| BOILER DESIGN SOFTWARE FOR FIRE-TUBE BOILERS |
| THEORETICAL BACKGROUND |
| DOWNLOAD (pdf file) Calculation of mean radiant temperature (article in Journal of Institute of Energy, UK, September 2000) |
| DOWNLOAD (pdf file) Equations used in algorithm |
| DOWNLOAD (pdf file) Research work information |
| Algorithm had been developed in framework of multi year research work as part of doctoral thesis at University of Ljubljana, Slovenia, Department for Mechanical Engineering (Internet address: http://www.fs.uni-lj.si/eng) in cooperation with Institute for Thermal Engineering at Technical University Vienna, Austria (Internet address: http://www.ite.tuwien.ac.at/ite/home) under comentorship of Prof. Wladimir Linzer (Internet address: http://whitepages.tuwien.ac.at/oid/633357.html). |
The new theoretical findings incorporated in algorithm
1. Heat transfer by thermal radiation in boilers
To determine the radiant and convective part of heat transfer in the fire-tube boilers the
effective temperatures in each boiler section must be known. In case of convection that
temperature is transferred to mean logarithmic temperature difference. For the radiation
from the hot flame and flue-gases to a cooled enclosure of simple geometry, which is the
case with cylindrical shape of the furnace in fire-tube boilers, the literature quotes an
approximate approach to calculate the total radiant heat flow from hot flame and gas to
the boiler walls by defining an effective flame temperature. That approach is not based
on the laws of physics, and it was introduced solely due to its simplicity compared with
other methods suitable for large boilers. However, it tends to be less accurate since it
cannot accurately assess a combination of flame and gas radiation.
New algorithm incorporates a more accurate equation for mean radiant temperature.
2. Turbulators in boiler tubes
Turbulence promoters (hereafter refereed to as turbulators) are inserts, which increase
the rate of convection in the tubes compared to those, which are empty. As their name
implies, their function is to increase the turbulence of the hot gases flow by breaking up
the laminar boundary layer and thereby increasing convection. These devices appear in
different shapes. Meanwhile, a more accurate heat transfer calculation in boilers calls for
a more accurate analytical assessment of turbulator's effect on the total heat transfer.
This is of particular importance in order to appropriately operate the boiler system,
including the burner. An inappropriate assessment of turbulators' impact on pressure
drop can cause the choking of the burner because its fan would be unable to overcome
the increased pressure drop in the boiler due to an inaccurate assessment of the
turbulators combined effect on heat exchange and pressure drop. There are published
results of tests on differently shaped turbulators. Further, a thorough analytical solution
for twisted-tape turbulators has been produced which is not true for coiled-wire ones.
Coiled-wire turbulators are, today, the most widely used because of following advantages:
* simple and very effective;
* compared to heat exchange intensification, such turbulators do not cause high
* pressure drop increases;
* easy to produce, maintain, and replace.
Literature dealing with the analytical assessment of the impact on heat exchange and
pressure drop in tubes with coiled-wire turbulators is very scarce. This area has not yet
been researched enough, and the available theory needs to be extended. The latest
published results of experiments and theory is limited to coiled-wire turbulators with
diameters not exceeding 25 mm and wire diameters not exceeding 3 mm. This is not
applicable to boilers where the tubes are of a diameter, which usually starts at 50 mm.
New algorithm incorporates a more accurate yet simple procedure for analytical
assessment of the impact of coiled-wire turbulators on heat transfer and pressure drop in
boiler tubes.
3. Heat transfer by convection in boilers
Convection in boilers takes place simultaneously with radiation. In tubes of fire-tube
boilers more than 90% of heat exchange takes place by the convection. In the furnaces
the radiant part is greater than in tubes. Calculation of convection is conducted by
standard equations for flows in straight tubes and channels. This is also true for the
boiler furnaces, whether they are circular or rectangular in cross-section. This picture
totally changes when gas flow directly hits the surface involved in convection, such as in
the case of the cooled rear of the furnace. The rate of convection is much higher in these
cases and cannot be assessed by classical equations for straight flow in tubes and
channels. Tests showed much lower gas exit temperatures from the furnace than
calculated, which was found to be attributable to lacking of taking into account the heat
transfer from impinging jet of the flue-gases. This type of the heat transfer is known, but
its role in boilers has not yet been investigated.
