Fouling in Plate Heat Exchangers: Some Practical Experience

Due to their compact size, Plate Heat Exchangers (PHEs) are widely used in industrial processes. They have higher heat-transfer performance, lower temperature gradient, higher turbulence, and easier maintenance in comparison with shell and tube heat exchangers. For minimizing material consumption and space requirements compact models have been developed over the last years. By using thin plates forming a small gap, these compact models impress with larger heat transfer coefficients and, thus, smaller required heat transfer area. 

The advantages of compact heat exchangers over shell and tube ones at a glance:

• larger heat transfer coefficients
• smaller heat transfer surfaces required
• lower fouling due to high fluid turbulences (self-cleaning effect)
•  significantly smaller required installation and maintenance space
•  lighter weight
•  simplified cleanability especially for GPHE
•  lower investment costs
•  closer temperature approach
•  pure counter-flow operation for GPHE

In Figure 1, plate heat exchangers are compared with shell and tube heat exchangers regarding effectiveness, space, weight and cleaning time.

Deposits create an insulating layer over the surface of the heat exchanger that decreases the heat transfer between fluids and increases the pressure drop. The pressure drop increases as a result of the narrowing of the flow area, which increases the gap velocity (Wang et al., 2009). Therefore, the thermal performance of the heat exchanger decreases with time, resulting in an undersized heat exchanger and causing the process efficiency to be reduced.

Heat exchangers are often oversized by 70 to 80%, of which 30 to 50% is assigned to fouling.

While the addition of excess surface to the heat exchanger may extend the operation time of the unit, it can cause fouling as a result of the over-performance caused by excess heat transfer area; because the process stream temperature change greater than desired, requiring that the flow rate of the utility stream be reduced (Müller-Steinhagen, 1999). The deposits

must be removed by regular and intensive cleaning procedures in order to maintain production efficiency.

As a result of the effects of fouling on the thermal and hydraulic performance of the heat exchanger, an additional cost is added to the industrial processes. Energy losses, lost productivity, manpower and cleaning expenses cause immense costs. The annual cost of dealing with fouling in the USA has been estimated at over $4 billion (Wang et al., 2009).

The manner in which fouling and fouling factors apply to plate exchangers is different from tubular heat exchangers. There is a high degree of turbulence in plate heat exchanger, which increases the rate of deposit removal and, in effect, makes the plate heat exchanger less prone to fouling. In addition, there is a more uniform velocity profile in a plate heat exchanger than in most shell and tube heat exchanger designs, eliminating zones of low velocity which are particularly prone to fouling. Figure 2 shows the fouling resistances for cooling water inside a plate heat exchanger in comparison with fouling resistances on the tube-side inside a shell and tube heat exchanger for the same velocity. A dramatic difference in the fouling resistances can be seen. The fouling resistances inside the PHE are much lower than that inside the shell and tube heat exchanger.

Fouling inside heat exchanger can be reduced by:

•  Appropriate heat-exchanger design
•  Proper selection of heat-exchanger type
•  Mitigation methods (mechanical and/or chemical)
•  Heat exchanger surface modification

The mechanics of deposits build-up and the impact of operating conditions on the deposition rate should be understood in order to select the appropriate method to reduce fouling (Müller-Steinhagen, 1999).

Deposits were accumulated at an area about 20cm from the plate inlet and selectively covered the plate surface, as can be seen in Figure 5. They could plug the channels and restrict the water flow over the plate. These deposits accumulated due to the reduction of the gap velocity (shear stress) which increased the surface temperature.

 Solving fouling problems by heat exchanger design modification: Redesigning of the PHEs

The surface area of the CO2 cooler was reduced by removing 86 plates out of 254 plates (the surface area was reduced by 34%). The average cooling water velocity inside the gaps was increased from 0.30 to 0.42 m/s, as can be seen in Table 4.

The deposits formed on the surface of the plates were decreased as a result of the increase in the shear stress and the decrease of the surface temperature from 72 to 69°C. The surface temperature was calculated from the fluids temperatures, thermal conductivities and duties on both sides. The operation time for the cooler was increased from 30 days to 43 days and the plates were cleaned after more than 40 days of operation, as shown in Figure 6.

The CO2 outlet temperature started to increase after about 23 days of operation due to the accumulation of deposits on the cooling water side which led to a reduction in the cooling water flow rate. The unit was opened after about 43 days for mechanical cleaning.

Credits:

https://www.researchgate.net/publication/221927711_Fouling_in_Plate_Heat_Exchangers_Some_Practical_Experience

https://www.researchgate.net/profile/Ali-Bani-Kananeh