Current Development Status and Energy-Saving Benefits of Condensate Recovery Systems

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This post was last edited by li*ch1968 on 2012-5-29 at 18:19. Based on an introduction to the recovery methods of condensate recovery systems as well as the commonly used recovery equipment both domestically and internationally, it discusses in detail how to select such systems and equipment. The thermoeconomic efficiency of the condensate recovery system was discussed through examples, demonstrating that condensate recovery offers significant energy-saving benefits. 1 Overview: As a heat transfer medium, steam is widely used in various industrial fields such as power generation, the petroleum industry, the chemical industry, printing and dyeing, papermaking, light textiles, brewing, the rubber industry, and ceramics. After releasing its latent heat of vaporization in various devices that use steam, it turns into saturated condensate at nearly the same temperature and pressure. Since the pressure at which steam is used is higher than atmospheric pressure, the heat contained in the condensate can account for 20%-30% of the total heat of the steam. The higher the pressure and temperature, the more heat is present in the condensate, and thus the larger its proportion of the total heat of the steam. It can be seen that recovering 100% of the heat from condensed water and making effective use of it holds great potential for energy savings. Currently, many enterprises are paying attention to the condensate water generated in their production processes, driven by the increasingly evident economic and social benefits of condensate recovery, as well as the problems caused by energy shortages. They aim to save energy, protect the environment, and reduce production costs through effective utilization of waste heat. Technical improvement projects for the recycling of condensate water are being implemented continuously, but how to effectively treat and utilize this water, as well as how to achieve optimal economic benefits, are issues that warrant further study. 2 Characteristics of the condensate recovery system Generally speaking, condensate recovery systems can be divided into two main categories: open and closed types [2]. 2.1 Open recovery system: An open recovery system is one in which the condensate water is recycled back into the boiler’s feedwater tank. During the process of recovering and utilizing this condensate water, one end of the recovery pipeline is open to the atmosphere; typically, the condensate collection tank is open to the atmosphere. When the pressure of the condensate is low and it cannot reach the reuse location by its own pressure, a pump can be used to pump the condensate. The advantage of this system is its simple equipment, easy operation, and low initial investment ； However, the economic benefits obtained by the system are poor, and since the condensate comes into direct contact with the atmosphere, the dissolved oxygen concentration in it increases, which can lead to equipment corrosion. This type of system is suitable for small steam supply systems with low condensate production and a small amount of secondary steam. When this system is adopted, the amount of steam generation should be minimized in order to reduce thermal pollution as well as losses of working fluid and energy. 2.2 Closed recovery system: In a closed recovery system, the condensate collection tank and all pipelines are under a constant positive pressure, making the system sealed. Most of the energy contained in the condensate in the system is directly recovered and returned to the boiler through specialized recovery equipment; the only energy loss occurs due to the cooling of the piping network. Thanks to the closed system, the water quality is maintained, which reduces the costs associated with treating the water before it is fed into the boiler. Its advantages are good economic benefits from condensed water recovery and a long service life for the equipment, but the initial investment for the system is high and operation is inconvenient. Open and closed recovery systems are merely general classifications of condensate recovery systems; specific condensate recovery systems vary depending on the particular project conditions, such as the on-site conditions of the condensate, its physical parameters, and the objectives of recovery. 3 Determination of recovery methods and equipment: For different condensate treatment projects, choosing the appropriate recovery methods and equipment is a crucial step in ensuring that the project achieves its investment objectives. Firstly, to properly select a condensate recovery system, it is essential to accurately determine the amount of condensate generated by the system as well as the volume of condensate water that needs to be discharged. If the amount of condensate water is calculated incorrectly, it will result in the condensate pipe diameter being chosen either too large or too small. Secondly, it is necessary to accurately understand the pressure and temperature of the condensate water, as these parameters are key in selecting a condensate water recovery system. Issues such as the method to be used in the recovery system, the equipment to be employed, how to arrange the piping network, whether to utilize secondary steam, and whether it is necessary to recover all the heat from the condensate are all related to the pressure and temperature of the condensate. Third, the selection of steam traps in the condensate recovery system is also something that the recovery system needs to take into consideration. Different choices of steam traps can affect the pressure and temperature of the condensate when it is utilized, as well as the steam leakage in the recovery system. Thanks to the continuous improvement and refinement of condensate recovery technologies, as well as the ongoing advancements in condensate recovery equipment and the development of new, high-performance recovery systems, the possibility of effectively utilizing various resources derived from condensate in condensate recovery systems has **increased**. In the past, the recovery equipment used in condensate recovery systems was generally steam traps, collection tanks, and ordinary water pumps. The problems existing in such systems – such as water hammer in the pipelines caused by the coexistence of steam and water, steam leakage resulting from improper selection of steam traps, cavitation issues arising during the operation of ordinary water pumps, and the inability to make effective use of condensate – have been gradually resolved through the development of new recovery equipment. To make full use of the vapor content in the condensate and its energy, a condensate expansion tank is installed in the pipeline; this causes the condensate to flash and generate secondary steam, which is then recovered. In this way, the energy can be utilized fully, and the water hammer problem in the pipeline can be resolved. Furthermore, to address the cavitation problem in pumps handling high-temperature saturated condensate, a sealed recovery device for such condensate has been developed by utilizing the principle of jet boosting [3], and building