Chlorine, in gaseous form or as hypochlorine, is often used to inhibit the growth of such organisms. However, the doses normally used only limit growth and do not destroy the resistant biological forms which these organisms can adopt in conditions temporarily unfavorable to their survival. It is best to introduce the chlorine product in massive doses and at intervals; the actual dose and the frequency of application depend on the particular case and the time of year. The use of chlorinated materials is not recommended in a scale inhibiting system: the higher the pH, the less effective they are, and in addition, phosphonates will react with them. A suitable biocide should therefore be used.

Periodic short and drastic biocide treatment (once to three times a year) is also advisable, particularly at the start of the growing season, between February and June, in order to destroy the resistant forms. Besides the question of cost-effectiveness, it is necessary to take into account:

• the compatibility of the biocide with the corrosion-inhibitors or dispersants;
• the effects on the receiving medium of the deconcentration below-down.

Each day brings confirmation of the efficiency of by-pass filtration in dealing with suspended matter and colloids; this is quite understandable; but this effectiveness is also seen to extend to biological phenomena, which is not obvious.

Lack of care at the start-up a new cooling tower system may lead to fouling which will be difficult to eliminate; whereas by-pass filtration, probably used, will form the start bring about a considerable and lasting reduction in the biological activity of the system.

Where towers and heat exchangers are used, chlorination is required to control slime and algae in the cold water basin and fill of the tower, as well as the exchanger tubes and total pipe network. Chemical dosage vary, but are usually within the one to three part per million range for continuous feed and three to eight part per million for shock treatment. Residuals are measured in the hot return line to the tower, determining effectiveness and demand for chlorination. Residuals should not exceed one part per million free chlorine over long periods of time. Keeping residual low eliminates the needless waste of chlorine and prevents damage to wooden towers.

One of the most widely used chemicals for biofouling control in industrial cooling water systems. Also it possesses very considerable residual oxidizing capacity and is therefore useful for the destruction of a pitting corrosion bacteria. Chlorine dissolved in water reacts with its solvent according to the reaction: which is accompanied by the secondary reaction:

HClO <---> ClO^- + H⁺

The direction of these equilibrium reactions depends on the pH value of the medium. If the pH is below 2, all the chlorine occurs in molecular form. At pH 5, the molecular chlorine has entirely disappeared and recurs as hypochlorous acid (HClO). At pH 10, the chlorine is combined in the form of hypochlorite ions (ClO^-). If the pH value lies between 5 and 10, which is usually the case with water subjected to chlorination, we encounter a mixture of hypochlorous acid and hypochlorite ions.

Chlorine has an atomic number of 17 (number of excess positive charges on the atomic nucleus) and an atomic weight of 35.456. Molecular chlorine, Cl₂, has a weight of 70.914. Two isotopes of chlorine, Cl³⁵ and Cl³⁷, occur naturally, and at least five other isotopes have been artificially produced. Ordinary atomic chlorine consists of a mixture of about 75.4 percent Cl³⁵ and 24.6 percent Cl³⁷. Chlorine usually forms univalent compounds, but it can also combine with a valence of Proteins, Total organic carbon, Nitrites, an Manganese.

In its elemental form, chlorine is a greenish yellow gas that can be readily compressed into a clear, amber-colored liquid which solidifies at atmospheric pressure at about 150^oF. Chlorine gas forms into a soft ice upon contact with moisture at 49.3^oF and at atmospheric pressure. (This is chlorine hydrate, Cl₂?8H₂O.)

In commerce, chlorine is always packaged as a liquefied gas under pressure in steel containers. The liquid is about one and one-half times as heavy as water (denser) and the gas is about two and one-half times as heavy as air. The liquid vaporizes readily at normal atmospheric temperature and pressure. It has an unmistakable irritating, penetrating, and pungent odor. The properties of chlorine gas and liquid are listed in Tables 6-1 and 6-2 respectively.

Table 6-2 (properties of liquid chlorine)
Critical Temperature	291.2oF (144oC)
Critical Pressure	1118.36psia
Critical Density	35.77 lb/ft3 (573 g/l)
Compressibility	0.0118% per unit volume per atm at 68oF
Density	91.67 lb/ft3 @32oF
Specific Gravity	1.41 (water = 1) @68oF
Boling Point	-30.1oF (-34.5oC) @ 1atm
Freezing Point	-149.76oF (-100.98oC)
Viscosity	0.345 centipose @68oF (Approx 0.35 x water @ 68oF)
Specific Heat	0.226 Btu/lb/oF
Latent Heat of Vap.	123.8 Btu/lb @-29.3oF
Heat of Fusion	41.2 Btu/lb @-150.7oF

1) Critical Properties

The critical temperature is that above which chlorine exists only as a gas (291.2^oF), despite the pressure. The critical density is the vapor pressure of liquid chlorine at this critical temperature. The critical density is the mass of a unit volume of chlorine at the critical pressure and temperature.

