Reduction of Heat Losses for the in-line Induction

INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 6, pp. 903-909 JUNE 2013 / 903

KSPE and Springer 2013

Reduction of Heat Losses for the In-line InductionHeating System by Optimization of Thermal Insulation

Hong-Seok Park1,# and Xuan-Phuong Dang1

1 Laboratory for Production Engineering, School of Mechanical and Automotive Engineering, University of Ulsan, Korea, 680-749# Corresponding Author / E-mail: [email protected], TEL: +82-52-259-1458, FAX: +82-52-259-1680

KEYWORDS: Thermal insulation, Induction heating, Energy-savings, Optimization

Improving the electrical energy efficiency of the manufacturing process is the imperative task to resolve the cost-saving pressure and

environmental legislations. This paper focuses on a study on the thermal efficiency of the in-line induction heating systems that are

using in the hot forging applications. Besides optimizing the process parameters that increase the electromagnetic and thermal

efficiency, reducing the heat losses to the surrounding air is one of the practical ways to save the operating energy. The weakness of

the current in-line induction heating systems were pointed out, and we proposed an insulating system to reduce the heat losses caused

by convection and radiation. The analytical model of the heat transfer and the simulation model were built to calculate and verify the

thermal efficiency of the insulating covers. The results show that using insulating covers at the open spaces between adjacent heaters

of an in-line induction heating for automotive crankshaft forging can approximately reduce 9% of heat losses compared with the

energy stored in the workpiece. The best values of the geometrical design parameters of the insulating cover were determined by

solving the constrained optimization that considers some technological aspects of the proposed insulating system. This work is intended

as a contribution to make the hot forging industry become greener and more efficient in terms of saving operating energy.

Manuscript received: October 2, 2012 / Accepted: February 17, 2013

1. Introduction

Energy-savings considerations in manufacturing processes have

been drawing a great attention to the researchers and manufacturers

because of the ecological issues, cost-saving pressure, and new

environmental legislations.1 Improvement of machining and process

efficiency for manufacturing is one of the promising solutions.2,3 For

hot forging industry, induction heating process has been considered as

a high productivity, repeatable quality, and green heating technology

compared to fuel-fired furnaces. This is the reason why induction

heating, a best available heating technology, is preferred in forging

manufacturing.4,5 Induction heating prior to hot forging requires a huge

amount of electrical energy for heating a steel workpiece with large

volume from the ambient temperature to approximately 1150 ~ 1250oC.

Therefore, the heat losses account for a great number, and the increase

in the thermal efficiency of the heating system significantly saves the

consumed energy.

Solutions for saving energy for industrial induction heating may

include the energy management, innovative components of induction

devices, energy recovery, and adaptive control.6 Diverse published

works devoted to optimization of induction heating,7-13 but most of

them focused on how to minimize the temperature deviation at the end

of the heating process. Studies on minimizing the energy consumption

for particular manufacturing processes have been still dimmed although

NOMENCLATURE

= heat-transfer coefficient (W/m2oC)

= thermal conduction (W/moC)

= emissivity

= kinematic viscosity (m2/s)

= Stefan-Boltzmann constant (W/m2.K4)

q = heat transfer rate per unit length (W/m)

t = temperature (oC)

Gr = Grashof number

Nu = Nusselt number

Pr = Prandtl number

Ra = Raleigh number

DOI: 10.1007/s12541-013-0119-6

904 / JUNE 2013 INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 6

the producers of induction heaters are trying to increase the efficiency

of their products. Besides the optimization of process parameters,

practical solution that reduces the heat losses caused by radiation and

convection is an efficient way for saving the energy consumption.14

However, the study of reducing heat losses in the induction heating line

has not been focused adequately. This work tried to analyze the heat

losses through radiation and convection in order to find the solution for

saving energy in the in-line induction heating system.

2. State of the Research

This work aims at increasing energy efficiency for a hot forging

production of the automotive crankshaft in which the reduction of

thermal losses from the heating system is one of the important targets. As

previously mentioned, induction heating step is the one that consumes a

great amount of energy compared with other stages in the forging

process. Therefore, reduction of heat loss in the induction heating system

will increase the energy efficiency of the whole forging line.

