基于分级超结构的换热网络改造优化

建立了基于分级超结构模型的换热网络改造同步优化数学模型。该模型不依赖于夹点约束,不需要预先给定最小传热温差,能够有效权衡改造投资费用与运行费用之间的关系。改造投资费用中考虑了现有换热器的重新配置费用、现有换热器新增传热面积费用和新增换热器费用,符合工程实际要求。针对换热网络改造优化数学模型具有不连续和非线性的特点,数学模型的求解采用双层优化策略,其中表示网络结构调整的离散变量优化利用遗传算法,而表示操作参数的连续变量优化利用粒子群算法优化。2个不同规模的换热网络改造算例用于验证所提出方法的有效性。     

Application of intensified heat transfer for the retrofit of heat exchanger network

A number of design methods have been proposed for the retrofit of heat exchanger networks (HEN) during the last three decades. Although considerable potential for energy savings can be identified from conventional retrofit approaches, the proposed solutions have rarely been adopted in practice, due to significant topology modifications required and resulting engineering complexities during implementation. The intensification of heat transfer for conventional shell-and-tube heat exchangers can eliminate the difficulties of implementing retrofit in HEN which are commonly restricted by topology, safety and maintenance constraints, and includes high capital costs for replacing equipment and pipelines. This paper presents a novel design approach to solve HEN retrofit problems based on heat transfer enhancement. A mathematical model has been developed to evaluate shell-and-tube heat exchanger performances, with which heat-transfer coefficients and pressure drops for both fluids in tube and shell sides are obtained. The developed models have been compared with the Bell-Delaware, simplified Tinker and Wills-Johnston methods and tested with the HTRI® and HEXTRAN® software packages. This demonstrates that the new model is much simpler but can give reliable results in most cases. For the debottlenecking of HEN, four heuristic rules are proposed to identify the most appropriate heat exchangers requiring heat transfer enhancements in the HEN. The application of this new design approach allows a significant improvement in energy recovery without fundamental structural modifications to the network.     

Plant energy saving through efficient retrofit of furnaces

Tubular process furnaces belong to energy demanding equipment in the process industry, especially in the chemical and petrochemical process plants and refineries. Several ways of energy saving in such plants usually exist. Retrofit of furnaces can be considered as one of the straightforward and efficient ways. However, operational and geometrical constraints of an existing furnace are the reasons due to which the retrofit of a furnace is a very difficult task. Therefore, the process retrofit is usually focused on heat exchanger network (HEN) retrofit considering maximum furnace duty. Nevertheless, the furnace retrofit should be considered wherever possible. In some older plants, the placement of new shells or topology changes in HENs can be expensive due to various reasons and only minimum topology modifications are usually allowed. The furnace retrofit procedure described in this paper is based on an advanced furnace integration approach using some principles of Pinch Analysis and considering furnace limitations. It can bring surprising results. This method combines principles of an effective design of both processes and equipment. An efficient methodology for furnaces retrofit, using optimization of both stack temperature and air preheating system, is applied. An advantage of this approach is demonstrated through a case study — retrofit of furnace in petrol hydrogenation refining plant for energy efficiency improvement.     

Direct heat transfer considerations for improving energy efficiency in pulp and paper Kraft mills

The success of any process improvement study depends on the quality of the available data and the way in which the plant-specific characteristics are incorporated in the applied conceptual models; in the context of process integration studies these issues are directly related to the rules followed during the data extraction stage. Improving energy efficiency in a pulp and paper Kraft mill requires the identification of the most promising heat recovery network retrofit projects. In a retrofit analysis using pinch technology/process integration methods, only the process streams associated to the existing heat exchangers and some outlet streams (such as wastewater/effluent streams and vents) with high potential for heat recovery are usually included, while the energy exchanged through non-isothermal stream mixing (NIM) or direct heat transfer (DHT) is often assumed fixed and is not considered in the analysis. Relaxing this assumption requires extracting more data to represent the DHT design configuration that exists in the plant. However, different data extraction options can be considered to represent the DHT configuration depending on the associated process/operation constraints. This work describes a systematic procedure to extract and analyse the impacts of DHT on the overall energy efficiency of a Kraft process with a specific focus on mixing along the pulp line and in water tanks.     

