STEAM CONDENSER,VACUUM AND CALCULATIONS

A steam condenser is device or an appliance in which steam condenses and heat released by steam is absorbed by water. Heat is basically shell & tube type heat exchanger, where cooling water passes through tubes & steam condenses in shell.

The functions of the condensers are:

• It condenses the steam exhausted from Turbine last stage

• Increase the thermal efficiency of the plant reducing the exhaust pressure and thereby reducing the exhaust temperature

• It maintains a very low back pressure on the exhaust side of the Turbine

• Supplies feed water to Boiler through deaerator

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Condenser related components:

• Hot well
• Cooling water inlet & outlet system
• Cooling tower
• Support springs or expansion neck
• Air Ejector system
• Condensate extraction system
• Cooling water tubes & tube sheet
• Vacuum breaker valve
• Safety valve or rapture disc
• Water box
• Air & water vent lines

Types of steam condensers:

• Surface Condenser

• Air Cooled Condenser

• Jet Type Condenser

Types of surface condensers

• Down flow type

• Central flow type

• Inverted flow type

• Regenerative type

• Evaporative type

Design considerations of surface steam condensers

Design code XEI-IX

• Quality & Quantity of steam to be condensed
• Exhaust steam pressure and temperatures
• Steam velocity
• Quantity & quality of cooling water
• Operating pressure and temperature of cooling water
• Fouling factor
• Corrosion allowance

Effects of air leakage in condenser:

Lower Thermal Efficiency: The leaked air in the condenser results in increased back pressure on the turbine this means there is loss of heat drop consequently thermal efficiency of plant will decrease.

Increased Requirement of Cooling Water: The leaked air in the condenser lowers the partial pressure of steam due to this, saturation temperature of steam lowers and latent heat increases. So it requires more cooling water to condense more latent heat steam.

Reduced Heat Transfer: Due to poor conductivity of air heat transfer is poor.

Corrosion: The presence of air in the condenser increases the corrosion rate.

Functions of rapture disc & vacuum breaker valves in surface condensers:

Rapture disc is used to release the high pressure from steam condenser during excess pressure in condenser. Excess pressure may be due to cooling water pump failure or vacuum breaker valve failure etc. Rapture disc is thin steel foil designed to with stand condenser operating pressure, it raptures at high condenser pressure.

Vacuum breaker valve is used to bring down the turbine speed quickly to zero in case of emergency trip of turbine. Valve can be manually or auto opened.

Vacuum:

Vacuum is a sub-atmospheric pressure. It is measured pressure depression below the atmospheric. The condensation of steam in closed vessel produces a partial vacuum by reason of the great reduction in the volume of the low pressure steam or vapour. The back pressure on the steam turbine can be lowered from 1.013 to 0.1bar abs.

Reason for vacuum creation in condensers:

Condenser is mainly used to convert the low pressure steam at the end of the turbine to liquid so that the process is continued. As the pressure in the last phase of turbine is very low so the pressure in the condenser must be lower than that so that the low pressure steam can flow to condenser and get liquefied.

Generally vacuum pumps or ejectors are used to create vacuum in the condenser. Specific volume of water is much lower than the steam. Hence when the condensing process happens, volume of steam reduces and basic vacuum is created.

As liquids takes up less volume than gases whenever a steam is liquefied..there is a huge pressure drop as the drastic decrease in volume of liquids. The volume change takes place in the multiples of thousand at a certain pressure from the original volume of the gas.

Effect of under/low vacuum:

• Lesser work done by Turbine & hence lesser power output
• Higher steam consumption
• Higher exhaust steam temperature

Effect of over/higher vacuum:

• It causes sub-cooling effect where hot well temperature reduces more than design which required to be added in boiler leads to heat loss.
• Erosion of last stage LP blades due to lower exhaust steam temperature

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Vacuum & Condenser efficiency:

It is the ratio of actual vacuum to the maximum obtainable vacuum.

Vacuum efficiency in % =Actual vacuum X 100 / (Atmospheric pressure or barometric pressure-Absolute pressure)

Condenser efficiency =Difference in cooling water inlet & outlet temperatures X 100/(Vacuum temperature-condenser Inlet temperature of cooling water)

Condenser efficiency = (T2 - T1) X 100/(T3 - T1)

T2: Condenser outlet cooling water temperature,

T1: Condenser inlet cooling water temperature,

T3: Temperature corresponding to the vacuum or absolute pressure in the condenser.

