TOWER OPERATION
In a cooling tower, heat is removed from cooling water by a
combination of loss of sensible heat and by means of evaporation. As was
discussed earlier, we can estimate the evaporation in a cooling tower to be
equal to 17% of the recirculation rate for every 10°F T (the temperature
difference between the hot water in and the cold water out in a cooling tower).
In order to better understand the operation of a cooling tower there are some
other terms with which you should be familiar. One of these is the approach of
the tower. The approach is defined as the difference between the cold water
temperature of the tower and the wet bulb
temperature of the atmospheric air near the tower.
On the surface it would appear that evaporation in a cooling tower would not
have a negative affect on the cooling water chemistry. However, we must remember
that most of the water we encounter is not "pure". Specifically, when
a raindrop falls through the atmosphere it collects a variety of dissolved
gases. Once that drop contacts the earth’s surface, it picks up their
contaminants such as dissolved and suspended solids.
In essence, by the time the water is used for cooling tower makeup, it
contains a variety of contaminants. These contaminants, coupled with process
leaks or other surface contaminants, intensify the potential water problems in a
cooling system.
Specifically, as water evaporates, it leaves behind almost all of the
contaminants it contained. The net effect is that the contaminants are
"concentrated" in the cooling water.
As an example, let's say you fill a beaker with 100 ml of water that has a
chloride concentration of 50 ppm. You then put the beaker on a table and allow
water to evaporate to the 50 ml mark. If you then analyzed the water, the
chloride concentration would be 100 ppm. The reason is simple: one half of the
water evaporated and the dissolved chlorides it contained were left behind.
Therefore, the chlorides were concentrated in the beaker 2 times. In a cooling
tower we refer to this as "2 cycles of concentration". The term cycles
of concentration or simply cycles, is used to define the number of times the
tower water has been concentrated over that of the makeup water. If we had
allowed the water in the beaker to evaporate to 25 ml, the chloride
concentration would have been 200 ppm and the cycles of concentration 4 (200 ppm
divided by 50 ppm). ultimately exceed their solubility limit. When this happens,
scale can form on heat exchange surfaces or precipitated solids can drop out in
low flow areas.
We can control this concentration mechanism by removing a portion of the
concentrated water from the cooling tower and replacing it with makeup water.
This process called "bleedoff", is practiced in most cooling towers.
In most instances, enough makeup water must be added to compensate for that lost
by evaporation, windage and drift as well as bleedoff.
As the quantity of bleedoff in a cooling tower is decreased, the cycles of
concentration are increased, the amount of makeup water needed is decreased, and
the quantity of chemical treatment is decreased. Therefore, in order to prepare
a competitive water treatment program, we must determine the maximum number of
cycles at which the tower can be safely operated.
Choice of proper cycles of concentration for a cooling tower system is
determined by its design, water characteristics, operating parameters and
treatment program; these factors are discussed later in this manual.
The nomenclature employed in the following discussion is shown below.
E = Evaporative water loss, as a percent of the recirculation rate
W = Windage and drift water losses, as a percent of the recirculation rate
B = Water lost by bleedoff (and other losses to waste), as a percent of the
recirculation rate
Q = Circulation rate, in gpm
V = Volume of the system, in gallons
M = Makeup water, needed, as a percent of recirculation rate
C = Cycles of concentration
T = Overall retention time, in days
P  Dosage of treatment chemical required, in parts per million (ppm)
Cycles of concentration can be calculated based on a number of different
variables such as chlorides, total dissolved solids (TDS) or calcium. The
variable used will depend on the operating conditions of the tower. For example,
the use of chlorides for cycles determination in a tower using chlorine may not
be reliable because the chlorine will contribute chlorides to the water.
However, as an example, if cycles are based on chlorides, the formula for
calculating the cycles of concentration is as follows:
Cycles of concentration= water chlorides(ppm)/
makeup water chlorides (ppm)
As mentioned earlier, makeup is added to compensate for other water
losses in a system. Therefore,
M = E + H + B
Makeup also compensates for such undesirable water losses as leaks. Makeup,
in gallons per minute, is obtained by multiplying percentage of makeup by Q, the
system circulation rate.
The amount of makeup needed is governed by a cooling tower's operating cycles
of concentration. A material balance around the cooling tower yields the
following relationship:
B + W = E/ (C – 1)
A cooling tower with specific evaporation and windage
losses can therefore be regulated in a manner that bleedoff, or makeup, is
actually a function of the tower's operating cycles of concentration.
This is shown graphically in Figure on the left, in which
the curves show makeup requirements as a function of cycles of concentration for
various range (evaporation) parameters.
These date are based on assumed windage and drift of 0.2%
of the recirculation rate  a typical average for older mechanical draft cooling
towers  newer units go as low as 0.005%.
An increase in
cycles of concentration in the range of one to five can save significant
amounts of makeup water, and, as will
be demonstrated, also results in significant treatment chemical
savings. Operation
of a cooling tower above six cycles does not result in
large water savings to the plant, however.
The overall retention time in a cooling water system can be calculated by the
formula:
This states that by dividing the overall
volume of a cooling water system by its water losses of windage, drift, bleed
and leaks (if applicable), one can determine the theoretical length of time that
one drop of water will remain in the system.
Retention time is important because if
affects the time a slug treatment will remain in a system and is also a function
of cycles of concentration. Bleedoff decreases as cycles of concentration
increase ; lower bleedoff in turn causes longer retention time.
Treatment chemicals are applied to a cooling
water system in two ways. Corrosion inhibitors and most antifoulants are fed to
the system continuously, at a rate based on makeup requirements. Microbiocides
usually work best, however, by being slug feed to the system to cause rapid
microbial reduction; the slug feed is determined
by the system's volume.