Global algorithm for systematic retrofit of tubular process furnaces

Tubular process furnaces are widely used especially in the chemical and petrochemical process plants and refineries. Retrofit of a furnace is usually recommended for increasing plant capacity or improvement of the furnace (and/or plant) energy efficiency. Many different constraints of existing furnace design and operation are the reasons for which the retrofit of a furnace is a very difficult task. Main operational and geometrical constraints are taken into account in furnace retrofit integration approach created earlier. This approach was developed primarily for improvement of the furnace energy efficiency. When furnace retrofit for increasing the plant capacity is performed additional criteria have to be considered. Then fluid hydrodynamic behaviour plays an important role and influences the retrofit strategy. This problem is discussed in the paper and results in suggestion of the global conceptual algorithm for furnace retrofit strategy. Practical case studies demonstrate applicability of the proposed global furnace retrofit algorithm.     

膜全热交换器在空调设计系统中的应用

膜全热交换器作为一种新型的热交换设备,不仅能回收室内空气的显热和潜热,降低空调系统新风负荷,又能引进新风改善室内空气品质。介绍了膜全热交换器的工作原理和优点,结合实际工程进行了运行性能分析,得出其具有很好的应用前景。     

Upgrading mixed ventilation systems in industrial conditioning

Mixing ventilation systems (MVS) involve injecting cold air from the upper part of the room. The resulting turbulent mixing of the cold stream with ambient air determines a cooling of the whole volume and establishes an uniform temperature level with a very low vertical temperature gradient. While this method is commonly practiced it may not be energy efficient as the entire room volume gets cooled while often only the lower room volume, occupied by personnel at the shop floor level, needs conditioning. This may be a severe drawback especially in high-ceiling buildings and when process equipment with high thermal loads are utilized. In this paper an easily retrofitted improvement solution to existing MVS, called retrofit hybrid displacement ventilation system (RHDVS) is suggested and compared with a traditional MVS taking into account technical and economic performances. The study was carried out resorting to experimental measurements on a single-diffuser pilot installation of the RHDVS in an actual industrial facility, to characterize the existing MVS and estimate the performance improvement obtainable with the proposed RHDVS. An economic analysis was finally performed to assess the retrofit economic feasibility in case it is extended to the entire plant. Respect mixing ventilation, a reduction of the air flow rate cooling requirement from 12 to 6 kJ/kg resulted in the considered application, with an average cost saving of 50,000 €/yr. A pay back time of less than 1 year followed, making the upgrade to RHDVS an interesting alternative.     

Use of advanced composite curves for assessing cost-effective HEN retrofit II. Case studies

In this paper a graphical method for cost-effective heat exchanger network (HEN) retrofit is applied in two cases. The method uses information about the placement of heaters/coolers in the existing HEN to identify the potential for cost-effective retrofit. In Case 1 detailed matrix calculations are compared with the results from the graphical method, and in Case 2 the graphical method is compared with earlier published results.In Case 1, the retrofit investment cost for a certain heat recovery was more than doubled for a HEN with heaters placed at high temperature, in comparison to a HEN with heaters placed at low temperature, although both HENs have the same stream data and utility requirements. In a HEN with heaters placed at both low and high temperatures, the heaters placed low could be detached as cheaply as the case with all heaters at low temperatures. The heaters at high temperatures were cheaper to detach than in the case with all heaters at high temperatures, since the initial changes to heaters at low temperatures keep the total investment cost down.Case 2 demonstrates how the graphical method is used to identify potential heat recovery. The conclusions from this method were compared with detailed calculations. Results showed that the PBP increased by a factor of four, when heat saving increased from the level suggested by the advanced curves to the maximum possible heat recovery.     

Thermographic analysis of a building integrated photovoltaic system

A residential-scale building integrated photovoltaic (BiPV) cogeneration system has been thermographically investigated. The results are useful in calibrating the numerical models created to predict the system's operational temperatures. The combined heat and power system is based on existing BiPV roofing technology with the addition of a modular heat recovery unit. The convection of the air behind the panels will serve both to cool the photovoltaic panels and provide a heat source for the residence. The analysis allows for the interpretation of the surface emissivities and operating temperatures, as well as qualitative graphic analysis of temperature gradients.     

