The Dependence of the Heat Energy Consumption upon the VVorking lntensity and the Frequency of the Isolation Maintenance of a Pusher-type Furnace
Ovisnost utroška toplinske energije od intenziteta rada i učestalosti održavanja izolacije potisne peči
J. Črnko, Metalurški fakultet Sisak, Aleja narodnih heroja 3, 44103 Sisak
The paper covers investigations of the specific heat energy consumption dependence upon the productivity and frequency of the isolation maintenance in a pusher-type furnace of a strip and billet rolling mili.
U okviru ovog rada istraživana je ovisnost specifičnog utroška toplinske energije od produktivnosti i učestalosti odražavanja izolacije na primjeru potisne peči u valjaonici traka i gredica.
1	Introduction
Systematic decreasing of the fuel consumption per unit of a product should be one of the most important tendencies in Yugoslav rolling mills. However, the dynamics of the specific fuel consumption decreasing has recently been stopped in some heating furnaces and has assumed, for the various reasons (mostly objective), an inversed trend. Thus, even a decreased vvorking intensity of heating furnaces leads to an inereased specific fuel consumption. This often happens to appear in the rolling mills mentioned, especially lately. This is the reason that the paper deals with the investigation of the specific heat energy consumption dependence upon the productivity of heating furnaces and, especially, upon the frequency of isolation maintenance on the example of one of the two pusher-type furnaces of a Sisak Ironwork's strip and billet rolling mili (Sisak, Croatia).
2	Heat energv balance and fuel consumption
Rough values for specific fuel consumption decreasing in the čase of inereased working intensity of heating furnaces can be obtained by heat energy balance1'2. The total heat energy consumption (]T E,) can be derived into two groups:
1.	one depending on the mass of steel being heated in a certain heating furnace:
•	heat energy of steel being heated (Ei),
•	heat energy of scrap developing in the course of steel heating (E2),
•	heat energy losses (through the walls to the out-side, by eooling water etc.) independently to the mass of steel being heated (£3);
2.	one depending on the level of fuel utilization in a certain heating furnace:
•	heat energy losses because of mechanical un-burning, incomplete chemieal combustion and by fuel gases going out (E4).
Obviously, it is (per unit share)
Ex + E2 + E3 + E4 = l
(1)
First, let's take that a quantity of conditional fuel A„ is consumed to heat a mass of steel Mn, i.e.
E1Xn + Eo Xn + E3 Xn + E4 Xn = Xn (2)
If the intensity of steel heating inereases 2 times, the consumption of the conditional fuel inereases up to the value of A'2, and the equation (2) takes the follovving form:
(3)
z(L\ + E2) Xn + E3Xn + E4Xt = Xz
From the equation (3) we can now derive a quantity of the conditional fuel X,, i.e.
A 2 — A,
z{Ex + E2) + E3
1 - E4
(4)
Based on these, it is possible to define a decrease of the specific fuel consumption, as a result of the intensity of steel heating inereased r times, as follovvs:
Ax =
A, AL A.
zMr,
■ 100 =
• 100 =
A,

+ b1) + e3\
1 -
zMn
z{El+E2) + E*
z{ 1 - E4)
100
100 (5)
Consequently, as a result of the inereased vvorking inten-sity of the heating furnaces, i.e. the process of steel heating being intensified ; times, the the specific fuel consumption (in %) vvill be decreased in a ratio:
z(Ex + E2) + E3 r(l " E4)
• 100.