The in-line induction system consists of seven heaters (research

objective) for heating the round steel bars with 97 mm of diameter and

6000 mm long as shown in Fig. 1. The steel bar moves continuously

through the in-line heaters with a designated velocity according to the

cycle time of 25 seconds. In every cycle time, the heated steel bar is cut

into 460-mm-long workpiece by the hot shearing machine before

moving to the crankshaft forging die. The rating power of the heating

line is 4250 kW.

To increase the energy efficiency of the in-line induction heating,

optimization the heating parameters and reduction of thermal losses are

the two potential methods. The way of increasing the energy efficiency

by process parameter optimization have successfully studied.14 The

investigation of the real induction heating system was carried out, and

it was found that the heat losses caused by convection and radiation at

the open spaces where the heated billet exposes to the ambient air

account for a significant amount of energy. The Figure 2 illustrates the

diagram of energy flow in the induction heating process in which

radiation accounts for an important portion. Table 1 and Figure 3 shows

the thermal losses of an induction heating line with seven heaters as

shown in the Figure 1.14 The notation t1 in the Table 1 is the

temperature at the surface of the heated workpiece obtained by using

a pyrometer at seven open spaces in the heating line. The heat flux

caused by convection and radiation per unit length (qconv and qrad) are

estimated by popular heat transfer formulas. In the in-line induction

heating, because the induction heaters are connected one by one, and

there are open spaces between heaters, the losses caused by radiation

and convection are remarkable. The total heat losses at the open spaces

between the heaters account for 82.2 KW, equivalent to 9.4% of the

heat energy stored in the SCM440 steel workpiece with 460 mm when

heating from 25oC up to 1220oC. However, the forging companies

dont always pay much attention on this issue due to the lack of the

study of how to reduce these heat losses and how is the efficient of the

using the insulation covers. The task of this research focuses on the

previously mentioned problem. Furthermore, an optimum design of the

insulating system was proposed to reduce the heat loses with a

maximum efficiency.

3. Development of the Insulating Covers to Reduce Heat

Losses for the In-Line Induction Heating System

3.1 The proposed thermal insulating system

The real in-line induction heating system shows that the distance

between two adjacent heaters usually accounts for 30% of the heater

Fig. 1 In-line induction heating of a long steel bar prior to hot forging

Fig. 2 The diagram of energy flow in the induction heating processes

Table 1 Rate of heat transfer in the case of without insulating cover

Position t1 (C) qconv (W/m) qrad (W/m) qsum (W/m)

1 812 2894 15496 18,390

2 889 3,208 20,413 23,621

3 950 3,458 25,069 28,527

4 1030 3,786 32,326 36,113

5 1160 4,322 47,330 51,653

6 1200 4,488 52,851 57,339

7 1220 4,571 55,785 60,356

Sum of heat losses of seven positions (W/m) 275,999

Fig. 3 The power of radiation and convection losses at the open spaces

between heaters without using the insulating covers

INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 14, No. 6 JUNE 2013 / 905

length because they need spaces for the roller supporting and

translating the hot and heavy steel bar continuously (see Fig. 1). At

these open spaces, the extreme hot billet (approximate 800 ~ 1220oC)

directly exposes to the ambient air. Due to the very high temperature

at the billet surface, the heat losses caused by radiation and convection

are considerable. To reduce these losses, the insulating covers at the

open spaces were proposed as shown in Figure 4.

Due to the high temperature of the billet, it is found that the special

materials that can withstand this temperature as shown in Fig. 5. In

addition, these materials have a good manufacturability with the

available large size and the ability that can be machined into the

required shape.

3.2 Mathematical model for calculating heat transfer

The insulating device is considered as the multi-layer cylindrical

wall, including an annulus cylindrical air gap, a ceramic tube, and a

thin stainless steel cover. In fact, there is an open space below the

insulating cover; however, this area accounts for a small portion.

Consequently, this open area is ignored, and the insulating device is

treated as perfect cylindrical walls. The schematic axisymmetric layout

of the insulating cover is illustrated in Fig. 6. In addition, other

assumptions were made:

- The steady-state working conditions exist,

- The ceramic insulating layer and the metal cover are isothermal,

- Air is an ideal gas.

The heat flux transfers through each layer of insulating cover is

conservative.

q = q12 = q24 = q4 (1)

where q12, q24 and q4 are the heat flux through the air layer, ceramic

and steel cover layers, and from the outmost cover to the ambient air,

respectively.

