Cooling water requirements

A number of people have expressed concern about the volume of cooling water required to condense the vapour from a 750 watt heater operating over many hours. It is not all that great, but if water is scarce or expensive where you live you will be interested in the following calculations.

The calculations cannot be exact because there are many imponderables. For example, the temperature of the cooling water, the permitted rise of cooling water temperature, the desired drop in the temperature of condensed alcohol, the rate of heat transfer between the cooling water and the alcohol (affected by thermal conductivity of coil material, e.g. copper, stainless steel, glass, and the thickness of the coil walls), so please read the following with these things in mind.

We are going to assume the following: The cooling water enters the coil at 10°C. and leaves it at 30°C., a 20° rise in temperature. By increasing the flow of cooling water you could decrease this rise in temperature, and by accepting a greater temperature rise you could reduce the flow of water. We also assume that the alcohol vapour is condensed in the stillhead and, following condensation, is cooled from 78.1°C. to 68.1°C., a drop of 10°C., before withdrawal.

The cooling water in the stillhead is condensing 45 g/min of a 95% w/w alcohol-water mixture (see Appendix II). The latent heat of this mixture is such that 10,856 calories per minute of energy must be drained off by the cooling water. The latent heat of vaporization of the cooling water is not involved, only its sensible heat, and this is 1 calorie per gram per degree C., the specific heat of water. So, just to condense the vapour without changing its temperature we require 10860 grams of water per degree C. per minute. Let's call it 10 litres. The collection of alcohol from a particular run will occupy (let's say) 20 hours. So the number of litres of cooling water would be 20 x 60 x 10 litre = 12,000 litres. This is just to condense the alcohol, not cool it. If we decrease cooling water flow so that its temperature rises, not by 1o C. but by 20o C. then the volume of water would be reduced to 12,000 -f 20 = 600 litres.

You might wonder why the stillhead doesn't cool the alcohol to room temperature. It is a matter of experience that, using the type of stillhead with cooling coil described in this book the alcohol vapour condenses on the lower turns of the coil, turns into liquid, and immediately drops off, avoiding further cooling. It is so hot, in fact, that some people suggest cooling it further by having the condensed liquid flow through a secondary heat exchanger before dropping into the collection bottle. Otherwise, they say, a lot of alcohol will be lost by evaporation. There is some truth in this but we have found it sufficient to draw off the hot alcohol and let it fall through a copper tube before entering the collection bottle. In effect, this is an air-cooled condenser.

We have calculated that 600 litres of cooling water are required just to condense the vapour. Now let us assume that the condensed liquid, before dropping off the bottom turns of the cooling coil, is further reduced in temperature by 10°C., i.e. from 78.1°C. to 68.1°C. This will require additional cooling water as follows:

We are concerned here with, not latent heat of condensation but the specific heat of alcohol. This varies a bit with temperature but is about 0.6 calories per gram per degree C. So the number of calories to be withdrawn for a 10° C. drop in temperature is: 10 x 0.6 x 46 grams per minute = 276 g/min or 330 litres of cooling water over a 20 hour distillation period.

Therefore, 600 + 330 = 930 litres of cooling water are required in toto. To this, of course, must be added the water consumed while the column is being equilibrated. And then there's the water consumed during beer stripping. Whether or not you consider this a lot of water depends on your particular circumstances. If you feel it is a lot then you might wish to try air cooling by circulating the cooling water through a car radiator and blowing air through it. This would also avoid the need for a drain. And if you wished to get really fancy you could experiment with circulating freon through the cooling coil and refrigerating it.

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