Physical Water Conditioning Technic

 Nielsen Technical Trading
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water conditioning physical, magnetic lime scale treatment water surface tension reduction lime scale deposit prevention, hard water descaler

Physical water treatment

By Professor Dipl.Eng. H. Schnell, Germany

Uncertainty exists about which appropriate water treatment to select for the many different applications: The new physical or conventional chemical procedures. Further which will lead to the desired success, is economically and at the same time environmental acceptable. Particularly the physical methods are considered disputed, since only rarely or insufficient information is available.

The industry, project engineers, plumbers, and estate responsible have the same problem: how to treat the water, the hard water, which increases the detergent requirement strongly and leaves deposits in the pipelines, hot water plants and hydrogen equipment (Abb1). This thermal contamination, which causes economical losses at billions dollars warldwide, was for years judged to be a just acceptable phenomenon, although it in reality is related to well known physical-chemical processes, when even inadvertently. 

To minimise such effects both physical and chemical procedures are implemented, whose effectiveness however is much dependent on the knowledge of the basic reasons.

Beyond discussion, the water on our planet earth is indispensable to all human daily life.

Water is available and occurs as:

The utilities of the water are: Drinking water, industrial water (process, heating and cooling water), and finally wastewater - depending on its material properties, which for water mainly is:

Different substances in the drinking water improve its quality as, e.g. calcium, magnesium, but cause technical problems as these substances are direct related to the hardness of the water.

Table 1: Methods of water treatment (index)

Chemical procedures:

Physical procedures:

Pre-treatment

Methods for clarifying:

Coagulation, flocculation, sedimentation to clear floating and grey particles.

(applying chemical reacting substances)

Filtrations of the sub-soil water predominantly sand as filtering medium, in pressure and gravity filters (back flushing of the filters necessary).

Main handling

Softening methods:

Lime milk - / soda principle,

Cations exchange (full softening),

Acid dosage (partly softening).

Demineralisation method:

Cation and anion exchanges

(present most effective and economical method).

Hardness stabilisation: Inhibitor dosage, also as dispersion and corrosion protection agents.

Reverse osmosis for demineralisation by use of diaphragms.

Methods for the reduction of the water surface tension and improvement of ion activity, resulting transformation of the crystal structures of the hardening causing substances:

- Magnetic field method by means of electrical alternating or permanent magnet,

- Electrostatic method by applied active anodes.

Subsequent treatment

Acid and caustic solution for cleaning of polluted thermal systems inclusively the neutralisation of applied chemical detergents.

Automatic cleaning of tubes heat exchangers by sponge rubber balls or brushes without operating interrupt of the plant.

 

In table 1 the methods of the water treatment are presented, which can be divided into the two major groups:

Both methods operate effectively under constant flow conditions and reduce their effectiveness at an intermittent operation. Their most important influence on substances is:

There is a close relationship between the contamination of thermal equipment and the corrosion factor on the materials. Therefore, the criteria for a successful water treatment are:

There exist no generally valid solution with regard to water treatments. The specific conditions of any water supply are too different, even supplied from just few foods away. In addition the demands to the applications are outnumbered different. The basis for all evaluation of the water quality can only be determinated by a chemical water analysis.

 

The basic water chemistry

Water molecules have a uniform asymmetrical structure (fig. 2). They are positive charged at the hydrogen elements and at the oxygen element negative charged, thus the water molecule is a dipole. The water molecules tends to form a strong accumulation on smallest space (Agglomerisation) and they are held together by the hydrogen binding forces. This is so powerful, that no water molecule can escape from the surface or evaporate, without applying an external energy

 

Due to its intermolecular forces has water a great surface tension. Water molecules, which gets contact with crystals, neutralise the attraction forces in the crystal structure and force the ions into a hydrate covering, which prevent the reunification and renewed crystallisation. Hereby the water receives its high dielektric constant and becomes a universal solvent fluid. Water is almost not ionizationable, possesses therefore no electrical conductivity and behaves like an insulator. Only when ionising materials is disolves in the water, the conductivity is improved and it becomes an electrolyte. External energy forces, as oscillations, heating, electrical and magnetic fields are required, in order to change the surface tension and viscosity, i.e. to change the internal friction of the molecules in the water (fig. 3).

