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 |
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%
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?
- 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.
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
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
Why can't the attemperator flow be too high?
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