Sensors are generally used as monitoring devices in cooling water systems. Comparison between the measuring results and the set points for specific parameter are useful to determine the control action required for normal operation.
Different type of sensors to be selected is relied on the control strategy. Common types of sensors are described as follows.
Flow meter:
Chemical feeding requirements are proportional to the make up water flow. Many chemical feeding systems are designed according this basis, which is the simplest automatic dosing method. Flow meters including orifices venture, flow tubes and turbine meters can be used to suit the design.
Level sensor:
Make up water supply to cooling tower sump depends on water level of cooling tower basin. Hence, chemical dosing can be controlled by monitoring of the water level. However, this dosing method is not accurate and cannot control the concentration of chemicals contained in the cooling system water precisely.
Conductivity sensor:
Electrical conductivity of water relies on total ionic concentration of water, which indicates the concentration of total dissolved solids (TDS) in water. Both corrosion rate and scale formation potential for any specific systems are normally proportional to the conductivity of water. Conductivity sensor is frequently employed as chemical detection device. The sensor is usually used for operation of blowdown system for the control of cycles of concentration.
pH sensor:
Carbon steel corrosion rate decreases with an increase in pH value, and the scale potential for most scale forming compounds increases with pH value. Also, water treatment chemicals work in specific range of pH value. Hence, pH value measurement is often the basic principle for acid dosing to maintain effective water treatment program.
ORP sensor:
ORP stands for Oxidation-Reduction Potential and is a measure of the cleanliness of the water and its ability to break down contaminants”. It has a range of –2,000 to + 2,000 and units are in “mV” (millivolts). ORP sensors work by measuring the dissolved oxygen. More contaminants in the water result in less dissolved oxygen because the organics are consuming the oxygen and therefore, the lower the ORP level. The higher the ORP level, the more ability the water has to destroy foreign contaminants such as microbes, or carbon based contaminants.
ORP sensors are used as real time monitoring and recording of oxidation reduction potential, which can be utilized to monitor chlorine residuals in cooling tower systems. It measures the inorganic and organic particles remaining in cooling water so as to facilitate chemical dosing. ORP sensor shall be used together with pH sensor since ORP values vary with pH value.
Chlorine residuals sensor:
Chlorine is commonly employed biocide for water-cooled air conditioning system.
Continuously measuring chlorine residual analyzer is commercially available to measure either free or total chlorine residual.
Turbidity sensor:
Turbidity measurement provides an indication of the amount of suspended matter in cooling water, which is useful in determining deposit potential. Therefore, it can be used for bleed-off control.
Corrosion rate sensors:
Corrosion rate sensors are available in the market to provide continuous, instantaneous measurement of corrosion rates for any alloy. These sensors are normally based on linear polarization resistance between a pair of electrodes.
Corrosion coupon test method can also be used to determine corrosion rate.
Sensors for specific ions and Compounds:
Sensors are commercially available to measure ion concentrations in water. Many analyzers are also available to measure either compounds or classes of compounds or classes of compounds dissolve in water.
The selection of proper location of sensors in the water-cooled air conditioning system is very important. Sensors measuring treatment chemicals shall be located at a point after the treatment is well mixed. Corrosion rate increases with increasing temperature. Therefore, corrosion monitoring device should be installed at the outlet of the heat exchanger where water with the highest temperature is passing through. Requirement of measurement and reliability of sensor shall also be considered in selecting an appropriate device.
Control Devices:
In order to achieve maximum effectiveness of water treatment program, chemicals must be dosed into the system in an appropriate concentration periodically. Since handling of chemicals may be dangerous, it is always recommended to perform chemical dosing by means of automatic monitoring and control strategy. Chemical injection can be facilitated by different control devices.
Timer:
It is a simple device allowing operator to set the operation of chemical dosing valve, hence, chemical dosing frequency, in a fix interval. Also, circulation of water during system intermittent down time can be achieved by using timer control.
Dosing pump:
Dosing pump can be operated manually or automatically. For automatic control, dosing pump activates upon receiving signal from timer or sensors. It runs for a certain period to inject chemicals into the cooling tower water circuit.
Motorized valve:
Motorized valve is an essential component for automatic controlled chemical dosing and bleed-off. It will switch to open / close position upon receipt of signal from monitoring devices, such as water meter and conductivity sensor.
