessary for appropriate heat transfer from steam to cooling water in the condenser and, consequently, to maintain high vacuum in the condenser. This increases the amount of energy extracted from the steam exhausted from the steam turbine and increases the plant’s energy production. Without a vacuum system, air and other non-condensable gases would severely reduce the heat transfer in the condenser, and the plant would require a condenser with significantly more surface area for the same thermal load. Because of this, vacuum system must exist in condensers.
What Will Happen If Vacuum Is Increased or Removed?
Although the working pressure of a condenser do have many advantages including faster condensation, better heat exchange, avoid air to get dissolved in water and increased overall efficiency of the unit, there is a limit to which it is feasible and when it starts to make more trouble than it solves. Generally, maintaining perfect sealing condition in a condenser as the pressure reduces below one atmosphere is a hard goal to accomplish. This increase on the cost and maintenance requirement makes it ineffective in long run. Furthermore, as we reduce the pressure; the steam starts to condense at lower and lower temperature. As the temperature of condensing steam decreases with decrease in pressure, different problems emerge to maintain the same heat exchange between the medium. Ideally, the greater the temperature difference between the two medium, the better is the heat exchange efficiency of the condenser. Similarly, if we remove the vacuum from the system it not only increases the amount of heat energy lost unused; but also causes a rapid drop in PH of the boiler water which will lead to sudden drop in system efficiency in short run and complete damage of the turbine assembly in long run. So a condenser must never be run without vacuum unless it is safe.
What Is the Vacuum Efficiency of the Condenser?
Vacuum efficiency is the ratio of actual (measured by gauge) to ideal vacuum in a condenser. Ideal pressure is defined as having the same pressure as the condensate temperature (pi) or with the temperature of steam flowing into the condenser.
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I really enjoyed this piece on condensers. If I have understood you correctly, a turbine is ‘blown’ at the h.p stages and ‘sucked’ at the l.p. end by one atmosphere. The 1/1600th fraction amazed me. On an aesthetic note, I love the sight of a bank of six or eight cooling towers in the sunset 😉 things of great beauty…
We’re glad you enjoyed this. And your understanding of what happens is right. We do agree with you that these are ‘things of great beauty’ 🙂
Thank you for the piece.
What are the causes of loss of vacuum in main condenser of steam turbine?
1- Excessive air leakage into the vacuum system
a) Sealing steam is not adequate to glands of turbine and stop valves
b) Gland sealing steam is not exhausted into the condenser
c) Gland steam makeup and spill controllers are defective
d) Gland steam return needle valve not adjusted correctly
e) Air leaks from other lines connected to vacuum condenser
f) Leaking exhaust bellow at the condenser inlet
g) Condenser safety valve or any other penetration leak
h) Air leaking into the system reduces the condenser efficiency
i) Air leaking system increases insulation to condense steam
j) Air bubbling may be observed in vacuum condenser sight glass
k) Gland steam inlet steam traps are fouled
l) Function of a gland drain is to draw water from sealing-gland cavities created by the condensation of sealing steam to keep clear to vacuum condenser
m) Gland steam blowing should be avoided because the steam usually blows into the bearing, destroying the lubrication oil in the main bearing. Steam blowing from a turbine gland also creates condensate, causing undue moisture in plant equipment.
2-Improper functioning of the air removal equipment
a) In the case of the vacuum pump unit, dirty nozzle in ejector unit
b) Condenser cooling is not adequate
c) Condenser is leaking
d) Multiple pumps interconnection and N/R valves leaking
e) Pump running directions are not correct
f) Line valves may be not correctly set
g) Dirty circulating water- corrosive, corroded tank and piping system
h) The nozzle provided is very small, necessary to maintain a clean system
i) The suction line from the condenser obstructed
j) The air discharge line from the ejector to the tank is restricted
k) The tank vent line obstructed
3- Improper drainage of condensate from the condenser
a) Condensate pump suction line vent restricted
b) Condensate pump re-circulating controller erratic
c) Air leaking from the Nonrunning unit mountings
d) The condensate temperature may be too hot
e) Condensate pumps clearances may be high
f) The valves to and from the hot well are throttled
g) Flooding of vacuum condenser and g/glass defective
h) Condenser level controller is erratic
4- Insufficient flow of circulating water
a) Dirty condenser
b) Division plate leaks
c) Ideal temperature of seawater outlet is 5-8 degrees centigrade below the corresponding vacuum temperature of steam
This is a comprehensive article about the importance, creation and maintenance of Vacuum in the condenser. Thanks to the writer.