An open heat pump process for moist gases

A heat pump process is proposed for the recovery of the latent heat of water vapour in waste heat gases. The process includes a humidifier where the moist waste gas is additionally humidified. The moist gas leaving the humidifier is pressurized in a turbocompressor. The dew point temperature of the gas is increased by the humidification and the compression. This is utilized by indirect heat exchange for the production of low pressure steam and for the required heat of vaporization in the humidifier. It should be possible to use the process for waste heat recovery from moist flue gases and from process gases such as dryer exhaust air. Performance data have been calculated for production of low pressure steam at 1.2 bar from moist air at atmospheric pressure, dry temperature 100°C and the absolute humidity 0.19 kg water vapour per kg dry air. With these data a coefficient of performance for the heat pump process has been calculated to be 2.8. 85% (m/m) of the waste heat has been recovered from the moisture content. For these calculations the pressurization in the compressor has been set to 4.0 bar. The process should be further investigated to find performance data under optimal operating conditions.     

Achievements and suggestions of heat metering and energy efficiency retrofit for existing residential buildings in northern heating regions of China

In order to promote energy efficiency and emission reduction, the importance of improving building energy efficiency received sufficient attention from Chinese Government. The heat metering and energy efficiency retrofit for existing residential buildings of 0.15 billion m² in northern heating regions of China was initiated in 2007 and completed successfully at the end of 2010. This article introduced the background and outline of the retrofit project during the period of 11th five-year plan. Numerous achievements that received by retrofit such as environmental protection effect, improvement of indoor environment, improvement of heating system, investment guidance effect, promotion of relevant industries and increasing chances of employment were concluded. Valuable experience that acquired from the retrofit project during the period of 11th five-year plan was also summarized in this article. By analyzing the main problems emerged in the past, pertinent suggestions were put forward to promote a larger scale and more efficient retrofit project in the period of 12th five-year plan.     

APPLICATION OF HEAT PUMP SYSTEMS FOR ENERGY CONSERVATION IN PAPER DRYING

Drying is one of the most energy intensive and common operations in the chemical and process industries. Scope for energy recovery is substantial, particularly from the latent heat of the exhaust moist air. Using real operating data from a major Swedish mill, optimal energy conservation strategies were investigated using different heat pump systems in paper drying. Simulation results are compared for compressor-driven and absorption heat pump systems. An absorption heat transformer was also investigated. A CH₃OH–LiBr double-lift cycle would have a low COP value due to the low temperature of the moist air stream and the restricted temperature of the cooling water available. A total of 30 MW thermal equivalent is currently needed in the mill at a temperature of 75°C for mixing-pits, district heating and a log store. Exhaust humid air at a temperature of 54°C from only three of the paper machines was used in this study. SHPUMP simulations revealed that installing a mechanical heat pump unit using HFC 134a would result in a recovery of 22 MW due to the temperature level of this application. On the other hand, 12 MW can be recovered with an absorption heat pump. To optimize the operating conditions, H₂O–NaOH was selected as the best of three based on exergy index criteria. Assuming a steam cost of 22 $/MW h and an electricity cost of 32 $/MW h, the pay-off periods would be 3·3 and 2·9 years for compressor-driven and absorption heat pump alternatives, respectively. © 1997 by John Wiley & Sons, Ltd.     

Thermal energy recovery of air conditioning system––heat recovery system calculation and phase change materials development

Latent heat thermal energy storage systems can be used to recover the rejected heat from air conditioning systems, which can be used to generate low-temperature hot water. It decreases not only the consumption of primary energy for heating domestic hot water but also the calefaction to the surroundings due to the rejection of heat from air conditioning systems. A recovery system using phase change materials (PCMs) to store the rejected (sensible and condensation) heat from air conditioning system has been developed and studied, making up the shortage of other sensible heat storage system. Also, PCMs compliant for heat recovery of air conditioning system should be developed. Technical grade paraffin wax has been discussed in this paper in order to develop a paraffin wax based PCM for the recovery of rejected heat from air conditioning systems. The thermal properties of technical grade paraffin wax and the mixtures of paraffin wax with lauric acid and with liquid paraffin (paraffin oil) are investigated and discussed, including volume expansion during the phase change process, the freezing point and the heat of fusion.     

