Integrating Waste Heat into a Steam Network for Industry
Introduction
Condensate is one of the most valuable resources that can be used to considerably improve the overall efficiency of steam plant in the modern energy conscious environment (Akelaitis, 2000). In many industrial applications, steam provides an efficient and effective medium for conveying heat into several premises and processes. To ensure hot water and energy in the steam are used optimally, there must be an effective condensate recovery. According to Bisram, (2008), condensate, defined as hot and treated water obtained as steam, releases its energy is estimated to contain approximately 25% of the total useful energy initially contained in the original steam. Thus, it is important to return the condensate to the boiler as opposed to disposing it off to drain. For instance, when 100kg/hour of steam is generated and supplied either for heating industrial processes or generation of electrical power, a similar quantity of the condensate should be discharged.
Condensate recovery, a process that reuses the water and sensible heat in the discharged condensate, offers several advantages such as energy savings, reduction in fuel costs, reduced water-related expenses, e.g. Water supply and chemical treatment costs and positive impacts on safety and the environment (Spirax Sharco Uk, 2016).
Figure 1: A typical steam and condensate circuit (Spirax Sharco Uk, 2016)
Although condensate recovery is associated with these benefits, Kully (2010) purports that most industries such as refineries are afraid of the high risk of condensate contamination that could result in serious boiler damages hence opting to drain this condensate. To overcome this challenge, heat exchangers are incorporated in condensate recovery systems to extract energy from contaminated condensate. This will ensure that a higher percentage of the thermal energy is recovered.
Industries facing condensate contamination challenge
Modern industries use steam in several processes for physical changes and chemical reactions in their raw materials as well as maintaining cleaning and sterile conditions for food handling and processing industries such as refineries and dairy processing industries. For refineries, steam and hot water play a major role in processing crude oil to final products. Contamination of steam and hot water in refineries occurs as a result of common feedwater contaminants such as excessive boiler treatment chemicals, oils, resins, oxygen and other dissolved gases and miscellaneous chemical and metal compounds. This contamination can cause corrosion in boilers, pipes and heat exchangers. This may result into mineral oils leaking into the return condensate causing condensate contamination in refineries.
In addition to refineries, condensate contamination is also a common challenge in dairy industries where steam is used for direct and indirect heating and sterilization processes. The use of high quality steam and condensate plays a major role in the production of high quality dairy products. The type of contaminants in the condensate varies from metals e.g. iron (originating from materials of condensate system and process equipment) to oils and process chemicals due to leakage in pumps, gland seals and heat exchangers. Condensate contamination occurs in automobile industries with water tube boilers. Condensate from different sections of the plant is collected into collection tank before flowing into the deaerator. Condensate contamination in automobile industries is caused with chemicals used in treating boiler water and metals from process equipment. The contaminated condensate if allowed to enter the boilers, it can result in metal corrosion of the boiler components. Although condensate has some useful thermal energy, the problem of its contamination has been the main challenge in exploiting such energy potential. Therefore, focus should be directed to design of condensate recovery system that will take advantage of the thermal energy of the condensate by avoiding the risk of any damage to the boilers. This can be achieved by use of heat exchangers.
Elements of condensate recovery system
In order to design an effective condensate recovery system that will take advantage of the condensate thermal energy, one should understand the functions of each component and the design specifications for effective functioning.
Pumps in the condensate recovery system should be made of 304-stainless steel. During pumping of the condensate, vapor pressure should be taken into consideration. Since the temperature of the condensate is similar to its boiling point, centrifugal pumps cannot be applied in this system due to the presence of an area of lower pressure at the center of the impeller (Zhang, 2013). For this reason, electric pumps are highly recommended to pump condensate in the condensate recovery system. The design of the pumping system considers the following elements: head (h), pressure head (hp), static head (hs), net static head, friction head (hf) and the total delivery head. The total delivery head of the pump, i.e. the head against which the pump needs to operate is givens as:
Total delivery head (hd) = hs + hf + hp ………………. (1)
Where hs defines, the pressure need to raise the water to the required static head, hf defines the friction head, i.e. the pressure to move water through the pipe and hp describes the pressure in the condensate system.
Moreover, the size of the mechanical condensate pump takes into consideration the maximum flow rate to the receiver, steam pressure or air for driving the pump, the available filing head between the pump and receiver and the total delivery head of the receiver (Bendig, Maréchal & Favrat, 2015) as calculated above (Eqn 1).
The piping system for transporting condensate is referred as condensate return piping whose design should comprise of two-phase flow. In two-phase flow, steam as either live steam, flash steam or mixture of the two flows through the pipe with liquid condensate in the same layer. The main pipe material used in making pipes and condensate line for steam system is carbon steel in accordance with ASME B 16.9 A106 standards (Lukosevicius, Akelaitis & Cicinskas, 2000). The ASME 31.1 and EN 45510 standards are used in selecting piping material and corresponding thickness of the wall for a given installation. There should be provided for additional frictional resistance for pipe fitting of between 5-20% as pipe length from 50 meters to 100 meters (Kirk, 2005).
