How to Clean Scale From a Heat Exchanger

Scale is an unwanted material that accumulates onto the internal surfaces of a heat exchanger — this deposition is otherwise known as fouling. If untreated, fouling will harden and prevent the system from operating at its intended, energy-efficient state.

Scaling and fouling will increase the system’s resistance to heat transfer, which is undesirable for water heating and cooling equipment. Even thin layers of scale will create effective insulation since the percentage of energy required to heat or cool the water increases as scale buildup does.

Periodic heat exchanger cleanings are essential because scale formation can quickly result in overheating, tube failures, increased energy consumption and a rise in operational costs. Scale may even corrode equipment if left without proper care, requiring intensive repairs or total replacement to correct.

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. Scale occurs when the dissolved minerals found in a water source fall out of suspension and form a continuous deposit layer on the surface of a heat exchanger.

Calcium, magnesium and silica are common minerals found in water supplies, so they are typical components of scale. Hard water is a primary source of this type of scale. The hardness of a water source is generally expressed as having a higher amount of dissolved minerals — hardness is usually measured in parts per million (PPM).

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, they combine to form calcium carbonate scale.

Scale buildup on walls, tubes and other heating and cooling equipment surfaces is more likely in hard water. Scaling issues tend to accumulate faster when there is a high mineral content in the water. Other particulates that may add to the overall scale composition include lime and rust deposits, though calcium and magnesium are the most prevalent.

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.

The most effective way to mitigate scale buildup is to prevent its formation in the first place. Scale can be controlled in a heat exchanger by:

  • Pretreating boiler make-up water: Demineralizers, water softeners and reverse osmosis can target and remove minerals that are likely to form scale.
  • Establishing an ongoing water treatment program: Perform regular preventative maintenance to the heat exchanger and schedule periodic chemical cleaning of any water cooling or heating equipment.
  • Reducing the mineral content in water sources: Make the water “softer” and slow scale formation with various solutions, including chemical softening, membrane separation softening and cation exchange softening.

If scale has already formed in a heat exchanger, you can restore the system to an energy-efficient state by cleaning scale buildup and establishing measures for future prevention.

How Do I Know if Cleaning the Heat Exchanger Is Necessary?

A heat exchanger lets heat flow through material, such as a stainless steel plate or copper tube. The heat moves from the hot side to the cool side. Scale and other accumulating materials insulate the heat exchange’s surface and reduce its efficiency.

Most heat exchangers are designed to operate at a specific temperature range called ΔT(Delta T). ΔT describes the difference between the temperature going into and coming out of the heat exchanger. A reduction in ΔT is an indication of a reduction in heat transfer efficiency. Thisdecrease is typically due to scale formation, microbiological fouling or mud settling in or on the tubes. It is more likely for minerals to fall out of suspension at higher water temperatures and settle into scale deposits on a heat exchanger’s interior surface.

For early detection of scale buildup, we recommend keeping a daily operational log for every piece of equipment.

Monitor the system’s heating and cooling performance for efficiency indicators and identify functional abnormalities before they become significant hazards.

Which Product Should I Use for Cleaning a Heat Exchanger?

Scalzo is the most effective product for descaling heat exchangers. While some exchangers may require unique products, Scalzo works on many systems. Scalzo contains hydrochloric acid for efficient cleaning, while corrosion inhibitors and dispersants 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.

Ensure the chosen product will completely dissolve the scale by performing a preliminary test on a sample of the deposit. This test can verify the scale composition and may help you prevent future formations, too.

What Is the Step-By-Step Procedure for Cleaning a Heat Exchanger?

Follow these steps for effective tank cleaning.

  1. Isolate the chiller by closing valves as close to the unit as possible. Online cleanings are never as effective as offline 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. A plate-frame heat exchanger will be about 40% tower water, while shell and tube heat exchangers are about 30% tower water. Ensure the cleaning solution tank is at least double the heat exchanger’s volume.
  3. Set up the chemical feed pump to ensure the unit is full of cleaning solution. The return line that leads to the cleaning solution tank needs to come off the top of the heat exchanger. The above diagram can help you achieve an ideal setup.
  4. Add water to the cleaning solution tank and turn on the circulation pump. Operate the pump and keep adding water until it 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 five minutes. Add additional acid if the pH increases to greater than 3.
  9. Repeat step 8 until the pH remains between 2 and 3 for 30 minutes. The heat exchanger is now clean. Neutralize the cleaning solution to a pH of 5 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 for final-rinse to passivate raw metal surfaces. Circulate for 15 minutes and drain or leave chemicals in the system if it’s to be put online.
  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.

Prevent Scale in Heat Exchangers With Treatment From Chardon Laboratories

At Chardon Labs, we’re an industrial and commercial water treatment company dedicated to quality customer service. Our treatment programs prevent scale and corrosion from fouling your heat exchanger, saving you time, money and hassle. If you need to restore any piece of water heating or cooling equipment to an energy-efficient state, contact Chardon Laboratories for your scale prevention needs.

matt welsh
Matt Welsh
Vice President, Water Consultant at | Website | + posts

Matt Welsh is the Vice President and Water Consultant at Chardon Labs.  He helps consult a wide range of customers utilizing various methods of water treatment, from chemical to chemical-free approaches, large and small applications, and across a wide range of geographical influences.  With 20 years of water treatment experience, including a wide range of troubleshooting and service in potable water and non-potable HVAC and industrial applications, he is an expert in water treatment chemistry for cooling towers, boilers, and closed-loop systems.

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