In the 2. main group of the periodic system of chemical elements (1868/70: L Meyer, D. Mendeljew) we find the alkaline earth metals, like calcium, magnesium etc., which have a special inportance for water chemistry.

By the disolving of minerals in the water, the atoms which leave the minerals create new materials with uniform chemical characteristics and relatively low moll masses, being the sum of the total atomic mass (). In water chemistry however the term used is predominantly:

Equivalent mass = moll mass / valence

Chemical reactions form cations (+) and anions (-) with the condition: å cations å anions.

- cations: allkali metals (+1), alkaline earth metals (+2), ammonium (NH4 / +1) etc.

- anions: carbonate (C03 / -2), bicarbonate (HCO3 / -1), chloride (Cl / -1), nitrate (NO3 / -1), phosphate (PO4 / -3), sulphate (SO4 / -2) etc. (in parentheses the chemical formula / valence)

Generally the dissolving ability of the minerals increases with rising water temperature. Remarkable exceptions are the chemical formations of the alkaline earth metals, e.g. calcium carbonate CaCO3 (limestone), calcium sulphate CaSO4 (gypsum), magnesium carbonate MgCO3, magnesiumhydroxid Mg (OH)2 and others. Due to the reciprocal dissolving behaviour of alkaline earth metals their ability to dissolve is reduced with increasing water temperature.

Water analysis

The use and applications of water is determined by its contents and the amount in a chemical water analyse:

 

Water hardness

The total sum of dissolved alkaline earth metals (Ca + Mg) is defined as total hardness GH.

(mmol/l)

(°dH = German hardnes - see conversion table below)

Previously the carbonate hardness (KH) corresponded to the Ca-hardness, but is today international defined as the hydrogene carbonate HCO3 content, respectivily derived at an acid capacity of the pH-value = 4.3.

(mmol/l)

(°dH = German hardness, conversion scale table see below)

 

Table 2: Comparison of hardness units

Hardness units

1 mval/l

German

1 dH

French

1 fH

English

1 eH

American.

1 ppm

International unit

1 mmol/l

 

28 mg CaO

or

50 mg CaCO3

each 1 l H2O

10 mg CaO

each 1 l H2O

10 mg CaCO3

each 1 l H2O

1 grain CaCO3 per gallon 14,3 mg CaCO3

each 1 l H2O

1 part per million

1 mg CaCO3

each 1 l H2O

100 mg CaCO3

each 1 l H2O

1 mval/l

1

2,8

5

3,51

50

0,5

1 °dH

0,357

1

1,786

1,25

17,86

0,1786

1 °fH

0,2

0,5599

1

0,7

10

0,1

1 °eH

0,285

0,7999

1,429

1

14,29

0,1429

1 ppm

0,02

0,056

0,1

0,07

1

0,01

1 mmol/l

2

5,6

10

7

100

1

The international hardness scale (mmol/l) is to be preferred to the national hardness scales in table 2.

For natural water the relation KH / GH » 2/3 can be assumed. The basic unit for the evaluation of water is therefore:

1 mmol/l 100 mg/l CaCO3

and the basic value of electrical conductivity:

1 m S/cm 1 mg/l CaCO3.

The relation of the equivalent mass of substances contained in water to calcium carbonate CaCO3 (=100; / v = 50) leads to a uniform determination of the ion current. As a control of a chemical water analysis it indicates if the datas given in the report are correct.

 

The crystal structure

The sedimentated products from the water form amorphous or crystalline structures, whereby amorphous structures are developed without special a form. They usually remain unstable, but may at a later time convert into crystalline structures. Crystals are limited by even, homogeneous surfaces as anisotropy bodies. Crystal building blocks consist of atoms, ions or molecules, which form a well defined grid structure (fig. 4)

Substances of the same chemical compound may appear in different crystalline forms: calcium carbonate (CaCO3) as Calcite = trigonal which is the stabile structure and as aragonite = rhombic, the unstable structure. Any modification is particularly stable in a fixed thermal status (pressure or temperature).

The crystal building structure will deform if influenced by external electrical forces causing a polarisation. The polarising effect is initiated by the ions, which are becoming polarised.

