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.     

Retrofit of pressure drop constrained heat exchanger networks

The study of retrofit of heat exchanger networks is usually restricted to constant heat transfer coefficients, and pressure drop constraints due to the additional exchanger area is neglected. In this work, a new approach considering the distribution of heat transfer area and pressure drop in retrofit is presented. The problem is described as a non-linear model, and the additional area required for the new network condition and available pressure drop are estimated based on economical optimisation (or process requirements).     

Economic considerations and cost comparisons between the heat pumps and solar collectors for the application of plume control from wet cooling towers of commercial buildings

This communication presents a case study based on the economic considerations and comparisons between the heat pump and solar collector heating systems for the application and utility to control the visible plume from wet cooling towers of a huge commercial building in Hong Kong. A detail economic study for both cases, i.e. for heat pumps as well as for solar collectors is done and compared using different (capital and operational) costs, taking other constraints into account. The capital cost is the actual cost of the device, for example, for a heat pump it is the cost of the heat pump machine. For a solar collector it is the cost of all the components like the collector, pipes, pump, heat exchanger, etc. On the other hand, the operational cost is the cost that keeps the system working in good condition. For a heat pump, the cost of the input power to the compressor is the running cost, while the necessary maintenance and replacement of parts comes under other cost. Similarly, for a solar collector, the cost of the power consumed by the pump/compressor to circulate the working fluid is the running cost which is very less as compared to the former. It is found that all the costs are much lesser for a solar collector system while it is reverse in the case of an air-cooled geothermal heat pump system. Other comparisons between the electric and geothermal heat pump systems are also given among different possible options.     

Heat Transfer Enhancement for Heat Exchanger Network Retrofit

Heat exchanger networks (HEN) play important roles in a chemical plant. In a plant lifetime, it may be required to retrofit a HEN several times in order to improve the energy efficiency or to accommodate the increase in throughput. The network pinch method developed by Asante and Zhu [1] can identify bottlenecks, which limit the increase in heat recovery for an existing HEN and also indicate promising structure changes to overcome the bottlenecks. As a result of HEN retrofit, additional surface area is required for some heat exchangers. There are a number of options to provide additional area, such as installing new shells or new units, adding new tubes to an existing bundle, etc. If heat transfer enhancement (HTE) is applied, additional area can be reduced significantly. This can result in a great reduction in capital cost and implementation time for modifications. However, in practice, heat transfer enhancement techniques have not been applied extensively, particularly in the petroleum refining industry. Several main aspects need to be addressed when HTE is taken into consideration for HEN retrofit. The first is how to determine which heat exchangers are suitable to apply HTE in the network and the second issue is to determine what level of augmentation of heat transfer performances is required. The last is about how to select a particular enhancement technique that can fulfil the enhancement requirement. A new strategy for applying HTE in HEN retrofit at the conceptual design stage has been developed. The above issues can be addressed properly by this new method. The new procedure is demonstrated using a case study.     

Paths combination for HENs retrofit

With the significant increase in fuel price, energy conservation projects should be reviewed and analyzed properly. Retrofit of heat exchanger networks (HENs) is among the common projects to reduce the plant operational cost. This paper introduced a new procedure using the path analysis approach for HEN retrofit. The developed procedure attempts at generating options for retrofit solution. These options are created by combining the available utility paths in HEN systematically. Instead of relying on a single path, the heat load could be shifted from HEN utilities using a set of paths simultaneously. To ensure feasible heat transfer between the hot and cold streams throughout, Exchanger Minimum Approach Temperature (EMAT) is maintained while shifting the heat load. The available exchangers’ pressure drop is considered in calculating the heat transfer coefficients. HEN devices are subjected to some additional area without any topological modification. In addition, the investment of such area could be recovered in a short span of time. The calculation of this approach encountered some iteration which has been overcome by mathematical programming. Demonstration example showed some options to be reasonable retrofit solutions. Energy savings ranged between $150 K and $450 K per year with payback time of less than 2 years were possible.     

Recent research development of process integration in analysis and optimisation of energy systems

The design of energy systems in a process plant requires a good understanding of each subsystem (e.g. processes, heat exchanger networks, utility systems) and their interactions in the context of an overall plant. An effective design method should be able to explore the synergy between the subsystems to the maximum extent and allow users to interact with the design process. To achieve this, the effective way is to combine physical insights with mathematical optimisation techniques. Physical insights are used as a wise man’s brain and eyes, while optimisation techniques are employed as a superman’s power in searching for optimal solutions. In the past, concepts and methods have been developed for handling grassroots design, operational management, retrofit and debottlenecking scenarios. This paper describes the recent research progress at UMIST in developing fundamental concepts and methodologies for analysis and optimisation of energy systems.     

