Conventional wisdom is to locate digester heating systems on or around the floor of the main digester. While this results in good initial heat transfer and creates thermal currents in the digestate to assist mixing, the accumulation of sediment quickly diminishes these properties and regular cleaning is necessary to maintain satisfactory performance with such systems. In some cases, heating units have actually been incorporated into the concrete walls or floor of digesters adding to the potential problems and expense should something go wrong.
The solution is to locate the heating unit outside of the digester, an approach which is not new. One of the earliest uses of an external heater and heat exchanger for a digester dates back to 1946. Since then, external digester heating based on heat exchangers has been used successfully in AD plants around the world. Over the years the design of heat exchangers has improved considerable and is epitomized by the HRS DTI Series of corrugated tube heat exchangers which are ideally suited for the external heating of digester fluids and sludge – and which have been successfully used in a number of such applications.
The biggest advantage of external heating is that it can be checked, cleaned, or serviced at any time without the need to empty (or enter) the digester, a process that is both costly and potentially dangerous. However, there are a number of other benefits, including the fact that external systems can be designed so that one heat exchanger array heats more than one digester, and that the improved thermal performance reduces heating requirements and improves the overall energy efficiency of the AD plant. Where units are constructed of materials such as stainless steel, operating life is considerably improved compared to internal heating units, and routine maintenance is straightforward.
The HRS DTI Series is a true counter-current heat exchanger, meaning that the product flows through the inner tube and the service fluid flows through the surrounding shell. HRS’s corrugation technology increases both heat transfer and efficiency over standard smooth tube heat exchanger designs and minimizes fouling, and important consideration when dealing with materials such as digester sludge. Multiple units can be interconnected and have the options of frame mounting, insulated, and clad in stainless steel.