What is vacuum efficiency?

Short Answer:

Vacuum efficiency is the ratio of the actual vacuum created in a condenser to the maximum possible vacuum that could be achieved under ideal conditions. It shows how effectively a condenser removes steam and non-condensable gases to maintain a proper vacuum.

In simple words, vacuum efficiency measures the performance of a condenser in maintaining low pressure. A higher vacuum efficiency indicates that the condenser is working efficiently with minimum air leakage and better removal of gases, leading to improved turbine performance and higher plant efficiency.

Detailed Explanation :

Vacuum Efficiency

A vacuum in a steam condenser is created to maintain a low pressure at the turbine exhaust so that the steam can expand more and produce maximum work. However, in actual operation, air leakage and non-condensable gases enter the condenser through joints, glands, or cracks, reducing the vacuum level. To measure how efficiently the condenser maintains this vacuum, the term vacuum efficiency is used.

It is defined as the ratio of the actual vacuum obtained in the condenser to the ideal or maximum possible vacuum that could exist if no air or other gases were present.

Mathematically,

Vacuum efficiency is usually expressed as a percentage. For a perfectly airtight condenser, the efficiency would be 100%. In practical systems, it generally ranges between 90% and 98%, depending on the condition and maintenance of the equipment.

Concept of Vacuum in Condenser

In a steam condenser, the exhaust steam from the turbine is condensed into water by cooling. When steam condenses, the pressure inside the condenser drops, creating a partial vacuum. This vacuum is essential because it:

  1. Reduces the back pressure on the turbine.
  2. Increases the expansion of steam and turbine output.
  3. Improves thermal efficiency of the power plant.

However, the presence of air and other non-condensable gases in the condenser reduces the effectiveness of this vacuum. These gases occupy space, increase pressure, and lower the overall efficiency. Therefore, maintaining a high vacuum is essential for condenser performance, and vacuum efficiency is the measure of how effectively this condition is maintained.

Ideal and Actual Vacuum

  1. Ideal Vacuum:
    The ideal vacuum corresponds to the total absence of air and gases in the condenser. Under this condition, the pressure inside the condenser is equal to the saturation pressure of the steam at the existing temperature.
  1. Actual Vacuum:
    The actual vacuum is the pressure difference obtained in real operating conditions. Due to air leakage and incomplete removal of non-condensable gases, the pressure inside the condenser is slightly higher than the saturation pressure.

The difference between the ideal and actual vacuum is caused by the presence of air and other gases.

Expression for Vacuum Efficiency

By definition,

Substituting the respective expressions,

Where,

  • Barometric Pressure: Atmospheric pressure at the location (usually around 760 mm of Hg).
  • Saturation Pressure of Steam: Theoretical pressure corresponding to the steam’s temperature inside the condenser.
  • Actual Pressure: The real pressure recorded due to the presence of air and vapor mixture.

Factors Affecting Vacuum Efficiency

  1. Air Leakage:
    The most common reason for reduced vacuum efficiency. Air enters through joints, glands, valve openings, or cracks, increasing the condenser pressure.
  2. Temperature of Cooling Water:
    Higher cooling water temperature raises the saturation pressure of steam, which reduces the ideal vacuum.
  3. Efficiency of Air Extraction Pump:
    If the air pump fails to remove air and non-condensable gases effectively, the vacuum will drop.
  4. Condensate Temperature:
    If the condensate is not properly cooled, the partial pressure of steam increases, reducing vacuum efficiency.
  5. Load on Turbine:
    Sudden changes in turbine load affect steam flow and pressure, temporarily influencing condenser vacuum.
  6. Fouling of Tubes:
    Deposits or scaling on condenser tubes hinder heat transfer, preventing complete condensation of steam and increasing the pressure inside the condenser.

Methods to Improve Vacuum Efficiency

  1. Ensure Air-Tight Construction:
    All joints, flanges, and glands must be properly sealed to prevent air entry.
  2. Efficient Air Removal:
    Install properly sized and well-maintained air extraction pumps to continuously remove non-condensable gases.
  3. Use of Superheated or Dry Steam:
    Reduces the amount of moisture entering the condenser, improving vacuum stability.
  4. Maintain Clean Condenser Tubes:
    Regular cleaning of tubes prevents scaling and ensures good heat transfer.
  5. Proper Cooling Water Supply:
    Maintain an adequate flow rate and temperature of cooling water for effective condensation.
  6. Monitor Vacuum Regularly:
    Use accurate vacuum gauges to check performance and detect any drop early.

Importance of Vacuum Efficiency

  • Indicates the health and performance of the condenser.
  • Helps in identifying air leakage and maintenance needs.
  • Directly affects the thermal efficiency of the power plant.
  • Ensures low back pressure on the turbine, increasing power output.
  • Reduces steam consumption and operational cost.

A condenser with high vacuum efficiency ensures smooth, economical, and reliable operation of the entire power plant.

Typical Values

  • Ideal Vacuum: Around 760 mm of Hg (atmospheric pressure).
  • Actual Vacuum: Between 720–740 mm of Hg (depending on leakage and efficiency).
  • Vacuum Efficiency: Normally ranges between 90% to 98% in well-maintained systems.

Example:
If the barometric pressure = 760 mm Hg,
saturation pressure = 40 mm Hg,
and actual pressure = 60 mm Hg,

Then,

Hence, the condenser’s vacuum efficiency is 97.2%.

Conclusion

In conclusion, vacuum efficiency is a key indicator of a condenser’s ability to maintain the desired vacuum for effective steam condensation. It is defined as the ratio of actual vacuum obtained to the ideal vacuum possible. High vacuum efficiency means that air leakage and heat losses are minimal, leading to better turbine performance and increased plant efficiency. Regular inspection, efficient air removal, and proper maintenance of condenser parts are essential for maintaining high vacuum efficiency in power plant operations.