Recent development in the retrofit of heat exchanger networks

This work presents a methodology for heat exchanger network (HEN) retrofit, which is applicable to complex industrial revamps, considering existing networks and constraining the number of modifications. The network pinch approach [N.D.K. Asante, X.X. Zhu, An automated approach for heat exchanger network retrofit featuring minimal topology modifications, Comp. Chem. Eng. 20 (1996) S7–S12.] has been modified and extended to apply to the HEN design in which the thermal properties of streams are temperature-dependent. The modified network pinch approach combines structural modifications and cost optimisation in a single step to avoid missing cost-effective design solutions.     

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.     

相变储热预热式热泵热水器系统性能研究

对集储热、预热与放热于一体的相变储热预热式热泵热水器系统的工作性能进行了研究,并对储、放热过程中相变材料、制冷介质的温度变化及压缩机的功耗情况进行了分析,该储热式热泵热水器系统采用分段加热,利用储能材料对水的预热作用,减小水与系统工作介质之间的换热温差,降低压缩机的功率消耗,使系统COP提高,若储热阶段结合谷电的应用,系统节能效果将更加显著,该试验研究为此类热泵热水器的理论模拟及性能优化奠定了基础.     

Operation Characteristics of Heat Pump Systems With Ground Heat Exchangers

This article examines the merits of heating systems with a vapor compressor heat pump unit for small residential homes. In this case, the ground is assumed to serve as the low heat source, and the ground heat exchanger may be horizontal or vertical in form. A mathematical model for all vapor compressor heat pump (VCHP)–ground heat exchanger (GHE) systems is briefly presented. The model consists of two elements. The first contains the calculation model of the ground heat exchanger with the adjoining area of the ground. The equations for the elements of the VCHP, supplemented by the state equations for the working medium, form the second element of the general model. This model was used to perform thermal calculations for typical ground and VCHP parameters. The cumulative energy and ecological results of the application of VCHP–GHE systems were also estimated.     

Detailed equipment design in heat exchanger networks synthesis and optimisation

In heat exchanger network synthesis, important features like pressure drop and fouling effects are usually neglected. In this work a new methodology is proposed to include these effects in grassroots as in retrofit designs. Heat exchangers are detailed designed during the heat exchanger network synthesis. Pinch analysis is used to obtain the heat exchangers network with the maximum energy recovery, and a new systematic procedure is proposed to the identification and loop breaking. Bell–Delaware method for the shell side is used to design the heat exchangers. An example of the literature was studied and the results show differences between heat exchangers, with and without the detailed design, relative to heat transfer area, fouling and pressure drop. The great contribution of this work is that individual and global heat transfer coefficients are always calculated, in despite of the current literature, where these value are assumed in the design step. Moreover, the methodology proposed to the heat exchangers design assures the minor heat exchanger according to TEMA standards, contributing to the minimisation of the heat exchanger network global annual cost. Finely, the new heat exchanger network considering pressure drops and fouling effects presents values more realistic then those one neglecting the equipment detailed design.     

供暖用CO2空气源热泵变频运行性能研究

采用压缩机变频、设置回热器与气液分离器辅助加热等技术途径,设计与构建一种供暖用CO2空气源热泵系统。在此基础上,建立响应面模型对供暖用CO2空气源热泵的压缩机运行频率进行优化,以提高供暖用CO2空气源热泵的低温性能。响应曲面法分析结果表明,低温环境下压缩机合理升频运行可有效提高供暖用CO2空气源热泵制热量,虽压缩比增大,但仍能保证压缩机稳定运行。为提高供暖用CO2空气源热泵的性能系数(COP),在低温环境下压缩机可分段变频运行。当环境温度依次为-5、-10及-15℃时,COP最大时对应的压缩机运行频率分别为55、58及60 Hz。     

一种新的换热网络改造方法探析——基于原有网络拓扑结构和换热器的最优应用

通过对原有换热网络的拓扑结构和换热器匹配进行分析,在有分流的分级超结构物理模型的基础上,提出了一种新的可满足一般技改要求的换热网络改造方法.该方法能够在对原有换热网络的拓扑结构和原有换热器面积充分利用并满足冷热流股进出口温度需求的同时,实现对原有换热网络的节能改造.同时建立了换热网络改造的数学模型,并应用混合遗传算法对其进行了求解.实例分析结果表明,该方法具有操作简单、节能潜力大以及投资回收期短等优点.     