3.2.1 The heat transfer at the outside of the insulating cover

The total rate of heat loss from the outmost cover is:

q4 = q4conv + q4rad (2)

where q4con and q4rad are the heat losses caused by convection and

radiation, respectively. q4 is the heat flux per unit length transfers from

the outmost of the insulating cover to the ambient air (natural

convection in the infinite space) and can be calculated by assuming that

the temperature t4 is known.

(3)

where d4 is the diameter of the outer of the cover, t4 is the temperature

at the surface of the cover, and t is the temperature of ambient air as

shown in Fig. 6.

(4)

and Nu is calculated by the formulas15

(5)

The Grashof number is calculated as:

(6)

where and is the mean film temperature

The thermal properties of air are assumed to be constant and are

taken to be the values at the mean film temperature.

The heat loss caused by radiation is calculated as

(7)

q4conv d4 t4 t( ) 4Nu t4 t( )= =

4Nu

d4

------------=

Nu 0.360.518 GrPr( )1 4

1 0.559 Pr( )9 16+[ ]4 9

---------------------------------------------------+=

Grg t

4t

( )d4

3

v2

-----------------------------=

1

tf---= tf

t4

t

+

2-------------=

q4rad d4 T4

4T

4( )=

Fig. 4 Proposed thermal insulating covers at the open spaces between

adjacent heaters

Fig. 5 The selected insulating materials Fig. 6 Schematic axisymmetric layout of the insulating cover


3.2.2 The heat transfer through the insulating cover

The relation between the temperature t2 and t4 can be identified by

the theory of heat conduction through the cylindrical walls (ceramic

layer and stainless steel cover) in the steady-state.

(8)

in which it is assumed that q24 = q4 is already obtained as shown in

Section 3.2.1. The meaning of notations in formula (8) is shown in the

nomenclature and Fig. 6.

Temperature distribution in the cylindrical wall is

(9)

where the subscripts i and i + 1 denote the inside and outside of a

cylindrical layer.

3.2.3 The heat transfer through the air gap layer

To determine the relation between the temperatures t1 and t2 at the

billets surface and the inner side of insulating cover, respectively (see

Fig. 6), the equations of heat transfer in the air gap are figured out.

(10)

where qconv, qcond and qrad are the heat transfer due to convection,

conduction, and radiation, respectively.

The heat flux through the enclosure can be determined from

(11)

where is the characteristic length of the enclosure that is the gap or

the spacing between the hot billet and the ceramic cover

(12)

The theory of heat transfer indicates that the air in cylindrical annuli

(enclosure) behaves like a fluid whose thermal conductivity is Nu as

the results of convection. The quantity Nu is called the effective or

apparent thermal conductivity of the cylindrical annuli or enclosure.

That is16

or (13)

The calculation is started by assuming that the temperature inside

the ceramic cylinder is known (t2), and thus an average temperature

(t1 + t2)/2 is identified. The property of air is evaluated at this

temperature.

The recommended relation for convective thermal conductivity is16

(14)

where the geometric factor for concentric cylinders Fcyl is

(15)

Then the rate of heat transfer per unit length between the cylindrical

billet enclosed in a concentric the ceramic tube by convection becomes

(16)

The heat transfer per unit length caused by conduction in the air gap

is calculated as

(17)

(18)

where = 5.67*10-8 (W/m2.K4) is the Stefan-Boltzmann constant; 1

and 2 are the emissivity of the billet and ceramic, respectively.

3.3 Solution strategy for calculating the temperatures and the

rate of heat transfer

The known temperatures are t1 of the hot billet and the temperature

t of the ambient air. The rate heat transfer through the insulating cover

and the temperature distribution depend on the thermal properties of

insulating materials and their geometrical parameter (thickness or

diameter, see Fig. 6). The governing equation for determining the heat

transfer rate and the temperature is the energy balance as shown in

equation (1). From the equation (1), two equations can be derived as:

q12 = q24 (19)

and

q24 = q4 (20)

All of the components in Eq. (19) and (20) are identified as shown

in Section 3.2.

Assuming that the thickness of the insulating layers are

predetermined, the system of two equations (19) and (20) can be used

to determine the temperature t2 and t4 (two equations, two unknown

variables). All of the components in Eq. (19) and (20) are identified as

shown in Section 3.2. Whenever t2 or t4 is known, the rate of heat

transfer (rate of heat losses) is obtained. Then, the heat losses in the

case of with and without insulating cover can be compared.

