Cooling Tower Automation: Bleed, Dosing and Corrosion Monitoring with a Controller
The logic of cooling tower automation with Walchem controllers: which controller, which sensor, how to install it, and what each alarm means.
Tower Automation Combines Five Tasks in One Controller
- Bleed: the bleed valve opens when the conductivity sensor exceeds the setpoint.
- Inhibitor: proportional feed based on bleed duration, time or a water meter.
- Biocide: scheduled addition from daily up to 4-week cycles, with pre-bleed beforehand and bleed lockout afterward.
- Feedback: 4-20 mA and direct inputs from pH and ORP or free chlorine, PTSA and LPR corrosion sensors.
- Alarms and logging: email alarms, data logging, and transmission to building or SCADA systems via Modbus TCP or BACnet.
The chemical program itself (which chemical for which risk) is covered in our cooling tower water treatment guide; the product catalog is on the cooling tower control systems page. This page focuses on controller logic, selection, installation and troubleshooting.
What Does the Controller Do, with Which Input and Output?
According to the Walchem WCT Intuition-6 technical specification and WCT400/410 manual.
| Task | Input | Output | Control mode |
|---|---|---|---|
| Bleed | Contacting or electrodeless conductivity sensor (temperature-compensated); flow switch on the sample line alongside the sensor | Bleed valve (solenoid) relay | On/off setpoint; bleed time limit (runaway protection); bleed based on water meter or paddlewheel |
| Inhibitor | Bleed duration, time, water meter or paddlewheel flow meter pulse | Inhibitor pump relay | With bleed (with or without lockout); percentage of bleed; percentage of cycle time; proportional to meter or paddlewheel |
| Biocide | Schedule; conductivity for pre-bleed | Biocide pump relay (two separate outputs, WCT410) | Daily, weekly, 2-week, 4-week timer; pre-bleed; post-addition bleed lockout |
| Acid / pH | pH electrode (optional second sensor) | Acid pump relay | On/off, proportional, PID and dual setpoint options |
| Oxidizer (ORP, chlorine) | ORP, free or total chlorine, chlorine dioxide, ozone, peracetic acid or hydrogen peroxide sensor | Biocide pump relay | On/off or proportional control with setpoint |
| Monitoring | 4-20 mA inputs (fluorometer, corrosion monitor, level, flow), digital inputs (level switch, flow meter) | Alarm relay, 4-20 mA retransmission, email | Retransmission of sensor value; each relay can be linked to another input |
The WCT Intuition-6 can operate each of its six relay outputs using one of the following algorithms: on/off setpoint, time proportional, pulse proportional (with pulse-output solid-state relay option), flow proportional, PID, dual setpoint, bleed or feed based on water meter or paddlewheel, feed and bleed, feed and bleed with lockout, feed as a percentage of bleed, feed as a percentage of elapsed time, flow meter ratio, timed switch to an alternate setpoint, sensor wash timer, and biocide timers. All relays have a maximum on-time limit to prevent runaway control; any relay can be interlocked based on a digital input or the state of another relay.
Biocide Program and Inhibitor Feed Modes
Biocide program (WCT400/410 manual)
- The two biocide outputs are independent; each can be programmed for one addition per day in a 1-, 2- or 4-week cycle, or up to ten additions per day in a daily cycle. Two biocides may be added on the same day, but not at the same time.
- Pre-bleed: conductivity must be below this setpoint before addition starts; if it is higher, the bleed output opens to lower conductivity. If set to zero, pre-bleed is disabled.
- Lockout time: defines in hours and minutes how long bleed and new biocide additions remain locked out after an addition.
- Once a biocide cycle starts, the bleed relay (except during the pre-bleed phase) is locked out; if the biocide duration is too long or too frequent, little time remains to control conductivity.
- All outputs are interlocked with the flow switch input.
Inhibitor feed modes
- With bleed: feed turns on and off simultaneously with bleed; an optional lockout time sets the maximum on-time of the feed output.
- Percentage of bleed: once bleed closes, feed runs for the set percentage of the accumulated bleed duration. Example: 10 minutes of bleed and a 50% setting gives 5 minutes of feed.
- Percentage of time: feed runs for a set percentage of a cycle adjustable between 10 and 60 minutes. Example: a 10-minute cycle and a 40% setting runs 4 minutes on and 6 minutes off.
- Water meter: feed runs for the set duration on each meter pulse; pulses are accumulated.
- Paddlewheel: feed runs when a defined volume has passed; works with Hall-effect flow meters using a programmable K-factor.
- During biocide addition, inhibitor feed is either locked out or deferred depending on the mode; in meter and paddlewheel modes, flow is stored in memory for up to 256 pulses and fed after the biocide cycle.
