For an accurate bleed-off control system, automatic control by conductivity sensor is recommended to regulate the amount of bleed-off required. Conductivity is a measure of total ionic concentration in water, hence, concentration of total dissolved solids (TDS). Blowdown control is a critical part to ensure scale prevention in cooling water systems.
Blowdown rate is related to the cycle of concentration, which shall be determined by water treatment methods being adopted. If a comprehensive water treatment program, including both chemical and physical methods, is implemented to control scale, corrosion and micro-biological growth effectively, blowdown rate can be significantly reduced.
There are a number of methods to control blowdown, including:
1. Manual control: a blowdown valve is opened in response to test measurements
2. Timer / intermittent control: a simple interval timer is set to open and close the blowdown valve intermittently
3. Continuous bleed: an orifice plate or pre-set valve is set to release water continuously
4. Proportional control: an impulse water meter on the make up line actuates the blowdown valve
5. Conductivity control: the cooling water conductivity is continuously monitored and the blowdown valve is opened at a pre-set conductivity.
A number of points shall be considered for a blowdown system.
1. To ensure the bleed assembly can be isolated from the system for maintenance purposes;
2. To ensure the head of the conductivity sensing probe is positioned in the main water flow, but not where air pockets can develop;
3. To place the conductivity sensing probe upstream of the blowdown system solenoid valve;
4. To clean the conductivity sensing probe regularly;
5. To regulate valve in the blowdown line so that the flow cannot exceed the make up water to the system;
6. To provide an alarm signal when solenoid valve and flow switch signals in the blowdown line do not correspond.
Advantages of blowdown control:
This control strategy may have the following advantages:
1. Accurate dosing control is assured which optimizes chemical usages;
2. Blowdown and chemical dosage would not occur at the same time so that wastage of chemicals on drainage can be avoided;
3. Adjustment of water treatment programme in accordance with cooling tower system operation can be performed in the central control system;
4. Minimal chemical handling is required to reduce the risks of the operators’ health and safe;
5. Water consumption, energy consumption, chemicals consumption are recorded accurately;
6. Any malfunction of water treatment equipment can be reported immediately.
