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Thermal Efficiency and Heat Balance of Reheating Furnace of Rolling Mills

Article  in  International Journal of Ambient Energy · December 2018

DOI: 10.1080/01430750.2018.1563819

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 International Journal of Ambient Energy

ISSN: 0143-0750 (Print) 2162-8246 (Online) Journal homepage: http://www.tandfonline.com/loi/taen20

Thermal Efficiency and Heat Balance of Reheating

Furnace of Rolling Mills

Gandhi Mallela, Pallavi Paturu, P.Kishore Kumar, M Komaleswararao,

Shubham Sharma & G. Harsha Vardhan

To cite this article: Gandhi Mallela, Pallavi Paturu, P.Kishore Kumar, M Komaleswararao,
Shubham Sharma & G. Harsha Vardhan (2018): Thermal Efficiency and Heat Balance of Reheating
Furnace of Rolling Mills, International Journal of Ambient Energy

To link to this article: https://doi.org/10.1080/01430750.2018.1563819

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 Thermal Efficiency and Heat Balance of Reheating Furnace of Rolling Mills
Gandhi Mallela1, Pallavi Paturu1, P.Kishore Kumar1, Komaleswararao M2, Shubham
Sharma*1, G. Harsha Vardhan*1
Department of Mechanical Engineering,
1*
Vel Tech Rangarajan Dr.Sagunthala R&D Institute of Science and Technology, Chennai, India.
2
JNTUK University College of Engineering, Vizianagaram, India.
Email Id: gandhimallela@veltech.edu.in, pallavipaturu@veltechuniv.edu.in

THERMAL EFFICIENCY AND HEAT BALANCE OF REHEATING

FURNACE OF ROLLING MILLS
Abstract
In this research paper inside the metal making process, rolling generators play an
essential function in reworking the steel to finished products. The blossoms from metal dissolve
shop are rushed into reheating furnace for heating up and resulting rolling. Reheating furnace at
is strolling with these derivative energies as gas and yielding a restrained output of the steel
metal for rolling. In this reduction in the furnace output changed into to test through performance
heat stability. Additionally, to recoup the waste heat, go flow recuperator turned into mounted
for preheating air. As a result, in this research paper aspirations at calculating the heat loss with a
purpose to increase the reheating furnace efficiency of light and medium of the steel plant.

Key Words: furnace, reheating, thermal efficiency and heat balance.

1. Introduction

The steel industry occupies a pivotal place in the economic infrastructure of a country and is
also main consumer of energy considering to 15% of power consumption in the factory sector and
10% of the power consumption in all the sectors [1]. The metallic industry occupies pivotal vicinity
inside the economic infrastructure of a country and is likewise foremost customer of power accounting
to 15% of power consumption inside the commercial quarter and approximately 10% of the energy
consumption in all the sectors.

In a steel plant, energy accounts for 40% of the total manufacturing costs. Since much can’t
be done for lessen the product costs by reduction of inputs as the prices of the inputs cannot be
controlled [2]. Any saving in energy can therefore make the difference in profit and loss of a particular
plant. While developing countries, who have awakened to this reality, are periodically reviewing
their energy scenario by the introduction of new and efficient energy technologies and
effective energy conservation measures, right from the project concept stage to the needs of a
developing economic.
The vitality recuperation from the results if the coordinated steel plants ought to act
naturally supporting and independent to work and keep up the entire plant to bode well and
efficient [3]. These by-products are produced in different phase like vitality recuperation from
gas of blast furnace, gas of coke oven and LD converter [4]. In this manner it’s basic that the
recovered vitality inside the industry should be used adequately, controlled proficiently and
inspected consistently to accomplish the greatest effectiveness and higher creation rate alongside
profitability [3,4].

1.1 Classifications of Furnaces

Primarily depending on the approach of producing warmness, Heaters are (furnace)
widely categorized into two sorts specifically combustion and electric type energy kind [5]. For
this situation of burning kind furnace, contingent on the type of ignition, it can be broadly named
oil let go, coal let go or fuel let go. Basically in view of the method of arraign of materials heater
can be categorized as; (i) Batch furnace and (ii) Continuous furnace. The block diagram is shown
in fig 1.
 Fig 1 Block diagram of classification of furnace
1.2 Walking Beam Furnaces; The strolling beam furnaces beat a significant more number of
the issues of pusher furnace and allow heating of the base face of the bench stock. This permit
stock warming surrounding and length of the heater, therefore better to control the warming
rates, much the same stock pull out of the fire temperatures and operational adaptability [5]. In
this same fashion as exceptional and base let go pusher furnaces, be that as it manage, a suited
part of the oven is inferior the laid on the line of the plant; this conceivable a limitation in more
or less applications.

