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•
Ryznar index = 2 pHs - pHc between 6 and 7. Where pHs is the pH of saturation and pHc is the true level
measured in the circuit.
•
The product of sulphates and calcium concentration in the circuit water, (both expressed in mg/l Co3 Ca),
should be less than 500,000.
•
Silica content should be lower than 150 mg/l.
6.2.1.2 Suspended solids.
These can be brought into the secondary circuit of the cooling tower by the make-up water, by the air or by
contamination during the process.
In the secondary circuit of the cooling towers, 100 to 150 p.p.m. of suspended solids can be admitted.
6.2.1.3 Biological growth
The ambient conditions existing in a cooling tower favour biological growth.
It is usually necessary to periodically treat circuits with chlorine and/or other biocides in order to prevent these
growths.
Treatment of this kind is particularly necessary when the circuit might suffer casual (accidental) pollution caused by
process fluids, as happens in refineries, sugar refineries, etc..
6.2.2
Corrosion
Besides keeping the Ryznar index in the stable or slightly corrosive zone, corrosion inhibitors must be added to the secondary
circuit. Several varieties are commercially available and the most suitable should be selected in consultation with specialized
firms.
The above leads to a limitation on the maximum number of concentrations acceptable in the secondary circuit.
The number of concentrations is called "Concentration Cycles" and is represented by the letter N.
If we call:
E: Evaporated water flow in the tower as a percentage of nominal water flow.
P: Total blow down (drift in tower plus losses in circuit plus deconcentration blow down) as a percentage of nominal water
flow.
The following ratios are obtained:
Average make-up water flow as a percentage of circulating water flow:
𝑁𝑥𝐸
𝑁 −
1
Total blow down needed in the circuit as a percentage of circulating water flow:
𝑃
=
𝐸
𝑁 −
1
Which are used to control the number of concentrations, usually determined by dividing the concentration of chlorides in the
circuit by the chlorides in the make-up water.
N values (Concentration Cycles) higher than 5 (five) are not usually practical, even when the make-up water quality is good.
ALWAYS MAKE SURE THAT ANY PRODUCT USED FOR WATER TREATMENT AND CLEANING IS COMPATIBLE, AND IN THE PROPER
CONCENTRATION, WITH THE MATERIALS OF THE UNIT (GALVANISED STEEL, PP, STAINLESS STEEL, ETC) AND WILL NOT DAMAGE THE
COMPONENTS OF THE UNIT IT SELF
6.3
Electrical locking
To ensure this, we recommend that a key-operated stop switch should be fitted, making it impossible to operate when the key is not in
the lock.
VERY IMPORTANT! BEFORE REMOVING THE FANGUARD OR THE ACCESS DOOR TO THE MECHANICAL EQUIPMENT, IN ORDER TO CARRY
OUT MAINTENANCE OPERATIONS, ALWAYS MAKE ABSOLUTELY SURE THAT THE FAN IS STOPPED AND THAT IT CAN NOT BE RESTARTED
UNTIL THESE OPERATIONS HAVE FINISHED.
6.4
Cold weather operation
Cooling tower operation at temperatures below 0°C might give rise to the formation of layers of ice at the air intakes.
Ice formation can be reduced, and even prevented, by taking the following measures:
•
Install heating resistances and a level sensor
•
Add an antifreeze solution in the primary circuit
Summary of Contents for RAA Series
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