**3.1 Introduction**

542 Heat Exchangers – Basics Design Applications

Nile water treated with NALCO inhibitors caused fouling problems inside CO2 coolers in different Egyptian ammonia plants. Technical solutions including redesigning of the PHEs and new plate geometries were investigated. Reducing the surface area of the CO2 coolers by 34% increased the gap velocity from 0.30 to 0.42 m/s (shear stress from 5.31 to 10.84 Pa) and hence decreased fouling. The operation time for the cooler was increased from 30 days to 43 days. NT-plates with asymmetric flow distribution over the channels decreased the

In a recent study, Nano-composite coatings were used to reduce fouling inside gasketed plate heat exchangers involved in food production. An antifouling coating with low surface energy (low wettability) led to a hydrophobic and oleophobic effect. The goal of the project was the application of new surface coatings (nanotechnology) to avoid or minimize adhesion, improve process management, simplify cleaning processes with lesser resources

The test facility constructed by the Institute of Environmental Process Engineering (IUV) at the University of Bremen in Germany used for the investigation of milk adhesion and the stability of the coatings on small cylindrical ducts. A number of coatings and surface treatments were tested. A pilot plant including a milk pasteurizer at the Institute of Food Quality LUFA Nord-West in Oldenburg-Germany was used for the thermal treatment of whey protein solutions. Heat exchanger plates coated with different nano-composites as well as electropolished plates installed in the heating section of the pasteurizer were tested. Significant differences were observed between coated and uncoated plates. The coated plates showed reduced deposit buildup in comparison with the uncoated stainless steel plates. Polyurethane-coated plates exhibited the thinnest deposit layer. Electro-polished

Fig. 8. Deposits formed on the surface of NT-plate.

rate of deposition on the surface of the plates.

**3. Solving fouling problems by surface modification** 

and chemical use, and increase the product reliability.

**2.5 Conclusions** 

Production problems, like decrease of production rate and increase in the intensity of cleaning procedure, arise in the dairy industry as a result of the deposit adhesion to the plate surface. The deposits must be removed by regular and intensive cleaning procedures in order to comply with hygiene and quality regulations for the dairy industry (Augustin et al., 2007). If not controlled carefully, deposits can cause deterioration in the product quality because milk cannot be heated up to the required pasteurization temperature. Milk deposits generally form so fast that heat exchangers must be cleaned regularly to maintain production efficiency and meet strict hygiene standards and regulations (Bansal and Chen, 2006). Energy losses, lost productivity, manpower and cleaning expenses cause immense costs (Beuf et al., 2003). In the dairy industry, fouling and the resulting cleaning of the process equipment account for about 80% of the total production costs (Bansal and Chen, 2006).

Gasketed plate heat exchangers with stainless steel plates are commonly used in the dairy industry. Stainless steel surfaces have high surface energies. The adhesion of product on solid surfaces is determined by the surface roughness and surface energy. The adhesion of deposits could be reduced by either decreasing the surface energy of the metal or by coating the metal surface with high anti-adhesion effect (low surface energy) materials, such as those made of nanoparticles (Gerwann et al., 2002). The application of nano-coatings with their anti-adhesion effects reduces the buildup of deposits on the surface of heat exchanger plates due to the reduction of adhesive forces. The operation efficiency of the plant can be significantly improved and the general hygienic situation of the product can increase. Additionally, intensity and frequency of cleaning can be substantially reduced to achieve the desired degree of product quality (Kück et al., 2007).

Beuf et al. (2003) studied the fouling of dairy product on modified stainless steel surfaces in a plate and frame heat exchanger. Different surface modifications, such as coatings (diamond like carbon [DLC], silica, SiOX, Ni-P-PTFE, Excalibur, Xylan) and ion implantation (SiF+, MoS2) were analyzed. No significant difference was found between the modified stainless steels and the unmodified one. The cleaning efficiency of plates coated with Ni-P-PTFE was the best. The experimental results of Zhao et al. (2007) showed that the surface free energy of the Ni–P–PTFE coating had a significant influence on the adhesion of bacterial, protein and mineral deposits. The Ni–P–PTFE coating reduced the adhesion of these deposits significantly.

The fouling behavior of whey protein solutions on modified stainless steel (SS) surfaces coated with diamond-like carbon (DLC) and titanium nitride (TiN) have been studied by Premathilaka et al. (2007). They concluded that fouling decreased in the order DLC > SS > TiN and cleaning time decreased in the order TiN > SS > DLC.

The goal of the present work is to assess new surface coatings (developed by the Institute of New Materials, INM, in Germany) with low surface energy and low roughness to avoid or minimize adhesion of deposits, simplify cleaning processes, reduce resource and chemical

Fouling in Plate Heat Exchangers: Some Practical Experience 545

Fig. 9. Flowchart of laboratory heat exchanger apparatus (Institute of Environmental Process

For the experiments, a 10% (by weight) aqueous whey protein solution was set in the receiver tank. The solution was prepared by solving a whey protein concentrate WPC35 in water until the required concentration was obtained. The pH was adjusted to 6.0 using a 0.1 mol/liter HCl solution. Pre-heating was carried out to about 43 °C. The solution was pumped in the closed cycle of the experimental setup, the electric heater of the test channel was activated and the measuring procedure was started. After each trial, the whey protein solution was replaced to exclude any effect of heating on the ingredients. After each run, the tube was cleaned with 0.1 molar NaOH solution with cross flow velocity of 0.6 m/s. The

Industrial tests with milk were carried out on a small plant by the Institute of Food Quality LUFA-Oldenburg-Germany, with the support of the company GEA PHE Systems (Figure 10). The pilot plant can produce almost all dairy products. It is used for training purposes as

well as technological support and procedure development to the food industry.

Engineering IUV, Universität Bremen).

experimental parameters were:

Experimental time: 15 to 30 min.

Heat flux: 20 kW/m²

**3.2.3 Pilot plant testing** 

Volumetric flow rate: 0.036 – 0.37 m3/h Whey protein concentration: 10% (by weight) Average flow velocity in annulus: 0.2 m/s Fluid temperature (measuring section): 45 °C Temperature of the heating element: 230 °C

requirements, and increase product quality and consistency. The work will assess the deposit buildup during the thermal treatment of milk.
