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10.18 CHAPTER TEN

In addition to affecting TTHM concentrations and speciation, water containing bro-
mide will interfere with chlorine residual measurements. A common method of deter-
mining chlorine residuals utilizes DPD (N,N-diethyl-p-phenylenediamine). Valentine
(1986) studied the use of DPD in the presence of bromochloramine. He concluded that
bromochloramine reacts as half bromamine and half chloramines. It is therefore measured
as both free chlorine and total chlorine.

Chlorine Gas Systems
Each chlorine system consists of the following system components (see Figure 10.8): stor-
age media, vacuum regulator, gas feeder, and an injector or gas induction unit. For some
systems evaporators may be used in the system. Chlorine in the system upstream of the
vacuum regulator (storage systems and evaporators) is pressurized, and chlorine down-
stream of the vacuum regulator is maintained at a vacuum.

Storage, Evaporators, and Vacuum Regulators
Chlorine is delivered to the site in the storage media, which can be 150-1b (68-kg) cylin-
ders, 1-ton (1,0t6-kg) containers, tank trucks, or railroad tank cars. In all these vessels, liq-
uid chlorine occupies approximately 85% of the volume when the product is delivered. This
is to provide room for the expansion of liquid chlorine if the container should be heated.
Chlorine cylinders and containers should never be directly heated. As a safety pre-
caution, their outlet valves are equipped with a small fusible plug that melts at approxi-
mately 158 ° F (70 ° C) and releases some chlorine to cool the cylinder before a more se-
rious accident can occur.
Small to medium-size water systems generally withdraw gas from the top of the con-
tainer. The maximum withdrawal rate with this method is about 40 lb (18 kg) per day for
a 150-1b (68-kg) cylinder and 400 lb (180 kg) per day for a 1-ton (1,016-kg) container.
Higher feed rates can be obtained by connecting two or more cylinders or containers to
feed simultaneously. The temperature of the chlorine feed room should be maintained at
about 65 ° F (18 ° C).
Provision must also be made for a weighing device. Weighing devices are usually a
small platform scale on load cells for cylinders and trunnions on load cells for ton con-
tainers. Frequently, two weigh scales are provided to facilitate the determination of chlo-
rine consumption when empty containers need replacing.
If the containers being used are 1 ton (1,016 kg) or larger and the withdrawal rates ex-
ceed those available with the direct evaporation method described earlier, chlorine evap-
orators may be used. Evaporators are available in several capacities, but they are all on
the same 10,000 lb (4,535 kg) per day chassis. Smaller evaporators simply have smaller
heaters. When an evaporator is used, liquid chlorine is withdrawn from the bottom of the
container and transported to the evaporator, where it is converted to a gas. The most com-
mon type of evaporator uses an electric resistance heater in a hot water bath surrounding
a vessel in which the liquid chlorine is converted to gas.
The heat of vaporization of chlorine is very low, approximately 69 cal/g, compared
with 540 cal/g for water. Commercially available chlorine evaporators are designed with
extra capacity to ensure that the existing gas is superheated and does not recondense on
the downstream side. When an evaporator is being used beyond its capacity, misting oc-
curs. Misting is a severe problem in a chlorine system as the pipe materials generally used
downstream of the vacuum regulator (and some vacuum regulators) cannot withstand con-
tact with liquid chlorine and will rapidly degrade and fail under those conditions. A chlo-
rine gas filter should be installed on the exit gas line from the evaporators to remove ira-