A case study in the replacement of Spiral Heat Exchangers with Sacome’s technically advanced Shell and Tube Heat Exchangers.
The problem on site was the continual blocking of the existing Spiral Heat Exchanger used to heat the sludge prior to digestion. This resulted in unacceptable downtime and frustration for all concerned.
The Waste Water Treatment Plant (WWTP) concerned processes approximately 100 m3/hour of waste water. The temperature of the sludge as it enters the digester is critical because it affects the amount of time required for digestion. Studies show that at 30ºC 10 days are required digestion, while at 20ºC 25 days are required. So, the selection of the right heat exchanger is essential.
No two sewage treatment plants are alike since the sludge is an organic product. The thermal properties of these products, specifically the viscosity, can change significantly. Other thermal properties such as density, specific heat and thermal conductivity are also taken into account in the design of the heat exchanger for the particular plant. Once the sludge specification had been defined this was compared to Sacome’s in-house database where records of comparable dry solids equivalents are kept for this work. In this particular case there was no need for rheological testing (study of liquid flow characteristics) but such tests are routinely carried out in the labs at Sacome’s head office especially where process guarantees are required for a particular heat exchanger.
There are a number of different geometries of heat exchangers available today including Plate Heat Exchangers and Tubular Heat Exchangers. Plate Heat Exchangers are generally not suitable for sewage sludge which is why spiral and tubular types are in widespread use in the WWTP’s. Thicker, more viscous sludges with higher solids and fibers contents can block the channels requiring frequent cleaning and inevitable shutdowns. Sacome’s Shell and Tube Heat Exchangers have a very much larger process channel, avoiding the risk of blocking.
Due to the high levels of solids in the sludge Sacome’s model TF20 were selected as the ideal solution because:
• Mono-tubular design. The TF20 model consists of 2 concentric tubes in which the water used as heating medium flows through the annular space, while the sewage sludge flows through the inner tube. Unlike the traditional multi-tube, composed of a tube bundle with smaller inner tubes, there is only one process channel.
• Modular design. As it is a modular design, it is easy to dismantle the units for cleaning or to add more modules to handle additional capacity.
• Process duty. The heat exchanger can process a wide range of capacities and can be configured with several lines in parallel so there is no need to stop production for cleaning.
• Corrugation. The inner tubes are hard corrugated in order to increase the turbulence of the product. This reduces the risk of fouling and increases.
• Materials of Construction. Due to the presence of phosphate and ammonium precipitated (struvite) 316L stainless steel was recommended. This is more resistant to corrosion than 304L.
• Insulation. In this particular case the heat exchangers were insulated with 40 mm of mineral wool clad in aluminum to reduce the heat losses to the environment.
Installation and start-up
The equipment was installed on site by the clients engineers with technical support from Sacome. Since start-up it has been running successfully. With over 100 reference sites for similar WWTP projects Sacome can be trusted to find the right solution.