Compact heat Exchanger
Generally, compact heat exchangers are used in heat integrating processes and provide advantages over shell-and-tube heat exchangers, such as compactness, large surface area per volume ratio, low temperature differential, and can be applied as MHEX.
Compact plate-fin heat exchangers are one type of compact exchanger, and they are normally used in MHEX systems because they have the ability to handle a large number of hot and cold streams in the same unit. However, the MHEX structure is complex because it contains many channels for a number of hot and cold streams to flow through an interaction between them.
Picon-Nunez et al. (1999) demonstrated a methodology for designing compact plate-fin heat exchangers with the objective of minimising pressure drop. A thermo-hydraulic model was developed and the volume performance index (VPI) corresponding to Reynolds number was used to select the best fin surface.
According to Reynolds number assumptions, fin selection could not provide effective results. Picon-Nunez et al. (2002) then proposed the thermal design of a multi-stream plate-fin heat exchanger. To analyse the relationship between volume, heat transfer coefficients, and pressure drops, they developed composite curves related to enthalpy intervals and used a thermo-hydraulic model. A new design algorithm for counter-current plate-fin heat exchangers was proposed by Guo et al.
(2015). Basic fin geometries were treated as continuous variables to reduce computational time compared to the discrete design problem from previous work, and thermal hydraulic performance was applied together with it to minimise the total volume.
In this study, we present a methodology for designing multi-stream plate-fin heat exchangers by considering pressure drop utilisation, optimising basic fin geometry and minimising the total volume of the exchanger by using uniform block heights and widths across all block sections.
In the chemical process industry, heat exchangers are used for both heating and cooling. CHEs are designed for efficient heat transfer from one medium to another, with large heat transfer areas-to-volume ratios (minimum 300 m2/m3), high heat transfer coefficients (up to 5000 W/m2 K), small flow passages, and laminar flow.
CHEsare often used to achieve large heat rates per unit volume, especially when phase, composition, temperature, pressures, density, viscosity and other physical properties (Shah et al., 1990). A section on compact and non-tubular heat exchangers can be found in the 8th edition of Perry’s Chemical Engineers’ Handbook (Green and Perry, 2008).
The following are the most common types of CHE:
Plate heat exchangers (PHE) use metal plates to transfer heat between two fluids over a much larger surface area. The thin, corrugated plates used in PHE are either brazed, welded or gasketed together depending on the application. As a result of the compression of the plates in a rigid frame, parallel flow channels with alternating hot and cold fluids are formed. Adding or removing plates from the stack increases and decreases heat transfer area.
Plate-fin heat exchangers (PFHE) transfer heat between fluids using finned chambers and plates. In a PFHE, corrugated sheets are separated by flat metal plates which form a series of finned chambers. A series of hot and cold fluid streams flow through alternating layers of the HE and are enclosed by side bars. Additionally, the fins increase the structural strength of the PFHE, allowing it to withstand high pressures while providing extended heat transfer areas.
In addition to gas and liquid, PFHE can operate with any combination of two-phase or three-phase fluids. Moreover, it can accommodate the heat transfer between multiple process streams by using a variety of fin heights and types as different entry and exit points for each flow.
There are four main types of fins: plain (triangular or rectangular simple straight fins), herringbone (fins placed sideways to form a zig-zag path), serrated and perforated (cuts and perforations to reduce drag and enhance heat transfer). The proneness to fouling and difficulty of mechanical cleaning are two disadvantages of PFHE.
Spiral heat exchangers (SHE) are composed of two flat surfaces that are coiled together in a counter-flow arrangement, such as helical/coiled tubes. The space she uses is highly efficient, so she has a small footprint and low capital costs. She is commonly used for handling slurries. SHEs use three main types of flow patterns: (1) spiral-spiral flow used for all heating and cooling services, (2) spiral-cross flow (one fluid is in spiral flow while the other is in cross flow) used for condensers and reboilers, and (3) distributed vapour-spiral flow that can condense and subcool in the same unit.
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