on advanced foreign technologies. This device eliminates the cavitation issue that occurs when centrifugal pumps transport high-temperature saturated condensate, as well as the cavitation problem that arises during the jet boosting process in jet pumps. It enables closed-loop recovery systems to make full use of the thermal energy contained in the condensate, recover the greatest possible amount of it, save fuel and softened water, and thereby improve the economic efficiency of these recovery systems. Of course, there are other devices as well; for example, the JCRS-type heat-pump condensate recovery system without a check valve, which was developed using the technology of pumping off flash vapor with heat pumps [4]. This system utilizes a steam-jet heat pump to raise the pressure of the flash vapor from the condensate, thereby enabling its reuse. This approach allows for the recovery of both steam and water, resulting in an available amount of steam that exceeds the amount supplied by the boiler. It can also lower the temperature of the condensate as the flash steam is drawn away, and pump it back for reuse with an anti-cavitation pump, resulting in significant energy savings. Furthermore, a steam recovery compressor equipped with a self-pressurizing ring can directly press steam and high-temperature condensate into the boiler at high temperatures; such recovery equipment boasts a high heat recovery efficiency. The improvement of the condensate recovery devices has **increased** the recovery efficiency of the condensate recovery system. When selecting such devices, it is necessary to take into account the specific conditions of the recovery system on site, as well as the actual steam usage conditions such as steam pressure and temperature, the method of recovering flash vapor, and the type of drain valves. When selecting a system, it isn’t the case that the higher the system’s recovery efficiency, the better. While ensuring that the system achieves its recovery objectives, it is also necessary to consider its thermal economy – that is, in addition to taking into account the efficiency of waste heat utilization, the initial investment must also be considered, meaning an economic and technical comparison of the project. Only by conducting a proper economic and technical comparison to determine a reasonable ratio between investment and recovery can an optimized solution for the engineering project be achieved. Due to their high efficiency and low environmental impact, closed-loop recycling systems are often given priority and adopted in recycling projects. 4 Thermal economic analysis of the condensate recovery project: For a closed-loop condensate recovery system, the total investment mainly consists of the replacement or addition of steam-using equipment’s drain valves ； Recovery equipment – such as pumps, water collection tanks, heat exchangers, expansion vessels, high-performance recovery units, etc., as well as insulation and pipeline materials, technical services, and project construction costs. The cumulative amount of several costs constitutes the total investment for the project, and this investment amount needs to be determined based on the site conditions and a feasibility analysis of the project. The economic benefits of the recovery project are analyzed from the following aspects: (1) Due to the use of a closed-loop recovery system, the system operates in a closed manner, which increases the back pressure and reduces steam leakage, thereby generating benefits ； (2) The value saved by recycling condensate water in terms of softened water usage ； (3) The increase in the condensate recovery temperature raises the boiler feedwater temperature, resulting in savings in fuel consumption and corresponding benefits. Of course, there are also certain social benefits; the recovery of condensate helps to reduce pollution caused by leaks of steam and condensate, as well as wastewater discharge. It is obvious that there is a certain relationship between the benefits and costs of a condensate recovery system. Determining how to achieve an optimal balance between benefits and costs is an economic issue that must be considered in such systems. Typically, we use the payback period commonly employed in engineering and technical fields to determine the rationality and feasibility of investing in a project. After taking into account the energy-saving and economic aspects of the condensate recovery project, the social and economic benefits of such a recovery system are often significant. For example, in a certain dyeing and printing factory, the latent heat of vaporization released by steam is utilized to meet the requirements of processes such as drying and cooking. After being released, this steam is not recycled but is directly discharged into the sewer through a drain valve. The softened boiler feed water enters the deaerator at very low temperatures, requiring a large amount of high-temperature steam; this increases the boiler’s steam production and thus leads to high fuel consumption and elevated production costs. To reduce production costs and save water and energy, the factory adopts a closed-cycle condensate recovery system. The steam generated in the dyeing tanks is converted into condensate with a certain temperature and pressure through steam traps, and this condensate enters a collection tank. The flash vapor produced in this tank goes to the boiler deaerator, where it can be used to heat the feed water ； The condensate in the water collection tank is fed directly into the deaerator via an anti-cavitation pump, thereby recovering the condensate and its heat. An additional bypass can be used to force the condensate into the soft water tank, from where it then enters the boiler. After adopting this project, the printing and dyeing factory has reaped tremendous economic benefits. If the benefits are calculated on an annual basis, after the renovation, the factory will see a reduction in steam leakage from its steam-using equipment of 864 tons per year. At a price of 70 yuan per ton, this reduction can generate annual benefits of 61,000 yuan for the factory ； The benefits resulting from saving softened water amount to 58,000 yuan per year ； The benefit resulting from saving fuel due to increasing the boiler inlet temperature is 302,000 yuan per year. Its annual total revenue is 420,000 yuan per year. Considering that the total cost of equipment investment and various expenses amounts to 350,000 yuan per year, economic and technical analyses show that the investment was recouped in less than ten months after the project was put into operation. Moreover, environmental pollution caused by leaks was completely eliminated through the use of this system, and its social benefits are immeasurable. 5 Conclusion As global energy shortages become increasingly severe, energy conservation becomes more and more important. Condensate recovery, as an important energy-saving measure, will receive greater attention. Furthermore, due to the widespread use of steam energy, the continuous improvement of condensate recovery technologies, and the development of equipment for condensate recovery, the energy-saving benefits of condensate recovery will become even more prominent.