**Cooling Tower Cycles of Concentration**

As pure water is
evaporated, minerals are left behind in the recirculating water. As
evaporation continues, the water becomes more concentrated than the
original make up water. This eventually can lead to saturated
conditions. The term cycles of concentration compares the level of
solids of the recirculating cooling tower to the level of solids of the
original raw make up water. If the circulating water has four times the
solids concentration than that of the make up water, then the cycles are
4.

*Bleed off*
is the process of removing a portion of concentrated recirculating
water, which is obviously replaced with fresh make up water. By
specifying a certain amount of bleed off we limit the cycles of
concentration the system can operate at, thus controlling scale
formation. Various treatments will let us operate at various cycles
depending on the make up water analysis and heat loading of the tower.
Bleed off is critical to a successful treatment program. The preferred
method of bleed off control is with the use of automated bleed off
control. This includes a contacting head water meter, dual timer,
chemical pump and solenoid controlled bleed off line.

A)
*Evaporation Loss:* from a cooling tower (E) = .001 (C_{r})
(DT)
where C_{r} = circulation rate in gallons per minute and D
T = temperature differential
between hot and cold water in °F. The evaporation rate amounts to 1% of
the recirculation rate for every 10°F DT.

B)
*Windage Loss**:* This is a relatively small amount of
entrained water lost as fine droplets in the air discharge from a tower.
Unlike evaporation which does not contain dissolved impurities windage
carries these impurities with it and reduces dissolved solids in the
circulating water. Typical values are 0.1% to 0.3% of the circulating
rate for mechanical draft towers.

C)
*Bleed off**: *In the evaporation process the non
volatile impurities in the make up water are concentrated. To prevent
excessive concentration some of the circulating water must be removed
from the system. This is commonly referred to as bleed off or blowdown.
The amount of bleed off can be determined from the following equations:

%
B (bleed off) + % (windage) = % E (evaporation loss) / ( cycles -1)

D)
*Cycles**:* The previous equation added a new turn to
the mix . CYCLES!!!!!!!! This is one of the most difficult terms to
understand. I will now attempt to define cycles of concentration. The
NACE defines it as: A measure of the degree to which dissolved solids
are being concentrated in the circulating water. It is in effect the
inverse of blowdown. As pure water is evaporated all of the solids are
left behind so they concentrate in the cooling tower water. If we start
out with fresh water the tower has one cycle of concentration. When all
the fresh water is evaporated ( solids are still in the tower) the tower
then refills itself with more fresh water and we have twice the solids
in the tower. We now have two cycles of concentration. This process will
proceed and scale will form at a certain point. We use bleed off to
prevent the tower from reaching the point of scale formation. In an
actual tower, this is an ongoing process with continuous addition of
make up water and continuous bleed off. The allowable cycles of
concentration will be determined by the laboratory. They are based on
water analysis, heat transfer rate of the equipment, and feed equipment
present. We normally use chloride to determine the number of cycles a
tower is operating at. The following equation can be used for this
calculation:

Chlorides
in tower water / Chlorides in make up water = Cycles of concentration

For
example, a tower operating at a 450 gallons per minute recirculating
rate (C_{r}) , 5 cycles of concentration, 10° DT,
and 0.1% windage loss:

**%B
+ .1% = 1% / (5-1)**

which
is %B = .25% - .1% therefore %B = .15% of the circulating rate.

%B
= 0.15% ( 450 gallons/minute) which is 0.675 gallons of bleed off
/minute.

Total
make up water requirements for the system are:

Bleed
off 0.675 gallons per minute

Evaporation 1% (450 gallons/minute) = 4.5
gallons/minute Windage 0.1% (450 gallons/minute) =.45 gallons/minute

Total
make up water = 5.625 gallons/minute

Total
make up water per hour is = 60 minutes/hr. ( 5.625 gallons per minute)

=
337.5 gallons per hour.

As
you can see the higher the cycles the less make up water and blowdown
are required. This is true to a certain point where additional cycles do
not decrease the amount of make up water and blowdown as drastically.
The following graph illustrates the point. As you can see at about 4.5
cycles the curve starts leveling off which indicates higher cycles will
not effect the quantity of make up water drastically.