How To Clean Scale From a Heat Exchanger

diagram of condenser/heat exchanger having an influx and outflux of water and cleaning solution

Where Does The Scale Come From In A Heat Exchanger?

Scale comes in a variety of forms, but the most common is calcium carbonate or CaCO3. Calcium scale precipitates when the threshold solubility of calcium and carbonate is exceeded.

Calcium and alkalinity exist in different concentrations in virtually all make-up sources. As evaporation removes pure water from the cooling tower, the dissolved solids remaining increase in concentration. If the concentration becomes too great, the combine to form calcium carbonate scale.

How Is Scale Usually Controlled in a Heat Exchanger?

Preventing scale formation is a combination of controlling cycles of concentration and modifying the solubility of calcium carbonate with chemical threshold inhibitors or other means.  By knowing the threshold solubility, or the concentration at which scale begins to form, we can control the conditions at a safe margin below this point.

How Do I Know If Cleaning The Heat Exchanger Is Necessary?

Heat exchangers allow heat to flow through a material, generally a copper tube or stainless steel plate, from the hot side to the cool side. Scale or any other material that accumulates on the heat exchange surface acts as an insulator and reduces the efficiency of the heat exchanger.

Most heat exchangers are designed to operate at a specific temperature range called DT. The DT describes the difference between the temperature going into and coming out of the heat exchanger. A reduction in the DT is an indication of a reduction in heat transfer efficiency. This is typically due to scale formation, microbiological fouling or mud settling in or on the tubes.

Which Product Should I Use For Cleaning a Heat Exchanger?

Scalzo is the most effective product for cleaning heat exchangers, although some require special products. Scalzo contains hydrochloric acid for the most effective cleaning, plus corrosion inhibitors and dispersants to ensure the metal is protected and post-cleaning fouling does not occur.

Hydrochloric acid is not recommended for some materials, stainless steel in particular. CA-100 should be used for heat exchangers with stainless steel components. The citric acid is less aggressive and will not pit stainless steel.

What Is The Step By Step Procedure for Cleaning a Heat Exchanger?

Follow these steps for effective cleaning.

  1. Isolate the chiller by closing valves as close to the unit as possible. On-line cleanings are never as effective as off-line cleanings because the acid requires enough contact time to dissolve the scale.
  2. Estimate the total number of gallons of water in the heat exchanger and the isolated section of pipe. The volume of a plate-frame heat exchanger is approximately 40% tower water. Shell and tube heat exchangers contain approximately 30% of the total volume as tower water. The cleaning solution tank should be at least twice the volume of the heat exchanger.
  3. Set up the chemical feed pump as illustrated in the diagram above. The return line back to the cleaning solution tank must come off the top of the heat exchanger to ensure the unit is full of cleaning solution.
  4. Add water to the cleaning solution tank and turn on the circulation pump. Operate the pump and keep adding water until water comes out of the return line back into the cleaning solution tank.
  5. With the circulation pump running, add 8 ounces of CTA-800 or an alternate anti-foam directly into the cleaning solution tank.
  6. Now add one gallon of Scalzo, Ox-Sol, CA-100, or the cleaning product recommended by your Chardon representative to the cleaning solution tank.
  7. Measure the pH of the solution by dipping a pH paper into the stream of water returning from the heat exchanger. The pH should be 2-3.  If the pH does not decrease into this range, continue adding acid until the returning pH is in the 2-3 range.
  8. Continue circulating the cleaning solution. Check the pH every 5 minutes. Add additional acid if the pH increases to greater than 3.
  9. Repeat step 8 until pH remains between 2 and 3 for 30 minutes. The heat exchanger is now clean. Neutralize the cleaning solution to a pH of 5.0 with BD-6 by adding it to the tank and circulating the mixture.
  10. Drain the tank and heat exchanger to a sanitary sewer.
  11. Add fresh water to the tank and circulate the fresh water until a pH of 6 or 7 is reached and maintained.
  12. Add 1/2 gallon of Tube Bright to final-rinse in order to passivate raw metal surfaces. Circulate for 15 minutes and drain or, if the system is to be put online, leave chemicals in the System.
  13. If required, remove end bells and inspect tube sheets, tubes, and end bells to determine the desired cleaning. If debris remains, manually remove debris and flush areas with treating water.

If the system is still not satisfactorily cleaned, replace the end bells and repeat the procedure 1-10 again.