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Heat recovery in a ceramic kiln with an organic rankine cycle engine
Heat Recto'cryS)'slem.~Vol. Printed in Great Britain
I. No
~ ~ : ~25 to 131. 1981
0198-7fO3/gl/020125-07$02.00/0
Persamon Press Lid
HEAT RECOVERY IN A CERAMIC KILN WITH AN ORGANIC RANKINE CYCLE ENGINE C. CASCL G. ANOELINO, P. F ~ g R ~ I , M. G~dA, G. GIGLIOLI and E. M A c c m CNPM*flstituto di Macchine, Politecnico di Milano, Italy Abstract--Tbe process of ceramics firing, as well as many other high temperature processes, is highly inefficient and it is possible to recover an important fraction of the heat input. However, the recovery of heat is not practical because heat finds little use in the factory and its transport to other users is expensive. A recovery system has been proposed by the authors and funded by EEC, which transforms part of the heat of the kiln exhaust gases into electricity by means of an Organic Rankine Cycle Engine. The generated electricity can often be utilized directly in the oven for driving the air and exhaust fans. A thermodynamic study has led to the selection of tetrachlorocthylene as the working fluid, with a rated power output of 40 kW at the turbine shall at an evaporation temperature of the work fluid of 110°C. In the paper, the design criteria for the most significant plant components are briefly outlined; some experimental results, obtained in a series of tests carried out at Gemmindustria, are presented: They show that most of the technical goals of the research (engine efficiency, power output, behaviour in transient conditions, etc.) have been met. Recently the engine has been installed on the oven of a large ceramic industry, and will be continuously operated for testing the engine reliability in an industrial environment.
I. INTRODUCTION
HUGE quantities of heat are discharged in processes which make use of ovens like brick and ceramics firing, glass production, metal refining and forming. The waste heat temperature is variable but it is generally higher than 200°C. The recovery of such heat has been often proposed but seldom performed, even after the increase of energy costs in the 1970s, due to the fact that it is difficult to find a use for heat in the facility itself. The recovered heat could often be distributed for space heating purposes in the district around the plant facility, but the cost and difficulties of this operation are high and a too-long term capit.al investment is required, hence the direct use of heat is generally not viable. The recovery of discharged thermal energy for producing mechanical or electric energy can be much more interesting, mechanical power and electricity being used in the plant itself. In any case, the production of power in a thermal engine does not prevent the use of the heat released from the engine. The aim of the research was to demonstrate the technical feasibility of Organic Rankine Cycle Engines (ORCE) for this task. Though the theoretical approach to the organic fluid Rankine cycle has been assessed for many years, few practical constructions have been perfected. Moreover, only in U.S.A., Israel and Japan are some engines commercially available, and also these are not well adapted for the considered application, due to fluid selection or power level. While there is an almost general agreement that these engines cannot be economically viable for power outputs lower than 500 kW, a much lower capacity plant (40 kW at turbine shaft) was selected. It is, in fact, the authors' opinion that it is possible to build reliable and low cost ORCE in this range of power. Owing to the lower capital cost required and to the great availability of potential energy sources, a low power output could greatly help the dissemination of this new class of engines. 2. PLANT DESIGN
The oven scheme is represented in Fig. 1; although the recovery could be applied to several heat sources of the kiln [11 only the exhaust gases were considered for this application. The scheme of the plant is represented in Fig. 2. *
Centro di Studio per Ricerche sulla Propulsione e sull'Energetica. Peschiera B., Italy.
125
126
C. C^scl et aL Exhousl
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I 0 0 ,~00Bu r
Hot goses .I---.-
00000
I
I
'
I
Pre -heeling
F'r'n~
I
Coo, m
...
i
Fig. I. Scheme of a tile tunnel kiln.
