Plate heat exchangers

A plate heat exchanger, PHE, is a compact heat exchanger where thin corrugated plates (some 0.5 mm thick, bended 1 or 2 mm) are stacked in contact with each other, and the two fluids made to flow separately along adjacent channels in the corrugation (Fig). 

PHE Plates

The closure of the staked plates may be by clamped gaskets, brazing (usually copper-brazed stainless steel), or welding (stainless steel, copper, titanium), the most common type being the first, for ease of inspection and cleaning. Additionally, a frame (end-plates and fixing rods) secures together the plate stack and connectors (sometimes PFHE, standing for plate-and-frame heat exchanger, is used instead of PHE).

Plate assembly is sketched in Fig. 2. Suitable channels, sometimes helped by the gaskets, control the flow of the two fluids, and allow parallel flow or cross flow, in any desired number of passes, one pass being most used. 

They have large conductance coefficients (up to K=6000 W/(m2·K) for liquid-to-liquid use), are ideally suited for low-viscosity fluids, the number of plates can be adjusted to the needs, and the transfer surface accessible to cleaning (the latter two advantages only for gasket assemblies; in any case, the gaskets should be changed if dismounted). 

The projected area of plates is usually taken as nominal heat transfer area, in spite of the real curved surfaces and lost space in gaskets and ports.

Plate heat exchanger flow pattern

Major limitations in PHEs are: maximum allowed pressure (usually below 1 MPa, although there are designs with 4 MPa), temperature range (usually limited to 150 ºC by the gasket material, although there are designs allowing 400 ºC), and prize (but brazed PHEs are about half prize of serviceable PHEs). 

Although typical PHE application is in liquid-to-liquid heat-transfer, special plate designs have been developed for phase-change applications. Higher working pressures and still goof thermal performance can be achieved with hybrid plate-shell heat exchangers, where a plate stack is welded inside a shell (i.e. a kind of STHE with plates instead of tubes).

The PHE was developed in the 1920s in the food industry (for the pasteurization of milk), but they are taking over all markets now because of its compactness and efficiency (3 to 10 times more than STHE).

They are used for process heating, cooling, in all cryogenic applications, and as an intermediate step in domestic water heaters, where consumable hot water (hot tap water) is produced in an intermediate heat exchanger from closed-loop fuel-fired hot water, to minimise solid depositions. 

PHE are often named CHE (compact heat exchangers), although the word compact can be added to any type of heat or mass transfer unit with specific area >103 m2/m3.

Thanks for reading - Plate heat exchangers
Naitik Patel
Industrial Guide

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