Heating energy savings potential from retrofitting old apartments with an advanced double-skin façade system in cold climate

Apartments account for over 60% of total residential buildings and consume a significant portion of primary energy in South Korea. Various energy efficiency measures have been implemented for both new apartment constructions and existing apartment retrofits. Old apartment structures have poor thermal performances, resulting in a high energy consumption. The South Korean government initiated retrofitting projects to improve the energy efficiency in old apartments. Apartment owners typically replace old windows with high-performance windows; however, there is still a demand for better and more innovative retrofit methods for a highly improved energy efficiency. This paper proposes an advanced double-skin façade (DSF) system to replace existing balcony windows in old apartments. Considering the cold climate conditions of Seoul, South Korea, it mainly discusses heating energy savings. Three case models were developed: Base-Case with existing apartment, Case-1 with typical retrofitting, and Case-2 with the proposed DSF system. The EnergyPlus simulation program was used to develop simulation models for a floor radiant heating system. A typical gas boiler was selected for low-temperature radiant system modeling. The air flow network method was used to model the proposed DSF system. Five heating months, i.e., November to March, and one representative day, i.e., January 24, were selected for detailed analysis. The main heat loss areas consist of windows, walls, and infiltration. The results reveal that the apartment with the DSF retrofit saves 38.8% on the annual heating energy compared to the Base-Case and 35.2% compared to Case-1.     

Studies on the retrofit of heat exchanger network based on the hybrid genetic algorithm

The retrofit of heat exchanger networks (HENs) is an important branch of investigation for systematic heat integration. The studies on the economical and efficient retrofit techniques are very important for the high energy-consumption enterprises to save energy, protect environment and improve their market competitiveness. Because the retrofit of HEN is an optimization problem normally solved by a mixed integer nonlinear program (MINLP) which requires enormous solution space, it is very difficult to solve it with the traditional optimization methods. In this paper, by the analysis of an existing heat exchanger network, the hybrid genetic algorithm is applied to obtain the optimal retrofitted HEN with full utilization of the existing heat exchangers and structures. Two examples are taken to show the better effect of the retrofit method with the optimal new heat exchangers and re-piping cost and energy saving.     

Heat integrated heat pumping for biomass gasification processing

The main part of this paper is an industrial case study. It deals with an application of a heat pump in energy systems for biomass gasification in a wood processing plant. Process integration methodology is applied to deal with complex design interactions as many streams requiring heating and cooling are involved in the energy recovery. A refrigeration cycle maintains low temperature in the scrubber where the production gas (or synthesis gas–syngas) is cooled and undesirable contaminants are removed before the syngas is introduced into the engine. In addition to electricity generation, a large amount of waste heat is available in the biomass gasification system studied in the paper, and its appropriate heat integration with utility systems within a plant allows the available heat to be efficiently utilized for the site. The conceptual understanding gained from the case study provides systematic design guidelines for further process development and industrial implementation in practice.     

Performance of ejector heat pumps

An ejector-compression heat pump can use low-grade thermal energy in the neighbourhood of 93.3°C (200°F) to provide space cooling and heating. This paper applies the existing ejector theory to estimate the performance of an ejector heat pump system at various operating conditions. The study includes parametric, sensitivity and off-design analyses of the heat pump performance. The performance enhancement options and desired ejector geometry are also examined. Refrigerants 11, 113 and 114 are three of the halocarbons most suitable for the ejector heat pump system. The estimated coefficients of performance for a simple ejector heat pump are 0.3 for the cooling mode and 1.3 for the heating mode at a sample operating condition in which the refrigerant (R-11) boiling temperature is 93.3°C (200°F), condensing temperature 43.3°C (110°F) and evaporating temperature 10°C (50°F). A 24 per cent performance improvement is predicted for a heat pump with two-stage ejectors and regenerative heat exchangers. The off-design performance is relatively insensitive to the evaporator temperature variations.     

