Plate Fin Heat Exchangers

These heat exchangers are widely used in cryogenic application because of their low cost, small size, low weight, high thermal capacity and effectiveness relative to other types of heat exchangers. The result of the improved effectiveness is the achievement of true counter current flow where there is an increase in the temperature spread and a closer approach to ideality. This means that the refrigerant cooling curve is closer to the natural gas cooling curve. 
The exchanger is made up of manifolds or headers which consist of elements. A manifold and an element are shown below, Figures 4 and 5 respectively. An element is made up of a corrugated die-formed fin plate placed between flat metal separator plates. There are side bars along the outside of the fin sections. A stack of the elements is welded to form a rigid matrix and can be designed to meet any configuration and size. The stacks are welded onto the manifolds. Depending on the application, a number of manifolds can be assembled to form the heat exchanger.

 

Manifold or Header (xchanger.com)

An element

The wavy configuration of the fin promotes turbulence and therefore improves heat transfer. This increase in heat transfer is accompanied by an increase in pressure drop. This is a problem with low density fluids like gases because of the extra work required to surmount the pressure drop. 

Fin Configurations

This work is often much higher than the increase in heat transfer acquired from the fins. For applications where it is proposed any fin configuration other than the simplest, a thorough analysis of the effect on the system should be conducted.  

In designing the plate fin heat exchanger, it is possible to have different heights of the alternating fin plates. There is no requirement to have the same height or spacing of separator plates. This is a useful freedom to have in situations where the difference in density of the hot and cold fluids is large. In cryogenic systems, the refrigerant stream entering the expander has a higher density than the stream coming out of the expander. In such a case as this, it is necessary to use a larger height for the lower density stream so that a common Reynolds number and therefore heat transfer coefficient, U, can be attained.

Aluminum is the usual material of construction of plate fin heat exchangers for cryogenic applications. (Walker, 104-110)

Source:
Evaluation of LNG Technologies (by Valerie Rivera, Ayema Aduku, Oluwaseun Harris)
April 30, 2008