Heat exchangers - basics for efficient heat integration Introduction to modeling of heat exchangers

Heat exchangers - basics for efficient heat integration

If you are just a beginner in process simulation with the aim to get perspective on modeling of heat exchangers, a process engineer involved with energy efficiency project looking to find out more information about the calculation details, or you are involved with operational problems of your plant and would like to gain more information and be able to analyse and solve operational problem, this article might give you some insights on how to approach the heat exchanger analysis. 

Energy efficiency in heat exchanger design

Due to increased need for energy efficiency over the last years, more attention has been put tp the optimization of heat exchanger network. Heat exchangers in numerous engineering applications are just one of many components of a system. Thus, the design of a heat exchanger is inevitably influenced by system requirements and should be based on system optimization rather than component optimization. 
Heat exchanger network optimization is a subject of process design and the basis is done through steady state modeling and simulation. Influence of possible disturbances and control is analyzed through dynamic simulation. However, before a system-based optimization can be carried out, a good understanding of the exchanger as a component must be gained.

Steady state modelling

The steady state simulation is defining dependence of specific heat exchanger variables. For a given set of input data (e.g., flow rates and inlet temperatures), exchanger geometry, and other information, the output data (e.g., the outlet temperatures) will depend on heat transfer and fluid flow phenomena that take place within the boundaries of the heat exchanger. So even though one seeks a system optimum, in the process of determining that optimum, one must fully understand the features of the exchanger as a component.

The model of a heat exchanger in process simulation software may be used to heat or cool a single process stream, exchange heat between two process streams, or exchange heat between a process stream and a utility stream. Rigorous calculations may be performed for vapour-liquid systems. It is also possible to attach an exchanger to any of a distillation column and exchange heat between a process stream, either liquid or vapour.

Simulation in practice

When one is starting to build a process model that includes heat exchangers or HE networks, the general path is different in cases:

  • Designing a new section or unit,
  • Analyzing operating conditions of existing section or unit.

Both of these disciplines are important for process engineers when approaching improvements of heat transfer and estimating process and economic options and benefits.

Heat exchanger design

When developing a process model for a new section or unit, most important task is to define required heat transfer which is sufficient for a defined process. This is usually done through the steps of:

  • Definition of required inlet and/or outlet variables (flow, temperature, vapour fraction),
  • Estimation of pressure drop.

Inlet and/or outlet variables are defined by the heat exchange process itself and other operations of the system (columns, other heat exchangers, utilities etc.). Most process simulator programs indicate the necessary data which should be defined to solve the heat exchanger. The solution includes exchanged heat and definition of all process streams. 

The basic calculation becomes the inlet data for next step which defines:

  • Size,
  • Geometry,
  • Materials,
  • Temperature distributions,
  • Exact pressure drop.

Through this step, the optimal design is defined having in mind the purpose of the heat exchanger (cooler, heater, condenser, reboiler), required heat, type of fluids handled and economics. 
In addition to rating or sizing, this may include information about temperature distributions, local temperature differences, hot and cold spots, pressure drops, and sources of local irreversibility—all as functions of possible changes of design process variables and parameters.

For detailed design and calculations of heat exchangers, most process simulation software has their specialized tools for this purpose, such as: 
Specialised packages for implementing Pinch Point Analysis are available, as SUPERTARGET TM (Linnhoff/KBC), ASPEN Pinch TM, HEXTRANTM (Simsci). The synthesis of a heat exchanger network by mathematical programming may be handled by means of packages based on the generic environment GAMS TM: 

Heat exchanger in operation

When approaching operating condition analysis of the heat exchanger, the basis for the calculation is changed because the focus is put to variables that show unideal behavior and disturbing normal heat exchanger operation. Very often those analyses include corrosion effects, fouling, increased pressure drop or inadequate vapour-liquid distribution. The goal of this analysis is to recognize the difference of the heat exchanger operation in reality and the heat exchanger model. To be able to perform this analysis, the operations surrounding the heat exchanger should be modelled too.
Building a model for analysis of the heat exchanger in operation:

  • Definition of steady state conditions and gathering operational data from the plant and design data from the HE data sheet (HE surface, sizing, geometry)
  • Modelling of stranding operations and solving the heat exchanger by defining the conditions from the plant 
  • Analysis of parameters of the heat exchanger, such as fouling factors, heat transfer factors and finding the parameters which are different in heat exchanger operating conditions compared to the model.
  • Analyzing ideal pressure drop for the geometry, size and material compared to reality.

This analysis should give the answers about heat exchanger operation and decisions such as when to clean heat-transfer surfaces etc.

Design of a heat exchanger as a component is to a large extent an engineering art. So, despite high sophistication in heat exchanger thermal modelling, some of the final decisions (in particular those related to optimization) are based on qualitative judgments due to nonquantifiable variables associated with exchanger manufacturing and other evaluation criteria. Still, analytical modelling—a very valuable tool—is crucial to understand the relevant thermal–hydraulic phenomena and design options and various venues for design improvements.

Heat exchanger network optimization

In a typical process, we deal with many fluid streams to heat, cool, condense, vaporize, distill, concentrate, and so on. It takes a number of heat exchangers in a network to be able to heat, cool, or change of phase of the process streams with available utilities. This network is analysed based on the pinch analysis or pinch technology to ensure that all exchangers in a system meet the requirements of the process streams based on performance targets. 

With the currently available very sophisticated commercial software packages for optimization, the methodologies can be combined and the heat exchanger optimum dimensions and operating conditions can be obtained directly for an optimum system being characterized by the least cost, least energy consumption, or other set of criteria for optimization.