The condensate velocity through the pipe is obtained using the equation:
Condensate velocity (u) = ………. (2)
Similarly, cross sectional area (A) = ………. (3)
=
è Diameter of the pipe, d = ……. (4)
Heat exchanger is important in a condensate recovery system for extraction of thermal energy from contaminated condensate. The main types of heat exchangers that can be considered in the design of the condensate recovery system are: plate, brazed, gasketed (plate and framed), welded and shell and tube heat exchangers (Meyer, 2004). In the design of heat exchangers for the condensate recovery system, design steam load and the mean rate at which steam is condensed should be determined mathematically. The two are calculated as follows:
Design steam load = * measured steam load ……… (5)
Mean rate of steam condensation = …………………. (6)
Where m is the mass of water heated cp represents the specific heat of water, ΔT change in water temperature and hfg is the enthalpy of evaporation of steam.
References
Akelaitis, S. (2000). Process integration and waste heat recovery in Lithuanian and Danish industry: Case study: Sugar factory
Bendig, M., Maréchal, F., & Favrat, D. (2015). Integration of Organic Rankine Cycles for Waste Heat Recovery in Industrial Processes.
Bisram, S. (2008). Automated control system for condensate recovery (Unpublished master's thesis). University of Trinidad and Tobago.
Kirk., D. S. (2005). Constructal multi-scale heat exchangers (Unpublished master's thesis)
Kully, R. M. (2010). A study of the economical feasability of installing heating, ventilation, and air-conditioning condensate recovery system (Unpublished master's thesis)
Lukosevicius, V., Akelaitis, S., & Cicinskas, D. (2000). Process integration and waste heat recovery in Lithuanian and Danish industry: Process integration in Eastern Europe.
Meyer, A. (2004). Development of a range of air-to-air heat pipe heat recovery heat exchangers (Unpublished master's thesis).
Spirax Sharco Uk, 2016. Pipe and pipe sizing, available online at http://www.spiraxsarco.com/Resources/Pages/Steam-Engineering-Tutorials/steam-distribution/pipes-and-pipe-sizing.aspx
Spirax Sharco Uk, 2016. Condensate recovery solutions sales brochure, Reduce your operating costs through the effective management of condensate. Available at http://www.spiraxsarco.com/Documents/Condensate_Recovery_Solutions-Sales_Brochure
Zhang, F. (2013). Process integration of organic rankine cycle for waste heat recovery in industrial process (Unpublished master's thesis).
Condensate is one of the most valuable resources that can be used to considerably improve the overall efficiency of steam plant in the modern energy conscious environment (Akelaitis, 2000). In many industrial applications, steam provides an efficient and effective medium for conveying heat into several premises and processes. To ensure hot water and energy in the steam are used optimally, there must be an effective condensate recovery. According to Bisram, (2008), condensate, defined as hot and treated water obtained as steam, releases its energy is estimated to contain approximately 25% of the total useful energy initially contained in the original steam. Thus, it is important to return the condensate to the boiler as opposed to disposing it off to drain. For instance, when 100kg/hour of steam is generated and supplied either for heating industrial processes or generation of electrical power, a similar quantity of the condensate should be discharged.
Condensate recovery, a process that reuses the water and sensible heat in the discharged condensate, offers several advantages such as energy savings, reduction in fuel costs, reduced water-related expenses, e.g. Water supply and chemical treatment costs and positive impacts on safety and the environment (Spirax Sharco Uk, 2016).
Figure 1: A typical steam and condensate circuit (Spirax Sharco Uk, 2016)
Although condensate recovery is associated with these benefits, Kully (2010) purports that most industries such as refineries are afraid of the high risk of condensate contamination that could result in serious boiler damages hence opting to drain this condensate. To overcome this challenge, heat exchangers are incorporated in condensate recovery systems to extract energy from contaminated condensate. This will ensure that a higher percentage of the thermal energy is recovered.
Industries facing condensate contamination challenge
Modern industries use steam in several processes for physical changes and chemical reactions in their raw materials as well as maintaining cleaning and sterile conditions for food handling and processing industries such as refineries and dairy processing industries. For refineries, steam and hot water play a major role in processing crude oil to final products. Contamination of steam and hot water in refineries occurs as a result of common feedwater contaminants such as excessive boiler treatment chemicals, oils, resins, oxygen and other dissolved gases and miscellaneous chemical and metal compounds. This contamination can cause corrosion in boilers, pipes and heat exchangers. This may result into mineral oils leaking into the return condensate causing condensate contamination in refineries.