Cations are less deformable than anions. The number of attracted anions does not depend on the valence, but by the action sphere of the cation. Due to different load in crystal structures, the attraction and repulsive forces in the crystal grid keeps its ballance status within the system. The binding forces will determine the physical characteristics of the crystal grid and the electrical conductivity in the crystalline form is generally low. The grid stability corresponds to the degree of polarisation of the ions. Conclusion: Increasing the polarisation will proportional decrease the crystal stability[3]

Physical mechanisms

The behaviour of water is technical a subject to different physical actions, which may be:

Under normal conditions the crystal disposal products and other contamination particles will, (Table 3):

The flow processes are described by the Reynolds number RH, which in its simplified form reads:

Laminar flow < Re » 2300 < Turbulence flow.

 

According to the flow-arrows left- (fig. 5) the laminar flow is parallel within the entire tube area, whereby the speed is reduced towards zero near the inside walls due to friction and leaving possibilities for deposits to remain. In a turbulent stream (right) the flow-arrows constantly cross and provide good mixing of the water, thus the possibilities of deposits is less likely. The following approximate values of flow conditions apply:

Corrosion danger, particulary pit corrosion.

From the described external energy forces to improve the ion activity, fulfil magnetic fields the mentioned procedure performance and criteria. According to physical rules will permanent magnetic fields only have an effect on moving charge forces, whereas electrical fields also have influence on charges at rest (table 4). The water flow becomes therefore of equal importance for the treatment.

The carriers of charges in the substances of a water stream are diverted according to the Lorenz Force FL formula:

FL = Q × c × B (N)

(Q = electrical charge (As), c = flow rate (m/s), B = induction (Vs/m2)

Before the cations and anions enter into the magnetic field, they move parallel in stream of water flow and are diverted by the magnetic field in such a way, that they move towards each other. Hereby collisions take place between the different ions with a short-term transformation of unstable CaCO3 - molecules, which by unchanged balance will return into their original conditions again. Only by adding external forces and by the attempt of the system to avoid such reaction, the development of a new ballance status is formed. The flow rate of the water determines the mobility of the ions and the impact of the applied energy, thus the frequency of the collisions. The magnetic field has naturaly no influence on the flow rate, thus not involved in this magnitude of their reaction.

The created CaCO3 - molecules of minor stability (aragonite) passes the magnetic field as uncharges crystal germs. The greater the guantity of germs, the more fine amorphous crystal mud is formed, which transported by the water stream form an effective corrosion protection on the inside tube walls. This thin film can manualy easily be removed and does not create lime scale deposits.

Table 3: Contamination classes

Index criteria:

Causes and effects:

Sedimentation -

Contamination

Deposit of solid substances

from the disolved particles in water stream.

 

Deposit on the pipe walls and heating systems, formation of contamination layers.

Crystallisation

Contamination

Deposit of hardness substances from molecules in water stream due to the reciprocal dissolving behaviour of alkaline earth metals.

 

Encrustation on the tube walls by large thickness layer with of thick crystalline structure, power losses of heat transfer.

Chemical reaction

contamination

Self-oxidation and polymerisation of hydrocarbons due to oxygen intrusion.

 

Hard encrustation on tube walls

Corrosion

contamination

Chemical or electro-chemical attack by aggressive substances to the material.

 

Emergence of different corrosion forms,

e.g. Surface erosion, pitting corrosion, intergranular corrosion etc.

and to final material destruction.

Biological growth

Microorganisms (algae, bacteria) and macro organisms (shells, mushrooms) produce biological films with substantial thermal resistance in pipelines and heat exchangers.

 

Power loss by reduced flow of heat stream, interruption of the function within heating systems.

These causes and effects are prevented by effective permanent magnets, which operates free of external power supply. They are avaible for integrated installation into the water pipes by means of thread or flanges, with different constructions and energy forces, according to the standard pipe diameters and flow rates. The permanent magnet is thoosen according to application, shall be shielded against external radiation. The pressure loss in the water stream is extremely small and the units operates maintenance-free and without subsequent costs. The efficiency of the permanent magnets remains - if not exspelled to external influences - practically unchanged for years to come[ 5].

