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Optimisation of installations for cooling-down industrial water
Energy 26 (2001) 1101–1107 www.elsevier.com/locate/energy
Optimisation of installations for cooling-down industrial water J. Kozioł *, J.K. Chwiolka Institute of Thermal Technology, Technical University of Silesia, Konarskiego 22, 44-101 Gliwice, Poland
Abstract This paper presents a way of optimising installations for cooling industrial water, taking into account the influence of atmospheric conditions, the operational characteristics of the cooling systems and technological systems. 2001 Published by Elsevier Science Ltd.
1. Introduction: aim of the investigations The fundamental aim of optimising cooling systems is the maximisation of economical profits of production, and consequently also an increase of the market value of the firm. Sometimes the aim of optimisation is a minimisation of losses connected with the required investment outlay. The aim of modernisation may be the following: 앫 A reduction of the costs of production. At a time when free-market competition is stiff, every feasible way of reducing the costs of production may help to raise the profits of the enterprise and enable it to survive on the market. 앫 Amendments to the actual laws (e.g. in Poland the law of 25 April 1997 [1], which changes the “Water laws” bill of 24 October 1974 [2], and introduces changes essential for the operation of open systems for the cooling of industrial water). Although open cooling systems are very expedient from the viewpoint of their operation (this concerns particularly the application of water from abyssal wells with favourable parameters of cooling water at the inlet to the cooling system), as well as from the economical point of view (due to the low investment outlay and low charges for their operation), now such systems cannot be applied because of the legal restrictions mentioned above, and also because of the high charges for operation.
* Corresponding author. Tel.: +4832-237-1672; fax: +4832-237-2872. E-mail address: [email protected] (J. Kozioł).
0360-5442/01/$ - see front matter 2001 Published by Elsevier Science Ltd. PII: S 0 3 6 0 - 5 4 4 2 ( 0 1 ) 0 0 0 7 2 - X
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앫 Depletion of the water intake providing cold water to the open cooling system. Thus, for example, a depression crater may be formed if the abyssal water intake is used excessively. Normally there are various variants of rationalisation, and the set of rights which warrants the highest profit should be chosen, taking into account the investment funds being at our disposal. The present paper deals only with the possibilities of improving the cooling systems, but not with the rationalisation of the whole industrial plant, i.e. with the rationalisation of the technological lines. By optimising the cooling system, however, we simultaneously take into consideration its effect on the technology of production.
2. Elaboration of a method of optimisation The economical effect comprising the whole time of the operation of any installation is usually expressed by the net present value, NPV [3]. In fact, this is the total forecast economical profit discounted to the year zero and expressed in monetary units of the year zero:
冘 n
NPV⫽
NCFt ⇒max (1+r)t t⫽0
(1)
where NCFt is the annual net of cash flow in the respective years; t is the consecutive number of the year from the year the investments had been started to the end of operation; n is the duration of the operation; and r is the discount rate. This is the fundamental objective function in the considered optimisation, but not the only one. The optimisation of cooling systems is rather difficult, due to the necessity of complex preliminary information and the mutual feedbacks between the cooling systems and the technological systems. The input data are mainly based on probabilistic information, e.g. the distribution of temperature, the relative humidity of the atmospheric air and the atmospheric pressure. The assumption of a deterministic character of input information may, however, lead to erroneous conclusions when problems of optimisation are being solved. The main reason for the occurrence of such errors in the course of the optimisation of cooling systems is the assumption of inadequate data concerning the weather conditions (climatic conditions). The occurrence of annual anomalies, as in recent years, may in the case of short-time investments lead to wrong conclusions. The existing cooling systems installed in Polish industrial plants are based on wet cooling towers. The temperature of the water cooled in the cooling tower depends mainly on the temperature of the atmospheric air and its humidity. In order to transfer the heat to the environment on hot, sultry days, the temperature of the water supplied to the cooling tower must be raised. This is connected with the temperature rise in the given technological process and may lead to an increased consumption of raw materials and/or electric energy. Another stimulant for the optimisation of cooling systems may be the change of legal dispositions concerning the protection of the environment, the consumption of water (particularly subterranean water). There are also situations when an excessive exploitation of the water intake results in its early depletion.
