1. Bleed-off control
Controls the accumulation or build up of dissolved solids in the recirculating water caused by the evaporation. The bleed-off control operates a bleed-off valve to drain water from the system, allowing additional make-up to dilute the system water and reduce the dissolved solids level.
Conductivity bleed-off systems measure the electrical conductivity of the recirculating water, which is directly related to the level of dissolved solids. As the conductivity rises above a set point, the bleed-off valve is opened, when the conductivity level drops below the set point the bleed-off valve is closed. This ensures the bleed-off rate is controlled in proportion to the duty or load on the cooling tower to minimise wastage of water.
Proportional bleed-off systems operate by opening the bleed-off valve in proportion to the make-up water volume. By definition, the quantity of bleed-off required is a direct proportion of the make-up rate. For example, to maintain a concentration factor of 2 the quantity of bleed-off required will be 50%, or one half, of the make-up rate and to maintain a concentration factor of 4 the quantity of bleed-off required will be 25%, or one quarter, of the make-up.
Proportional bleed systems operate from a water meter in the make up supply to the cooling tower and open the bleed-off valve for a set time duration relative to a set volume of make-up.
Both methods have their benefits and disadvantages:-
Conductivity bleed-off systems require conductivity instruments and cannot differentiate between dissolved solids from the make-up and water treatment chemicals. When shot doses of biocides are added they cause a rise in the conductivity activating the bleed-off. A bleed lockout timer is required to prevent bleed-off for a set period following biocide dosage to prevent wastage of expensive biocide chemicals.
Proportional bleed-off systems cannot compensate for any leakage or other unforeseen water losses from the system, nor will they detect any process contamination. Proportional systems do not, however, require bleed lockout systems as they are unaffected by slug doses of biocides and other chemicals.
2. Inhibitor dosage
Corrosion and scale inhibitors are dosed in proportion to the make-up to maintain a set level in the recirculating water. Inhibitor dosage systems therefore use a water meter in the make-up supply to the cooling tower to operate chemical dosing pumps to add chemicals in direct proportion to the make up water flow.
3. Biocide dosage
Two options are available:-
Non-Oxidising Biocides – Normally shot dosed on an alternating basis, usually once or twice per week using timer controlled dosing pumps. Typically, non-oxidising biocides require a contact time of 4 hours at a set concentration to destroy bacteria. As previously indicated, where conductivity bleed-off systems are fitted a bleed lockout system is required to prevent dilution and loss of biocide to bleed-off.
Oxidising biocides – Chlorine or Bromine. Chlorine should only be used where pH levels are between 6.5 and 7.8. At higher pH, chlorine is relatively unreactive and a poor biocide. Its use is therefore confined to industrial cooling towers incorporating pH control. Bromine is effective at pH levels up to 9.0, above which its biocidal effect drops significantly requiring excessive dosage rates.
Oxidising biocides are normally dosed continuously at a low level to maintain a set disinfectant level in the recirculating water. The level of disinfectant can be monitored electrically using Redox (Oxidation Reduction Potential – ORP) or a specific chlorine monitor. Because of their cost and sensitivity to contamination specific chlorine or bromine monitors are rarely used.
Redox (ORP) control measures the oxidation and reduction potential in millivolts in the water over a range of 0 to 1000 mV. This is related to the level of bromine oxidant present and uses simple technology similar to pH measurement. Whilst Redox is affected by changes in pH this has no significant effect on their performance in cooling water treatment as most cooling towers operate at a stable pH level.
Bromine biocides are available in two basic forms, as liquid or slow dissolving tablets. Liquid bromine is dosed using chemical dosing pumps. Bromine tablets or briquettes are dosed using a brominator or erosion feeder device.
Chemical dosing pumps
Two basic types of dosing pumps are available:-
Diaphragm pumps using either an electromagnetic solenoid operator or motor. Diaphragm pumps are capable of injecting into high pressures up to 20 bar depending on the motor driver used. If allowed to pump dry they will lose their prime and will require manual repriming.
Peristaltic pumps use a system of rollers compressing a tube to create a flow. These have the advantage that they do not require non return valves to operate and will readily self prime even if run dry. The output pressure of peristaltic pumps is however limited by their design to 1.5 bar on most models and 5 bar on high pressure units.
Towerpack MTD units
Use higher pressure diaphragm pumps (5 bar injection pressure on standard units with 10 bar option) together with conductivity bleed-off control using electrodeless maintenance free probe.
Bleed lockout or bleed inhibit timer included as standard. Redox control on oxidising biocide units using liquid or tablet bromine.
Supplied with a sample and injection manifold to simplify installation, can be installed remotely from the cooling tower in the chiller plant room or pump room provided the injection pressure of the dosing pumps is not exceeded.
Volt free contacts for BMS output for chemical low level and system fault as standard.