2) Compressibility Coefficient

The compressibility coefficient of liquid chlorine is greater than that of any other liquid element. It represents the percent decrease in volume corresponding to a unit increase in pressure when the liquid is held at constant temperature. This physical characteristic is the reason why the volume-temperature relationship of chlorine is so important.

3) Volume-Temperature Relationship

The volume of liquid chlorine increases rapidly as its temperature increases. Because of this characteristic, coupled with non-compressibility, extreme care must be taken to prevent the possibility of hydrostatic rupture of containers or pipe lines by expanding liquid chlorine due to a rise in temperature. All containers are filled to their prescribed weight of chlorine at 60^oF so that 15 percent of the container volume is vapor space. Actually the Chlorine Institute Manual (4th edition, 1969) says on page 5, paragraph 2.1.5b, "The maximum permitted filling density is defined by the D.O.T. [173.300 (g)] as.... the percent ratio of the weight of gas in a container to the weight of water that the container will hold at 60^oF. (One pound of water equals 27.737 cubic inches at 60^oF.)" The practical approach to an overly complicated D.O.T. definition is to know the density of water at 60^oF (62.366 lb/ft³) and the water volume of the container.

The vapor space provided by the above requirement is designed to accommodate a temperature rise sufficient to melt the fusible plug in the 150-lb cylinders and ton containers. The fusible plug metal is designed to melt between 158^oF and 165^oF, thus relieving pressure and preventing rupture of the container in case of fire or other exposure to high temperature.

4) Density of Chlorine Vapor

The density of chlorine vapor varies widely over changes in pressure and modestly over changes in temperature. This is a most important variable when calculating gas flow pressure drop in both vacuum and pressure systems.

5) Density of Chlorine Liquid

The density of liquid chlorine varies only slightly with temperature. At 40^oF it is 90.85 lb/ft³, and at 140o F it is 79.65 lb/ft³.

6) Viscosity of Chlorine

This is the measure of internal molecular friction when a substance is in motion. it is necessary to know this property for both liquid and gaseous chlorine, as it is a variable in calculating the Reynolds number for the determination of friction losses in pipeline.

7) Latent Heat of Vaporization

This is the heat required to change one mass of liquid to vapor without a change of temperature. If the liquid chlorine is at 70^oF it requires about 100 BTU to vaporize one pound of liquid chlorine.

8) Vapor Pressure

This is the pressure of chlorine gas above liquid chlorine when the vapor and liquid are in equilibrium. This pressure varies widely with temperature. It is necessary to know this relationship particularly when the consumer is transferring chlorine from tank cars to vaporizing equipment.

9) Specific Heat

This is the amount of heat required to raise the temperature of a unit weight of chlorine vapor one degree F. At atmospheric pressure and 59^oF it requires 0.085 BTU/lb.

10) Solubility of Chlorine Gas in Water

Chlorine gas a limited solubility in water. At atmospheric pressure and 68^oF its solubility in water is 7.29 g/l. This does mot represent the conditions that surround the application of chlorine in this text. Operation of chlorination equipment which produces chlorine solution is always at partial pressure (vacuum). At the vacuum levels currently being used the maximum solubility is about 5,000 mg/l. The upper limit of solubility recommended by all chlorinator manufacturers is 3,500 mg/l. This arbitrary figure has been successful in preventing solution discharge systems from being adversely affected by gas pockets in the solution piping and off-gassing at the point of application.

11) Chemical Reactions

Liquid chlorine in the absence of moisture will not attack ferrous metals; hence the use of steel containers. Since there is no such things absolutely "dry" liquid chlorine, extra wall thickness is provided to offset corrosion. Liquid chlorine will attack and very quickly destroy PVC materials and rubber, hard or soft. Dry chlorine gas will not attack ferrous metal, cooper, or ferrous alloys. Dry chlorine gas will support combustion of carbon steel at 483^oF. Chlorine exists only as a gas above 291.2^oF regardless of pressure (critical temperature).

Moist chlorine gas will destroy all ferrous metals including stainless steel, copper, and ferrous alloys. Gold, platinum, and tantalum are only metals that are totally inert to moist chlorine gas. Silver is widely used with moist chlorine gas, because the silver chloride formed upon contact with the moist gas is inert.

Aqueous solutions of chlorine are extremely corrosive. For this reason PVC, Fiberglass, Kynar, Polyethylene, certain types of rubber, Saran, Kel-F, Viton, and Teflon are commonly used where both moist gas and chlorine solutions are encountered.

Chlorine reacts with ethyl alcohol and ether in trace mounts to form solid, waxy hexachloriethane. It also reacts with grease and oils to form a voluminous frothy substance. Solid complex hydrocarbons are formed by the reaction of chlorine and the various petroleum distillates. At normal temperatures there are no reactions between chlorine gas and the methane derivatives, chloroform, wood alcohol, and carbon tetrachloride.