New algorithm incorporates a procedure for the analytical assessment of the heat
transfer from the impinging jet as it appears in boilers.
1. Heat transfer by thermal radiation in boilers
To determine the radiant and convective part of heat transfer in the fire-tube boilers the
effective temperatures in each boiler section must be known. In case of convection that
temperature is transferred to mean logarithmic temperature difference. For the radiation
from the hot flame and flue-gases to a cooled enclosure of simple geometry, which is the
case with cylindrical shape of the furnace in fire-tube boilers, the literature quotes an
approximate approach to calculate the total radiant heat flow from hot flame and gas to
the boiler walls by defining an effective flame temperature. That approach is not based
on the laws of physics, and it was introduced solely due to its simplicity compared with
other methods suitable for large boilers. However, it tends to be less accurate since it
cannot accurately assess a combination of flame and gas radiation.
New algorithm incorporates a more accurate equation for mean radiant temperature.
2. Turbulators in boiler tubes
Turbulence promoters (hereafter refereed to as turbulators) are inserts, which increase
the rate of convection in the tubes compared to those, which are empty. As their name
implies, their function is to increase the turbulence of the hot gases flow by breaking up
the laminar boundary layer and thereby increasing convection. These devices appear in
different shapes. Meanwhile, a more accurate heat transfer calculation in boilers calls for
a more accurate analytical assessment of turbulator's effect on the total heat transfer.
This is of particular importance in order to appropriately operate the boiler system,
including the burner. An inappropriate assessment of turbulators' impact on pressure
drop can cause the choking of the burner because its fan would be unable to overcome
the increased pressure drop in the boiler due to an inaccurate assessment of the
turbulators combined effect on heat exchange and pressure drop. There are published
results of tests on differently shaped turbulators. Further, a thorough analytical solution
for twisted-tape turbulators has been produced which is not true for coiled-wire ones.
Coiled-wire turbulators are, today, the most widely used because of following advantages:
* simple and very effective;
* compared to heat exchange intensification, such turbulators do not cause high
* pressure drop increases;
* easy to produce, maintain, and replace.
Literature dealing with the analytical assessment of the impact on heat exchange and
pressure drop in tubes with coiled-wire turbulators is very scarce. This area has not yet
been researched enough, and the available theory needs to be extended. The latest
published results of experiments and theory is limited to coiled-wire turbulators with
diameters not exceeding 25 mm and wire diameters not exceeding 3 mm. This is not
applicable to boilers where the tubes are of a diameter, which usually starts at 50 mm.
New algorithm incorporates a more accurate yet simple procedure for analytical
assessment of the impact of coiled-wire turbulators on heat transfer and pressure drop in
boiler tubes.
3. Heat transfer by convection in boilers
Convection in boilers takes place simultaneously with radiation. In tubes of fire-tube
boilers more than 90% of heat exchange takes place by the convection. In the furnaces
the radiant part is greater than in tubes. Calculation of convection is conducted by
standard equations for flows in straight tubes and channels. This is also true for the
boiler furnaces, whether they are circular or rectangular in cross-section. This picture
totally changes when gas flow directly hits the surface involved in convection, such as in
the case of the cooled rear of the furnace. The rate of convection is much higher in these
cases and cannot be assessed by classical equations for straight flow in tubes and
channels. Tests showed much lower gas exit temperatures from the furnace than
calculated, which was found to be attributable to lacking of taking into account the heat
transfer from impinging jet of the flue-gases. This type of the heat transfer is known, but
its role in boilers has not yet been investigated.
New algorithm incorporates a procedure for the analytical assessment of the heat
transfer from the impinging jet as it appears in boilers.
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