The Condenser

The function of the condenser is to convert the steam and oil vapour that issues from the still to water and liquid oil at the desired temperature. The commonest types use water as the coolant although air-cooled condensers can be used in some circumstances, as can refrigeration.

If flowing water is available, very little equipment is required to construct an effective condenser. For field stills, or small plantation stills, metal pipes running from the still to and under a stream or pond are quite effective. The above-water section of the pipe should be short if possible, or if it has to be long, should be at least 75mm diameter for a still of 3m3 or more capacity. Once under water a 50mm diameter tube is sufficient. This needs to be at least 20m long. With this system the fall in the stream has to be sufficient to allow the construction of a dam through which the condenser pipe runs so that the outlet is above water. As the condenser pipe must run downwards throughout its length, the receiver into which the condensate flows must be below the water level of the stream or pond.

This type of condenser, which is very simple and cheap is widely used in hilly country, but is not suitable for flat or swampy country. For flat country a condenser can be set up close to the still, and consist of a tank containing a volume of water with a pipe running through it, usually as a descending spiral. In this type the cooling water is static, requiring no pump. If the volume of water is sufficient, after cooling overnight or between distillations, it will cope with the next distillation. Occasional topping-up to replace water evaporated from the tank is all that is required. This is an effective type of condenser, easily made and cheap. About 20m of tube beneath the water is sufficient; say 3 lengths of galvanised 50mm tube.

A more compact and more portable condenser is the multitube type in which the hot distillate flows through a number of tubes while the coolant is pumped in a jacket around the tubes. In some cases the vapours flow in the space around the tubes while the coolant flows through the tubes. The advantages of this type of condenser are that it is much more compact, easily established and transported, easily cleaned and, because the hot vapours contact a large area of cold surface immediately on entering the condenser and collapse to liquid at that point, the resulting diminution in volume tends to pull the vapour from the still to the condenser.

The tubes, through which or around which the vapours flow are nowadays usually stainless steel. Other metals can be used provided they have reasonable conductivity and do not react with the oil.

The disadvantage of multi-tube condensers are the high cost and the necessity for a pump and motor to push the coolant through the condenser, unless it is possible to gravitate the water from a stream as is often done in hilly country. Where electric power is available, the necessity to pump is not much of a disadvantage, although it is a cost since the pump must run continuously during the distillation. Where no electric power is available from the grid, small petrol motors are the usual source of power. These are not always reliable and therefore the static tank type condenser is more attractive.

The capacity of the condenser has to be considered in relation to size of still and speed of distillation. With the static tank type it is not much more expensive to set up a 10,000 litre tank than a 5,000 litre. The cooling coil is the same. With the extra volume of water, the only time the condenser can be inadequate is when several distillations are done in rapid succession in hot weather. Even then, all that is necessary to get effective condensation is to push some cold water into the tank, displacing the hot water.

With a multi-tube condenser, if it is vapour-in-tube type, it may be mounted vertically or at any angle down to almost horizontal. There must always be enough slope to enable the condensed vapour to flow to the outlet. If the condenser is coolant-in-tube type, it is best mounted vertically. Unless it is very well designed a horizontally installed condenser of this type is likely to condense but not cool the distillate.

The effectiveness of a condenser depends upon the surface area available for heat transfer, the heat conductivity of the metal cooling surface, the difference in temperature of the distillate and the coolant, and the rate of vapour flow into the condenser. The third factor can be controlled to some extent by the rate at which the coolant (water) is pumped through or around the tubes. The first two factors are fixed for any condenser. It is therefore essential to make sure that the condenser is big enough before installation. If it is not, the rate of distillation will be limited.

Receiver or Separator

For tea tree the liquid oil and water flowing from the condenser during distillation are, for practical purposes, non-miscible and of different densities. Therefore separation can be achieved by allowing the oil to rise to the top of the mixture. There are essential oils with a higher density than water, and some with a density close to that of water. These require different apparatus or treatment, but for tea tree oil simple flotation is an effective means of separating oil and water.

The essential features of this type of separator are that it must be able to collect the distillate flow as it comes and be of sufficient volume to allow the liquids to settle so that there is no current flowing through. This can be done by introducing a second vessel within the main one, but if the volume of the primary vessel is large enough effective separation will occur. If cohobation is employed, any oil not separated will return to the still and come over later in the distillation.

An effective separator can be made by using a drum, e.g. 200 litre, into which the distillate flows, with an outlet for water from close to the bottom taken up to 3/4 of the height of the drum, and another outlet for the oil directly through the wall at the same height. Figure 1 illustrates this. For a lighter-than-water oil such as tea tree, introducing the distillate through a funnel with a turned up end has some merit as this directs the flow of oil globules upward.

If the condenser is of stainless steel, a separator of the same material tends to keep the oil free from colour, although stainless steel is not essential for either apparatus. Where the densities of the two liquids of the distillate are close, the temperature of the distillate is important, as, for oils lighter than water, difference in densities will increase as the temperature rises. However, for tea tree oil the difference at ambient temperature is sufficient for effective separation. There is no advantage in operating at other than ambient temperature.

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