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WSN 87 (2017) 150-162 EISSN 2392-2192

Numerical analysis of convection along hot surface of equipment in the selected boiler room

Robert Cichowicz1 & Artur Wacław Stelęgowski2

Department Environmental Engineering and Building Construction Installations, Faculty of Architecture, Civil and Environmental Engineering, Lodz University of Technology,

Al. Politechniki 6, 90-924 Lodz, Poland

1,2E-mail address: [email protected] , [email protected]

Phone +48 42 631 20 20, Fax +48 42 631 35 16

ABSTRACT

The combustion processes, that are taking place in combustion units in boiler plants, result in

heat production. Some of the heat is being exchanged between the thermal installation equipment and

the air in such type of a room. In consequence, both the temperature of equipment’s surface and of the

indoor air rises. This results in natural convection effect, in which the air heated up from the

equipment rises upwards (along with that part of the heat generated by combustion), and in its place

flows the air of lower temperature. As a result of the phenomenon, there is a change in airflow in the

room and a removal of part of heat gains from equipment. The numerical analysis of convection along

hot surface of technological facilities and equipment was made on the basis of the numerical

calculations of air parameters in the selected boiler room. Boundary conditions for the calculations

were determined using the results of building energy simulation and the results of experimental

measurements.

Keywords: boiler plant, air parameters, CFD technique

1. INTRODUCTION

Boiler plants are buildings equipped with thermal installations, in which the process of

fuel combustion is taking place and results in generation of heat [1-3]. Afterwards, the heat is

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distributed to the receiver, e.g. heating installation. The heat generation and distribution is

associated with losing heat that is being exchanged between the installation elements and air

in the room [4-6]. Unfortunately, thermal insulation of the technological equipment and pipes

does not provide full hermeticity of the process, which results in an increase in the surface

temperature of the equipment and transfer of heat to the indoor air. A source of heat losses of

the thermal system in a boiler room is not only a furnace or boiler, but also linked facilities

and equipment, such as heat exchangers, filters, chimneys and pipes. Air heats up from the

equipment and goes up along with a part of the heat produced during combustion process.

And in its place flows air of lower temperature. This phenomenon is called natural convection

and results from a difference in the fluid densities [7]. The motion of air can be also made by

an external devices, such as ventilation fans, and this mechanism is called a forced convection

[8]. Convection of air around hot surfaces of technological equipment causes local

disturbances in the air temperature and velocity distribution. Cognition of these changes

allows to determine the effect of the thermal system on the heat conditions in the boiler room

[9].

The analysis of a convection is possible using the knowledge of the air temperature and

velocity distribution, the determination of which is based on experimental measurements or

numerical calculations. The numerical calculations of the air parameters [10-11] can be based

on the computational fluid dynamic methods (CFD), supported by inter alia the computer

programs such as: Ansys Fluent, AutoCAD CFD or Design Builder. The CFD analysis

software can be used for modelling both the natural convection [12-14] and the forced

convection [15-17], as well as the thermal processes occurring inside the combustion units

[18-20]. The results of calculations of the air parameters in the surroundings of the thermal

equipment constitute a basis for the interpretation of the physical phenomena that is resulting

from a combustion process taking place in the boiler plant.

2. RESULTS

The boiler room analyzed is located in the city over 500,000 inhabitants (in Lodz), in

central Poland. Height of the boiler room is approximately 16.90 m. The thermal installation

consists of two independent, symmetrical process lines each of which contains inter alia

industrial furnace (fig. 1), recuperator (fig. 2), multicyclone (fig. 3) and chimney. The

temperature of the thermal process in the furnace is 870 °C and the maximum thermal power

of the installation is 8.14 MW. There is a mechanical exhaust ventilation installed in the room

and implemented by roof fans, while the air supply is carried out by the air intakes mounted in

the external walls.

The numerical analysis of air parameters in surroundings of the thermal installation was

made using the DesignBuilder software with a CFD module. The basis for the calculations

was the execution of the geometrical model and introduction of boundary conditions into the

program. Boundary conditions were determined using results of the building energy

simulation and of the experimental measurements. Additionally, the operation of only one

technological line was assumed for the analysis purposes. The following parameters were

introduced into the computer program: surface temperature of the equipment (37.0 ÷ 117.6

°C), temperature of partitions (18.0 ÷ 33.3 °C), outdoor temperature (23.3

°C) and the

ventilation airflow (97,020 m3/h). Surface temperature of the installation elements rises along


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with height and it was around 55÷118 °C for the furnace and recuperator, and 40÷60

°C for

the multicyclone and chimney. The zone average temperature was around 27 °C. Results of

the numerical calculations were obtained in a form of the air temperature and velocity

distribution.

