Desalination

Under desalination means the discharge of boiler or circulating water from z. B. steam boilers or evaporative coolers with the aim of reducing the salt content of the water and thus corrosion and deposits.

background

In the case of steam boilers, the feed water pumps constantly replenish fresh water in order to compensate for the water loss caused by evaporation. In the case of evaporative coolers, the loss due to evaporation must also be constantly compensated for by adding fresh water. Only chemically pure water is removed from the water, salts and hardness components remain behind. As a result, their concentration in the remaining water increases steadily, which can lead to increased corrosion and deposits. Therefore, the salt content must be regularly reduced by discharging water in order to keep it within certain limits. This is done manually or automatically using so-called continuous blowdown valves.

Important sizes

Thickening number

The thickening factor is a dimensionless quantity that describes the relationship between the permissible salt content of the water in the process and the salt content of the water supplied: ${\ displaystyle EZ}$ ${\ displaystyle S}$ ${\ displaystyle s}$

${\ displaystyle EZ = {\ frac {S} {s}}}$

Additional water volume

The additional amount of water that is necessary to avoid exceeding, calculated from the evaporating or vaporizing amount of water and the concentration factor for ${\ displaystyle Q_ {to}}$ ${\ displaystyle S}$ ${\ displaystyle Q_ {v}}$ ${\ displaystyle EZ}$

${\ displaystyle Q_ {zu} = Q_ {v} {\ frac {EZ} {EZ-1}}}$ .

The additional water thus on the one hand compensates for the water loss, on the other hand it "dilutes" the remaining water so that it is not exceeded. ${\ displaystyle Q_ {to}}$ ${\ displaystyle Q_ {v}}$ ${\ displaystyle S}$

Bleeding loss

The desalination loss is the difference between the supplied water and the escaping water , which must be removed from the process in order to keep the total amount of water constant: ${\ displaystyle Q_ {a}}$ ${\ displaystyle Q_ {to}}$ ${\ displaystyle Q_ {v}}$

${\ displaystyle Q_ {a} = Q_ {to} -Q_ {v}}$

The desalination loss can also be expressed as a function of the thickening number:

${\ displaystyle Q_ {a} = Q_ {v} {\ frac {1} {EZ-1}}}$

Minimizing the need for water

Required make-up water depending on the thickening number with different evaporation rates

The formulas show that by increasing the thickening number, i.e. by reducing the salt content in the make-up water , the amount of water to be added can be reduced and thus water can be saved. In particular for thickening numbers less than approx. 2, the amount of additional water required increases sharply. For large so if very low-salt water is added compared to the water in the process, approaches to on. Almost only the loss is then compensated, the desalination loss then approaches zero. ${\ displaystyle s}$ ${\ displaystyle Q_ {to}}$ ${\ displaystyle Q_ {to}}$ ${\ displaystyle EZ}$ ${\ displaystyle Q_ {to}}$ ${\ displaystyle Q_ {v}}$ ${\ displaystyle Q_ {v}}$ ${\ displaystyle Q_ {a}}$

In practice it is not possible to choose arbitrarily small, but depends on the supply water available. The thickening number then indicates by which factor the salt content in the process is higher. As a rule, a thickening number of 2 to 4 is then chosen, on the one hand to keep the amount of additional water as low as possible and, on the other hand , to prevent the salt content from increasing too much. A higher and thus a higher one would only lead to insignificant further water savings. ${\ displaystyle s}$ ${\ displaystyle EZ}$ ${\ displaystyle S}$ ${\ displaystyle Q_ {to}}$ ${\ displaystyle S}$ ${\ displaystyle EZ}$ ${\ displaystyle S}$

Desalination valves

Desalination valve

Continuous desalination valves are used for the controlled discharge of the desalination loss . Their function is thus similar to that of blowdown valves . With permanent boiler water monitoring, the fittings make a significant contribution to system and personal protection. ${\ displaystyle Q_ {a}}$

Your preferred areas of application are:

Shell boiler
Waste heat boiler
Pure steam generator

Manual desalination valves

With manual desalination valves, the scale is used to set the continuous desalination volume using the hand lever. With a suitable design of the cone of the continuous blowdown valves, the step nozzle, the discharge can be carried out with little noise and low wear. A free choice of the valve characteristic enables an optimal adaptation to the feed water and boiler water quality as well as to the boiler output. For manual sampling, there is usually a sampling valve below the valve on the pressure side.

Automatic continuous blowdown valves

With the automatic continuous blowdown valves, the blowdown volume is set using an electric actuator that is mounted on the manual blowdown valve. The actuator is controlled by a conductivity controller . Whether and how far the valve is opened depends on the conductivity in the boiler water, which is measured by a conductivity electrode with automatic temperature compensation. The automatic continuous blowdown valves are particularly used in systems that are operated idle.

literature

Georg Haberberger, Dieter Blanck: Cooling process . In: Christof Lechner, Jörg Seume (Ed.): Stationary gas turbines . 2nd Edition. Springer, Heidelberg / Dordrecht / London / New York 2010, ISBN 978-3-540-92787-7 , chapter 5.5.3, p. 228 ( limited preview in Google Book search).

Water losses and water treatment in cooling circuits. (PDF; 39 kB) AV technology, accessed on October 20, 2012 .

GESTRA AG Bremen: GESTRA brochure Equipment of energy centers
GESTRA AG Bremen: GESTRA Information B 1-2