Water Authority Improves Efficiency of Sludge Heating
28 September 2012, News release from HRS Heat Exchangers
"Heat exchangers that are designed to handle raggy sludge and bring better efficiency and reliability to waste water treatment plants."
Covering the Midlands and mid-Wales, Severn Trent Water is one of the largest of the 10 regulated water and sewerage companies in England and Wales, serving over four million households and businesses. It has numerous wastewater treatment facilities where digesters are used to treat sewage and waste products. The digesters use naturally occurring anaerobic microorganisms, which break down the sludge to produce methane and carbon dioxide. The biological process also breaks down organic solids within the sludge and greatly reduces the volume of final waste and associated cost for its disposal. The methane biogas formed is a renewable energy source and used to provide heat and power across the water treatment plants.
Mesophilic Digesters operate at around 37degC. HRS heat exchangers maintain these digesters at this temperature in the most thermally efficient manner with minimum fouling and maintenance.
The Problem: Existing heat exchangers prone to blocking, resulting in low efficiencies and high maintenance
Spiral heat exchangers have been used across many water treatment sites, selected for their small footprint and easy installation within restricted available space. The units have two pathways, which are very close together: one for hot water (used for heating) and one for the sludge requiring heating. The 'raggy' sludge (containing tissue and other fibrous solids), which passes through one of the tubes, has a tendency to catch around the studs within the spiral chamber and create blockages. These blockages reduce the heat transfer area and so prevent the unit from achieving its designed thermal duty. As the units block up the pressure drop across them also increases which, in turn, stops the pumps from working effectively and so the process degrades.
Blockages within installed spiral heat exchangers led to increasingly inefficient operation and high maintenance costs for Severn Trent. In the worst case, one site had a unit that was working so inefficiently it required unblocking on a daily basis. As units were not performing their required thermal duty, there was added concern that significant temperature drops within the digester tanks would make the sludge inactive, no longer producing methane, and necessitating costly, alternative disposal.
The units needed to be replaced but, to make this cost effective, consideration also needed to be given to other piping and machinery that had proven reliability and was functioning well.
The Challenge: Effective replacement of unreliable units working within the small footprint available
Inefficiencies and large ongoing maintenance costs necessitated the replacement of the spiral heat exchangers on the digesters. This represented a number of challenges for the project team:
1.The spiral units were selected for their small footprint and replacement units needed to fit within the space available on the sites.
2.Severn Trent did not wish to replace existing, reliable water and sludge pumps so both sides of the replacement heat exchangers would need to produce pressures that would suit existing equipment.
3.The heat exchanger design needed to have minimal impact on existing site pipework which was robust and reliable.
4.A heat exchanger needed to be provided that did not block with raggy sludge and met the required thermal duty with high efficiency, high reliability and minimal maintenance requirements.
The Solution: Industrial tube in tube heat exchangers in various formats to meet individual site needs
The Severn Trent Digester Team contacted HRS Heat Exchangers to discuss the problems they were experiencing with the sludge heating. HRS recommended its DTI series industrial double tube heat exchangers. The DTI is a tube in tube design with tubes sized to allow large particles to pass through and keep pathways clear, ideal for the raggy sludge Severn Trent were handling. The internal tube is corrugated which creates a turbulence in the flow. This turbulence improves the heat transfer coefficient because it mixes the fluid whereas a laminar flow relies solely on the thermal conductivity of the fluid to transfer heat from the middle of the flow to the heat exchanger walls. For raggy sludge this turbulence also has the significant added benefit of naturally reducing the risk of any fouling within the tube.
HRS designed each DTI heat exchanger specifically to the individual Severn Trent site requirements based on detailed surveys. Drawings were produced showing how and where individual units would fit on site and highlighting any other equipment that would need to be moved. This was a real team effort between the Severn Trent Digester Team, the installation contractors and HRS.
The first site at Claymills had three spiral heat exchangers installed. HRS attended site and conducted surveys on the worst performing of these units. A replacement unit was designed and installed on site and left to run for a few months. The DTI unit worked perfectly and Severn Trent proceeded to replace all of the spiral units on site and then roll out the replacement programme across other sites over a three and half year period.
The time to complete individual installations varied. At Severn Trent Barston where the spiral heat exchanger was blocking daily and had become a significant maintenance overhead, HRS delivered the customised unit within around seven weeks of initial site survey.
The heat exchangers were designed in various forms to meet site constraints and provide the required thermal duty. Some units were in vertical form with heights of up to six metres, others were wide with a very low height or in a zigzag form to gain the required thermal length with the restricted space available. Water connections were designed to match existing pipework and, where required, split water flows were used with two inlets and two outlets to match pump specifications. HRS upgraded 11 units across five Severn Trent sites with installations as follows:
Severn Trent Claymills: Three units, 1x9 HRSDTI 3 metres long (27 metres of thermal length)
Severn Trent Rugby: One unit, 1x6 HRSDTI 4 metres long (24 metres of thermal length with a special frame to suite site concrete foundations)
Severn Trent Wanlip: Four units, all 1x8 HRSDTI 6 metres long (24 metres of thermal length with Zigzag, Standard and Vertical layouts to match requirements and site space available)
Severn Trent Hayden: Two units, both 1x12 HRSDTI 2 metres long (24 metres of thermal Length in a Vertical layout)
Severn Trent Barston: One unit, 1x8 HRSDTI 3 metres long (24 metres of thermal length)
Space-saving zig-zag HRS DTI unit supplied to Severn Trent Wanlip
One of two vertically mounted HRS DTI units supplied to Severn Trent Hayden
The Results: Reduced costs, improved efficiency and no blockages
Over the three and half year total project period HRS has successfully upgraded most of the high maintenance spiral units on the Severn Trent sites. With spaces as tight as a mere 50mm gap remaining between the top of the heat exchanger and the roof on one installation, all units have been customised and carefully designed to meet the required thermal duty with minimal impact on other site equipment and pipework.
The overall project was a huge success through a close working partnership with Severn Trent, the installer and HRS. The corrugated surface within the DTI heat exchangers is the result of many trials into enhancing flow patterns within the tube without adding resistance, which creates, increased pumping requirements. The design improves heat transfer and the turbulence it creates produces a lower heat transfer coefficient and better handles the raggy sludge.
Severn Trent has experienced a marked improvement in the efficiency of its digesters and there has not been a single blockage on any of the new heat exchangers to date. The HRS DTI units have dramatically cut maintenance costs and improved biogas recovery.
The DTI series of industrial, single tube heat exchangers are increasingly accepted as the class leading unit for thick, raggy, difficult fluids, especially within the sewage sludge heating application.
For further information please email HRS Heat Exchangers