**1. Introduction**

532 Heat Exchangers – Basics Design Applications

[68] Kazi, S. N., Duffy, G. G. And Chen, X. D., Mineral scale formation and mitigation on

metals and a polymeric heat exchanger surface. Applied Thermal Engineering, 30

[67] Eberhard, J. F. and Rosene, R. B., Removal of Scale Deposits. 1961: U. S. A.

[66] Reich, C. F., Scale Removal. 1961: U. S. A.

(2010): p. 2236-2242.

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:


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

Fouling in Plate Heat Exchangers: Some Practical Experience 535

Fig. 2. Comparison of fouling resistance in PHE to tube-side fouling resistance (Müller-

intensity and frequency of cleaning can be substantially reduced.

**2. Solving fouling problems by heat exchanger design modification** 

Fouling problems with cooling water inside CO2 coolers in different Egyptian fertilizer plants were investigated. Thermodynamic and hydraulic solutions were proposed, which included redesign of the existing PHEs and new plate geometries. The main problems arose from the large surface margins required to meet pressure drop limits on the CO2 side. Reducing the surface area of the heat exchanger increased the fluid velocity (shear stress from 5.31 to 10.84 Pa) inside the gaps and hence decreased fouling. Using computer-

This chapter focuses on solving fouling problems in some industrial applications. The first section presents fouling problems with cooling water inside CO2 coolers in different Egyptian fertilizer plants. The effect of heat exchanger geometry and flow patterns on the fouling behavior will be shown. Thermodynamic and hydraulic solutions are proposed like, redesigning of the plate heat exchangers and new plate geometries. The second section explains how fouling can be reduced inside gasketed plate heat exchangers used in food production using Nano-composite coatings. An antifouling coating with low surface energy (low wettability) can be used to avoid or minimize adhesion, improve process management, simplify cleaning processes with less resources and chemical use, and increase product reliability. The operational efficiency of the plant can be significantly improved and the

Steinhagen, 2006).

Fig. 1. Comparison of plate heat exchanger with shell and tube heat exchanger.

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:


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).

Fig. 1. Comparison of plate heat exchanger with shell and tube heat exchanger.

much lower than that inside the shell and tube heat exchanger.

Fouling inside heat exchanger can be reduced by:

 Mitigation methods (mechanical and/or chemical) Heat exchanger surface modification/coating

 Appropriate heat-exchanger design Proper selection of heat-exchanger type

fouling (Müller-Steinhagen, 1999).

production efficiency.

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

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

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

Fig. 2. Comparison of fouling resistance in PHE to tube-side fouling resistance (Müller-Steinhagen, 2006).

This chapter focuses on solving fouling problems in some industrial applications. The first section presents fouling problems with cooling water inside CO2 coolers in different Egyptian fertilizer plants. The effect of heat exchanger geometry and flow patterns on the fouling behavior will be shown. Thermodynamic and hydraulic solutions are proposed like, redesigning of the plate heat exchangers and new plate geometries. The second section explains how fouling can be reduced inside gasketed plate heat exchangers used in food production using Nano-composite coatings. An antifouling coating with low surface energy (low wettability) can be used to avoid or minimize adhesion, improve process management, simplify cleaning processes with less resources and chemical use, and increase product reliability. The operational efficiency of the plant can be significantly improved and the intensity and frequency of cleaning can be substantially reduced.

## **2. Solving fouling problems by heat exchanger design modification**

Fouling problems with cooling water inside CO2 coolers in different Egyptian fertilizer plants were investigated. Thermodynamic and hydraulic solutions were proposed, which included redesign of the existing PHEs and new plate geometries. The main problems arose from the large surface margins required to meet pressure drop limits on the CO2 side. Reducing the surface area of the heat exchanger increased the fluid velocity (shear stress from 5.31 to 10.84 Pa) inside the gaps and hence decreased fouling. Using computer-

Fouling in Plate Heat Exchangers: Some Practical Experience 537

In the CO2 scrubbing process three plate heat exchangers are switched in parallel, two in operation (A and B) and one in standby (C). Figure 4 shows the three coolers with their operating conditions. The CO2 flows into the PHEs as a gas-steam mixture at 94 °C and is cooled down in a countercurrent process to 33 °C. Water at 30 °C is used as coolant. Each of the 10 tons and 3 meter high PHEs has 1000 m² of high-performance stainless steel (1.4539;

AISI 904 L) VT-plates. The transferred heat capacity is 14.5 megawatts.

Fig. 4. CO2 coolers used in the scrubbing process.

Fig. 3. Ammonia process [Uhde].

modeled plate geometries from the new technology (NT) series with larger gap velocities due to better fluid distribution over the plates could decrease fouling and increase the availability of fertilizer plants.