As explained in more detail in f2], tetrachloroethylene was selected as working fluid due to its favourable vapour pressure level, molecular mass and saturation curve shape. In fact, though a fluid having the critical point near to the minimum exhaust gas temperature would allow a better matching of the heat release curve of the exhaust gases and consequently a reduction of the entropy generation in the primary exchanger, a low pressure cycle is more suitable for a low power system, due to the higher turbine efficiency which can be obtained in low pressure, high volume flow engines and to the lower influence of pump efficiency on cycle performance. Moreover, tetrachloroethylene is a well known fluid, extensively used for dry cleaning of clothes, is not flammable, inexpensive and allows the design of a heat engine completely at sub-atmospheric pressure, yielding an easy detection of leaks and minimizing the danger of fluid losses. The final design data are summarized in Table 1. The most significant features adopted in the design of the plant components, will now be briefly recalled. As shown in Fig. 3, heat is transferred to the working fluid in two different heat exchangers: a preheater, which brings the fluid to saturation temperature, and an evaporator. The two heat exchangers are inserted into a secondary exhaust duct, in parallel with the main one, to allow the continuous firing required by the kiln, also in case of recovery system shutdown. The layout is such that exhaust gases first cross the evaporator tube bank, in a single-pass cross-flow arrangement, external to the tubes, then the pre-heater tube bank, which conveys the working fluid in a multi-pass cross-counter flow arrangement. Both heat exchangers consist of bare-tubes banks, with outside tube diameters of 15 mm. In designing these components, care was taken to the danger of compromising the thermal stability of the fluid; for this reason low gas velocities were adopted and the use of
L
H
P
F
,
,
I
I
L...J
Fig. 2. Simplified scheme of the recovery plant: A ffi Feed p u m p ; B = Pre-heater; C ffi Vaporizer; D = Turbine; E -- Condenser; F = Constant level hotwell; G = Auxiliary fan; H = Stack; L -- By-pass stack; M = By-pass valve; N = Cooling tower; P = Alternator.
Heat recovery in a ceramic kiln
127
Table 1 (a)
Kiln Data Exhaust gas flow rate Exhaust gas inlet temperature Exhaust gas outlet temperature
(b)
Thermal enoine overall data Working fluid Evaporation temperature Condensation temperature Power at the turbine shaft Heat input Efficiency
(c)
C,C14 110°C 40°C 40 kW 350 kW O.115
Heat exchanger data Primary heat exchanger surface Condenser surface De-superheater surface Pre-heater surface
(d)
5.0 kg/s 220°C 1500C
37 m 2 102 m 2 40 m 2 14 m 2
Turbine data Isentropic enthalpy drop Mass flow rate Angular velocity Stator mean radius Rotor mean radius No. of stator blades No. of rotor blades Stator blade height Rotor blade height Material
46.4 kJ/kg 1.23 kg/s 702 r/s 0.243 m 0.243 m 83 56 16 m m 19-56 m m
Light alloy
externally finned tubes was discarded. The presence of dust in the exhaust gases precluded the use of more compact matrices (the selected tube pitch is 18.7 by 30 mm, in longitudinal and transverse direction respectively). Eventually, the evaporator was designed with large liquid recirculation ratios, and therefore low quality variation through the boiling channels. The water cooled condenser is of the shell-and-tube type, with externally finned tubes (external/internal surface ratio = 3.6), and is preceded by a desuperheater, which is designed for off-design operation of the engine.
~
E
M'
/ / / /
,25 /////
1
i
Fig. 3. Layout of the plant: A = Hot gases from the oven; B = G u flow regulation valves; C = Tetrachloroethylene evaporator; D = Liquid tetrachloroethylene prehenter; E = Turbine control valve; F = Turbine casing; O = De-superheater casing; H = Condenser casing; L = Speed reducer; M = Alternator; N = Exhaust duct, O = Constant level hot well, P = Feed pump.
128
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IlO
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Fig. 4. Pressure ratio and turbine efficiency prediction as a function of the cycle evaporation temperature.
The turbine has rather peculiar characteristics: a very large pressure ratio (13.7), a relatively small enthalpy drop and, owing to the low fluid discharge pressure, the turbine handles large volume flow rates. Accounting for all these features, the best solution was found to be a full admission, single-stage turbine, with a moderate (0.20) degree of reaction, highly supersonic stator nozzles, and transonic rotor blades. The chosen turbine speed of revolution (6700 rev/min) allows a favourable blade aspect ratio, with blade height/mean radius ratios low enough to adopt untwisted blades without significant loss increase. Detailed aerodynamic studies were carried out, which show that a total-to-static efficiency of the turbine of 80% could be achieved. The predicted turbine efficiency curve at various operating conditions and at the nominal speed is shown in Fig. 4. As d i ~ u s ~ l in the next chapter, the turbine was tested at pressure ratios between 9 and 12, and exhibited the expected efficiency.