J. Kozioł, J.K. Chwiolka / Energy 26 (2001) 1101–1107
1103
3. Elaboration of programs for computer-aided calculation 3.1. Parameters of the environment As mentioned above, optimisation is based on probabilistic data. If there are many variables in the duration curve, it is not feasible to analyse all the possible sets of values of these parameters. The number of sets resulting from the division of the duration curve into finite difference points would then be too large. Therefore, the Monte Carlo method was applied [4]. The subject of sampling is the value of the reduced co-ordinate of time on the duration curve of the temperature of atmospheric air (Fig. 1). As the correlation factor between the temperature of the air and its humidity is positive, no direct sampling of the relative humidity of the air from the annual duration curve was carried out. Auxiliary time–distribution functions of relative humidity were plotted for their corresponding intervals of temperature (temperature ranges). Thus, after the sampling of the value of temperature the respective time–distribution function of the relative humidity within the given range of temperatures must be found (Fig. 1). Then, based on this diagram the value of
Fig. 1.
Time–distribution functions of the relative humidity of the air for their corresponding ranges of temperature.
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the relative humidity is sampled. Based on these data the temperature of the wet thermometer is determined. 3.2. Energy characteristics of cooling installations Any optimisation requires a knowledge of the energy characteristics of all the cooling installations already operating or those which are to be installed. Below, exemplary characteristics of cooling installations will be presented. 3.2.1. Characteristics of a wet ventilator cooling tower These characteristics have been set up based on a one-dimensional model of a wet cooling tower (Fig. 2) [5]. An interesting parameter is the temperature of the water leaving the cooling tower. It depends on the temperature of the atmospheric air, its humidity, the atmospheric pressure, the temperature of the water at the inlet, the flux of the flowing water and, of course, the structural parameters of the cooling tower as well as the angle of inclination of the blades of the fan. The
Fig. 2. Distribution of the thermal parameters over the height of the cooling tower when the temperature of the air entering the cooling tower amounts to 12°C, the relative humidity to 90% and the temperature of the water flowing into the cooling tower to 38°C.
J. Kozioł, J.K. Chwiolka / Energy 26 (2001) 1101–1107
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consumption of water by the cooling system influences the operational costs, this means that this is a quantity which — related to the given unit of time — appears in the objective function. 3.2.2. Characteristics of evaporative spray condensers Evaporative spray condensers are a part of air cooled chillers. The characteristics of this kind of installation are contained in the catalogue data provided by the producers. As an example the characteristics of the condenser SWC-18 is presented here (Fig. 3). 3.2.3. Characteristics of a cooling system based on absorption liquid chillers The basis for the elaboration of the characteristics of a system with absorption liquid chillers is the knowledge of the dependence of its efficiency on the temperature of the water at the inlet to the condenser and absorber. Fig. 4 illustrates this type of dependence in the case of a bromolithium chiller fed with saturated dry steam at a pressure of 150 kPa. Knowing this relation we can determine the flux of heat which must be transfered to the environment:
冉
˙ odd⫽Q ˙ chl 1⫹ Q
冊
1 e(two chl)
(2)
˙ odd is the heat flux transfered to the environment; Q ˙ chl is the heat flux taken over from where Q the cooled water; and e(two chl) is the efficiency of the chiller. Choosing an adequate cooling tower or using an already existing cooling tower and knowing the energy characteristics of these cooling tower, we get the characteristics of the cooling system based on an absorption chiller. 3.3. Characteristics of a technological line The knowledge of the technological characteristics of any given process and of the consumption of the driving energy in the function of technological parameters is indispensable when optimising
Fig. 3. Simplified characteristics of the evaporative spray condenser SWC-18.
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J. Kozioł, J.K. Chwiolka / Energy 26 (2001) 1101–1107
Fig. 4. Dependence of the efficiency of the cooler on the temperature of water at the inlet to the cooling system of an absorption cooler.
a cooling system. These quantities are components of the objective function. Fig. 5 presents an exemplary dependence of the consumption of electric energy as a function of a technological parameter.
Fig. 5. Consumption of electric energy depending on the mean temperature of the electrolyte.
