Fluorometers In Water Treatment
As we all know, scale and corrosion on cooling tower system surfaces reduce heat transfer efficiency. While scale and corrosion inhibitors added to the system water can minimize excessive buildup, they aren’t cheap and maintaining the minimum appropriate levels can be difficult.
The most common way of feeding inhibitors is in proportion to the volume of make-up water to the tower. This method works well if the conductivity of the make-up water is consistent. However, if it varies, or there are multiple supply sources, feeding inhibitors based on make-up volume may result in fluctuating chemical levels. If the make-up water conductivity is on the low end, the overall system conductivity will be lower, resulting in less blowdown, less make-up volume and less inhibitor added. This can lead to corrosion and scale. On the contrary, as make-up water conductivity rises, more blowdown will occur requiring higher make-up water usage and more inhibitor feed. This is wasteful and if overly excessive can result in residual deposits. Somewhere in between is the optimal chemical level.
Let’s take a look at a way of controlling inhibitor levels that works independently from other controller variables.
Refer to the image below. Fluorescent dye can be added to the inhibitor, serving as a level tracer. PTSA levels can then be accurately measured by an in-line fluorometer in the sample line, which will vary in direct proportion the the level of inhibitor.
What Is a Fluorometer?
Simply put, a fluorometer is a device which measures the fluorescence level of a medium, typically liquid, which can be correlated to specific molecular presence, amount or density.
The device exposes the sample to a light source of a particular spectrum and matches the resulting fluorescence intensity as a scaled output. They are used in many industries and fields of research, such as verifying dairy milk pasteurization, monitoring algae levels in ocean water and DNA quantification. As such, there are several types of these meters that range in complexity and precision based on the application.
Fluorescent Tracing in Cooling Water Systems
Fluorescent tracing became practical for use in water treatment due to the discovery of the fluorescent chemical called PTSA (pyrenetetrasulfonic acid). PTSA is more environmentally friendly than other tracers so there are typically no restrictions for blowdown drainage. Its composition also allows for a more affordable sensing device compared to the components needed for the fluorescent tracing applications mentioned above.
For use in cooling tower water treatment systems, we recommend the Little Dipper 2 In-Line Fluorometer, manufactured by Turner Designs. The Little Dipper™ 2 is an accurate, single-channel, light industrial fluorometer which installs directly into the cooling tower system sample line. It provides a 4-20 mA signal output proportional to the concentration of the PTSA dye being measured. The Little Dipper 2 is a rugged, 24/7 sampling device that provides maximum performance, minimal maintenance, and solid-state reliability.
How Can Fluorometers Simplify Water Treatment?
The Little Dipper 2 can be used with Lakewood Instruments controller models 1575e, 3175 and NexSys. The kit comes pre-wired with a plug-in power supply and junction box. The junction box contains a DC signal passive isolator which prevents any electrical noise resulting in a precise, non-fluctuating 4-20mA output to the controller. All you have to do is a little bit of plumbing to install the sensor housing, insert and secure the fluorometer, attach the cable to the back of the fluorometer, terminate 2 wires on the controller and change a few controller settings.
The 1575e and 3175 only have one 4-20mA input. The image above shows where to land the input wires. On a NexSys N420 node they can be wired to any of the four inputs, red to (+) and black to (-).
The sensor range of the Little Dipper 2 is 0 to 300 ppb, so the controller range needs to be configured accordingly (4mA = 0 ppb and 20mA = 300 ppb). This is done on the “4-20 MA IN/OUT” Main Menu selection on the 1575e and 3175 and on the “PROCESS PARAMETER” screen for the N420I node on the NexSys.
Now you can use the “SETPOINT” control option for the inhibitor feed pump relay. Be sure to choose “BELOW SETPOINT” for “WHEN TO FEED” on the 1575e and 3175 and “REVERSE ACTING” on the NexSys. Once you have calculated the concentration of PTSA dye in the chemical container and desired system inhibitor level (the setpoint), you’re done. The associated inhibitor feed pump will activate when the concentration level falls below your setpoint. You can be confident that your system always has enough, and none will be wasted.
Questions About Fluorometers?
Fluorescent tracing with setpoint control is the most accurate way of maintaining a consistent chemical level in your cooling tower system but it must be set up correctly. Give us a call and we’ll help you with all of the necessary controller input parameters, calibration process and relay configuration to make it happen for you!
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