Design History of Sentinel



The “Sentinel for Cooling Water” was designed and built to fit a need in the monitoring of industrial and comfort cooling water performance that has existed for many years.  Previous methods of simulating a heat transfer surface resulted in poor, unrealistic results due to non-uniform contact between the electric heating elements being used and the inside of the tubes.  This was overcome in Sentinel by using a liquid simulated process that provides uniform contact and heat transfer to the tubes and therefore much more realistic results.
Knew Value has an extensive data base of cooling water exchanger design and actual performance conditions as a result of being in the cooling water exchanger monitoring business for a number of years.  This data base was used to create the design parameters we had to meet to create a tool that would realistically meet industry’s needs.  We sampled 1,000 exchangers in refineries and chemical plants in North America to determine the following conditions:

Design versus Actual conditions for

• Skin Temperature
• Velocity
• Heat Flux

We then compared average, minimum and maximum values for each criteria.

From this analysis we determined the design criteria for the skid and set out to achieve our objective.

The first design was a multiple tube, removable bundle exchanger approximately 3 inches in diameter containing from 10 to 20 tubes which were 18 to 20 inches long.  This very large surface area presented some unique challenges in heating the simulated process stream sufficiently to achieve the desired cooling water skin temperature.  In order to accomplish this a 480 Volt, 3 Phase heater was required.  While this model achieved all of our design criteria the resulting tool was very expensive and a challenge to get plant acceptance for its use.
In an attempt to make the skid more user friendly and cost effective we modified our exchanger design from a multiple tube, removable bundle to an annular exchanger.  This is a 0.5 inch tube inside a 1.0 inch outer tube which is a clear polycarbonate material which allows users to observe the cooling water results during the exposure period.  This significantly reduced the surface area of the exchanger and allowed us to mount two exchangers in the skid which are piped in parallel.  This affords the opportunity to run simultaneous tests during the exposure period.  The much smaller surface area also resulted in a heater design which lowered the power requirement to 120 Volt AC, 30 amps.  The primary design of the skid is non-classified, however we can equip it with an air purge which would allow it to be used in classified areas.
The two exchanger design allows customers to set up a variety of evaluations.

• Two different metallurgies can be evaluated at identical skin temperatures and velocities.
• The same metallurgies can be evaluated at identical velocities and varying skin temperatures.
• The same metallurgies can be evaluated at identical skin temperatures and varying velocities.
• Two identical metallurgies at the same skin temperatures and velocities can be exposed for different periods (30 day and 90 day) similar to corrosion coupon exposure periods with the very significant difference of measuring the actual corrosion in the exchangers being modeled on the bulk water corrosion rate from a non-heated surface.

The Sentinel design was then changed after 3 years of field evaluations to simplify the operation, making the skid more reliable and easier to operate. The simulated process is now a chamber in the exchanger which contains an immersion heater and is controlled by a rheostat.

The overall objective of “Sentinel for Cooling Water” is to provide industry with an affordable method of visually evaluating the actual performance of their cooling water treatment and control system over a specific exposure period and simulate the conditions in the skid that are present in their most critical bundles.  This is the first method of monitoring scale and corrosion with a heat transfer surface operating at easily reproducible exchanger conditions.  It represents a “Quantum Shift” in the performance monitoring of cooling water. We are able to combine all the elements of actual exchanger inspection, corrosion measurements and deposit sampling on whatever frequency in between plant shutdowns. This allows us to modify the program to achieve the desired results before waiting for a failure or the end of a typical production run.