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Increasing the capacity of a evaporator by cleaning the system with Pursanova TMDisk


Company: THENAMARIS INC., Shipping Company, Greece

Description of the problem and its negative consequences
  In next to every sea-going ship there is at least one seawater boiler (evaporator) installed to produce drinking water. In an evaporator, the seawater partially evaporates (at low pressure due to low temperature) and the residual brine is conveyed back into the sea. The produced water vapors are condensed at a cold surface and feed the ship's fresh water tanks. Inside the heat exchanger of the evaporator a stony layer consisting of salt, lime and other solids contained in the seawater is gradually formed. This layer absorbs an essential part of the thermal energy, which is normally intended for the evaporation of the seawater. As a result, the evaporator's performance is day by day reduced and the fresh water production becomes insufficient for the ship's needs.
When this point is reached, expensive fresh water has to be purchased at the harbors. The traditional solutions are the chemical and the mechanical cleaning. Both processes are hard, tough, unhealthy and expensive. Also, they have the additional disadvantage that they presuppose that the evaporator has to be stopped for many hours once per one to two months, which is not always feasible.

Our solution

We adapted two Pursanova TM on the pipe which supplies with seawater to one boiler, which had been recently cleaned. At the same time, we installed two more Pursanova TM on the feed pipe of another evaporator, which had not been cleaned lately and its daily fresh water production had already dropped noticeably. We used two Pursanova TM instead of one for safety reasons only, in orders to make sure that the trial would be successful because the flow rate was several m3/h, which might exceed the capacity of one single Ring.
However, it is very probable that one Ring for each boiler would have been efficient, too.

In which way our customer checked the results of the trial

The first boiler did not need any cleaning again because its performance was not reduced. On the contrary, about one month after the installation of the Rings the tanker's engineer informed the company's technical department that the daily fresh water production of the evaporator, instead of dropping gradually, as it always happened until then, had increased from 28 to 31 tons. This obviously leads to the following two conclusions: First, the chemical and the mechanical cleaning can never remove the whole mass of the accumulated scale and, second, the Pursanova TM not only prevent the formation of new scale, but they also manage to dissolve the residual one, which cannot be properly cleaned with the traditional cleaning methods. Neither the second evaporator had to be cleaned again. Its performance increased at a quick rate and within a short time reached its highest limit, according to the evaporator's features. This means that the boiler had shaken off the scale, without the planned chemical cleaning, which is very often responsible for the corrosion and the leaks which happen to equipment cleaned in this way.