In order to maintain a stable chemical concentration in cooling water, a number of application methods can be adopted, including:
1. Shot / slug dosing
2. Continuous / intermittent dosing
3. Proportional dosing related to bleed-off volume
4. Proportional dosing related to make up water volume
5. Dosing controlled by sensor
1) Shot / slug dosing:
Chemicals are added to the system manually on a routine basis. Shot / slug dosing is the most economic and effective application method, which may be adopted in small-scale cooling tower installation and particularly for biocide application. However, it is not recommended for large systems because chemical concentration cannot be controlled accurately and large fluctuations in chemical levels are always found.
2) Continuous / intermittent dosing:
Continuous / intermittent dosing makes use of mechanical devices, such as timer and pump for chemical dosing. It is the simplest type of automatic dosing system. Since chemical dosing frequency and amount is regulated by the pre-set value, fluctuation of chemical concentration inside cooling tower water is always found.
3) Proportional dosing related to blowdown volume:
System bleed-off can be controlled by simple timer, conductivity sensor or make up water flow. Signal from the device can also be used to initiate chemical dosing.
When the conductivity of the system water reaches a pre-determined value, a timer can be actuated which allows a dosage pump to run for a period to add the requisite amount of chemicals, in relation to the make up water entered the system.
A delay timer shall be installed to prevent wasteful chemical addition during blowdown.
4) Proportional dosing related to make up water volume:
Proportional dosing maintains a nearly constant chemical level by dosing in proportion to a varying make up water rate. The treatment requirement is based on make up water quantity and injection rate varies as the water make up changes.
Impulse water meter installed in make-up line shall be used to activate a chemical dosing pump. Proportional dosing can be applied to all cooling tower systems continuously. Such dosing is particular benefit to systems which operate under conditions with great varieties.
5) Dosing controlled by sensor:
Dosing controlled by sensor is ideal for controlling chemical concentration in a system. Correct amount of chemical is continuously presented to the system once the dosing rate and frequency are gauged by operational parameters. Therefore, concentration of chemicals inside cooling tower water can be maintained within a designated range. pH sensor, redox probes and oxidation reduction potential (ORP) probes are commonly used to control dosing of acids and oxidizing biocides, respectively.