For each value of the thickness 1 and 2, the temperature t2 and t4

has a certain value. As the result, the value of heat transfer rate also

changes. Therefore, the optimum value of 1 and 2 that minimize the

heat losses through the cover in the design pace can be found.

4. Design Optimization

The optimization problem is minimizing the heat transferred

through the insulating cover to reduce the heat losses within some

constraints of design space of the input design parameters. The problem

is stated as follows:

q24

t2

t4

1

22

------------d3

d2

-----ln1

23

------------d4

d3

-----ln+

---------------------------------------------------=

t ti ti ti 1+( )d

di----

ln

di 1+

di---------

ln=

q12

q12conv q12cond q12rad+ +=

Q

A---- t

1t2

( ) Nu

--- t

1t2

( )= =

d2

d1

( )2

-------------------=

e Nu=e

----- Nu

=

e

----- 0.386

Pr

0.861 Pr+-----------------------

1 4

FcylRa( )1 4

=

Fcylln d

2d1

( )[ ]4

3

d2

3 5d1

3 5+( )

5--------------------------------------=

q2e

lnd2

d1

-----

----------- t1

t2

( )=

qQ

L----

21

T

lnd2

d1

-----

-------------------= =

q12 rad

d1

t1

4t2

4[ ]

1

1

----d1

d2

-----1

2

---- 1 +

--------------------------------=


Find 1 and 2 that

minimize q12

subject to:

q12 = q24

q24 = q4

t2 allowable temperature of ceramic

d1min d1 d1max

d2min d2 d2max

In this optimization problem, it is noted that the objective function

also be treated as a constraint due to the system of Eq. (19) and (20).

The input parameters (design variables) are 1, 2, t2 and t4. These

parameters change within their constraints during the optimization

process.

To facilitate the optimization process, the iSight optimization tool

was adopted. The optimization problem can also be solved by solver

tool of MS Excel. The optimization process for seven insulating covers

between seven induction heaters in the heating line is carried out. The

optimization results are shown in the Table 2. In this Table, t1 is the

temperature of the surface of the workpiece (input value); t2, t4 and q

are the considered outputs; 1 and 2 are design variables (thickness of

insulating covers).

5. Simulation Verification

The analytical result was verified by the simulation using ANSYS

software. Due to the restriction of the combination of convection and

radiation in the closed closure, only radiating heat transfer through the

air gap between the hot billet and the inner face of the ceramic was

done. Radiation heat transfer is dominated in the air gap, so the

convection and conduction in the air gap can be ignored. Figure 7

illustrates the FEM model for radiation simulation between the billet

and the cylindrical ceramic tube. The 2D-thermal element PLANE55

was used. The radiosity method of ANSYS was adopted for radiation

analysis. The radiation boundary conditions are the inner surface of the

ceramic cover and the outer surface of the heated workpiece. The

temperature of the workpiece is fixed as the temperature due to

induction heating. The ambient air is 25oC. The input parameters are

shown in Table 3. Fig. 8 shows the simulation results for the cover at

the last position of the heating line (the 7th or the hottest position with

1220oC of the billet). The simulations for other positions were done as

the same manner.

The simulation result shows that the temperature t2 inside the

ceramic tube is 1206oC, lower than the analytical result 4oC. This

difference is acceptable because the convection and conduction in the

air gap are ignored. The numerical simulation result implies that the

analytical model is accurate and believable. As the result, the

optimization process and the results are adequate. The result in the Fig. 8

also shows that the temperature in the inner surface of ceramic cover

Table 2 Temperature distribution, rate of heat transfer, and the

optimum thickness of insulating layers in the case of using covers

Pos. t1 (oC) t2 (

oC) t4 (oC) q (W/m) 1 (m) 2 (m)