 

Table 4. Magnetic sizes and units [ 4]

Size:

Symbol

SI - unit

Earlier unit

Relations:

Magnetic induction

(magn. flow density)

B

T

G

 1 T 104 G

1 G 1 Vs/m2

Magnetic field strength

(excitation)

H

A/m

Oe

1 Oe 79,6 A/m

Magnetic energy

B H

J/m3

G Oe

1 G × Oe 79,6 J/m3

Magnetic flow

F

Stock

M

1 Wb 1 Vs 108 M

Magnetic polarisation

J

Wb/m2

-

1 Wb/m2 1 Vs/m2

A = ampere, G = Gauss, J = joule, M = Maxwell, Oe = Oerstedt, T = Tesla, V = volt, Wb = Weber

Cathodic protection compensate the corrosion current, which leads to the metal erosion at the anodic places of the pipe walls, by means of an electric current which create a protective potential. Direction and voltage of this direct current must be regulated under operation in order to insure that the material is constanly preserved as a cathode. To achieve this funtion a self-active anode without current supply or an electrode, which is connected to the positive pole of a d.c. Supply, is applied. The necessary current density depends on the salt content (approx. 0,8 ... 1,0 mmol/l as CaCO3), respective the electrical conductivity in the water (approx. 600 m S/cm)

By the formation of hydrogen ions on the wall surface to be protected of minimum concentration as mentioned above, the calcium, respective hydrogene carbonate (CaCO3) in the crystalline form of aragonite (at pH > 10 also Mg(OH)2 ) is separated and disposed on the pipe walls as a corrosion protection film. Forming such, the protection current density is reduced. At reduced water flow (c ® 0) in addition to the Ca- and Mg- reaction also sulphates as well as phosphates will be precipitated, which together can produce hard disposals on the tube walls. At sufficient flow rate the hydroxid ions and the deposit substances are removed with the flow of the water [ 6].

As active anodes are metal alloys used with own low potential - e.g. magnesium, zinc, aluminium. Table 5 shows the advantage of Zn-anodes compared to other materials. In natural water the zinc behaves similar to calcium, but is less solutable (fig. 6), thus also makes it suitable for use in drinking water systems [ 7].

Table 5: Material behaviour of active anodes [ 6]

Material:

Magnesium

Aluminium

Zinc

Solidity r (kg/m3)

1740

2720

7400

Potential against Cu / CuSO4 - half cell

- 1,55 V

- 1,05 V

- 1,10 V

Hydrogen development

strongly

moderately

None

Current consumption efficiency

40... 50 %

50 %

90... 95 %

Effective current yield (Ah/kg)

1100

1600

760

Effective current yield (Ah/dm3)

2000

4400

5400

By experience, the thermal contamination achieves an iterated process after a certain working time; i.e. there always exist a ballance between increasing contamination and - erosion.

C = Water Speed; KH = Hardness; Klein = little flow; Gross = high flow; Opt. = Optimal; Vandabtrag = Erosion pipewall.

This relation can effectively be overcomed by automatic cleaning methods without interrupting the operation. In tubes heat exchangers - and in a similar form also in plate-type heat exchangers - sponge rubber balls or plastic brushes are used, which mechanically clean the tube walls when flowing through the heat transfer units and will avoid such contamination resistance which causes less efficiency [ 8; 9].

The described methods of the physical water treatment are well tested and proven by numberous of practical installation application, leaving the manufacturer to accept guarantee periods up to 5 years.

 Applications

The drinking water and the industrial process water applications are to be defined separately.

The water in Germany delivered througt the public supply enterprises - is strictly controlled by the health authorities - for a good water quality in accordance with the drinking water regulation and the EU - guidelines. The hardness values are usually high as calcium and magnesium are vitally necessary to life and health, thus the content of these alkaline earth metals must be of the value > 1.5 mmol/l. The drinking water in the households should therefore not subsequently be manipulated by use of chemicals or orther similar reacting substances.

The industrial methods of the water softening, e.g. ion exchange by means of common salt (NaCl), with a following dossage of additives to the necessary regeneration are not suitable for households - despite constant recommendation - both due to hygienic reasons and because of unwantet increased salt content in the waste water.

