The simplest method of continuous chlorination of systems less than 75 gpm is by the use of a hypochlorinator.  Hypochlorinators are motor driven pumps which are used to added hypochlorite solutions to water.  The pump pulls the hypochlorite solution out of a holding chamber and pumps it into the water to be treated.  Where the pipe from the pump joins the pipe carrying the raw water, the Venturi effect creates a small vacuum and pulls the chlorine solution into the water.

It is often necessary to increase or decrease the amount of chlorine added to the water as conditions change. Hypochlorinators allow you to adjust the amount of chlorine fed into the water in three ways. You can adjust the stroke length or machine speed by varying the pulley size. Both of these adjustments change the hypochlorinator feed rate - the speed at which the machine puts chlorine into the water. You can also adjust the amount of chlorine added by changing the strength of the hypochlorite solution.

Chlorinators and Cylinders

While hypochlorinators are usually used to perform continuous chlorination in smaller systems, chlorinators are more economical when the supply source is greater than 75 gpm and may sometimes be used in smaller systems as well.  Anticipated pumping periods and chlorine demand (based on the chlorine residual test) determine whether a hypochlorinator or chlorinator should be used in each situation.

Chlorinators are devices which introduce chlorine gas to water using liquid chlorine supplied in steel cylinders.  The following sections will explain how the proper quantity of chlorine is delivered from the cylinder to the source water.  But first we need to understand how the liquid chlorine is stored.

Chlorine cylinders

Liquid chlorine can be stored in 100 or 150 pound cylinders, ton containers, or 55 to 90 ton rail cars.  In each case, the chlorine has been condensed into a liquid form, but expands back into a gas as it leaves the cylinder.  Whenever a substance changes state from a liquid to a gaseous form, heat is required.  The heat which is absorbed by the chlorine as it changes state in the cylinder comes from the surrounding air.

If chlorine is drawn off from a cylinder too quickly, the temperature of the air surrounding the tank will drop and will cause frosting and lower gas flow.  To prevent frosting, the draw off rate should be no greater than 350 pounds of gas/day for a 100-150 pound cylinder.  If greater feed rate are required, several tanks can be connected using a manifold, which is a pipe joining the cylinders together so that chlorine gas is drawn from several cylinders at once. 

The only accurate way to determine the feed rate of chlorine from a cylinder is to weigh the cylinder over time.  By subtracting the tare weight (the weight of an empty cylinder), the operator can determine how much chlorine gas remains in the cylinder so that empty cylinders can be replaced in a timely manner.  If the cylinders are weighed over time, the feed rate of chlorine can be determined to ensure that the proper concentration of chlorine is being added to the water. 

Whenever dealing with gaseous chlorine, safety is an important issue.  Ammonia should be kept handy for checking for leaks and storage buildings should be well ventilated.  If the operator must walk through an area with chlorine in the air, he or she should use a breathing apparatus.  If no breathing apparatus is available, the operator should keep his head high since chlorine is 2.5 times as heavy as air and will tend to sink to the ground. 

Vacuum Chlorinators

The most typical kind of chlorinator, a vacuum chlorinator, is shown below:

In a vacuum chlorinator, chlorine gas is pulled from the cylinder into the source water by a vacuum.  The vacuum is created by water flowing through the injector and creating a negative head.  This negative head forces open the pressure regulating valve on the cylinder and allows chlorine gas to flow out of the cylinder and into the chlorinator. 

Once the gas has entered the chlorinator, the chlorine feed rate is measured using an indicator known as a rotameter.  Just beyond the rotameter, the chlorine gas flows past a regulating device (a V-notch plug or a valve) which is used to adjust the chlorine feed rate. 

Then the chlorine gas is pulled into the injector, also known as an ejector.  The injector consists of a pipe filled with flowing water.  The flowing water pulls chlorine into the water, both chlorinating the source water and creating a vacuum in the chlorine line which pulls more chlorine gas out of the cylinder.  This type of chlorinator is also known as a solution feeder since the chlorine gas is dissolved into a small amount of source water, which is then piped into the main line of water to be chlorinated. 

Chlorinators can be controlled manually (using the regulator) or with a controller.  The most common type of controller is the flow proportional controller which automatically feeds chlorine based on the flow rate of the water. 

Vacuum chlorinators are very safe since any break in the line with disrupt the vacuum and close the pressure regulating valve.  As a result, chlorine leaks are very uncommon. 

Direct Feed Chlorinators

In a few cases, direct feed chlorinators are used instead of vacuum chlorinators.  In a direct feed chlorinator, the chlorine gas is under pressure and is pumped directly into the main flow of water.  There, the chlorine is evenly dispersed into the water using a diffuser, like the one shown below.

Since the chlorine is under pressure, a pressurized water supply is not needed for use with a direct feed chlorinator.  However, the pressurized chlorine is prone to leakage, so safety issues limit direct feed chlorinators to small installations or for use as emergency equipment.