Let us have glance over following calculations to for more clear understanding of above script

Example:1

A down flow type surface condenser has vacuum -0.92 kg/cm2 condenses 100 TPH steam at cooling water inlet and outlet temperatures 27 °C and 37 °C respectively, calculate the condenser efficiency.

Given that,

T1 = 27 °C, T2= 37 °C

T3 at vacuum -0.87 kg/cm2 is 48 °C

We have,

Condenser efficiency = (T2 - T1)/(T3 – T1)

                                      = (37 - 27) X 100/(48-27) =47.61%

Example-2:

Exhaust steam from condenser enters at 42 °C, if the vacuum gauge of condenser reads -0.89 kg/cm2, find the vacuum efficiency.

Given that,

Condenser pressure =-0.89 kg/cm2

Exhaust steam temperature = 42 °C

From steam tables, partial pressure of steam at exhaust temperature Ps = 0.084 kg/cm2

Maximum obtainable vacuum by considering atmospheric pressure as 1.033 kg/cm2

 = 1.033 - 0.084 = 0.95 kg/cm2

Vacuum efficiency = (Actual vacuum in condenser X 100)/Max. Obtainable vacuum.

                                 = 0.89 X 100/0.95

                                 = 93.6%

Example-3:

The volume of condenser which contains 0.162 kg of air with steam is 4.2 m3.Temperature in the condenser is 42 deg C and there is some water in the condenser. Determine the pressure in the condenser. Take R for air=287/joules/kg K

Given that,

Mass of air Ma =0.162 kg

Volume of air =V=4.2 M3

T=42+273=315 K

We have relation, PaVa=MaRT

Pa X 4.2=0.162 X 287 X 315

Pa=(0.162 X 287 X 315) X 10-5/4.2

Pa=0.035 bar

Partial pressure of water vapour at condenser temperature 42 deg c, Ps=0.08 bar

Pressure in the condenser = Pa+Ps=0.035+0.08 =0.115 bar

Gauge pressure =1.03-0.115 = -0.915 Bar

Example-4:

The air leakage into the steam condenser is 0.721 kg/min.The vacuum near the outlet of ejector is 690 mm of Hg when the barometer reads 760 mm of Hg & temp.at this point is 20 deg c..Calculate the mass of steam condensed.

Solution:

Pressure in the condenser =760-690 =70 mm of Hg

Convert into bar =70 X 0.001333 =0.0931 bar

Partial pressure of steam at 20 deg C =0.022 bar

So partial pressure of air Pa=0.0931-0.022=0.0713 bar

Mass of air leakage =Ma =0.721 kg/min

V = Ma X R X T/Pa

V = 0.721 X 287 X (273+20)/(0.0713 X 105)

V=8.5 M3/min

From the Dalton’s law of partial pressure, volume of the steam is same as air = 8.5 M3/min

Ms =Volume of steam/Specific volume of steam at 0.022 bar

Ms=8.5/62.5 =0.136 kg/min

Example-5:

A down flow type surface condenser is designed to handle 110 TPH of steam, the steam enters the condenser at 0.12 kg/cm2 absolute pressure and 0.9 dryness fraction. Condensate leaves at 45 °C, calculate the quantity of cooling water required, condenser inlet and outlet cooling water temperatures are 29 °C and 37 °C respectively.

Given that,

Mass of exhaust steam Ms = 110 TPH

Condenser pressure = 0.12 kg/cm2a

Dryness fraction x = 0.9

Cooling water condenser inlet temperature T1 = 29 °C

Cooling water condenser outlet temperature T2 = 37 °C

Condensate leaves at temperature Tc = 45 °C

We have,

Latent heat and saturation temperature of steam at exhaust pressure are

Hfg = 569.54 kcal/kg and T3 =49 °C

Mw = (Ms X (hfg X dryness fraction(x) + Cpw (T3 - Tc)))/(Cpw X (T2 - T1))

Mw = 110 X ((569.54 X 0.9) + 1 X (49 - 45))/(1 X (37-29))

        Mass of cooling water Mw = 7048.55 M3/hr

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