(6)
3 Main characteristics of a pusher-type furnace
A three-zonal pusher-type furnace observed, designed by American Rust Furnace Co., vvas built in the year 1972. Beginning from the charging side the fumace has a pre-heating zone, a heating zone and a soaking zone. In the preheating and heating zone a charge vvas heated both from the upper and the lovves side, vvhereas in the zone of soaking it was heated only from the upper side. Burners vvere installed laterally on the furnace, above and belovv the charge in the preheating and heating zone. In the preheating zone five burners vvere installed above the charge and five burners belovv the charge. Tvvo burners vvere installed above the charge on one side and three on the other side of the furnace. The burners vvere also installed belovv the charge, but placed inversely to those above the charge. In the heating zone six burners vvere installed above the charge and six burners belovv the charge. On each side of the furnace three burners vvere placed above the charge and three belovv the charge. In the soaking zone six burners vvere placed on the frontal side of the fumace. Othervvise, the furnace is not of a conventional profile and is of a conventional temperature regime3. By operating measurements it vvas found that the firing from the lovver side and that from the upper side have the equal efficiency. The furnace profile, as vvell as its main dintensions are presented schematically in Fig. 1.
H 11 I §11 IU II 1 ffl
ITI
-I
Figure 1. Schematic presentation of a pusher-type furnace profile.
Slika 1. Shemalski prikaz profila potisne peči.
During the energetic balancing slabs of St 12 (per DIN) quality, dintensions of 430 x 190 x 3800 mm and mass of 2500 kg singly vvere heated in the pusher-type furnace. The furnace vvas fired by natural gas having the heating value of 37300 kJ/m3. Gas consumption vvas 2347 m3/h, and consumption of air necessary for its combustion vvas 24286 m3/h. Air temperature at the metal recuperator exit vvas 300°C. According to the request of the rolling mili train the furnace productivity vvas kept at 37 t/h, and the slabs vvere heated up to the final temperature of 1230-1250° C. Hovv-ever, the furnace vvas designed to achieve the productivity of 67 t/h vvhen heating slabs of stated quality and dintensions. This shovvs that the coefficient of capacity utilization of the furnace vvas about 0.55, vvhich indicates that vvorking intensity of the furnace can be increased 1.81 times.
4 Fuel consumption dependence upon the productivity and frequency of a pusher-type furnace maintenance
For a calculation of the specific fuel consumption decrease heat energy consumption in a pusher-type furnace vvas found.The heat energy consumption (per single items) in
the furnace vvhen heating the slabs of the stated quality and dimensions is given in Table 1.
Table 1. Heat energy consumption in a pusher-type furnace vvhen heating slabs of St 12 quality and dimensions of 430 X 190 x 3800 mm
Heat energy consumption items	Quantity of heat energy	
	MW	%
Ei	8.998	33.32
E-,	-	-
e3	6.108	22.61
Ea	11.902	44.07
LE,	27.008	100.00
The follovving step enables to find out hovv much the specific fuel consumption decreases if the vvorking intensity of the furnace increases 1.81 times.
Applying the equation (6) we get that the specific fuel consumption decreases for 18.09%, providing that, as stated, the vvorking intensity of the pusher-type furnace increases 1.81 times, i.e.
1.81 0.3332 + 0.2261 \ 1.81(1 - 0.4407) J
100 = 18.099?
The value obtained also anables calculating the specific fuel consumption in the čase that the furnace vvorks full capacity. Results of such a calculation shovv that the specific natural gas consumption in the furnace decreases from 63.43 to 51.96 m3/t, corresponding to the decrease of the specific heat energy consumption from 2366 to 1938 kJ/kg, i.e. for 428 kJ/kg.
To get a real picture of the dependence of the specific heat energy consumption upon the vvorking intensity of the puscher-type furnace, a three-years period of the furnace vvork has been analyzed. Only the data referring to the furnace vvork during the St 12 quality slabs of 430 x 190 x 3800 mm dimensions being heated vvere delt vvith. The dependences of the specific heat energy consumption upon the furnace productivity being got that way are presented in a form of a diagram in Fig. 2.
3500
3000
2500
. 2000
o 1500
1000L-30
35
_j__i__I__I____
i 0 45 50 55 50 65
Productivity, t/h
Volume in m3 refers to a nomial state.
Figure 2. The dependence of the specific heat energy consumption upon the pusher-type fumace productivity. Slika 2. Ovisnost specifičnog utroška toplinske energije od produktivnosti potisne peči.