. .

Figure 1. Furnace Figure 2. Recuperator

At the +7.60 m above the floor level (fig. 4), the air temperature in surroundings of

thermal equipment of the operating process line was much higher than on the rest of the area

and it exceeded 27 °C, while at a distance of about 1 m from the equipment the temperature

dropped to about 26 °C. At this height the temperature of the equipment oscillated from 40 °C

to 80 °C. The technological equipment in the boiler room (fig. 4) was marked as:

F – furnace;

R – recuperator;

B – boiler;

M – multicyclone;


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FI – filter;

CH – chimney.

Figure 3. Multicyclone

Figure 4. Air temperature at + 7.60 m


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In the fig. 5 the air velocity vectors were showed to indicate the flow of warm air heated

from the technological equipment (at the +12.70 m level). The convection phenomena was

noticeable in the surroundings of the furnace and recuperator, where the air velocity was

around 0.50 ÷ 1.30 m/s and the air temperature above 32 °C. While over the multicyclone and

boiler it was 0.50 ÷ 0.70 m/s and 30 ÷ 32 °C. The dynamics of the removal of heat gains from

the equipment increased along with the temperature of its surface. Therefore, the highest air

velocities occurred near such installation elements as the furnace and the recuperator (surface

temperature of 105 °C and 118

°C), and the lowest near the chimney (60

°C). This is a positive

phenomenon in terms of the thermal comfort in the room, because of the removal of the heat

gains from the combustion units. However, the significant increase of the air velocity in the

working area can lead to draughts, which can be assessed negatively by workers staying in the

room [21-23].

Figure 5. Air temperature and velocity at + 12.70 m

In the cross-section of the non-operating process line (fig. 6), the air temperature and

velocity was not affected by the technological equipment. There was a stratification of air in

the room, which means that the air temperature rose along with height and was approximately

constant at the same level. The occurrence of this kind of air distribution was related to the

significant impact of natural convection that was not disrupted in this cross-section. The air

turbulence was very low and the air velocity in the most of the area (approximately 90%) was

below 0.30 m/s. However, some fluctuations occurred near the location of elements of the

ventilation system. Also, the air temperature varied locally. This was because the changes in

the air conditions in this part of the boiler room were due to impact of heat gains from the

operating process line and due to operation of the industrial ventilation. Therefore the local


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increase of the air velocity above 1.00 m/s was caused by air supply through the air intakes in

the central part of the room and air exhaust through the roof fans.

Figure 6. Cross-section of the non-operating process line

In the cross-section of the operating process line (fig. 7), the air temperature and

velocity was much affected by operation of the boiler plant, and related heat gains. The

distribution of air temperature and velocity varied a lot in this cross-section. Significant

fluctuations of air parameters occurred in the vicinity of the ventilation elements as well as of

the technological equipment. The highest air temperatures (over 35 °C) were noticed above

the furnace and recuperator, which were the biggest sources of heat gains in the room. The

highest air velocities (over 1.00 m/s) were related to the operation of the ventilation system

and occurred near the location of air supply through the air intakes and air exhaust through the

roof fans. The convection streams occured in the case of the air heated up from the hot

surfaces of the technological equipment, which rose upwards along with a part of the heat

generated by combustion. The air velocity related to the convection phenomena exceeded

0.70 m/s in surroundings of the multicyclone, boiler, recuperator and furnace. However, the

stratification of air occurred only in the part of the boiler room with no combustion units.

In the cross-section A of the room (fig. 8), the air in surroundings of the operating

recuperator had a velocity of 0.27 ÷ 0.73 m/s and near the non-operating process line of below

0.27 m/s. Despite the air in the boiler room was exhausted by mechanical ventilation, there

was no significant influence of the operation of roof fans on the air velocity in the vicinity of

technological equipment. Therefore, it can be concluded that the influence of natural

convection caused an increase in the air velocity of approximately 0.50 ÷ 1.00 m/s (fig. 5).


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Figure 7. Cross-section of the operating process line

Figure 8. Air velocity in the cross-section A


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In the cross-section B of the recuperators (fig. 9), there was a visible difference between

the air parameters in the vicinity of the operating and non-operating process line. The air

temperature over the operating equipment exceed 35 °C and the air velocity was almost 1.00

m/s. However, in the case of the non-operating recuperator the air temperature was below

30 °C and the air velocity below 0.30 m/s. As the roof fans were located symmetrically, the

difference between both cases in the air temperature and velocity was mainly caused by the

natural convection phenomena. The impact of forced convection due to operation of the

ventilation system was not significant in this cross-section. In most of the area (70%), there

was air stratification with air velocity value below 0.30 m/s and approximately constant air

temperature at the same height. Therefore, the convection caused only a local disruption in the

distribution of air temperature and velocity in the vicinity of the operating recuperator.