1.3 Performance evaluation of the Furnace

Thermal ability of by the number heating products, a well-known as, heaters, ovens, and
furnaces is the aesthetic principle of light conveyed to furnishings and ignite provided to the
heating equipment [5,6]. The where one headed for a heating rite is to reveal a tenacious held a
candle to of elate vitality facing a factor, hoisting it to a specific temperature to apply it up for
additionally preparing or culmination it’s properties . To diligent this, the peripheral is warmed
in a heater. These outcomes in vitality misfortunes in various regions. For most warming
product, a great deal of the warmth gave is misused as exhaust gases [6] as shown in fig 2. (i)
These heater misfortunes included; Heat stockpiling structure of the heater, (ii) Outside wall
losses of the heater, (iii) Heat provided out of the warmer, (iv) Heat receiving from load
conveyor, (v) Radiation losses from opening of hot uncovered part, (vi) Heat passed on by the
cooling air infiltration into the heater, (vii) Heat passed on by the excess air used as a piece of the
burners.

Fig 2 Heat losses in industrial heating furnace [6]

2. Methodology

The system boundary for the purpose of carrying out heat balance of the walking beam
furnace was first defined. The heater and air recuperator were considered as different boundary
systems. Next, depending on each boundary system defined, the output and input was
considered. This research work mainly concentrates on the furnace efficiency and hence the input
and output were calculated for this system of furnaces. After, calculating a heat balance sheet
was also prepared [7].
The various components in the heat balance of the furnace (walking beam) are; (i) input
of the furnace is; heat given by the mixed gas and heat added by scaling. (ii) Output of the
furnace is; heat required for heating the material, heat lost to flue gases, E.C.S losses, heat lost
through cooling water, heat lost through conduction, heat lost through door openings,
unaccounted losses [8].
Once the heat balance sheet was prepared, it was revealed that the maximum loss of heat was
through fuel gas and cooled water [9]. This was causing flue gas passage to restrict the flow and
hence limit the load of the furnace as shown in Table 1 and 2. Analysis was then carried
out to know the percentage of the heat recovery in the air recuperator and it was observed that
the heat gained by air recuperator was reduced compared to the design value [10]. With a
specific end goal to increase the temperature of the preheated ignition air,
enlargement strategy inside the containers of the recuperator was recommended to
improve within heat transfer coefficient [11]. The dissertation work also includes
relationship of various parameters of furnace and with the rates.

Table 1 Values of the various parameters used for calculating the thermal efficiency of furnace 1
Sl. No. Parameters Values
1 Average gas flow per hour 22260 m3/hr
2 Specific heat of the gas 10194 KJ/kg K
3 Average blooms rolled per hour 41.2
4 Weight of each bloom 3.78 Tonnes
5 Total mass of the bloom 155.736 Tonnes
6 Specific heat of each bloom 0.690822 KJ/kg K
7 Temperature difference 1438 K

Table 2 Values of the various parameters used for calculating the thermal efficiency of furnace 2

Sl. No. Parameters Values

1 Average gas flow per hour 23305 m3/hr
2 Specific heat of the gas 10568.2 KJ/kg K
3 Average blooms rolled per hour 42.12
4 Weight of each bloom 3.78 Tonnes
5 Total mass of the bloom 159.22 Tonnes
6 Specific heat of each bloom 0.6890822 KJ/kg K
7 Temperature difference 1438 K

