How to Control Fouling Thermal Resistance of Coating Machine Finned Tube Heat Exchangers

How to Control Fouling Thermal Resistance of Coating Machine Finned Tube Heat Exchangers

The environment in which the finned tube heat exchanger for coating machine is used is usually dusty. The finned surface of the finned tube heat exchanger is prone to deposit dust, paint dust, fibers, and other impurities to form a fouling layer. Fouling will increase the fouling thermal resistance, resulting in a decrease in the total heat transfer coefficient, which ultimately reduces drying efficiency and increases energy consumption.

Through the design optimization of fins and runners, reducing the possibility of dust adhesion from the source is the core means of controlling fouling thermal resistance.

Targeted design of fin parameters

Enlargement of fin spacing: Under the premise of meeting the demand for heat transfer area, the fin spacing is appropriately increased (e.g., from the conventional 2-3mm to 4-6mm). Dust particles (especially paint dust that may exist in the coating environment, with a particle size of 10-50μm) are easier to be carried away by the airflow in a wider spacing, reducing the probability of getting stuck between the fins. However, it should be noted that too much spacing reduces the heat transfer area per unit volume, which should be compensated by increasing the fin height or length.

Fin shape optimization: Use flat or low corrugated fins (reduce sharp corners), avoid sawtooth fins, high corrugated fins with “notch” structure - these structures are prone to form airflow vortex zones, leading to dust deposition in the dead space. If enhanced heat transfer is required, complex fin shapes can be substituted by increasing the air flow rate.

Fin Surface Inclination: Design fins at an angle (e.g., 10-15°) to the base tube to allow gravity to assist in the natural sliding off of dust during shutdown (especially for dry, non-sticky dust).

Flow path layout and infusion design

Avoiding localized low velocity zones: CFD simulation is used to optimize the flow field of air in the heat exchanger to ensure that the airflow is uniformly swept over the surface of the fins and to reduce the “dead zones” (zones where the flow velocity is < 1m/s) - low velocity zones are the hardest hit areas for dust deposition. . A deflector plate can be added to the heat exchanger inlet to distribute the airflow evenly along the length of the fins and avoid localized vortices.

Reduce the height of fins: too high fins (e.g. > 15mm) will lead to an increase in the difference between the flow rate at the root and the top of the fins (low flow rate at the top, easy to accumulate dust), short fins (8-12mm) can be used with an increase in the number of fins to ensure that the total area of heat exchange while reducing the risk of deposition brought about by the uneven distribution of airflow.

Through filtration and purification means to reduce the concentration of dust in the air entering the heat exchanger, to reduce the source of dirt from the source, is the key antecedent measure to control the thermal resistance of dirt.

Multi-stage air filtration system

Fresh air filtration at the inlet of coating machine: Multi-stage filters are set up at the air inlet of the heat exchanger (usually connected to the drying room of the coating machine):

The first stage: coarse filter (e.g., G4, filtering particles of more than 5 μm), intercepting large particles of dust and fibers;

The second stage: intermediate filter (e.g., F7, filtering particles of more than 1 μm), aiming at the micro-dust of the paint generated in the process of coating (the size of 2-10 μm), and reducing the risk of depositing dirt and heat resistance. The second stage: medium efficiency filter (e.g. F7 grade), for coating dust (particle size mostly 2-10μm) generated during the coating process, significantly reducing the total amount of dust entering the heat exchanger.

The filter should be replaced periodically (according to the differential pressure gauge, when the resistance exceeds 1.5 times of the initial value), to avoid clogging of the filter itself, resulting in a decrease in air flow rate, which in turn aggravates the accumulation of dust on the fins.

Local airflow isolation

If there is an obvious source of dust near the coater (e.g., paint mixing area, winding area), a “positive pressure isolation hood” can be set up at the inlet of the heat exchanger to prevent dusty air from seeping in by introducing a small amount of clean air (e.g., fresh air from outside of the workshop) to create a positive pressure inside the hood.

By adjusting the operating conditions of the heat exchanger (e.g., air speed and temperature), the adhesion of dust on the fin surface is reduced to avoid hardening of the dirt layer (which is more difficult to remove after hardening).

Control air flow rate and turbulence

Maintain a high air velocity: the air flow rate outside the tube is controlled at 3-5m/s (conventional heat exchanger air velocity is mostly 2-3m/s). Higher flow rate can enhance the airflow on the fin surface of the “blowing” effect, so that the dust is difficult to settle; at the same time, increased turbulence can reduce the thickness of the boundary layer, partially offsetting the initial fouling caused by heat transfer losses.

Note: The wind speed needs to match the fan power, to avoid excessive wind speed leading to wind resistance surge, energy consumption exceeds the standard (need to calculate the fluid dynamics simulation to balance the wind speed and energy consumption).

Control the surface temperature of fins

The hot air temperature for drying of coating machine is usually 60-120°C. If the surface temperature of fins is too low (e.g., close to the dew point), the water vapor in the air will condense on the surface of the fins, causing the dust particles to stick together and form “wet and sticky dirt” (high hardness, difficult to remove). It is necessary to control the temperature of the heat medium (e.g. steam pressure) to ensure that the fin surface temperature is higher than the ambient dew point temperature by 5-10°C to reduce condensation and sticking.

For high temperature drying scenarios (>100℃), it is necessary to avoid localized overheating of the fins leading to carbonization of the dust (carbonization forms a hard layer of coke), and optimize the distribution of the heat medium (e.g., uniform distribution of steam flow) to ensure a uniform temperature of the fins.