J. Kozioł, J.K. Chwiolka / Energy 26 (2001) 1101–1107
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Table 1 Comparison of economic effects of cooling systems Relative value Characteristics of the suggested modernisation NPV 100.0% 33.5% 101.6% 44.9%
104.1%
106.6%
Basic system, being the reference system for other solutions. Basic system; the amendments of the legal dispositions concerning the protection of the enviroment have been taken into account. System with electric fan (ventilator) coolers; water from an abyssal well is drawn only to supplement the losses of water in the fan coolers. Fully automated system. A system abandoning fan coolers and water intakes from abyssal wells. Seven compressor stations are installed with a cooling capacity of 1500 kW each. Each station is provided with a screw compressor with an electric motor of 250 kW. Fully automated system. System with an attached fan cooler, water is supplied from an abyssal well only to supplement the loss of water in the fan coolers. Three compressor stations with a cooling capacity of 1500 kW/compressor station. These stations are provided with screw compressors with an electric motor of 250 kW. Fully automated system. System with already existing fan coolers, water is supplied from an abyssal well only to supplement the loss of water in the fan coolers. An absorption station with a cooling capacity of 6000 kW is installed. Fully automated system.
4. Conclusions The results of this analysis cannot be generelised because of the great variety of climatic zones and the specific microclimates (encountered in the environment of towns and on the premises of large industrial plants). Thus, the results presented should not be related even to a twin plant situated in another climatic zone. Also the peculiarity of the consumption of raw materials and/or energy by the technological part contribute to this. In Table 1 the results of an exemplary optimisation of a cooling system have been gathered taking into account some variants of cooling systems. References [1] The amendments — water laws. Official Journal. Dz. U. 97.47.299. Warszawa (Poland); 1997. [2] Water laws. Official Journal. Dz. U. 74.38.220. Warszawa (Poland); 1974. [3] Behrens W, Hawranek PM. Manual for the preparation of industrial feasibility studies. United Nations Industrial Development Organisation, 1991. [4] Szargut J. Thermodynamic and economic analysis in the industrial energetics. Warszawa: WNT; 1983. [5] Chwiolka J. Optimisation of installations for the cooling-down. Doctorate [in preparation].
Optimisation of installations for cooling-down industrial water J. Kozioł *, J.K. Chwiolka Institute of Thermal Technology, Technical University of Silesia, Konarskiego 22, 44-101 Gliwice, Poland
Abstract This paper presents a way of optimising installations for cooling industrial water, taking into account the influence of atmospheric conditions, the operational characteristics of the cooling systems and technological systems. 2001 Published by Elsevier Science Ltd.
1. Introduction: aim of the investigations The fundamental aim of optimising cooling systems is the maximisation of economical profits of production, and consequently also an increase of the market value of the firm. Sometimes the aim of optimisation is a minimisation of losses connected with the required investment outlay. The aim of modernisation may be the following: 앫 A reduction of the costs of production. At a time when free-market competition is stiff, every feasible way of reducing the costs of production may help to raise the profits of the enterprise and enable it to survive on the market. 앫 Amendments to the actual laws (e.g. in Poland the law of 25 April 1997 [1], which changes the “Water laws” bill of 24 October 1974 [2], and introduces changes essential for the operation of open systems for the cooling of industrial water). Although open cooling systems are very expedient from the viewpoint of their operation (this concerns particularly the application of water from abyssal wells with favourable parameters of cooling water at the inlet to the cooling system), as well as from the economical point of view (due to the low investment outlay and low charges for their operation), now such systems cannot be applied because of the legal restrictions mentioned above, and also because of the high charges for operation.
* Corresponding author. Tel.: +4832-237-1672; fax: +4832-237-2872. E-mail address: [email protected] (J. Kozioł).
0360-5442/01/$ - see front matter 2001 Published by Elsevier Science Ltd. PII: S 0 3 6 0 - 5 4 4 2 ( 0 1 ) 0 0 0 7 2 - X
1102
J. Kozioł, J.K. Chwiolka / Energy 26 (2001) 1101–1107
앫 Depletion of the water intake providing cold water to the open cooling system. Thus, for example, a depression crater may be formed if the abyssal water intake is used excessively. Normally there are various variants of rationalisation, and the set of rights which warrants the highest profit should be chosen, taking into account the investment funds being at our disposal. The present paper deals only with the possibilities of improving the cooling systems, but not with the rationalisation of the whole industrial plant, i.e. with the rationalisation of the technological lines. By optimising the cooling system, however, we simultaneously take into consideration its effect on the technology of production.