Heat through Scaling = Total tonnage rolled per hour x 13.5 (Factors for producing exothermic
heat)
Flue gas analysis: Mixed gas composition is (CO gas : BF gas : LD gas : 1.1 : 0.89 : 1)
CO2 = 13.56%, O2 = 0.087%, CO = 26.636 %, N2 = 25.49%, CH4 = 8.99%, CnHm = 0.945%
Combustion reaction:
H2+1/2O2 = H2O, CO+1/2O2 = CO2, CH4+2O2=2H2O, C2H4+3O2 = 2CO2+2H2O
3. Results and Discussions
3.1 Methods for Calculating Thermal Efficiency
3.1.1 Efficiency through Direct Method
Formula for calculating efficiency = Output/Input
= Heat utilized in heating the metal / (Heat given by mixed gas + Heat given by scaling)
Heat utilized in heating the metal = Total mass of blooms rolled x Specific heat of the metal x
Temperature of the bloom [2,6]
Heat given by mixed gas = Mixed gas volume consumed per hour x Calorific value of gas
Furnace - 1
Heat utilized in heating metal = 155.736 tonnes x 0.165Kcal/kg C x 1165oC = 29936.35 Mcal/hr
=125133.943 KJ/hr
Heat given by mixed gas = 22260 m3/hr x 2435Kcal/m3 = 54203 Mcal/hr = 226568.54 KJ/hr
Heat given by scaling = 155.736 x 13.5 = 2102.43 Mcal/hr = 8788.1574 KJ/hr
Efficiency = 125133.943 / (226568.54 + 8788.1574) x 100 = 0.531 x 100 = 53.16 %
Furnace - 2
Heat utilized in heating metal = 159.22 tonnes x 0.165KCal/kg oC x 1165oC = 30606.06 MCal/hr
= 128055.75 KJ/hr
Heat given by mixed gas = 23305 m3/hr x 2528.3KCal /m3 = 58924.01 MCal/hr = 246538.05
KJ/hr
Heat given by scaling = 159.22 x 13.5 = 2149.47 MCal/hr = 8993.38 KJ/hr
Efficiency = 128055.75 / (246538.05+8993.38) x 100 = 0.501 x 100 = 50.1%
3.1.2 Efficiency through Indirect Method
Indirect method = 100 – (Percentage of losses {loss through flue gas + steam generated + heat
loss through cooling water +heat loss due to convection and radiation + Unaccounted loss}) +
Heat recovered. The efficiency comparision between furnace 1 and 2 is shown in Fig.3. Table 3
describes the heat balance sheet of the furnace 1.
Furnace - 1
Total heat input = 243213.1871 KJ/hr
Heat utilized in metal = 125490.706 KJ/hr
Flue gas loss = Flue gas volume formed x Flue gas specific heat x Temperature difference
= 50531.627 KJ/hr = 28.78 % of total input
Volume of flue gas = 81460 m3/hr (Ratio of flue gas and air), Specific heat = 1.297KJ/Kg K,
Temperature difference = 625.6 K
Loss due to steam generation = Volume of steam generated per hour x Specific heat of steam
Volume of steam generated per hour = 12.4 T/hr, Specific heat of steam = 1842 KJ/kg K
= 22806.08 KJ/hr = 9.38% of total input
Loss due to indirect cooling water = Rate of flow of water x Temp. difference x Specific heat of
water
= 26.55 KJ/hr = 0.01 % of total input
Rate of flow of water = 128.9 m3/hr, Temperature difference = 298 K, Specific heat = 4.1868
KJ/kg K
Radiation and Convectional losses = Heat loss through (Mill wall + SMS wall + Roof
temperature + Hearth area)
= 5730.608 KJ/hr = 2.36% of total input
Mill side wall loss = Convectional loss + Radiational loss
Convectional loss: Mill side wall = Factor regarding natural convection x (Mill side wall temp –
Ambient temperature)1.25
Factor regarding natural convection = 2.20, Mill side wall temperature = 367.17 K, Ambient
temperature = 300 K
Radiational loss: Mill side wall = 4.88 x ε x {(MS wall temp)4 – (Ambient temp)4 }/ (100)4
ε = Emissivity factor, Mill side wall temperature = 367.17 K, Ambient temperature = 300 K

Heat loss through conduction = Surface temperature of (Roof + Mill wall + SMS wall +
Charging door + Discharging Door)
= 4749.650 KJ/hr = 1.95% of total input
Mill side wall: Area of the wall x {(Furnace temperature – Mill wall temperature) / Effective
resistance}
Area of wall = 444.82 m2, Furnace temp = 1473 K, Mill wall temp = 396 K, Effective resistance
= 1.713
Charging Door = 0.4229 x Door opening Time for hr x {(Temp of furnace)4/(100)7} x 1000.

Heat recovered from recuperator = Flue gas volume per hour + Flue gas specific heat x (Flue gas
inlet temperature – Flue gas out let temperature) x 1000
= 4117.262 KJ/hr = 1.69 % of total input
3
Volume of flue gas = 81460 m /hr (Ratio of flue gas and air), Specific heat = 1.297 KJ/kg K,
Flue gas inlet temperature = 921.6 K, Flue gas out let temperature = 751.8 K
Unaccounted loss: Input – (Output + Losses)
= 243213.1871 – (125490.706 + 87961.777)
= 29761.4101 KJ/hr = 12.23% as showing in the table 3 and fig 4.
 Fig. 3 Efficiency of direct and indirect method of furnace 1 & 2

Table 3. Heat balance sheet of the furnace 1

Description Value Percentage Description Value Percentage

(KJ/hr) (KJ/hr)
Chemical 234446.3352 96.40% Heat required to heat the 125490.710 51.60%
heat of fuel metal
combustion
Flue gas losses 50531.627 20.78%
Heat 8766.839 3.60%
formed by
scaling
E.C.S Steam generation 22806.08 9.38%