J. Črnko: Ovisnost utroška toplinske energije od inteziteta rada i učestalosti održavanja izolacije potisne peči
Comparing the calculated values for the specific heat energy consumption with those obtained by operating mea-surements for the stated quality and dimensions of the charge, the results of vvhich are presented in a form of a diagram in Fig. 2, we can see that the differences are relative^ small. Considerable differences are the consequence of including a consumption of natural gas for "blank" fir-ing to the operating data on the natural gas consumption. Also, the changes of energetic losses per items of energetic balance, in the course of the furnace utilization for its iso-lation reparation betvveen the two stoppings, can bring to some bigger differences betvveen the calculated and operating values for the specific heat energy consumption in a certain furnace productivity.
2500
0	2	4	6
Maintence frejuence, mth Figure 3. The dependence of ihe specific heat energy consumption upon the frequency of the isolation maintenance and the pusher-type furnace floor cleaning. Slika 3. Ovisnost specifičnog utroška toplinske energije od učestalosti održavanja izolacije i čiščenja poda potisne peči.
The analysis carried out for the furnace vvork betvveen its last tvvo stoppings, because of the beam cleaning and of the isolation reparation, shovved that vvith the produc-tivity of 50 t/h (realized annual productivity) the specific heat energy consumption increases from the initial 1480 to the final 2240 kJ/kg, in other vvords the average value is 1860 kJ/kg (Fig. 3). Beside others, the course of that is al-most up to 80% of refractory mass falls off the skid carrier so that energy losses by cooling vvater are considerable4. The increase of isolation maintenance frequency, as vvell as that of beam cleaning from the layers, to the half of tirne (6 months) of the previous (12 months) vvould decrease the
average heat energy consumption from 1860 to 1670 kJ/kg (Fig. 3), i.e. for 190 kJ/kg.
Hovvever, for a rather long period of time in some vvest-ern countries a quartal frequency of pusher-type fumaces5 maintenance has been practised, so regarding to this, there are no reasons to make the first step in our strip and billet rolling mills as vvell.
5	Summary
Results obtained analytically shovv that the specific heat en-ergy consumption decreases for about 18% if the produc-tivity of the pusher-type furnace observed increases from 37 to 67 t/h. Also, the operating data analysis of the furnace vvork does not shovv significant deviations from the results obtained analytically. Hovvever, because of the lack of coordination betvveen pusher-type furnaces and rolling mili train capacities, even in normal production conditions it is impossible to realize. The same way, the increase of the pusher-type furnace isolation maintenance frequency and that of the layers removal from the floor to the period of
6	from so far 12 months, vvould decrease the specific heat energy consumption for about 10%. This can be realized successfully by better month and vveek planning of rolling, vvhich vvould assure heating of the charge having the same quality and dimensions in the course of a fevv vveeks. In such a čase it could be possible to stop one of the furnaces if another's passing capacity per hour vvould satisfy the re-quests of the rolling mili train. Such periods sometimes occure now, too, as vvell as those in vvhich plans of rolling change from shift to shift together vvith plans of heating.
6 References
1	W. Heiligenstaedt: Warmetechnische Rechnungen fiir In-dustrieofen, Verlag Stahleisen M.B.H., Dusseldorf, 1956, s 6-13
2	A.N. Nesenčuk et al.: Ognetehničeskie ustanovki i toplivosnabženie, Izdatel'stvo "Vyšejšaja škola", Minsk, 1982, s 35^46
3	V.A. Krivandin, JU.R Filimonov: Teorija t konstrukcij metallurgičeskih pečej, Izdatel'stvo "Metallurgija", Moskva. 1986. s 360
4	Ž. Acs, Lj. Milič, M. Kundak: Zbornik IV. savetovanja "Energetika i zaštita čovjekove sredine u crnoj metalurgiji", UJŽ Beograd, Herceg Novi, 1985, s 119-125
6 L.J. Grafe: Improving energy utilization in steel reheat furnaces, Iron and Steel Engineer, Vol. 62, No 1, 1985, s 43-*7