To verify that the inhibitor is actually present in the water, fluorescent tracer (PTSA) and corrosion rate feedback are required; these are listed in the sensor table below. A cycles of concentration and bleed calculator is used to determine cycles of concentration and the conductivity setpoint.
Which Controller: Intuition-9, Intuition-6, W100 and Others
Values are taken from Mikron product pages.
| Controller | Inputs and outputs | When to use |
|---|---|---|
| Intuition-9 | Depending on model code: 0 – 24 analog sensor inputs, 0 – 12 relays, 12 digital inputs, 4 I/O slots (LPR corrosion sensor can be added); Ethernet or WiFi, USB, BACnet/Modbus-TCP; color touchscreen | Multi-tower or multi-parameter plant. Typically configured for up to 8 measurements in Mikron installations |
| Intuition-6 (WCT) | 6 relays, 0 – 2 sensor inputs, optional dual analog (4-20 mA) input, 6 digital inputs, optional two 4-20 mA outputs; 5-inch touchscreen; Ethernet (optional), USB; cooling tower (WCT), steam boiler (WBL) and pH/ORP (WPH) configurations | Single tower or compact system. Typically up to 2 measurements in Mikron installations; Intuition-9 for additional parameters |
| W100W | 3 outputs, touchscreen, universal sensor input, multi-language | Entry level, single measurement; standalone control such as biocide, pH or conductivity |
| WIND | Conductivity-based bleed and chemical dosing control | Straightforward tower or boiler application requiring only conductivity bleed and dosing |
| W600, W900 | Discontinued (Walchem 'Discontinued Products' list, verified 19.08.2026) | Existing sensors are retained; replaced by Intuition-9 or Intuition-6 |
| WebAlert | Ethernet-connected, web-based monitoring; email and SMS alarms | Remote status and alarm monitoring |
For an overview of the controller and sensor family, see the Walchem page. Mikron provides Walchem system integration in Turkey.
Which Sensors Connect to the Tower?
| Parameter | Sensor | Range and specification | Connection to controller |
|---|---|---|---|
| Conductivity | Walchem contacting conductivity | Cell constant 1.0 sensors: 0 – 30,000 µS/cm at 25 °C; polypropylene body, graphite or 316 SS electrode, up to 10 bar (high-pressure type up to 20 bar); typical tower application 0 – 70 °C | Direct sensor input; tee or flow-switch manifold on sample line |
| Conductivity | Hamilton Conducell | Contacting sensors for cycles of concentration tracking and bleed automation | Arc digital output models are listed on the Mikron product page |
| pH and ORP | Walchem WDS and WEL | pH for acid dosing; ORP as an indicator of biocide effectiveness | Preamplified signal; second sensor input |
| Free chlorine | Hefer CL series | Potentiostatic, 0 – 20 ppm | For biocide pump control |
| Chlorine dioxide, ozone, peracetic acid, H₂O₂ | Walchem amperometric | Reagent-free; ClO₂ and O₃ 0 – 2 / 0 – 20 ppm; PAA and H₂O₂ 0 – 200 / 0 – 2,000 / 0 – 20,000 ppm | Second sensor input |
| Corrosion rate | Pyxis CR (LPR) | General corrosion 0.001 – 995 MPY, localized (pitting) index; CR-200 wireless (Bluetooth), CR-300 4-20 mA and Modbus | Intuition-9 expansion slot or 4-20 mA analog input |
| Inhibitor tracer (PTSA) | Pyxis Little Dipper 2 | 1 – 200 ppb PTSA; 4-20 mA output; up to 100 psi, 50 °C | 4-20 mA analog input |
| Water usage | Water meter, paddlewheel flow meter | Make-up and bleed volume measurement | Digital counter input (Intuition-6: low speed 0 – 10 Hz, high speed 0 – 250 Hz) |
Installation Principles: Sample Line, Sensor, Wiring
According to the installation section of the Walchem WCT Intuition-6 technical specification.
- Sample line: taken from the discharge side of the recirculation pump and returned to the tower basin or pump suction.
- Sensor position: must remain submerged in the sample at all times, at a location with good flow and rapid response to changes.
- Cables: sensor cables are routed at least 6 inches (approximately 15 cm) away from AC voltage wiring. If the standard length is exceeded, 24 AWG, 3-twisted-pair shielded cable is used.
- 4-20 mA output and water meter: 22 – 26 AWG twisted-pair shielded cable is used.
- Small towers: the WCT400/410 manual states that timed sample mode can reduce installation cost in small towers by eliminating the need for a separate sample bypass line.
- Associated equipment: solenoid valve for bleed control, water meters for make-up and bleed, level switch or transmitter for chemical tank level, and dosing pumps for chemical control. For pump and material selection, see chemical pumps and our dosing pump maintenance guide.
Logging, alarms and integration
- Local or internet access via Ethernet, Walchem Fluent account management; CSV data logging and alarm delivery to up to eight email addresses.