In addition to refineries, condensate contamination is also a common challenge in dairy industries where steam is used for direct and indirect heating and sterilization processes. The use of high quality steam and condensate plays a major role in the production of high quality dairy products. The type of contaminants in the condensate varies from metals e.g. iron (originating from materials of condensate system and process equipment) to oils and process chemicals due to leakage in pumps, gland seals and heat exchangers. Condensate contamination occurs in automobile industries with water tube boilers. Condensate from different sections of the plant is collected into collection tank before flowing into the deaerator. Condensate contamination in automobile industries is caused with chemicals used in treating boiler water and metals from process equipment. The contaminated condensate if allowed to enter the boilers, it can result in metal corrosion of the boiler components. Although condensate has some useful thermal energy, the problem of its contamination has been the main challenge in exploiting such energy potential. Therefore, focus should be directed to design of condensate recovery system that will take advantage of the thermal energy of the condensate by avoiding the risk of any damage to the boilers. This can be achieved by use of heat exchangers.
Elements of condensate recovery system
In order to design an effective condensate recovery system that will take advantage of the condensate thermal energy, one should understand the functions of each component and the design specifications for effective functioning.
- Pumps
Pumps in the condensate recovery system should be made of 304-stainless steel. During pumping of the condensate, vapor pressure should be taken into consideration. Since the temperature of the condensate is similar to its boiling point, centrifugal pumps cannot be applied in this system due to the presence of an area of lower pressure at the center of the impeller (Zhang, 2013). For this reason, electric pumps are highly recommended to pump condensate in the condensate recovery system. The design of the pumping system considers the following elements: head (h), pressure head (hp), static head (hs), net static head, friction head (hf) and the total delivery head. The total delivery head of the pump, i.e. the head against which the pump needs to operate is givens as:
Total delivery head (hd) = hs + hf + hp ………………. (1)
Where hs defines, the pressure need to raise the water to the required static head, hf defines the friction head, i.e. the pressure to move water through the pipe and hp describes the pressure in the condensate system.
Moreover, the size of the mechanical condensate pump takes into consideration the maximum flow rate to the receiver, steam pressure or air for driving the pump, the available filing head between the pump and receiver and the total delivery head of the receiver (Bendig, Maréchal & Favrat, 2015) as calculated above (Eqn 1).
- Pipes and pipe sizing
The piping system for transporting condensate is referred as condensate return piping whose design should comprise of two-phase flow. In two-phase flow, steam as either live steam, flash steam or mixture of the two flows through the pipe with liquid condensate in the same layer. The main pipe material used in making pipes and condensate line for steam system is carbon steel in accordance with ASME B 16.9 A106 standards (Lukosevicius, Akelaitis & Cicinskas, 2000). The ASME 31.1 and EN 45510 standards are used in selecting piping material and corresponding thickness of the wall for a given installation. There should be provided for additional frictional resistance for pipe fitting of between 5-20% as pipe length from 50 meters to 100 meters (Kirk, 2005).
The condensate velocity through the pipe is obtained using the equation:
Condensate velocity (u) = ………. (2)
Similarly, cross sectional area (A) = ………. (3)
=
è Diameter of the pipe, d = ……. (4)
- Heat exchanger
Heat exchanger is important in a condensate recovery system for extraction of thermal energy from contaminated condensate. The main types of heat exchangers that can be considered in the design of the condensate recovery system are: plate, brazed, gasketed (plate and framed), welded and shell and tube heat exchangers (Meyer, 2004). In the design of heat exchangers for the condensate recovery system, design steam load and the mean rate at which steam is condensed should be determined mathematically. The two are calculated as follows:
Design steam load = * measured steam load ……… (5)
Mean rate of steam condensation = …………………. (6)
Where m is the mass of water heated cp represents the specific heat of water, ΔT change in water temperature and hfg is the enthalpy of evaporation of steam.
References
Akelaitis, S. (2000). Process integration and waste heat recovery in Lithuanian and Danish industry: Case study: Sugar factory
Bendig, M., Maréchal, F., & Favrat, D. (2015). Integration of Organic Rankine Cycles for Waste Heat Recovery in Industrial Processes.
Bisram, S. (2008). Automated control system for condensate recovery (Unpublished master's thesis). University of Trinidad and Tobago.
Kirk., D. S. (2005). Constructal multi-scale heat exchangers (Unpublished master's thesis)
Kully, R. M. (2010). A study of the economical feasability of installing heating, ventilation, and air-conditioning condensate recovery system (Unpublished master's thesis)
Lukosevicius, V., Akelaitis, S., & Cicinskas, D. (2000). Process integration and waste heat recovery in Lithuanian and Danish industry: Process integration in Eastern Europe.
Meyer, A. (2004). Development of a range of air-to-air heat pipe heat recovery heat exchangers (Unpublished master's thesis).
Spirax Sharco Uk, 2016. Pipe and pipe sizing, available online at http://www.spiraxsarco.com/Resources/Pages/Steam-Engineering-Tutorials/steam-distribution/pipes-and-pipe-sizing.aspx
Spirax Sharco Uk, 2016. Condensate recovery solutions sales brochure, Reduce your operating costs through the effective management of condensate. Available at http://www.spiraxsarco.com/Documents/Condensate_Recovery_Solutions-Sales_Brochure
Zhang, F. (2013). Process integration of organic rankine cycle for waste heat recovery in industrial process (Unpublished master's thesis).
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