Heat integration retrofit analysis of a heat exchanger network of a fluid catalytic cracking plant

The impact of a process system on environmental pollution has both a local and global effect. The performance of the heat exchanger network (HEN) in a plant is an important aspect of energy conservation. Pinch technology and its recent extensions offer an effective and practical method for designing the HEN for new and retrofit projects.The fluid catalytic cracking (FCC) is a dominant process in oil refineries and there has been a sustained effort to improve the efficiency and yield of the unit over the years. Nevertheless, benefits and scope for improvement can still be found. The HEN of the FCC process considered here consists of a main column and a gas concentration section. Appropriate data were extracted from the existing network, using flowsheeting simulation. The stream data consists of 23 hot and 11 cold streams and cost and economic data required for the analysis were specified. The incremental area efficiency methodology was used for the targeting stage of the design and the design was carried out using the network pinch method consisting of both a diagnosis and optimisation stage. In the diagnosis stage promising designs were generated using UMIST developed sprint software. The generated design was then optimised to trade-off capital cost and energy savings. The design options were compared and evaluated and the retrofit design suggested.The existing hot utility consumption of the process was 46.055 MW with a ΔT_min of 24°C. The area efficiency of existing design was 0.805. The targeting stage using incremental area efficiency sets the minimum approach temperature at 11.5°C, thereby establishing the scope for potential energy savings. To achieve a practical project, the number of modifications is limited. The selected retrofit design has 8.955 MW saving – 74% of the whole scope. This corresponds to 27% utility cost savings with a payback period of 1.5 years. The modifications include addition of four heat exchanger units and repiping of one existing exchanger.     

Future use of heat pumps in Swedish district heating systems: Short- and long-term impact of policy instruments and planned investments

Heat pumps constitute one of the major technologies used in district heating systems in Sweden. Totally about 6 TWh of heat are supplied annually by heat pumps, equivalent to 12% of the heat supplied in district heating systems. New policy instruments that have recently been introduced will change the conditions for technologies in the district heating systems. It is likely that the incentives for waste incineration and combined heat and power will be improved. This study estimates how different policy instruments, and new investments in waste incineration and combined heat and power, affect heat pumps in Swedish district heating systems. The results indicate that heat pumps are affected in both a short-term and a long-term perspective, and that heat pumps will play a less important role in district heating systems in the future. Depending on the policy instruments applied in the district heating sector, the long-term use is between 18% and 71% lower than current use. In a long-term perspective, it is in the systems which currently use heat pumps during a large part of the year that new investments in waste incineration and combined heat and power can be expected, resulting in a convergence between different district heating systems regarding how much heat is supplied by combined heat and power and waste incineration, and regarding the annual operating hours for heat pumps.     

Analysis of a new drying chamber for heat pump mint leaves dryer

Drying is an energy intensive and time consuming process, so reducing amount of demanded energy and drying time are important issues for drying technology. The main aim of this paper is to analyze the drying characteristics of mint leaves in a new cylindrical form of drying chamber at low drying air temperature and by emphasizing on energy analysis. The dryer consists of air source heat pump system, air to air heat recovery unit and proportional temperature controller. Experiments were performed at 2, 2.5 and 3 m/s air velocities and at 35 °C cabin inlet air temperature. Mint leaves were dried from 9 g water/g dry matter to 0.1 g water/g dry matter. Designed drying chamber, with three stainless steel cylinders in circular nested form, has a positive effect for drying technology. This system has some advantages such as: drying of product by accessing a uniform air flow and preventing spread of light weight samples like mint leaves over drying system. Calculations based on experimental data show that in the best case, by consuming 3.164 kWh energy in a heat pump with 3.94 coefficients of performance, 4.56 kWh energy had been gained by heat recovery unit. Average 48% of energy was saved by means of heat recovery unit. Effective moisture diffusivity values varied from 3.50E?11 to 5.88E?11 for mint leaves.