2. Elaboration of a method of optimisation The economical effect comprising the whole time of the operation of any installation is usually expressed by the net present value, NPV [3]. In fact, this is the total forecast economical profit discounted to the year zero and expressed in monetary units of the year zero:
冘 n
NPV⫽
NCFt ⇒max (1+r)t t⫽0
(1)
where NCFt is the annual net of cash flow in the respective years; t is the consecutive number of the year from the year the investments had been started to the end of operation; n is the duration of the operation; and r is the discount rate. This is the fundamental objective function in the considered optimisation, but not the only one. The optimisation of cooling systems is rather difficult, due to the necessity of complex preliminary information and the mutual feedbacks between the cooling systems and the technological systems. The input data are mainly based on probabilistic information, e.g. the distribution of temperature, the relative humidity of the atmospheric air and the atmospheric pressure. The assumption of a deterministic character of input information may, however, lead to erroneous conclusions when problems of optimisation are being solved. The main reason for the occurrence of such errors in the course of the optimisation of cooling systems is the assumption of inadequate data concerning the weather conditions (climatic conditions). The occurrence of annual anomalies, as in recent years, may in the case of short-time investments lead to wrong conclusions. The existing cooling systems installed in Polish industrial plants are based on wet cooling towers. The temperature of the water cooled in the cooling tower depends mainly on the temperature of the atmospheric air and its humidity. In order to transfer the heat to the environment on hot, sultry days, the temperature of the water supplied to the cooling tower must be raised. This is connected with the temperature rise in the given technological process and may lead to an increased consumption of raw materials and/or electric energy. Another stimulant for the optimisation of cooling systems may be the change of legal dispositions concerning the protection of the environment, the consumption of water (particularly subterranean water). There are also situations when an excessive exploitation of the water intake results in its early depletion.
J. Kozioł, J.K. Chwiolka / Energy 26 (2001) 1101–1107
1103
3. Elaboration of programs for computer-aided calculation 3.1. Parameters of the environment As mentioned above, optimisation is based on probabilistic data. If there are many variables in the duration curve, it is not feasible to analyse all the possible sets of values of these parameters. The number of sets resulting from the division of the duration curve into finite difference points would then be too large. Therefore, the Monte Carlo method was applied [4]. The subject of sampling is the value of the reduced co-ordinate of time on the duration curve of the temperature of atmospheric air (Fig. 1). As the correlation factor between the temperature of the air and its humidity is positive, no direct sampling of the relative humidity of the air from the annual duration curve was carried out. Auxiliary time–distribution functions of relative humidity were plotted for their corresponding intervals of temperature (temperature ranges). Thus, after the sampling of the value of temperature the respective time–distribution function of the relative humidity within the given range of temperatures must be found (Fig. 1). Then, based on this diagram the value of
Fig. 1.
Time–distribution functions of the relative humidity of the air for their corresponding ranges of temperature.
1104
J. Kozioł, J.K. Chwiolka / Energy 26 (2001) 1101–1107
the relative humidity is sampled. Based on these data the temperature of the wet thermometer is determined. 3.2. Energy characteristics of cooling installations Any optimisation requires a knowledge of the energy characteristics of all the cooling installations already operating or those which are to be installed. Below, exemplary characteristics of cooling installations will be presented. 3.2.1. Characteristics of a wet ventilator cooling tower These characteristics have been set up based on a one-dimensional model of a wet cooling tower (Fig. 2) [5]. An interesting parameter is the temperature of the water leaving the cooling tower. It depends on the temperature of the atmospheric air, its humidity, the atmospheric pressure, the temperature of the water at the inlet, the flux of the flowing water and, of course, the structural parameters of the cooling tower as well as the angle of inclination of the blades of the fan. The
Fig. 2. Distribution of the thermal parameters over the height of the cooling tower when the temperature of the air entering the cooling tower amounts to 12°C, the relative humidity to 90% and the temperature of the water flowing into the cooling tower to 38°C.
