Cryogenic Heat Exchange
For those less familiar with the terminology and concepts of cryogenics and cryogenic heat transfer or heat exchangers this article provides some background and context.
Overview
Cryogenic processes refers to those which at least partly operate at temperatures well below normal ambient temperature (about 15°C or 288K). An arbitrary criterion is below minus 50°C but below minus 100°C is also a common definition. These low temperatures are required to liquefy and or separate what used to be considered permanent gases - such as oxygen, nitrogen, argon, carbon monoxide, neon, hydrogen, helium and some light hydrocarbons, methane, ethane or ethylene. Liquid carbon dioxide can be legitimately included as many of the cryogenic technologies apply.
Thermodynamics and Energy demand
The branch of physics known as thermodynamics (basically meaning heat and energy) established some fundamental facts and rules during the past 200 years. One is the fairly obvious fact that heat flows ‘downhill’ from higher to a lower temperature, whereas the reverse cannot occur.
To achieve or maintain an operation under cryogenic conditions requires a continuous provision of refrigeration. This is commonly called ‘cold production’, cold being heat below ambient.
The minimum energy required to produce this refrigeration increases quite dramatically as the temperature is reduced towards absolute zero ( 0 degrees Kelvin or - 273.15°C).
Figure 1 - Minimum energy for refrigeration versus absolute temperature
Such low temperatures are approached when liquefying the smallest molecules helium and hydrogen at about 4K and 20K. To liquefy nitrogen at about 77K (-196C) clearly requires relatively less energy.
Obtaining and maintaining cryogenic conditions needs efficient thermal insulation to minimise the parasitic inleak of heat from the environment. Heat ingress is proportional to the size / external surface area of equipment. To minimise equipment size dictates efficient equipment for heat transfer between the process fluids.
In the context of cryogenic processes, it is also not hard to see that the temperature differences between the cooled and the coolant (refrigerant) streams must always be positive but also it should be kept as small as economics permit, to reduce the energy needed to provide the refrigeration.
There is a compromise between small temperature differences and small compact equipment. It can be shown that the optimum temperature difference decreases from several degrees at ambient to a fraction of a degree as liquid helium temperatures are approached. For more detail - see the article on ‘Exergy’.
Heat Transfer Equipment
These considerations led to development over the past 70 or so years of very efficient compact heat exchangers of which the brazed aluminium plate-fin exchangers have become the mainstay type.
Their basic construction is a stacked sandwich of layers between which the two or more process fluids pass. Each layer comprises two rectangular flat aluminium plates about 1mm thick and 5-7 mm apart. Between each pair of plates, called parting sheets sits a corrugated aluminium layer (0.2-0.3 mm thick) which is brazed to the plates where the corrugations are in contact with them. The un-brazed parts of the corrugations that are normal to the parting sheets are termed fins.
fig 2 - Enlarged view of channels with corrugations
The fins provide extra heat transfer surface area and are a critical part of the high efficiency of these exchangers.
Fig 3 - Enlarged view of a section of corrugation fins
The brazing takes place in a special vacuum furnace, at high temperature just below aluminium’s melting point. After brazing, header tanks and nozzles are welded to the block. These will distribute the process streams into and out of the block.
Fig 4 - Plate-fin block showing main components (Courtesy - Guide to Compact Heat exchangers - Energy Efficiency Office, DOE)
The two or more warming and cooling process streams respectively occupy alternating layers, configured and arranged in a pattern to match the streams heat transfer duty. One heat exchanger block can contain up to a few hundred layers. Large plants use multiple blocks piped up in parallel.
Fig 5 - Eight plate-fin exchanger blocks or cores mounted in parallel.
Types of corrugation
The performance of the basic plain corrugated fin is frequently enhanced in several ways:
· by perforating the flat sheets (about 5% open) before the sheets are corrugated and assembled in the block
· creating wavy fins (also called herringbone)
· creating serrated fins (also called lanced or multi-entry)
Fig 6 - Fin types: a) Plain, b) Perforated, c) Serrated, d) Herringbone
Compared to the plain fin each of these increases the turbulence of the flowing stream and therefore increases the local film heat transfer coefficient (reduces the resistance), but at the expense of increased frictional pressure drop.
The relative pressure drop, and heat transfer of fin types vary as follows:
Serrated > Wavy > Perforated > Plain.
Heat transfer and pressure drop performance for the fin types is often represented as a function of Reynolds number. Most of this information is proprietary to the exchanger manufacturers.
Cross flow arrangement.
The warming and cooling streams normally flow counter-current to each other, but a cross flow arrangement is sometimes preferred for example in sub-coolers where one or more liquid streams are subcooled against a flow of cold low-pressure gas.
Fig 5 - A small two-stream cross flow unit
Summary
The process and mechanical design and manufacture of brazed aluminium plate fin exchangers requires special expertise and facilities including vacuum brazing furnaces held by a limited number of organisations. These companies are in general members of ALPEMA (The Brazed Aluminium Plate-Fin Exchanger Manufacturers’ Association). The ALPEMA Guide contains comprehensive literature on all aspects of plate-fin exchanger design, manufacture and operation.
Another useful reference is ‘Plate-Fin Exchangers – Guide to their Specification and Use’. Ed. M.A. Taylor.
Further articles will discuss some aspects of the design of cryogenic plate-fin exchangers in different duties.