Condenser and Vacuum Efficiencies: Everything You Need To Know

Today, I am going to discuss Condenser and vacuum efficiencies.

If you looking for Condenser Efficiency and Vacuum Efficiency, side by side then you landed on the right platform.

I hope this journey with me, will end your search and deliver you a simple, sweet and desired answer with an ending.

So, without wasting too much of your time, let’s get started…

Condenser and Vacuum Efficiencies-

Before starting this article, let’s split these above terms into two topics and further talk about these efficiencies one by one.

So, what am I waiting for?

Let’s dive in…

Condenser Efficiency-

It is the ratio of temperature rise of cooling water and the difference of temperature of steam and inlet temperature of condenser. It is denoted by ηcond.

Where the Condenser cools the water by dissipating it.

A condenser is closed vessel in which high volume of steam is condensed and steam is converted into water.

In an ideal condenser, the steam should give only its latent heat to the circulating water so that the temperature of the condensate becomes equal to the saturation temperature corresponding to the condenser pressure.

It means there should be no undercooling of the condensate, Maximum temperature to which the cooling water can be raised is the condensate temperature.

The condenser efficiency is then defined as the ratio of actual rise in the temperature of the cooling water to the maximum possible rise.

Whereas, condenser efficiency cannot be mentioned as the ratio of output and input because both are unknown.

There is actually no standard method of determining the efficiency of a condenser but a method adopted by the well-known makers of steam turbines, Messers parson & Co. has been widely used in engineering practice.

Now, here is the formula of Condenser efficiency

Efficiency of condenser formula-

Efficiency of condenser formula

Here is the mathematical representation of this formula with diagram-

efficiency of condenser

Mathematical representation of Condenser efficiency formula

condenser efficiency formula

As you know, the purpose of a condenser is to drain the latent heat from the steam, also known as the enthalpy of the evaporation.

Here is the numerical condenser efficiency of the condenser-


Outlet & inlet temperature of cooling water to a condenser are 37.5°C and 30C respectively. If vacuum is 706mm of Hg with barometer reading 760mm of hg. Determine the condenser efficiency.


Data Given-

to = 37.5°C ,  ti = 30°C ,  Actual vacuum, Pν= 706mm of Hg , Standard atmospheric pressure, Patm= 760mm


We need to find ts and absolute pressure in the condenser, ie- Pc.

So, Here-

The absolute pressure of condenser,

Pc= Patm – Pν

From the data given,

= 760-706  

=54 mm of Hg

Now, this value will convert into the bar-


 760 mm of Hg= 71.01325 bar


1mm of Hg= 1.01325/760= 0.001333

So, here we need to find out 54mm of Hg-


54mm of Hg= 54×0.001333

=0.072 bar

From the steam table, you can find the saturation temperature corresponding to 0.072bar is 40°C.

So, from the formula-

condenser efficiency formula

nCond=  37.5-30/40-30

= 75%

Hence, the efficiency of the condenser will be 75%, at the given condition.

Vacuum Efficiency-

It is the ratio of the actual vacuum recorded by the gauge to the ideal vacuum.

As you know, the pressure less than atmospheric pressure is called vacuum.

Volume of water is highly reduced hence in a vessel due to reduction of volume, vacuum is created. Vacuum is measured in mm of Hg referred to atmospheric pressure.

The vacuum efficiency is a measure of the degree of perfection of maintaining the desired vacuum in the condenser.

The ideal vacuum means the vacuum due to steam alone when air is absent. Under such conditions, the total pressure in the condenser will reach the pressure of steam corresponding to the saturation temperature of the steam.

If there is no air leakage into the condenser then Pa=0 and hence the vacuum efficiency becomes 100%.

The vacuum efficiency depends upon the effectiveness of the air cooling and the rate at which the air is removed by the air pump.

Generally, the vacuum efficiency is about 98 to 99%.

Maximum vacuum can also be obtained from absolute pressure by using formula.

So, the Formula of vacuum efficiency will be-

Vacuum Efficiency,



Actual vacuum= Barometric pressure – Absolute pressure of steam


Ideal vacuum= Barometric pressure – Actual absolute pressure of the condenser




That’s it for now. I hope you gained the desired knowledge regarding Condenser Efficiency and Vacuum Efficiency.

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Thank you for being with me. i hope to see you in the next session.


Before you go, here are some MCQs, you should consider to having a look for while-


A vessel into which the steam is exhausted and condensed after work in an engine or turbine is known as-
A. Steam Boiler
B. Steam condenser
C. Steam-preheater
D. Economizer

Answer- B. Steam condenser

A condenser maintains a low pressure so as to obtain the maximum possible energy from steam and thus to secure a high efficiency-
A. Agree
B. Disagree

Answer- A. Agree

Condensers in thermal power plants are-
A. Exchangers
B. Generators
C. Conservators
D. Modifiers

Answer- A. Exchangers

Condenser enables obtaining-
A. Maximum efficiency of steam turbine
B. Steam at low temperature
C. Cooled water
D. Environmental control

Answer- A. Maximum efficiency of steam turbine

Which is not the function of the condenser-
A. Savings steam required per unit electricity
B. Convert exhaust steam to water for reuse in cycle
C. Cause slight fall in cycle efficiency
D. Obtaining reduced pressure and more turbine work

Answer- C. Cause slight fall in cycle efficiency

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