Provided the heat exchanger is not completely clogged, it is possible to clean the exchanger by circulating a cleaning liquid (Cleaning in Place, CIP). Heat exchangers should therefore be cleaned at regular intervals. If the installation operates under difficult conditions, for example with hard water, installation of a heat exchanger with extra connections on the back for CIP piping is recommended to facilitate maintenance (see Figure 8.42). This makes it possible to connect and circulate the CIP solution through the system without having to disassemble the ordinary installation. The choice of cleaning solution depends on the problem, but a weak acid is a good start. This could be 5% phosphoric acid or, if the exchanger is cleaned frequently, 5% oxalic acid. The cleaning liquid should be pumped through the exchanger. For optimal cleaning, the flow rate of the cleaning solution should be at least 1.5 times the normal flow rate. Preferably, the flow should be in a back flush mode, which has a better chance of dissolving the scale because it attacks the deposits from the opposite direction.
After cleaning, the heat exchanger should be rinsed carefully with clean water. A solution of 1-2% sodium hydroxide (NaOH) or sodium bicarbonate (NaHCO3) before the last rinse ensures that all acid is neutralized. One way to get an indication of the appropriate rinse time is to test the pH of the liquid at the outlet from the heat exchanger. A quick and easy method is to use litmus paper. The pH should be 6-9.
The circulation system could be a vertical peristaltic pump. In this type of pump, the liquid is forced forwards by an eccentrically rotating wheel connected to an engine (see Figure 8.43).
Important features of a CIP pump:
- The reservoir for the CIP solution should be manufactured in acidand alkali-resistant material.
- Hoses should be made in PVC.
- It is an advantage if the pump is provided with a reverse flow device.
- Using a model with a reverse flow device makes it possible to attack scale from both directions.
- It is an advantage if the pump is equipped with a heating device. Heating the CIP solution usually increases the cleaning effect.
- The required flow rate capacity depends on the size of the heat exchanger.
Eliminating the Scaling Problem: Principles
There are several ways to eliminate the scaling problem. Usually, a commercial product containing additives to enhance the effect and/or prevent corrosion can be employed. Do not use any product containing ammonia if the filler material of the brazed plate heat exchanger is copper. Take great care when using strong inorganic acids such as hydrochloric, nitric or sulfuric acids, because they are extremely hazardous. Under certain conditions, hydrochloric acid can corrode stainless steel in minutes, and nitric acid corrodes copper.
Chemical cleaning is the use of chemicals to dissolve or loosen deposits from process equipment and piping. Removal is uniform and generally at a lower overall cost. In principle, there are two steps in this process. The last step can sometimes be excluded.
Step 1: Chemical Cleaning Solutions
Mineral acids such as hydrochloric acid (HCl), sulfamic acid (NH2SO3H), nitric acid (HNO3), phosphoric acid (H3PO4) and sulfuric acid (H2SO4) have a good ability to dissolve scale. However, they can also corrode the stainless steel or copper if used improperly. Organic acids are much weaker than mineral acids, in terms both of their dissolving ability and their ability to corrode the base material of the brazed plate heat exchanger. This makes these acids more useful when attempting to remove scale from the brazed plate heat exchanger because they are potentially less dangerous. They are often used in combination with other chemicals to bind the scale into complexes. Another advantage of organic acids is that they can be disposed of by incineration. Organic acids include formic acid (HCOOH), acetic acid (CH3COOH) and citric acid (C3H4(OH)(COOH)3).
Phosphoric acid is sometimes used at 2% concentration and 50°C for 4-6 h to pickle and passivate steel piping. It is not as effective as HCl in removing iron oxide scale, but is preferred for cleaning stainless steels. Formic acid is generally used as a mixture with citric acid or HCl, because alone it is unable to remove iron oxide deposits. Formic acid can be used on stainless steel. It is relatively inexpensive and can be disposed of by incineration. Acetic acid is used to clean calcium carbonate scale, but it is ineffective in removing iron oxide deposits. Because it is weaker than formic acid, it may be preferred where extremely long contact times are necessary. Inhibitors are specific compounds that are added to cleaning chemicals to diminish their corrosive effect on metals. Finally, surfactants, or detergents, are added to chemical cleaning solutions to improve their wetting characteristics. They are also used to improve the performance of inhibitors, and act as detergents in alkaline and acidic solutions.
Step 2: Passivation
A passive surface is one where the corrosion rate is reduced due to the precipitation of corrosion products on the metal surface. These corrosion products usually consist of oxides that inhibit further corrosion in water or in air. The term passivation is applied to procedures that are used to remove surface iron contamination from stainless steel equipment. To passivate stainless steels, mild iron contamination may be removed using a mixture containing 1% each of citric and nitric acids. For more persistent contamination, strong nitric acid solutions must be used.