3. TESI" RESULTS
A series of tests was carried out at Gemmindustria.* before the final plant mounting on the stack of the ceramic kiln. The set up of the test rig is represented in Fig. 5, where the measuring points are also indicated. Two oil burners were used as the energy source; to obtain the correct temperature of the gases at the heat exchanger inlet, the combustion products were diluted with air, sucked in by a fan. The generated electric power was wasted on a series of lamps; the desired torque at the turbine shaft being obtained by connecting the proper number of lamps. The main purposes of the teSts were the following: to chock the correct behaviour of the whole loop at various operating conditions (startup, shutdown, etc.); to detect possible early failures of the various components of the plant and of the control system; to measure the engine performance, * The present research programme was undertaken in the frame of the EEC contract No. 198 EEl by Gemmindustria, Milano, Italy with the scientific support of the authors.
Heat recovery in a ceramic kiln
129
Fi B. 5. Scheme of the engine test Fig: (1) oil burners; (2) refractory combustion chamber; (3) air vents; (4) prebeater; (5) evaporator; (6) centrifugal fan; (7) turbine casing; (8) condenser; (9) constant level hot well. TI, PI = evaporation temperature and pressure; T2, P2 = turbine exhaust temperature and pressure; P3 == pump outlet pressure; P4 ,= preheater inlet pressure; T5 = inlet hot gas temperature; T6 = exhaust gas temperature; TT, "I'8 inlet and outlet cooling water temperatures; F -- water flow rate; No0 = generated electrical power. As far as the first t w o p o i n t s are concerned, n o m a j o r p r o b l e m s were encountered d u r i n g the v a r i o u s t r i a l runs (for a t o t a l o f a b o u t 100 o p e r a t i n g hours): T h e l o o p p r o v e d
to be satisfactorily sealed; all rotating parts ran smoothly, including the mechanical seal; the engine was very easy to operate, the control system provided good constancy of the generated electricity frequency, even for sudden load variations. Some problems were initially met with the centrifugal feed pump, which was often operating in cavitating conditions;, this situation has been avoided by inserting a control which keeps a constant level in the condenser hotwell, by throttling the pump outlet. In some preliminary runs, due to the said pump failures, the heat exchanger was exposed for several minutes to gases at about 300°C, without any fluid circulation: No serious fluid decomposition occurred; this fact made us more confident about the capability of tetrachloroethylene of withstanding high temperatures, at least for short periods. The engine performance is summarized in Fig. 6. Although the available energy source was not powerful enough to reach the design power level, the trend is evident: At the design evaporation temperature, the predicted power output will be reached. It is of interest the trend of the Net/N~ratio: At low power output, large fractions of the power available at the turbine blades are dissipated in mechanical losses (seals, bearings, gears, windage) and in electric losses. At larger outputs, rh, reaches satisfactorily high values. This is an indirect confirmation that the predicted turbine efficiency (~ 80%) was actually attained. Even larger efficiency values are obtained by the measurements of the thermal drop across the turbine. The good turbine efficiency is confirmed also by the experimental results concerning the overall engine effÉciency represented in Fig. 7: A referred efficiency, i.e. the ratio between the efficiency of the engine and the Carnot efficiency in the same temperature interval, of about 58% should be reached at the design point. The most probable breakdown of the remaining 42% is: 6% electric losses; 3% by-pass valve losses; 3% loss of the gear box; 3% in the turbine seal and bearing; 2% in the various pressure losses present in the loop; and eventually 16% in the fluid-dynamic irreversible processes occurring in the turbine blades. Besides these losses, related to the imperfection of the various components, a loss of 9% of the ideal efficiency takes place in the fluid preheater, where heat is introduced in the cycle at temperatures lower than the evaporation one, thus with an irreversible process.
130
C . C . ~ o et al. 40
30
Net = Electric power (exp)
Design
Nt • Mechoni¢ol i x ~ r
/
j"
I0 -
-- ~90 - -
%.
-
080 o+
--0 60 0 50
I
l
l
i
I
I
60
70
80
90
I00
I10
120
Evaporation temperature, "C
Fig. 6. Power and efficiency of the engine. The curve is computed according to turbine efficiency represented in Fig. 5, for the actual values of the condensing temperature.
