Heat exchanger section: [Weekly Topic] What are the methods for preventing scaling on heat exchange tube surfaces? (2011.7.25-7.31)

I. Manual or mechanical methods: When the tube bundle is slightly blocked or covered with scale, cleaning is carried out using manual or mechanical methods such as scraping and wire brushing, along with flushing using compressed air, high-pressure water, and steam. When the pipes are severely scaled or completely blocked, a tubular flushing drill (also known as a pipe cleaning machine) can be used for cleaning. II. Irrigation method: There are two types of irrigation methods. The first method is reverse flow flushing, which is generally used during operation or during short periods of shutdown. It does not require disassembling the device, but a reverse flow bypass line must be pre-installed in the equipment; this method is quite effective when scaling is not severe. The second method is high-pressure water gun flushing. Different rotating water nozzles are used for various heat exchangers; these nozzles can be rigid or rotatable, and the pressure can be adjusted freely from 10 MPa to 200 MPa. High-pressure water is used to remove dirt, and it is applicable to both between pipes and inside pipes as well as in the housing. High-pressure water guns are effective for cleaning heat exchangers. Widely applied. III. Chemical descaling: Scaling in the tube side of heat exchangers is mainly caused by poor water quality, which leads to the formation of scale as well as the deposition and adhesion of oil residues. To remove this scale using chemical methods, it is first necessary to conduct laboratory analysis of the scaling substances in order to determine their properties, thereby deciding which solvent should be used for cleaning. Generally, alkaline cleaning (such as soda ash, caustic soda, trisodium phosphate, etc.) is used for sulfate and silicate scale, while acidic cleaning (such as hydrochloric acid, nitric acid, phosphoric acid, hydrofluoric acid, etc.) is used for carbonate scale. To remove oil scale and carbonization, a cleaning solution can be prepared by mixing sodium hydroxide, sodium carbonate, laundry detergent, liquid detergent, sodium silicate, and water in certain proportions for cleaning purposes. Using chemical cleaning requires re-piping on site, which is time-consuming.

There are now many efficient heat exchange tubes that are inherently resistant to scaling, such as corrugated tubes and spiral tubes.

Some preventive and corrective measures (personal opinions): 1. Strictly monitor various parameters of the circulating water – pH value, concentration ratio, as well as the levels of Ca2+, Mg2+, Zn2+, PO43—, and CO32; the total alkalinity should be kept at ≤420 mg/L. 2. Depending on the conditions of the system, a certain amount of corrosion inhibitor and scale inhibitor should be added to the circulating water regularly (make sure to control the dosage properly). 3. Strictly control the turbidity of the circulating water; when the turbidity is high, it is necessary to increase the rate of water replacement, and at the same time activate the bypass filtration system to reduce the turbidity as quickly as possible. 4. Monitor and record the water volume and temperature in the shell-side heat exchanger; strive to increase the water volume and reduce the outlet temperature, keeping it below 40 degrees Celsius. 5. If scaling is detected on the heat exchanger, physical or chemical cleaning of the heat exchanger shall be carried out during maintenance.

1. Strictly monitor all indicators of circulating water. 2. Monitor and record the water volume and temperature in the shell-side heat exchanger; strive to increase the water volume and reduce the outlet temperature, keeping it below 40 degrees Celsius. 3. If scaling is detected on the heat exchanger, physical or chemical cleaning of the heat exchanger shall be carried out during maintenance. 4. Control the process parameters during use.

Proper use and control are highly beneficial for the lifespan and performance of heat exchangers

Chemical cleaning and high-pressure water jet cleaning can be used

Heat exchangers are designed to control the flow rate; selecting a slightly higher flow rate, as long as the pressure drop remains within acceptable limits, can effectively prevent scaling.