Steam is a useful heat transfer medium due to its ease of transport and the large amount of energy it contains per unit mass, among its other virtues. In transferring heat, saturated steam condenses to form condensate, which is essentially hot distilled water. This condensate is removed downstream of the site of heat transfer using steam traps, which “trap” steam and release condensate. Getting rid of the condensate quickly is important from a heat transfer point of view. It seems intuitive that this condensate be recovered, yet in many instances much of it is simply discharged to drain. Why then should it be recovered? The reasons are many.
Firstly, every drop of returned condensate reduces the amount of make-up water that has to be injected into the steam system. The net imports of water to the site concerned are therefore reduced when condensate return ratios are increased.
The volume of effluent generated is reduced with increasing levels of condensate return. While some flashing of condensate occurs when pressurised condensate encounters atmospheric pressure, most of the condensate will end up in the drainage system. Some would say that this helps to dilute pollutants from other processes on the site, but “the solution to pollution is not dilution” – the mass of contaminants remains the same, whether they are at a lower concentration or not. It is unfortunate that in many cases effluent tariff structures consider concentration as opposed to total load.
Provided the steam does not come into contact with contaminants during use, the condensate produced is water of very good quality, literally free of dissolved solids. Condensate is generally of better quality than make-up water. This means that re-use of condensate increases the quality of the feedwater used for steam generation. The upshot of this is that boiler blowdown, with its associated loss of energy and water, can be reduced. In addition, the amount of make-up water that has to be treated is reduced, reducing the costs of treatment chemicals and any energy required for treatment.
Condensate contains thermal energy, and condensate recovery therefore reduces fuel requirements, since the amount of energy to be added to boiler feedwater to produce steam is reduced. Condensate should be returned to steam generation at as high a temperature as possible. This can be achieved by insulating condensate return lines and storage tanks, and returning the condensate as soon as possible after it is produced.
It is unusual to find a site where no condensate recovery takes place, but it is also not always possible to recover all of the condensate produced. For example, where live steam is used and the steam becomes contaminated, the condensate cannot be returned. You should however be constantly measuring the condensate return ratio and doing everything possible to maximise it. Except for processes where very small quantities of condensate are produced, condensate recovery generally tends to be economically justifiable, with all of the environmental benefits, including emission reductions, an added bonus.
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