4. ECONOMIC ASSESSMENT
The ceramic tunnel kilns are generally operated at rated temperature for 7000--8000 h/yr, and the energy recovered can be completely used in the factory. According to the experience gained during the design and construction of this engine, the expe~ed price for the plant, in the case that a certain number (I0--I00) of ~ l a r plants (though not identical due to the singularities of each recovery situation) is pro-
011
06
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.=
-
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u
,,,
007
--
0.06
-
005
50
I
60
I
70
I
80
I
90
I
I00
1
110
120
EvopOrotion temperoture, °C Fig. 7. Measured engine efficiency as a function of evaporation temperature.
Heat recovery in a ceramic kiln
131
Table 2
Op. Time L/kW DM/kW PBP yr
7000 h/yr 500000 1160 2.86
700000 1630 4.00
8000 h/yr 500000 1160 2.5
700000 1630 3.5
duced for the same purpose, is 500 to 700 × 103 lire (about 50% of the actual cost of the prototype, comprehensive of R & D costs); the associated Pay Back Period is reported in Table 2. The rate of return of the investment, for a mean value of PBP and a lifetime of 5 yr would be 18%. The figures quoted demonstrate the economical potential of these plants. Another benefit is the possibility of supplying electricity to the ovens when there is a failure in the electric grid. 5. REFERENCES
I. M. Gala, Recupero del calore di scarico dai forni a tunnel per ceramica, (in Italian),Convegno sull'utilizzazione e resparmio di gas naturale nelrindustria delle piastrelle per ceramica. Modena (December 1977). 2. (3. Oiglioli et al., Tetrachlore~thylene Rankine cycle for waste heat recovery from ceramic tunnel kilns, Proceedings 13th Intersociety Energy Conversion Engineering Conference, pp. 1488-1492, SAE paper 789603.
I. No
~ ~ : ~25 to 131. 1981
0198-7fO3/gl/020125-07$02.00/0
Persamon Press Lid
HEAT RECOVERY IN A CERAMIC KILN WITH AN ORGANIC RANKINE CYCLE ENGINE C. CASCL G. ANOELINO, P. F ~ g R ~ I , M. G~dA, G. GIGLIOLI and E. M A c c m CNPM*flstituto di Macchine, Politecnico di Milano, Italy Abstract--Tbe process of ceramics firing, as well as many other high temperature processes, is highly inefficient and it is possible to recover an important fraction of the heat input. However, the recovery of heat is not practical because heat finds little use in the factory and its transport to other users is expensive. A recovery system has been proposed by the authors and funded by EEC, which transforms part of the heat of the kiln exhaust gases into electricity by means of an Organic Rankine Cycle Engine. The generated electricity can often be utilized directly in the oven for driving the air and exhaust fans. A thermodynamic study has led to the selection of tetrachlorocthylene as the working fluid, with a rated power output of 40 kW at the turbine shall at an evaporation temperature of the work fluid of 110°C. In the paper, the design criteria for the most significant plant components are briefly outlined; some experimental results, obtained in a series of tests carried out at Gemmindustria, are presented: They show that most of the technical goals of the research (engine efficiency, power output, behaviour in transient conditions, etc.) have been met. Recently the engine has been installed on the oven of a large ceramic industry, and will be continuously operated for testing the engine reliability in an industrial environment.
