Vapour Condensers For Efficient Heat Recovery

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This time, I would like to talk some more about energy efficiency and heat recovery using the latest liquid heat exchangers and vapour condensers.  As you might already know if you are a regular visitor to this blog, I have a green streak running right through me in engineering and process terms.   Why apply heat using fossil fuels when you can have it for free from the sun from on high, the air all around us and in the ground beneath us?

I recognise that the process and manufacturing industries cannot just rely on ‘free’ energy but it is clear that they are moving towards ever greater efficiency by energy recycling with the use of heat exchanger equipment such as plate vapour condensers and evaporators, liquid plate heat exchangers.

So how do plate heat exchangers work?  In essence, it is passing two fluids over a common plate or pipe wall but on opposite sides.  The hotter liquid flow is cooled and the cooler one is warmed.   You can immediately see that the heat transfer for given flows is dependent on the conductivity of the plate or pipe wall, the temperature differential between flows, the contact time on the heat transfer surfaces and the mass density of both fluids.

The only ways to increase the efficiency of heat transfer for two given fluids, is to increase the contact area, increase contact time, to reduce the thickness of the transfer plate/pipe wall and to use the most thermally efficient material for the  transfer surface.   By far the most significant factor is the area of contact surface area followed by the contact time.

The traditional shell and tube heat exchangers, such as can still be found on on steam locomotives, are designed to produce steam under pressure and the spent steam (after driving the loco and operating the brakes) is used to heat the feed water in the loco’s tank.  There are limitations to the contact area possible this form of  vapour condenser, especially as one fluid is a gas with it’s low thermal conductivity and the other is a liquid.  Such heat recovery is not particularly efficient in such equipment.  Nonetheless, the shell and tube heat exchanger has its uses of large power plant where space is not at a premium and the fluid pressures are high.

1st Heat Exchanger invented by Gustav Laval

Gustav Laval is credited with the first plate heat exchanger in 1890 to cool milk and, in essence, the principles have not changed, only the variety of applications the humble plate heat exchanger is now put to and innovative design solutions.  By using a stack of welded (or brazed) profiled pressed steel or alloy plate cassettes connected by gaskets to form a series of connected alternate chambers with two entirely separated flows in close proximity, a much more efficient solution is possible.    What is more, of course, the thin and convoluted spaces ensure that the contact time is maximised and the thin pressed metal plates ensure that the maximum heat transfer occurs.  Thus heat exchanger equipment has a tiny footprint compared to other heat exchange methods.

The now ubiquitous plate heat exchanger can be found in food and beverage production, pharmaceutical and chemical industries as well as forming the backbone of many industrial and commercial heating systems.

Alfavap Plate Evaporator

One of the more recent innovations, is the vapour condenser and the similar evaporator that uses two fluid phases, gas and liquid.  The principles are exactly the same as the liquid-to-liquid exchangers and it is only the relative flow rates, enclosed space capacities and the flow paths that have been optimised for the two-phase exchangers.  The Afavap from Alfa Laval has been especially developed for evaporation and crystallisation processes.

These exchangers are available in exotic materials such as SMO, titanium, nickel and Hastelloy™ to ensure no chemical interaction occurs during the heat exchange process.  The illustration gives a good illustration on how these particular plate heat exchangers work.

Flexibility is the watchword for plate heat exchangers in that if the process needs adjusting, cassettes can be added or removed, gas pressures can  amendeded and they can easily be cleaned by clean-in-place (CIP) systems.

Heat recovery to reduce our reliance on fossil fuels is therefore being optimised by use of plate heat exchangers and vapour condensers.  We will, no doubt, be returning to this subject in future posts.

Arrividerci