Martín Picón-Núñez

Thermal design of multi-stream heat exchangers

Abstract The thermal design of multi-stream heat exchangers of the plate and fin type is presented. Although originally used in low temperature processes, their application is extrapolated to above temperature processes and it is shown that, conceptually, multi-stream exchangers could replace whole heat recovery networks. The approach is based on the use of temperature vs. enthalpy diagrams or composite curves, which show how a multi-stream exchanger can be subdivided into block sections that correspond to enthalpy intervals and indicate the entry and exit points of the streams. A design methodology for plate and fin exchangers in countercurrent arrangement, characterized by the maximization of allowable pressure as a design objective is extended to the design of multi-fluid exchangers. The methodology uses a thermo-hydraulic model which relates pressure drop, heat transfer coefficient and exchanger volume. The potential applicability of the methodology is demonstrated on a case study.

Surface selection and design of plate-fin heat exchangers

Abstract This paper presents a methodology for the design of compact plate–fin heat exchangers where full pressure drop utilization is taken as a design objective. The methodology is based on the development of a thermo-hydraulic model that represents the relationship between pressure drop, heat transfer coefficient and exchanger volume. A simple approach to surface selection based on the concept of volume performance index (VPI) is also presented. Surfaces that result in the smallest volume will exhibit higher VPI. Surfaces are compared on the basis of VPI and envelopes for best performance are produced. Simultaneous surface selection and design for full pressure drop utilization can be achieved by using envelopes for best surface performance together with the thermo-hydraulic model. Design algorithms for cross-flow and counter-flow arrangements are presented and results tested by comparing them with case studies from the literature.

Thermal integration of heat pumping systems in distillation columns

A methodology for the thermal integration of electrically driven heat pumping systems between intermediate stages in distillation columns using the concept of column grand composite curve is proposed. An optimization procedure based on a four way trade off between energy, capital, temperature lift and electricity to fuel cost ratio is discussed.

Flow passage arrangement and surface selection in multistream plate-fin heat exchangers

This paper presents a general design methodology for multistream plate-fin heat exchangers that incorporates the consideration of operability aspects through the manipulation of stream flow passage arrangement. The main features of the design approach are uniform heat load per passage and secondary surface or fin selection. Surface selection is implemented as a means to achieve uniformity in the heat transfer rate of the various streams that take part in the heat exchange process. Uniform heat load content per passage is a design consideration through which an equal number of hot and cold passages is achieved. Under these conditions, the number of passages allocated to a given stream is directly proportional to its heat capacity mass flow rate. A simple model for the steady-state simulation of multistream exchangers is also presented. This model can be used to determine the exchanger response to changes in temperature and flow rate that may take place during operation. Results indicate that flow passage arrangement is a design consideration that can be manipulated to reduce the effect of these types of disturbances upon the target temperatures of specific streams.

Alternative Design Approach for Plate and Frame Heat Exchangers Using Parameter Plots

The simultaneous design and specification of heat exchangers of the plate-and-frame type is analyzed. A pictorial representation of the design space is used to guide the designer toward the selection of the geometry that best meets the heat duty within the limitations of pressure drop. The design space is represented by a bar plot where the number of thermal plates is plotted for three conditions: (1) for fully meeting the required heat load, (2) for fully absorbing the allowable pressure drop in the cold stream, and (3) for fully absorbing the allowable pressure drop in the hot stream. This type of plot is suitable for representing the design space, given the discrete nature of the plate geometrical characteristics, such as effective plate length and plate width. Applications of the use of bypasses as a design strategy are also presented.

Alternative Sizing Methodology for Compact Heat Exchangers of the Spiral Type

A rapid sizing methodology for compact heat exchangers of the spiral plate type is presented. The methodology allows for the determination of the exchanger geometry such that full pressure drop utilization is achieved on both streams. This is done by considering plate width and plate spacing as continuous variables. The resulting values are the basis for selecting the final exchanger dimensions according to standard dimensions. The design approach makes use of empirical correlations for the calculation of heat transfer coefficient and friction factor based on average curvature. The approach is demonstrated using two case studies.

Alternative Design Approach for Multipass and Multi-Stream Plate Heat Exchangers for Use in Heat Recovery Systems

This work presents an alternative approach for the preliminary design of plate and frame heat exchangers in complex flow arrangements and looks at the use of this type of exchanger in multi-stream applications. The need for a new design approach arises from the fact that most current design algorithms are industrially owned and thus are not readily available to the practicing engineer. The essential information required for the design of plate and frame heat exchangers are the heat transfer and friction performance data, which again are industrially owned. However, a few correlations for the most common type of plates have been reported in the open literature and, in this work, are used for the development of a design approach that exploits the concept of full utilization of allowable pressure drop. This method can be used as a first estimate for the determination of the surface area requirements of single-phase, two-stream heat exchangers. It is shown how the approach can be extended to the targeting and design of heat recovery systems or multi-stream exchangers where plate heat exchangers are a suitable option.

Steady state simulation for the de-bottlenecking of heat recovery networks

Abstract This work presents the use of a steady state simulator for the de-bottlenecking of heat recovery networks. It is shown how a heat exchanger network designed for fixed conditions can be de-bottlenecked when process streams undergo changes in operating conditions such as flow rate and supply temperature. A network is said to be flexible if it is capable of maintaining acceptable operation either during normal or under modified conditions. The de-bottlenecking of heat recovery networks can be considered as a special case of the design for improved flexibility. A simulation model for a single phase network of heat exchangers is presented. The model is based on the use of the thermal effectiveness (e) parameter for heat exchangers. Any type of exchanger configuration and flow arrangement can be modeled by using the appropriate e–number of transfer units relationships. A general methodology for improving network flexibility is proposed.

Application of Process Integration Techniques for the Efficient Use of Energy in a Urea Fertiliser Plant: A Case Study

Abstract: This chapter presents a practical application of Process Integration (PI) techniques for the efficient use of energy in a fertiliser plant. The case under study is the CO 2 stripping process for the production of urea in an existing plant in Mexico. The retrofit analysis of an existing process can take two possible forms of action: the modification of the existing heat recovery structure for increased heat recovery, and the identification of the appropriate changes to operating conditions for more efficient energy utilisation. In this chapter we show that the practical information obtained through the application of PI techniques brings about unseen opportunities for avoiding unnecessary energy waste.

Process Integration Techniques for Cogeneration and Trigeneration Systems

Abstract: The production of various energy outputs and other useful secondary by-products from one or more energy inputs is known as polygeneration. The application of polygeneration schemes implies a high degree of Process Integration. This is particularly the case when various utility services are provided from a single source; for instance, when using a waste stream as fuel, energy can be extracted via combustion; then steam can be raised and used for process heating, power production and even process cooling. When only heating and power are produced for use within the site, the system is referred to as cogeneration; when cooling is additionally produced, the system is referred to as trigeneration. This chapter focuses on trigeneration systems, describes their main components, establishes the criteria for its appropriate selection and looks at the use of Process Integration techniques for the design of such systems in the context of background processes. The design of trigeneration systems requires the development of simple and reliable thermodynamic models for evaluating the thermal performance of prime movers at full and part load. Such models are necessary to evaluate operation in situations where plant throughput changes due to seasonal marked demands.