The physical water treatment of drinking water represent an ideal method: The installation is problem-free and without any additional costs arise, but most of all: The water quality and the content of minerals remain unchanged. The advantages are the described ability to prevent lime deposits and corrosion processes in pipelines, heat exhangers and household machines.

The industrial process waters applications covers water supplies for many differently purposes. Depending upon the location, this may be drinking water, sub soil or surface water and any of these types are equally being used.

 

Cooling water:

The closed circuit water-cooling is a combined heat and substance transfer process, whereby approx. 70 % of the heat steam is evaporated and only approx. 30 % of the heat steam exhausts by convection to the ambient air. The closed loop cooling is performed in cooling towers (open water circulation) or in hybrid cooling towers (evaporation cooling) with two water circulations. The primary circuit is a closed loop and the internal water flow remains qualitative and quantitatively unchanged, but the first filling require a water treatment. The secondary circuit (spray water circuit) for the heat transfer to the ambient air is subject to the same conditions as aplly for cooling towers with open loop water circulation[10].

The evaporation process causes small loss of water (approx. 0,15 to 0,20 % of the circulating water flow per each degree Kelvin cooling). Due to the evaporation an upconcentration of mineral salts occour in the cooling water. In order to maintain a contant max. salt content, in particulary the KH -level = constant, a continuous and effective draining is neccessary.

Guidance value: Evaporation Draining. The water consumption (approx. 2 x the evaporation) is replaced by fresh water supply, which must be treated in advance. If so done by the means of softning (ionisation) or chemicals (Table 1), this often require dossage of additives for protection against corrosion, which again due to their mostly toxical carracter have to be neutralised before draining.

The application of physical treatment for cooling water cycles is to be strongly recommended, as the drained water may be reused, e.g. for the irrigation of gardens and agricultural areas. The physical treatment is without influence against biological growth, viz. table 3, thus in this respect not applicable.

Warm water systems:

In hot water systems, e.g. engine circulation ect., the basic water hardness is necessary for corrosion protection. The empirical the values are 0,35 mmol/l < KH < 0,7 mmol/l (2 °dH < KH < 4 °dH) at 7,5 < pH < 9. Further the content of chloride and sulphate must be limited to 50 mg/l each to avoid corrosion damage. In usual practice is the application of polyphosphate performing hardness stabilising and act anti corrosive. Over a longer period however, the danger of transformation into orthophosphate exists with harmful consequences. Therefore frequently inhibitors of organic basis are used; but with the same concerns relating the wastewater when drained as for cold water. (Phosphate in cold water is a good fertile soil for biological growth). The physical methods of water treatment are also here judged as more economical.

Hot water systems:

The condition of the water in heating systems is in Germany to be in accordance with the " TÜV specification TCh 1466 " e.g. " AGFW instruction 5/15" [11] and the "VDI recommendation 2035 " [12]. The instructions differentiates between poor salty water (< 100 m S/cm) and salty water (100...1500 m S/cm). The danger of corrosion by oxygen depends for poor salty water on the electrical conductivity; whereby an O2-content > 0,1 mg/l requires the adding of O2 binding agents. For salty water, the intake of oxygen should be avoided or appropriate corrosion protection measures be applied. For no- or low-alloy steel a sufficient alkalinity (9 < pH < 10,5) is necessary, in order to ensure the material preservations by a protective layer.

First time filling and adding make-up waters are to be demineralized on the total hardness GH < 0,02 mmol/l ( 0,11 °dH). Most economically is this low remainding hardness achieved by a full softening by means of ion exchange - for the steam boilers by complete softening. An additional water treatment using corrosion inhibitor is most probable. Hardness stabilisation only - including procedures of physical water treatment - is not sufficient for this application [13].

Sewage plants:

In waste water pipes similar physical-chemical conditions exist and a physical water treatment will contribute to the improvement of the flow conditions, but also require a more regulary maintenance.

Mixed applications:

Many washing processes as well as cool and lubrication systems in mechanical manufacturing plants operate with chemical additives, which are health-dangerous to the operating personnel. To assure humane working conditions the reduction of the surface tension by the physical water treatment will contribute to the denouncement of these additives.

The target of any water treatment must be to guarantee and constantly improve the environment.

To the German version {SNL/01-98}

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