Maintenance of Plate Heat Exchanger 2

Plate heat exchanger is in use. Due to improper operation of water treatment equipment. The water quality control is not up to the standard. The unqualified softened water is injected into the system, which makes the calcium, magnesium and carbonate in the water decompose into calcium carbonate and magnesium hydroxide precipitates after being heated and stick on the heating surface of the heat exchanger, forming a hard scale. Due to the poor thermal conductivity of scale, the heat exchange efficiency of heat exchanger is reduced and the heat energy is wasted seriously. Thus, the effect of heat transfer is affected.

Basic principle of scale removal:

1) Dissolution: the acid solution is easy to react with calcium, magnesium and carbonate saltwater scale to generate soluble compounds, which make the scale dissolve.

2) Stripping effect: acid solution can dissolve oxide on metal surface, destroy the combination with scale. Thus, the scale attached to the metal oxide surface is peeled off. And fall off.

3) Airlift: a large amount of carbon dioxide is produced after the acid solution reacts with calcium, magnesium and carbonate scale. Carbon dioxide gas is in the process of spilling. For the scale layer which is difficult to dissolve or slow to dissolve, it has a certain lifting power, which makes the scale fall off from the heating surface of the heat exchanger.

4) Looseness: for scale containing silicate and sulfate mixture, as the oxides of calcium, magnesium, carbonate and iron dissolve in the acid solution, the remaining scale will become loose and easily washed down by the flowing acid solution.

Specific steps of chemical descaling:

1) Flushing: before acid cleaning, the heat exchanger shall be flushed in an open way to ensure that there is no mud, scale and other impurities inside the heat exchanger, which can not only improve the effect of acid cleaning, but also reduce the acid consumption of acid cleaning.

2) Pour the cleaning solution into the cleaning equipment and then into the heat exchanger.

3) Pickling: soak the heat exchanger filled with acid solution in static state for 2h. Then the dynamic cycle was continued for 3-4 hours. The positive and negative cleaning was carried out alternately every 0.5 H. After pickling, if the pH value of the acid solution is greater than 2, the acid solution can be reused. Otherwise, the acid solution shall be diluted and neutralized and discharged.

4) Alkali washing: after acid washing, NaOH, Na3PO4 (trisodium phosphate) and softened water shall be prepared in a certain proportion, and the heat exchanger shall be alkali washed in the way of dynamic circulation to achieve acid-base neutralization, so that the plates of the heat exchanger are no longer corroded.

5) Water washing: after alkali washing, wash the heat exchanger with clean softened water for 0.5h repeatedly, and thoroughly wash the residue in the heat exchanger.

6) Record: During the cleaning process, the time of each step shall be strictly recorded to check the cleaning effect. In short, after cleaning, the heat exchanger should be pressure tested. It can only be used after it is qualified.