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.
% B (bleed off) + % (windage) = % E (evaporation
loss) / ( cycles -1)
A) Evaporation Loss: from a cooling
tower (E) = .001 (Cr) (DT) where
Cr = 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:
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
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 (Cr) , 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
%B = 0.15% ( 450 gallons/minute) which is 0.675
gallons of bleed off /minute.
Total make up water requirements for the system
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 quanity of make up water drastically.