I. INTRODUCTION
HUGE quantities of heat are discharged in processes which make use of ovens like brick and ceramics firing, glass production, metal refining and forming. The waste heat temperature is variable but it is generally higher than 200°C. The recovery of such heat has been often proposed but seldom performed, even after the increase of energy costs in the 1970s, due to the fact that it is difficult to find a use for heat in the facility itself. The recovered heat could often be distributed for space heating purposes in the district around the plant facility, but the cost and difficulties of this operation are high and a too-long term capit.al investment is required, hence the direct use of heat is generally not viable. The recovery of discharged thermal energy for producing mechanical or electric energy can be much more interesting, mechanical power and electricity being used in the plant itself. In any case, the production of power in a thermal engine does not prevent the use of the heat released from the engine. The aim of the research was to demonstrate the technical feasibility of Organic Rankine Cycle Engines (ORCE) for this task. Though the theoretical approach to the organic fluid Rankine cycle has been assessed for many years, few practical constructions have been perfected. Moreover, only in U.S.A., Israel and Japan are some engines commercially available, and also these are not well adapted for the considered application, due to fluid selection or power level. While there is an almost general agreement that these engines cannot be economically viable for power outputs lower than 500 kW, a much lower capacity plant (40 kW at turbine shaft) was selected. It is, in fact, the authors' opinion that it is possible to build reliable and low cost ORCE in this range of power. Owing to the lower capital cost required and to the great availability of potential energy sources, a low power output could greatly help the dissemination of this new class of engines. 2. PLANT DESIGN
The oven scheme is represented in Fig. 1; although the recovery could be applied to several heat sources of the kiln [11 only the exhaust gases were considered for this application. The scheme of the plant is represented in Fig. 2. *
Centro di Studio per Ricerche sulla Propulsione e sull'Energetica. Peschiera B., Italy.
125
126
C. C^scl et aL Exhousl
Cooling OIr exit
I 0 0 ,~00Bu r
Hot goses .I---.-
00000
I
I
'
I
Pre -heeling
F'r'n~
I
Coo, m
...
i
Fig. I. Scheme of a tile tunnel kiln.
As explained in more detail in f2], tetrachloroethylene was selected as working fluid due to its favourable vapour pressure level, molecular mass and saturation curve shape. In fact, though a fluid having the critical point near to the minimum exhaust gas temperature would allow a better matching of the heat release curve of the exhaust gases and consequently a reduction of the entropy generation in the primary exchanger, a low pressure cycle is more suitable for a low power system, due to the higher turbine efficiency which can be obtained in low pressure, high volume flow engines and to the lower influence of pump efficiency on cycle performance. Moreover, tetrachloroethylene is a well known fluid, extensively used for dry cleaning of clothes, is not flammable, inexpensive and allows the design of a heat engine completely at sub-atmospheric pressure, yielding an easy detection of leaks and minimizing the danger of fluid losses. The final design data are summarized in Table 1. The most significant features adopted in the design of the plant components, will now be briefly recalled. As shown in Fig. 3, heat is transferred to the working fluid in two different heat exchangers: a preheater, which brings the fluid to saturation temperature, and an evaporator. The two heat exchangers are inserted into a secondary exhaust duct, in parallel with the main one, to allow the continuous firing required by the kiln, also in case of recovery system shutdown. The layout is such that exhaust gases first cross the evaporator tube bank, in a single-pass cross-flow arrangement, external to the tubes, then the pre-heater tube bank, which conveys the working fluid in a multi-pass cross-counter flow arrangement. Both heat exchangers consist of bare-tubes banks, with outside tube diameters of 15 mm. In designing these components, care was taken to the danger of compromising the thermal stability of the fluid; for this reason low gas velocities were adopted and the use of
L
H
P
F
,
,
I
I
L...J
Fig. 2. Simplified scheme of the recovery plant: A ffi Feed p u m p ; B = Pre-heater; C ffi Vaporizer; D = Turbine; E -- Condenser; F = Constant level hotwell; G = Auxiliary fan; H = Stack; L -- By-pass stack; M = By-pass valve; N = Cooling tower; P = Alternator.
Heat recovery in a ceramic kiln
127
Table 1 (a)
Kiln Data Exhaust gas flow rate Exhaust gas inlet temperature Exhaust gas outlet temperature
(b)
Thermal enoine overall data Working fluid Evaporation temperature Condensation temperature Power at the turbine shaft Heat input Efficiency
(c)
C,C14 110°C 40°C 40 kW 350 kW O.115
Heat exchanger data Primary heat exchanger surface Condenser surface De-superheater surface Pre-heater surface
(d)
5.0 kg/s 220°C 1500C
37 m 2 102 m 2 40 m 2 14 m 2
Turbine data Isentropic enthalpy drop Mass flow rate Angular velocity Stator mean radius Rotor mean radius No. of stator blades No. of rotor blades Stator blade height Rotor blade height Material
46.4 kJ/kg 1.23 kg/s 702 r/s 0.243 m 0.243 m 83 56 16 m m 19-56 m m
Light alloy
externally finned tubes was discarded. The presence of dust in the exhaust gases precluded the use of more compact matrices (the selected tube pitch is 18.7 by 30 mm, in longitudinal and transverse direction respectively). Eventually, the evaporator was designed with large liquid recirculation ratios, and therefore low quality variation through the boiling channels. The water cooled condenser is of the shell-and-tube type, with externally finned tubes (external/internal surface ratio = 3.6), and is preceded by a desuperheater, which is designed for off-design operation of the engine.