基于压降二次分配的换热网络综合研究

在换热网络的综合过程中,依据单体换热设备的换热面积,进行压降约束的初步线性分配,根据单体换热设备的详细设计结果,按流体流动方向,对压降约束进行再次分配,并校核流股总压降是否在允许的范围内,给出了考虑压降分配前提下的换热器详细设计流程框图,算例结果表明,依此方法综合出的新的能量系统与现场能量系统相比节省换热面积30．87％,系统压降总和降低12．66％。     

Defrost improvement by heat pump refrigerant charge compensating

During winters, the air-source heat pump often operates with substantial frost formation on the outdoor heat exchanger, and the frost layer has to be melted away periodically to keep a high heat pump coefficient of performance (COP). Otherwise, the unmelted frost layer and water will become high density frost or ice layer in heating mode. However, it is difficult to melt the frost layer in the defrosting cycle, where the effective defrosting time plays an important role in improving the defrosting ability. Generally, the defrosting time can be decreased by the following ways: increasing the refrigerant flow rate effectively, and rapidly establishing the suction pressure, discharge pressure, and the compressor power. A new heat pump defrost system with a refrigerant charge compensator, instead of the accumulator which is a key component for the frosting cycle performance, is developed in this paper. Furthermore, test results showed that the improved frost system with the compensator worked as expected, and its suction and discharge pressures and the power of the compressor during the defrosting were much larger than before.     

Defrosting method adopting dual hot gas bypass for an air-to-air heat pump

A novel dual hot gas bypass defrosting (DHBD) method is developed to remove frost from the outside heat exchanger (HEX) of an air-to-air heat pump. The proposed method adopts two bypass lines of hot gas from the compressor: one is connected to the inlet of the outdoor HEX, and the other is connected to the outlet of the exchanger. We compare the dynamic performance and defrosting time of the conventional reverse cycle defrosting (RCD), hot gas bypass cycle defrosting (HGBD), and DHBD methods using a medium air-to-air 16 kW heat pump. The salient feature of the DHBD method is its ability to prevent a sharp decrease in the compressor outlet temperature at the melting frost stage after the HGBD process begins. Due to the additional bypass, the DHBD method sustained a higher compressor outlet pressure and reduced the defrosting time by 36% compared to the HGBD method. Compared to RCD, the defrosting time was comparable (126%); however, the amenity characteristics of the DHBD method were superior than those of the RCD method. The proposed DHBD method can overcome the main disadvantages of the RCD and HGBD methods, and showed excellent performance for an air-to-air heat pump in a defrosting operation.     

Application for pinch design of heat exchanger networks by use of a computer code employing an improved problem algorithm table

In this work, the methods used in pinch design were applied to a heat exchanger network with the aid of an improved problem algorithm table. This table enables one to compose composite and grand composite curves in a simplified way. A user friendly computer code entitled DarboTEK, compiled by using Visual Basic 3.0, was developed for the design of integrated heat exchanger networks and estimation of related capital costs. Based on the data obtained from the TÜPRA petroleum refinery at Izmit, a retrofit design of heat exchanger networks was accomplished using DarboTEK. An investment of $ 3,576,627 is needed which will be paid back in 1.69 years simply by energy conservation due to heat integration.     

Cost estimation and energy price forecasts for economic evaluation of retrofit projects

A cost estimate can have a major impact both on project profitability and influences the technical solution. This leads to a commitment of substantial amounts of money and manpower over an extended time. Various methods often provide different results. These differences can be rather significant towards the real cost of a project and selection of the right arrangements. It is therefore important to use a proper estimate that generates enough confidence to choose the right alternative.Another source of uncertainty during the economic analysis of design and retrofit projects is the future energy price value as part of operating costs. This value can significantly affect the project viability and profitability.This paper gives a summary of the most common methods used for cost estimation of heat exchange equipment in the process industry and the sources of energy price projections. It shows the relevance of the choice of the right method and the most reliable source of energy price forecast used when choosing between alternative retrofit projects or when trying to determine the viability of a retrofit project.Ten methods for heat exchanger costing procedure are considered. Oil, natural gas and electricity price projection figures published in reports of nine established analytical centres are discussed. Web sites of institutions and companies are also included.     

考虑压降的换热网络优化设计

提出考虑压降的有分流换热网络合成的优化策略,在分级超结构的基础上,建立换热网络同步综合的MINLP数学模型,以年度总费用最小为优化目标,采用改进粒子群算法对模型求解,并通过两个算例验证了模型和方法的有效性。     

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。