Scaling analysis of heat exchanger

Common forms of heat exchangers, working principles and heat transfer media

In different temperatures between the fluid transfer of heat energy device called a heat exchanger.
In the heat exchanger at least two different temperatures of the fluid, a fluid temperature is higher, releasing heat.
The other fluid is a lower temperature, absorbing heat. Heat exchanger can be divided into heaters, coolers, condensers, evaporators, reboilers, deep coolers, superheaters, etc. according to the different uses.

The influence of the chemical composition of the heat exchange medium on the formation of fouling

The heat exchange medium is the auxiliary medium which is used for heat exchange with process materials, commonly used are water, oil, air and so on.
Water is the most common heat exchange medium, its impurity composition has a great impact on the formation of heat exchanger scaling.
Specifically, there are: impurities dissolved in water in the ionic or molecular state: calcium salts, magnesium salts, sodium salts.
Impurities in a colloidal state: iron compounds, microorganisms, sludge in the cooling circulating water, from the dust in the air and suspended matter in the supplemental water, gradually deposited in the heat exchanger with low flow rate.
Sticky scale: mainly the secretions of microorganisms and water sediment, corrosion products, bacteria and algae remains bonded to become, often attached to the heat exchanger wall.
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The physical and chemical properties of heat exchanger scale

In the heat surface and heat transfer surface deposited on the adhesion layer is often called scale.
In the heat exchanger, especially in the compressed cold plate and other circulating cooling heat exchanger, containing bicarbonate decomposition products and microbial sludge.
Carbonate scale is the main type of scale on the heat transfer surface of circulating cooling water system and heat exchanger.
The basic properties of carbonate scale: carbonate scale is white or off-white in appearance.
If the equipment has corrosion, it will be colored with corrosion products, carbonate scale is hard and brittle, firmly attached, difficult to peel off and scrape.
For circulating cooling water, the water quality should be tested regularly so that the water quality conforms to GB50050 “Water Quality Standard for Circulating Cooling Water”, when the water quality cannot reach the standard, the water quality should be treated according to the national standard GBJ50 “Industrial Circulating Cooling Water Treatment Design Specification”, which can effectively prevent the influence of scale on the heat transfer effect of the equipment.

The influence of the flow rate of the medium on the formation of scale in the heat exchange process

Appropriately increase the flow rate of the fluid, so that the deposits in the fluid is not easy to deposit, scaling, but the pressure drop of the heat exchanger increases.
constantly changing the flow direction of the fluid so that the fluid keeps hitting the wall of the heat exchanger tube, so that various impurities in the fluid are less likely to stay on the wall.
The choice of corrosion-resistant smooth materials can also slow down the formation of fouling.
In practice, we often check the pressure difference to determine whether the heat exchanger is fouling. The company also does this before each equipment maintenance stop.

The influence of the temperature of the medium in the heat exchange process on the formation of scale

Heat exchanger import and export temperature changes directly reflect the changes in heat transfer capacity of the heat exchanger.
Regular measurement of heat exchanger import and export flow, temperature, when the heat transfer capacity is low to meet the process requirements, it should be improved through mechanical cleaning or chemical cleaning heat transfer capacity to meet and maintain the needs of process operation.
When using water as a cooling medium, the water export temperature is best controlled at 50 ℃, because more than 50 ℃ will make the tube corrosion, heat exchanger scaling is serious, affecting the heat transfer capacity, so the export water temperature should not exceed 65 ℃.

Heat exchanger structure form the impact of scale formation

Practice found that the structure of the shape of the pipe has a great impact on scaling.
For example, corrugated pipe, which not only can force the fluid according to the prescribed path of multiple misflux through the bundle, so that the degree of fluid turbulence continued to be strengthened, but also to improve the heat transfer efficiency, while having a good anti-scaling ability.
Its anti-scaling mechanism is: the high speed turbulence of fluid in the flow channel makes it difficult for particles in the fluid to deposit scaling, even if a small amount of scale is generated, due to the turbulent flow of medium inside and outside the tube, the scouring of the tube wall is strong and the anti-scaling ability is strong.
In addition, the bellows on the existence of the strain due to the temperature difference between the tube process and the shell process stress, so that the curvature of the bellows with elastic characteristics of microscopic changes, so that the bellows heat exchanger has the ability to prevent and remove scale.

The influence of local environmental conditions on the formation of scaling

Operating temperature is higher or lower than the ambient temperature, some heat exchangers need to be insulated or cooled, insulation or insulation layer intact state directly affects the heat transfer performance of the heat exchanger.
Such as insulation or cooling once the destruction of the layer, the local environmental conditions change, will also accelerate the gathering of scale, the formation of scale layer.

Heat exchanger out of service during the impact on the formation of scale

Heat exchanger equipment during the annual overhaul out of service, if the internal water or set of liquid is not discharged in a timely manner or excluded incomplete, in the relative static conditions, more likely to form scaling.
Therefore, in the equipment out of use, especially in a longer period of time out of use, attention should be paid to the drainage of heat exchangers and maintenance, and

Reduce, eliminate the formation of scaling methods and measures

(1) From the design perspective to reduce and eliminate the formation of scaling conditions
The design of the heat exchanger is determined by calculating the economic and reasonable heat transfer area and the relevant structural dimensions of the heat exchanger to achieve the required heat transfer purposes.
When designing the structure, it is advisable to consider using special structures, for example, structures designed to produce turbulence, important heat exchange equipment can also consider setting up electronic descaling devices, backwashing systems, etc. If water is used as the heat exchange medium, materials such as anti-scaling additives are used when considering corrosion.
Besides, following simple design principles can also reduce or even eliminate the external conditions of forming scaling, for example, unclean and scaling-prone fluids should take the pipe course, because it is more convenient to clean the pipe course.
(2) Small flow rate or viscosity of the fluid should take the shell process, because the fluid in the shell process with baffle flow, due to the flow rate and flow direction of the continuous change, you can achieve turbulence, to prevent the formation of scaling.
(3) Corrosive fluid should go to the tube process, so as to avoid corrosion of the tube and shell at the same time, and the tube process is convenient for maintenance and replacement.
(4) The fluid to be cooled should take the shell process, can use the shell of the external heat dissipation, while the tube process is easy to replace after scaling.
(5) Saturated steam should take the shell process, because the steam is cleaner, not easy to scale, do not need to clean. Fluid flow rate selection: the choice of fluid flow rate involves the heat transfer coefficient, flow resistance and heat exchanger structure.
Increase the flow rate, can increase the convective heat transfer coefficient, reduce the formation of fouling, so that the total heat transfer coefficient increases; but at the same time make the flow resistance increases, the power of the heat exchanger structure.
but at the same time make the flow resistance increases, power consumption increases.
Choose a high flow rate, so that the number of tubes to reduce the heat transfer area of a certain, have to use a longer tube or increase the number of process, the tube is too long is not conducive to cleaning, single process into multiple processes so that the average heat transfer temperature difference decreased.

Therefore, generally need to choose the appropriate flow rate through a variety of trade-offs. Temperature difference is not large, the shell process media fouling is not serious, the shell process can be used when the chemical cleaning, the choice of fixed tube plate heat exchanger.

Source: Network Arrangement – China Tube Sheet Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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