J. Kozioł, J.K. Chwiolka / Energy 26 (2001) 1101–1107
1105
consumption of water by the cooling system influences the operational costs, this means that this is a quantity which — related to the given unit of time — appears in the objective function. 3.2.2. Characteristics of evaporative spray condensers Evaporative spray condensers are a part of air cooled chillers. The characteristics of this kind of installation are contained in the catalogue data provided by the producers. As an example the characteristics of the condenser SWC-18 is presented here (Fig. 3). 3.2.3. Characteristics of a cooling system based on absorption liquid chillers The basis for the elaboration of the characteristics of a system with absorption liquid chillers is the knowledge of the dependence of its efficiency on the temperature of the water at the inlet to the condenser and absorber. Fig. 4 illustrates this type of dependence in the case of a bromolithium chiller fed with saturated dry steam at a pressure of 150 kPa. Knowing this relation we can determine the flux of heat which must be transfered to the environment:
冉
˙ odd⫽Q ˙ chl 1⫹ Q
冊
1 e(two chl)
(2)
˙ odd is the heat flux transfered to the environment; Q ˙ chl is the heat flux taken over from where Q the cooled water; and e(two chl) is the efficiency of the chiller. Choosing an adequate cooling tower or using an already existing cooling tower and knowing the energy characteristics of these cooling tower, we get the characteristics of the cooling system based on an absorption chiller. 3.3. Characteristics of a technological line The knowledge of the technological characteristics of any given process and of the consumption of the driving energy in the function of technological parameters is indispensable when optimising
Fig. 3. Simplified characteristics of the evaporative spray condenser SWC-18.
1106
J. Kozioł, J.K. Chwiolka / Energy 26 (2001) 1101–1107
Fig. 4. Dependence of the efficiency of the cooler on the temperature of water at the inlet to the cooling system of an absorption cooler.
a cooling system. These quantities are components of the objective function. Fig. 5 presents an exemplary dependence of the consumption of electric energy as a function of a technological parameter.
Fig. 5. Consumption of electric energy depending on the mean temperature of the electrolyte.
J. Kozioł, J.K. Chwiolka / Energy 26 (2001) 1101–1107
1107
Table 1 Comparison of economic effects of cooling systems Relative value Characteristics of the suggested modernisation NPV 100.0% 33.5% 101.6% 44.9%
104.1%
106.6%
Basic system, being the reference system for other solutions. Basic system; the amendments of the legal dispositions concerning the protection of the enviroment have been taken into account. System with electric fan (ventilator) coolers; water from an abyssal well is drawn only to supplement the losses of water in the fan coolers. Fully automated system. A system abandoning fan coolers and water intakes from abyssal wells. Seven compressor stations are installed with a cooling capacity of 1500 kW each. Each station is provided with a screw compressor with an electric motor of 250 kW. Fully automated system. System with an attached fan cooler, water is supplied from an abyssal well only to supplement the loss of water in the fan coolers. Three compressor stations with a cooling capacity of 1500 kW/compressor station. These stations are provided with screw compressors with an electric motor of 250 kW. Fully automated system. System with already existing fan coolers, water is supplied from an abyssal well only to supplement the loss of water in the fan coolers. An absorption station with a cooling capacity of 6000 kW is installed. Fully automated system.
4. Conclusions The results of this analysis cannot be generelised because of the great variety of climatic zones and the specific microclimates (encountered in the environment of towns and on the premises of large industrial plants). Thus, the results presented should not be related even to a twin plant situated in another climatic zone. Also the peculiarity of the consumption of raw materials and/or energy by the technological part contribute to this. In Table 1 the results of an exemplary optimisation of a cooling system have been gathered taking into account some variants of cooling systems. References [1] The amendments — water laws. Official Journal. Dz. U. 97.47.299. Warszawa (Poland); 1997. [2] Water laws. Official Journal. Dz. U. 74.38.220. Warszawa (Poland); 1974. [3] Behrens W, Hawranek PM. Manual for the preparation of industrial feasibility studies. United Nations Industrial Development Organisation, 1991. [4] Szargut J. Thermodynamic and economic analysis in the industrial energetics. Warszawa: WNT; 1983. [5] Chwiolka J. Optimisation of installations for the cooling-down. Doctorate [in preparation].