Indirect cooling water 13.45 0.01%

loss
Radiation and 5730.608 2.36%
convection losses
Loss due to conduction 4749.650 1.95%

Loss due to radiation 4117.262 1.69%

through doors
Unaccounted losses 29773.780 12.23%

Total 243213.1742 100.00 Total 243213.174 100.00

 Fig. 4 Heat utilizing and losses of the reheat furnace 1

Furnace - 2
Total heat input = 255531.43 KJ/hr
Heat utilized in metal = 128055.75 KJ/hr
Flue gas loss = Flue gas volume formed x Specific heat of the flue gas x Temperature difference
Volume of flue gas = 81460 m3/hr (Ratio of flue gas and air), Specific heat = 1.2958 KJ/kg K,
Temperature diference = 784 K
= 58848.88 KJ/ hr= 23.03% of total input
Loss due to steam generation = Volume of steam generated per hour x Specific heat of steam
Avg.
volume of steam generated per hour = 15.4 T/hr, Specific heat of steam = 1842 KJ/kg K
= 28363.98 KJ/hr = 11.10% of total input
Loss due to indirect cooling water = Rate of flow of water x Temp. difference x Specific heat of
water
Rate of flow of water = 124 m3/hr, Temperature difference = 298K, Specific heat = 4.1868 KJ/kg
K
= 25.55 KJ/hr = 0.01 % of total input
Radiation and Convectional losses = Heat loss through (Mill wall + SMS wall + roof temperature
+ floor area)
= 5775 KJ/hr = 2.26% of total input
Mill side wall loss = Convectional loss + Radiational loss
Convectional loss: Mill side wall = Factor regarding natural convection x (Mill side wall temp –
Ambient temperature)1.25
Factor regarding natural convection = 2.20, Mill side wall temperature = 367.17 K, Ambient
temperature = 300K
 Radiational loss: Mill side wall = 4.88 x ε x {(Mill side wall temp)4 – (Ambient temp)4 }/ (100)4
ε = Emissivity factor, Mill side wall temperature = 367.17 K, Ambient temperature = 300 K
Heat loss through conduction = Surface temperature of (Roof + Mill wall + SMS wall +
Charging door + Discharging Door)
= 4752.88 KJ/hr = 1.86% of total input
Mill Side wall: Area of the wall x {(Furnace temperature – Mill wall temperature) / Effective
resistance}
Area of wall = 444.82 m2, Furnace temp =1473 K, Mill wall temp = 396 K, Effective resistance
= 1.713
Charging Door = 0.4229 x Door opening Time for hour x {(Temp of furnace)4/(100)7} x 1000.
Heat recovered from recuperator = Flue gas volume per hour + Specific heat of the flue gas x
(flue gas inlet temp - flue gas outlet temp) x 1000

Volume of the flue gas = 84285 m3/hr (Ratio of flue gas and air), Specific heat = 1.297 KJ/kg K,
flue gas inlet temp = 1080 K, Flue gas outlet temp = 811 K
= 4140.252 KJ/hr = 1.62% of total input

Unaccounted loss: Input – (Output + Losses)

= 255531.43 – (128055.75 + 101906.54) = 25569.14 KJ/hr = 10.00% of total input as
showing in the table 4 and figure 5.
Table 4 Heat balance sheet of the furnace 2

Description Value KJ/hr Percentage Description Value KJ/hr Percentage

Chemical heat 255531.43 96.50 Heat required to heat the 127933.351 50.12
of fuel metal
combustion
Flue gas losses 58848.88 23.03
Heat formed by 8993.38 3.50
scaling
E.C.S Steam generation 28363.98 11.10

Indirect cooling water 25.55 0.01

loss
Radiation and convection 5775 2.26
losses
Loss due to conduction 4752.88 1.86

Loss due to radiation 4140.252 1.62

through doors
Unaccounted losses 25569.14 10.00

Total 255239.836 100.00 Total 255531.43 100.00

 Fig 5 Heat utilizing and losses of the reheat furnace 2

4. Conclusions
The two primary intent of this work is to lessen fuel and increase production. The various
parameters responsible for the loss of energy have been identified and recorded which prevail
over the two reheating furnace at the steel plant. The above described remedies can overcome the
losses and reduce the input of energy expenditure up to 25% and increase the efficiency up to 8-
10% through this thesis and can be further increase by targeting every aspect at depth which is
responsible for the losses. The main areas which gather more attention towards the losses in
reheating furnace are: Extraction of heat from flue gas going out of chimney, Overcoming
delays, Replacing normal gas burner with Regenerative burner, Insulation of Furnace walls,
Providing extra heating capacity for high grade steels rather using extra fuel.
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