~
E
M'
/ / / /
,25 /////
1
i
Fig. 3. Layout of the plant: A = Hot gases from the oven; B = G u flow regulation valves; C = Tetrachloroethylene evaporator; D = Liquid tetrachloroethylene prehenter; E = Turbine control valve; F = Turbine casing; O = De-superheater casing; H = Condenser casing; L = Speed reducer; M = Alternator; N = Exhaust duct, O = Constant level hot well, P = Feed pump.
128
C. C^scl et al. 20 I"'
I
/ /I
/
....
~ o~c
I
Tc "40"C
/
/ o
/ io-
/
# I
15 _
a_ ~
,
/
expllficne~s ore ovoilol~
,,
/
/ / / I
/ /
.C.,
.-N."
/ ..:.-.. .-:':." / /..:.:." "."
~.
/ 5--
/ / ///
- 0.90
~
.
~
O80
.
/
-- 070
TS
-- 0 6 0
,
i
I
/
I
7'0
I
I
80
90
I
1
I00
Evaporotion temperoture,
IlO
05o
120
°C
Fig. 4. Pressure ratio and turbine efficiency prediction as a function of the cycle evaporation temperature.
The turbine has rather peculiar characteristics: a very large pressure ratio (13.7), a relatively small enthalpy drop and, owing to the low fluid discharge pressure, the turbine handles large volume flow rates. Accounting for all these features, the best solution was found to be a full admission, single-stage turbine, with a moderate (0.20) degree of reaction, highly supersonic stator nozzles, and transonic rotor blades. The chosen turbine speed of revolution (6700 rev/min) allows a favourable blade aspect ratio, with blade height/mean radius ratios low enough to adopt untwisted blades without significant loss increase. Detailed aerodynamic studies were carried out, which show that a total-to-static efficiency of the turbine of 80% could be achieved. The predicted turbine efficiency curve at various operating conditions and at the nominal speed is shown in Fig. 4. As d i ~ u s ~ l in the next chapter, the turbine was tested at pressure ratios between 9 and 12, and exhibited the expected efficiency.
3. TESI" RESULTS
A series of tests was carried out at Gemmindustria.* before the final plant mounting on the stack of the ceramic kiln. The set up of the test rig is represented in Fig. 5, where the measuring points are also indicated. Two oil burners were used as the energy source; to obtain the correct temperature of the gases at the heat exchanger inlet, the combustion products were diluted with air, sucked in by a fan. The generated electric power was wasted on a series of lamps; the desired torque at the turbine shaft being obtained by connecting the proper number of lamps. The main purposes of the teSts were the following: to chock the correct behaviour of the whole loop at various operating conditions (startup, shutdown, etc.); to detect possible early failures of the various components of the plant and of the control system; to measure the engine performance, * The present research programme was undertaken in the frame of the EEC contract No. 198 EEl by Gemmindustria, Milano, Italy with the scientific support of the authors.
