How do you calculate the attemperator water consumption in Boilers?

 

1-What do you mean by attemperation?

Attemperation is a method employed for controlling the super-heated steam temperature

Attemperators are used to control the main steam (super-heated steam) temperature in Boilers.

2-What are the two basic types of attemperators?

Spray type attemperator

Surface type attemperator

3-What is the main differences between attemperator & Desuper heaters?

Sl No.	Attemperator	Desuper heater
1	An attemperator controls steam temperature	Desuperheater removes whatever superheat there is in steam and reduces the temperature to a point at or nearly at saturation temperature
2	Attemperators are generally found in and/or associated with boiler steam, in zones where too high temperature affects something downstream of that point	Desuperheaters are used for downstream use of saturated steam
3	Outlet of the attemperation will be superheated steam only	Outlet of desuperheater will be saturated steam
4	Used in super-heated lines	Used in MP or LP steam lines

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4-What type of attemperation method is used in modern high pressure boilers?

In modern high pressure boilers variable nozzle and spray type attemperators are generally used

5-What is the percentage of attmepration used in High pressure Boilers?

Generally varies from 8 to 15%

 6-What is the reason behind using stainless steel sleeve inside the attemperator header?

Generally SS sleeves are fitted inside the attemperator header, this sleeve performs following functions.

 

Acts as a thermal barrier, separates hot and cold working elements to mitigate the intensity of thermal cycles experienced by critical components.

Protects steam pipe from thermal shock, helps to improve secondary atomization.

 

7-What precautions should be taken during attemperator liner or sleeve design?

•  Minimum length of straight pipe upstream of the liner should be three times the pipe diameter.
• Length of the liner downstream from the spray nozzles should be between 3 and 6 ft, depending on the particular installation.
• Length of straight pipe downstream of the liner should allow a residence time of 0.067 second for spray water to evaporate before the first elbow.
• Location of the temperature sensor should be at a distance downstream of the liner that allows 0.2 seconds of residence time to ensure complete mixing of the evaporated water and superheated steam. However, if the mass flow of spray water is greater than 15% of the mass flow of superheated steam, the residence time should be increased to 0.3 seconds.

 8-What factors are considered for designing attemperator?

Following factors are considered:

• Feed water pressure, flow rate, and temperature at the spray water control valve during various load conditions
• Locations of temperature sensors at the upstream & downstream ends
• Water chemistry
• Residence time of steam & water mixture for sensing temperature at downstream end
• Type of attemperator spray nozzle
• Rate of atomization & size of droplets

9-What is the distance of temperature sensors from attemperator spray nozzles?

For proper controlling of steam temperature, the upstream & downstream distance of sensors should be minimum of 5D & 20D respectively for straight pipe, where D is the diameter of attemperator header

 How do you calculate the Attemperator water consumption?

1. An attemperator is used to control the 95 TPH super-heated steam temperature from 425 deg C to 395 deg C by using 110 deg C feed water. Consider the main steam & feed water pressure 87kg/cm2 & 100 kg/cm2 respectively. Calculate the quantity of attemperator water

Given data,

Mass of steam, Ms = 95 TPH

Enthalpy of steam before attemperation at pressure 87 kg/cm2 & temperature 425 deg C, H1 = 762.41 kcal/kg.

Enthalpy of steam after attemperation at pressure 87 kg/cm2 & temperature 395 deg C, H2 = 741.81 kcal/kg

Feed water enthalpy at temperature 110 deg C, Hf = 111.65 kcal/kg

For calculation of attemperator water Mw

We have the relation,

Heat lost by the super-heated steam = Heat gained by the feed water

Ms X (H1-H2) = Mw X (H2-Hf)

95 X (762.41-741.81) = Mw X (741.81-111.65)

Mw = 3.1 TPH

2. A 125 TPH Boiler having variable type attemperator control valve for controlling main steam temperature from 495 deg C to 425 deg C at pressure 67 kg/cm2. The feed water is used for attemperation is 105 deg C, calculate the quantity of water required for de-superheating.

In the above example, Boiler feed pump having head 1000 meter & efficiency 62% supplies attemperator water, then calculate the extra power consumption for attemperation. Consider motor efficiency 95%

Given data,

Mass of steam, Ms = 125 TPH

Enthalpy of steam before attemperation at pressure 67 kg/cm2 & temperature 495 deg C, H1 = 812 kcal/kg.

Enthalpy of steam after attemperation at pressure 87 kg/cm2 & temperature 425 deg C, H2 = 771 kcal/kg

Feed water enthalpy at temperature 105 deg C, Hf = 106 kcal/kg

For calculation of attemperator water Mw

We have the relation,

Heat lost by the super-heated steam = Heat gained by the feed water

Ms X (H1-H2) = Mw X (H2-Hf)

125 X (812-771) = Mw X (771-106)

Mw = 7.7 TPH

For calculation of power required for pumping 7.7 TPH of water, we have

Motor input power = Flow in m3/sec X Total head X Density of water X 9.81 / 1000 X Pump efficiency X Motor efficiency)

Density of attemperator water at temperature 105 deg C = 960 kg/m3

Attemperator flow in m3/sec = 7.7 X 1000 X / (960 X 3600) =0.022 m3/sec

Now,

Motor power = 0.0022 X 1000 X 960 X 9.81 / (1000 X 0.62 X 0.95)

Motor power = 35.17 KW

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