- Communication with PC-based applications, HMI/SCADA, building energy management and DCS via Modbus TCP and BACnet.
- USB: software updates, saving all setpoints to a flash drive to transfer to another Intuition-6, and downloading log files.
- Calibration: one-point process, two-point buffer or one-point buffer calibration; a two-level access code protects setpoints.
Alarm and Troubleshooting Table
Summarized from the troubleshooting section of the Walchem WCT400/410 manual.
| Symptom | Meaning | Possible causes and initial check |
|---|---|---|
| Sensor Error | Conductivity signal is invalid; conductivity control stops | Short circuit in electrode cable; faulty electrode (replace); faulty controller (self-test) |
| High Alarm (high conductivity) | Conductivity exceeded the high alarm setting; control continues | Dirty electrode (clean); faulty bleed solenoid valve; incorrect wiring; clogged Y-strainer; faulty bleed relay; conductivity rising during biocide lockout (allow normal bleed) |
| Low Alarm (low conductivity) | Conductivity is below the low alarm setting; alarm relay opens | Sensor disconnected; sensor dry (tee blockage, flow, orientation); pre-bleed set too low; solenoid valve stuck open; faulty electrode |
| Bleed Timeout | Bleed output remained open longer than the programmed bleed time limit; conductivity control stops | Time limit set too low for normal conditions; clogged strainer; low bleed flow or insufficient differential pressure; bleed valve not opening; valve wiring or relay fault |
| No Flow | Flow switch contact is open; all outputs disabled, alarm relay opens | Recirculation pump or valves; open circuit in flow switch or cable; insufficient pressure drop across sample line |
| Cal Fail | Controller accepts at most a cumulative 50% correction to conductivity | Example: if a 30% correction was made in one calibration, only an additional 20% is accepted in the next; inspect the electrode and replace if required |
| Reading does not change | Conductivity display remains constant | Apply the electrode evaluation procedure described in the dedicated section of the manual |
For Legionella risk and continuous biocide monitoring in cooling towers, as well as glycol concentration and freezing point in closed loops, see our solutions; for project engineering, commissioning and documentation of automation systems, see system validation and qualification.
Sources: Walchem WCT Intuition-6 technical specification; Walchem WCT400/410 manual; Mikron product pages (Intuition-9, Intuition-6, W100W, WIND, W600, W900, WebAlert, Walchem conductivity, Pyxis CR, Pyxis fluorometer).
Frequently Asked Questions About Cooling Tower Automation
Bleed control, biocide scheduling, inhibitor modes, controller selection and alarms.
What is cooling tower automation?
Cooling tower automation is a controller system that continuously measures tower water conductivity to open and close the bleed valve, operates inhibitor and biocide pumps according to a schedule or flow signal, and maintains alarms and data logs. Its core functions are conductivity-based bleed, inhibitor feed, scheduled biocide dosing, and monitoring of pH, ORP, chlorine and corrosion.
How does a bleed controller work?
The controller reads the conductivity sensor and energizes the bleed valve relay when the reading exceeds the setpoint; dilution with make-up water continues until conductivity drops below the setpoint. The Walchem WCT series manual states that in the event of a sensor error, conductivity control is stopped to prevent control based on a false reading, and that control also stops if the bleed time limit is exceeded.
How is biocide dosing programmed on the controller?
In the Walchem WCT Intuition-6 technical specification, daily, weekly, 2-week or 4-week biocide timers are defined. Prior to each addition, a conductivity-based or timed pre-bleed is applied, followed by a timed bleed lockout after the addition. The WCT400/410 manual describes up to ten additions per day for a biocide output in a daily cycle, and one addition per day in weekly cycles.
In which modes does the inhibitor pump operate?
The WCT400/410 manual lists six modes: feed with bleed (two modes, with and without lockout), feed as a percentage of bleed time, feed as a percentage of a defined cycle time, feed proportional to water meter pulses, and feed proportional to a paddlewheel flow meter. Which mode is selected depends on whether make-up and bleed flow metering is installed.
Which controller replaces the W600 and W900?
On Walchem's website, the W600 and W900 are listed under 'Discontinued Products'; they have been succeeded by the Intuition-9 and Intuition-6. Sensors used with the W100, W600, W900 and P1000 series are compatible with the Intuition-6 and Intuition-9 (confirmed by Mikron), meaning sensor investment does not need to be repeated when upgrading the controller.
Under what conditions does the controller trigger an alarm?
According to the WCT400/410 manual, high or low conductivity, lack of flow (No Flow), sensor error, bleed timeout and calibration failure are among the alarm or fault conditions. During a No Flow condition, all outputs are disabled. Calibration accepts a cumulative correction of at most 50% to the conductivity reading; if this is exceeded, a calibration error is reported.
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