Anti-corrosion knowledge of heat exchanger condenser evaporator

- Aug 17, 2018-

1. Scale: Heat exchangers generally use a cooling tower cooling cycle.


The evaporation of water causes the concentration ratio of circulating water to increase. The solubility of slightly soluble salts such as CaCo3, Ca(PO4)2 and MgCO3 decreases with the increase of temperature, and it is easy to reach supersaturation and precipitate from water. These crystal deposits are easy to deposit. Scale is formed on the surface of the heat exchanger. 

It is firmly attached to the heat exchange surface and pipeline of the condenser of the unit heat exchanger. Its structure is dense and hard, and it is not easy to be washed away by water. In addition, the cooling water is in contact with the outside through the cooling tower to create conditions for the reproduction of microorganisms.

These microorganisms and muds of insoluble salts in water form crystal nuclei of CaCo3 salt-like microcrystals, accelerate the precipitation process of crystals such as CaCo3, and are easily deposited on the surface of the heat exchanger condenser to form biological sludge, thereby reducing The heat transfer efficiency of the heat exchanger.

(The average thermal conductivity of copper is 384W/mk, the average thermal conductivity of brass is 393W/mk, the thermal conductivity of various soils is roughly 0.06~3.5W/mk, and the thermal conductivity of carbonate is 0.464~0.697W/mk. )As time passes, fouling and scale corrosion in the heat exchanger system will become more and more serious.

Heat exchanger condenser equipment will have a reduced service life due to corrosion of the system. Impurities in the heat exchanger system can easily block the heat exchanger condenser, resulting in a significant decrease in heat exchanger efficiency.

When the anti-corrosion measures are not correct, the heat exchange surface of the evaporator of the heat exchanger condenser often has rust tumors attached. They are often deposited on the heat transfer surface together with scale, microbial slime, and the like.

The deposit formed by this rust-like corrosion product, in addition to affecting heat transfer, is more serious to promote the reproduction of certain bacteria, which eventually leads to corrosion and perforation of the heat exchange surface of the heat exchanger condenser.

Therefore, it is necessary to perform scale cleaning, heat exchanger cleaning and condenser cleaning of the circulating cooling water system.

 

2. heat exchanger metal corrosion and oxide scale:

Corrosion caused by dissolved O2 and CO2 in water: Both cooling water and chilled water dissolve a certain concentration of O2 and CO2, and in the cooling water heat exchanger system, water and air can be in full contact.

Therefore, O2 in the water can reach saturation. When carbon steel is in contact with water in which O2 is dissolved, many corrosive microbatteries are formed on the surface of the carbon steel of the heat exchanger due to the heterogeneity of the metal surface and the conductivity of the water.

The oxidation reaction and the reduction reaction respectively generated in the cathode region and the anode region of the microbattery produce a loose structure of Fe2O3. Without protection, these reactions will cause the heat exchanger metal in the cathode region of the microbattery to be continuously dissolved and corroded.


3. heat exchanger microbial corrosion and algae slime:

Heat exchanger corrosion caused by microorganisms: In the cooling water circulation system, water containing microorganisms is continuously replenished. At the same time, the water sprayed from above in the cooling tower captures a large amount of microorganisms from the encountering air into the cooling water system.

The water temperature range of the cooling water is particularly beneficial for the growth of certain microorganisms. The cooling water dissolves a large amount of oxygen during the spray aeration in the cooling tower, which provides the necessary conditions for aerobic microorganisms, and some deposits such as dirt provide a shelter for anaerobic microorganisms.

The growth of microorganisms causes corrosion of the heat exchanger metal. This is because deposits formed by microorganisms and mucus and inorganic scales and sediments are attached to the metal surface of the heat exchanger to form a concentration battery of oxygen, which causes corrosion of the metal of the heat exchanger.

The corrosion of the heat exchanger directly affects the service life of the heat exchanger equipment, and the rust generated by it also seriously affects the heat transfer effect of the heat exchanger, so that the energy consumption is greatly increased.

Algae: Algae in circulating water are mainly green algae, cyanobacteria and diatoms. They form a metal surface in the water to erode the battery and cause corrosion under heat exchanger deposits. The block also blocks the tubing in the heat exchanger condenser.

Reduce the flow rate of the heat exchanger water, thereby reducing the heat exchange efficiency of the heat exchanger condenser.