However, the convection air streams caused an induction of the surrounding air, which effects

in the increase of its velocity.

Figure 9. Air temperature and velocity in the cross-section B


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In the cross-section C of room, related to the location of multicyclones (fig. 10), there

was a significant movement of air of low temperature (23.3 °C) supplied at the bottom of the

operating multicyclone, and a movement of air of higher temperature (30 °C) that rose from

its upper surface due to occurrence of convection. The flow of the air supplied through the air

intake with the velocity exceeding 1.50 m/s was caused by forced convection due to the

operation of exhaust fans located on the roof of the building. High speed of supplied air

caused a local turbulence of the air at the lower parts of the technological equipment.

However, the air movement over the operating multicyclone was caused by a natural

convection of warm (above 30 °C) air heated up from surface of the equipment. The rest of

the cross-section area, where impact of the air supply stream and hot surfaces was minute, had

a distribution of air characteristic for the stratification. In most of the area (70%), the air

velocity was below 0.30 m/s and the temperature was approximately constant at the same

height of the room.

Figure 10. Air temperature and velocity in the cross-section C


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This means that the disruptions in air temperature and velocity occurred only locally,

that is in the line of the air supply and over the operating facilities and equipment.

Nevertheless, the convection air streams caused an induction of the surrounding air.

Also, in the cross-section D of the room (fig. 11), the air near the operating chimney had

a higher velocity than in the rest of the area. The increase of air speed was conjunct with an

increase of air temperature near the chimney. Heat gains from hot (around 60 °C) surface of

the operating chimney were causing an increase of temperature of the surrounding air over

30 °C and velocity over 0.70 m/s. The convection phenomena caused that the air of lower

temperature (23.3 °C) flowed at the bottom of the chimney with low speed (approximately

0.10 m/s) and the air of higher temperature (above 30 °C) was exhausted at the top of the

room with high speed (over 0.70 m/s). Whereas, in the vicinity of non-operating chimney, as

well as in the most of the area, the velocity was below 0.20 m/s. However, at the bottom of

the room the air velocity exceeded 0.30 m/s, which was associated with an impact of supplied

air and related induction of air in the room.

Figure 11. Air temperature and velocity in the cross-section D


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3. CONCLUSIONS

High temperature of the combustion process in the boiler room analysed was causing

the increase of surface temperature of elements of the combustion plant. As a consequence, a

convection of air heated from the equipment occurred, resulting in a disruption of the air

temperature and velocity distribution. By the temperature of 60 °C of installation elements,

the air temperature was approximately 30 °C and the velocity 0.70 m/s. While by the

temperature 118 °C of the furnace, the air temperature exceeded 32

°C and the velocity was

above 1.30 m/s. The difference between the air speed near the operating and non-operating

process line was up to 1.00 m/s (fig. 5).

This means that the occurrence of the convection phenomenon in the surroundings of

the thermal installation had positively affected the removal of heat gains related to the

combustion processes. This is because the air velocity increased along with the temperature of

the equipment. In addition, the natural convection had supported the industrial ventilation, the

aim of which was to remove the contamination and heat derived from the combustion process

[24-26]. What is more, providing better thermal conditions in a room effects in greater

thermal comfort sensations of the workers [27-29]. On the other hand, the disruptions of the

air temperature and velocity occurred mostly in the vicinity of operating technological

equipment and near the location of ventilation elements, such as air intakes and roof fans. In

most of the area, the air temperature was approximately constant at the same height and the

air velocity was below 0.30 m/s, which indicates for air distribution characteristic for

stratification. Therefore, the convection occurred in the whole boiler room, with a strong

fluctuation of air parameters in the surroundings of the operating boiler plant elements and

stable temperature and velocity in the rest of the area.

Understanding an influence of operation of combustion plant on thermal conditions in a

boiler room is related to knowledge of the temperature and air velocity distribution.

Convection and other thermal phenomena can be modelled using numerical methods [12-17],

which allow to determine the air parameters at any point of the room. Therefore, the CFD

methods are widely used in the analysis of heat transfer issues [11,18-20].

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( Received 20 September 2017; accepted 10 October 2017 )

numerical analysis of convection along hot surface …€¦ · 1,2e-mail address:...

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