Heat recovery in a ceramic kiln
129
Fi B. 5. Scheme of the engine test Fig: (1) oil burners; (2) refractory combustion chamber; (3) air vents; (4) prebeater; (5) evaporator; (6) centrifugal fan; (7) turbine casing; (8) condenser; (9) constant level hot well. TI, PI = evaporation temperature and pressure; T2, P2 = turbine exhaust temperature and pressure; P3 == pump outlet pressure; P4 ,= preheater inlet pressure; T5 = inlet hot gas temperature; T6 = exhaust gas temperature; TT, "I'8 inlet and outlet cooling water temperatures; F -- water flow rate; No0 = generated electrical power. As far as the first t w o p o i n t s are concerned, n o m a j o r p r o b l e m s were encountered d u r i n g the v a r i o u s t r i a l runs (for a t o t a l o f a b o u t 100 o p e r a t i n g hours): T h e l o o p p r o v e d
to be satisfactorily sealed; all rotating parts ran smoothly, including the mechanical seal; the engine was very easy to operate, the control system provided good constancy of the generated electricity frequency, even for sudden load variations. Some problems were initially met with the centrifugal feed pump, which was often operating in cavitating conditions;, this situation has been avoided by inserting a control which keeps a constant level in the condenser hotwell, by throttling the pump outlet. In some preliminary runs, due to the said pump failures, the heat exchanger was exposed for several minutes to gases at about 300°C, without any fluid circulation: No serious fluid decomposition occurred; this fact made us more confident about the capability of tetrachloroethylene of withstanding high temperatures, at least for short periods. The engine performance is summarized in Fig. 6. Although the available energy source was not powerful enough to reach the design power level, the trend is evident: At the design evaporation temperature, the predicted power output will be reached. It is of interest the trend of the Net/N~ratio: At low power output, large fractions of the power available at the turbine blades are dissipated in mechanical losses (seals, bearings, gears, windage) and in electric losses. At larger outputs, rh, reaches satisfactorily high values. This is an indirect confirmation that the predicted turbine efficiency (~ 80%) was actually attained. Even larger efficiency values are obtained by the measurements of the thermal drop across the turbine. The good turbine efficiency is confirmed also by the experimental results concerning the overall engine effÉciency represented in Fig. 7: A referred efficiency, i.e. the ratio between the efficiency of the engine and the Carnot efficiency in the same temperature interval, of about 58% should be reached at the design point. The most probable breakdown of the remaining 42% is: 6% electric losses; 3% by-pass valve losses; 3% loss of the gear box; 3% in the turbine seal and bearing; 2% in the various pressure losses present in the loop; and eventually 16% in the fluid-dynamic irreversible processes occurring in the turbine blades. Besides these losses, related to the imperfection of the various components, a loss of 9% of the ideal efficiency takes place in the fluid preheater, where heat is introduced in the cycle at temperatures lower than the evaporation one, thus with an irreversible process.
130
C . C . ~ o et al. 40
30
Net = Electric power (exp)
Design
Nt • Mechoni¢ol i x ~ r
/
j"
I0 -
-- ~90 - -
%.
-
080 o+
--0 60 0 50
I
l
l
i
I
I
60
70
80
90
I00
I10
120
Evaporation temperature, "C
Fig. 6. Power and efficiency of the engine. The curve is computed according to turbine efficiency represented in Fig. 5, for the actual values of the condensing temperature.
4. ECONOMIC ASSESSMENT
The ceramic tunnel kilns are generally operated at rated temperature for 7000--8000 h/yr, and the energy recovered can be completely used in the factory. According to the experience gained during the design and construction of this engine, the expe~ed price for the plant, in the case that a certain number (I0--I00) of ~ l a r plants (though not identical due to the singularities of each recovery situation) is pro-
011
06
oo
o° ++ lie
009 -
.=
-
o4
u
,,,
007
--
0.06
-
005
50
I
60
I
70
I
80
I
90
I
I00
1
110
120
EvopOrotion temperoture, °C Fig. 7. Measured engine efficiency as a function of evaporation temperature.
Heat recovery in a ceramic kiln
131
Table 2
Op. Time L/kW DM/kW PBP yr
7000 h/yr 500000 1160 2.86
700000 1630 4.00
8000 h/yr 500000 1160 2.5
700000 1630 3.5
duced for the same purpose, is 500 to 700 × 103 lire (about 50% of the actual cost of the prototype, comprehensive of R & D costs); the associated Pay Back Period is reported in Table 2. The rate of return of the investment, for a mean value of PBP and a lifetime of 5 yr would be 18%. The figures quoted demonstrate the economical potential of these plants. Another benefit is the possibility of supplying electricity to the ovens when there is a failure in the electric grid. 5. REFERENCES
I. M. Gala, Recupero del calore di scarico dai forni a tunnel per ceramica, (in Italian),Convegno sull'utilizzazione e resparmio di gas naturale nelrindustria delle piastrelle per ceramica. Modena (December 1977). 2. (3. Oiglioli et al., Tetrachlore~thylene Rankine cycle for waste heat recovery from ceramic tunnel kilns, Proceedings 13th Intersociety Energy Conversion Engineering Conference, pp. 1488-1492, SAE paper 789603.