1 812 799.4 109.1 896.5 0.02 0.08

2 889 877.5 115.6 989.4 0.02 0.08

3 950 939.3 120.7 1,063.1 0.02 0.08

4 1,030 1,020.1 127.1 1,159.9 0.02 0.08

5 1,160 1,151.5 137.2 1,317.3 0.02 0.08

6 1,200 1,191.9 140.2 1,365.8 0.02 0.08

7 1,220 1,210.0 151.2 1,748.1 0.045 0.078

Sum of heat losses (W/m) 8540.1

Table 3 The values of parameters for heat transfer simulation in the

insulating cover

Parameter Value Unit Parameter Value Unit

t 25oC 4 0.0378 W/m

oC

t1 1220oC 1 0.79

t2 1210oC 2 0.9

tair 25oC Pr air 0.701

2 0.16 W/moC 4 0.0718 W/m

oC

3 16.3 W/moC air 0.000155 m

2/s

Fig. 7 The FEM model for radiation simulation between the billet and

the ceramic tube

Fig. 8 Simulation result of heat transfer (temperature distribution) by

radiation in the insulating cover

Table 4 Comparision of heat losses (kW) in two cases: without and

with insulating devices

Position 1 2 3 4 5 6 7 Sum

Without

insulation5.52 7.09 8.56 10.83 15.50 17.20 18.11 82.8

With

insulation0.27 0.30 0.32 0.35 0.40 0.41 0.52 2.6


is very high; however, the temperature at the outmost surface of the

cover drops dramatically, from 1206oC down to approximate 151oC.

This effect results in a good insulation or an effective reduction of heat

losses.

6. Results and Discussions

The geometrical parameters of the insulating covers at different

positions were obtained by solving the optimization problem as

mentioned in Section 5. Changing the input parameter t1 and the

constraint for the temperature t2 as well as the geometrical constraints

for 1 and 2 of different insulating covers at different positions in the

heating line, the optimum values of the 1 , 2 and the rate of heat

transfer are obtained. The final result shows that using insulating

covers at seven open spaces reduces the heat losses from 9.4% down

to 0.3% (82.8 kW down to 2.6 kW, see Table 4) compared with the

energy stored in the workpiece (saving around 9.1%). The data in Tale

4 were obtained by calculating the radiation and convection energy per

unit time in two cases. In the first case, the hot billet is exposed directly

to the ambient air without insulating cover. The second case uses the

insulating covers with the dimensions and temperature obtained from

the optimization results in Section 4.

It can be found that the increase of the temperature t2 can reduce the

heat loss caused by radiation but affect the service life of the insulating

cover. In addition, the smaller 1 thickness results in the higher t2 and

the smaller heat losses. It is easy to conclude that increasing the

thickness 2 of the insulating ceramic layer, the heat losses will be

reduced. However, if the upper range of the t2 is intercepted due to the

safety of the insulating material, the thickness 2 may not reach its

upper range. The optimum value of 1 tends to converge to the lower

margin because the lower value of 1, the higher of t2 and lower heat

losses caused by radiation (see equation 18). This trend of 1 brings the

advantage for the insulating system because the smaller air gap protects

the heater against overheat compared to the insulating cover with the

big air gap 1.

It is clear that the insulating system is very effective for saving the

energy. It is also simple in terms of structure, low cost, easy to

manufacture and assembly. The insulating ceramic material which can

withstand up to 1300oC helps the cover work safely without cooling

circuit. A thin stainless steel for the outmost layer is chosen in order to

lower the emissivity and protect the oxidization process of the

insulating system.

7. Conclusion and Future Work

In summary, this paper practically studies on the heat losses

reduction in the in-line induction heating. It is found that the heat losses

caused by the radiation and convection at the open spaces between

adjacent heaters of the induction heating line for forging the automotive

crankshaft account for 9.4% of the energy stored in the workpiece. A

proper insulating system with optimum structure and specification was

proposed and designed; the heat losses are reduced significantly as

shown by the results of calculation (9.1% reduction compared to

without the insulating system). The cost for making the insulating

system is not high, and it can work without maintenance. Therefore,

this is an effective way for saving energy for the induction heating

system. The working environment for the worker is improved due to

the reduction of the radiation and temperature in the shop floor.

Instead of using the simulation-based optimization, the analytical

approach for minimizing the heat losses and optimizing the design

parameters of the insulating system was adopted. This method

significantly reduces the computing time and obtains a reliable solution

because of the solid foundation of the theory of the engineering heat

transfer. The simulation tool was applied to verify the analytical results.

The real insulating system will be manufactured, implemented and

tested at the forging factory. At that time, the temperature of the heated

workpiece will increase due to the heat losses reduction. Consequently,

it is necessary to adjust and apply the optimization the processing

parameters of the induction heating line. The holistic combination of

new optimum heating parameters and a proper insulating system will

be implemented.

ACKNOWLEDGEMENT

This work was supported by the Ministry of Knowledge Economy,

Korea, under the International Collaborative R & D Program hosted by

the Korea Institute of Industrial Technology.

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Reduction of Heat Losses for the in-line Induction

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