Various types of heat exchangers are available on the market, but only few are suitable to be used with wastewater. Until recently, heat exchangers with automatically cleaning surfaces and a periodic sediment discharge were not at all available. HUBER specialists in the field of energy and mechanical engineering have therefore developed the HUBER wastewater heat exchanger RoWin.
1. Design and function
The HUBER wastewater heat exchanger RoWin consists of a welded stainless steel construction in which standardized pipe modules (1) are inserted. Cooling water (2) flows through the stainless steel pipes, whereas the wastewater (3) is directed through the tank at a higher temperature level. Due to the resulting difference in temperature heat transfer takes place at the pipe. The heat transfer rate is excellent due to the good thermal conductivity of stainless steel. The heated cooling water transports the thermal energy to the heat pump in which the requested heating temperature of up to 65° C is generated.
To prevent the deterioration of heat transfer efficiency caused by sediments on the pipes, the exchanger surfaces are kept clean by a timed cleaning system (4). This preventive measure guarantees constant heat transfer. Sediments, such as grit or similar materials, are removed by a screw conveyor (%) and via an automatic outlet valve (6) returned into the sewer along with the heated wastewater.
Air tubes installed in the modules generate a horizontal flow which, combined with the wastewater flow velocity, improves turbulences and optimizes heat transfer.
The lateral surfaces and covers are insulated to prevent the loss of thermal energy to the environment. Sealing elements eliminate odours and ensure that no explosive atmosphere develops outside the HUBER RoWin system. The air introduced by the turbulence generator is removed via a ventilation valve or the drain pipe of the system.
All moving system components operate at minimum velocity and with run times of approx. ten minutes daily to minimize energy demand and noise emission.
The data recording system integrated in the electrical control regime keeps records and archives all relevant process data, such as volume flows and temperatures, so that problems or changes in the process can better be traced and eliminated. The HUBER wastewater heat exchanger RoWin is equipped with additional safety devices that ensure trouble-free operation.
The RoWin heat exchanger is available in three sizes and can be charged with a wastewater flow of up to 100 m³/h. If the wastewater is cooled by ~2 kelvin, one plant can deliver approx. 250 kW to the heat pump, which is sufficient for the heat pump to provide more than 300 kW for the heating system. If the energy demand is higher, several RoWin units can be installed in parallel or in series.
The fully automatic process with very low velocities of moving parts reduces wear and maintenance requirements to a minimum. Heat recovery with a HUBER wastewater heat exchanger RoWin represents a reliable, low-cost and low-emission form of heating.
2. Fields of application
The HUBER wastewater heat exchanger RoWin is suitable to be applied where heat is to be recovered from a more or less polluted liquid (normally water) or heat is to be introduced. Typical applications are described below.
2.1 Raw wastewater from sewer
The wastewater flowing in our sewers holds an enormous heat potential. If treated with the HUBER ThermWin® system this potential can be utilized to heat buildings. The heart of the ThermWin® system is the HUBER wastewater heat exchanger RoWin which is installed above ground and fed with pre-treated sewer wastewater. The heat that is extracted from the wastewater with this treatment method is “pumped” to a higher temperature level. As sewers are always situated near buildings, short ways are guaranteed because heat suppliers and heat consumers are virtually neighbours. Interestingly, this principle works also vice versa. For cooling purposes, heat is extracted from the building and via the HUBER RoWin system introduced into the sewer.
2.2 WWTP effluent
The HUBER RoWin system represents an economical and climate-friendly energy source also for WWTP operators. For it is not only raw wastewater but also WWTP effluent that offers an enormous but previously unused heat source. The constantly high volume flow and the possibility of increased cooling open up new ways to increase the energy efficiency of the overall WWTP. In addition to heating buildings on a WWTP it is appropriate to operate for example a HUBER Solar Active Dryer (HUBER SRT sludge dryer). The HUBER SRT sludge dryer uses not only solar radiation as drying energy but also the heat potential of the WWTP effluent. The process chain WWTP effluent – RoWin heat exchanger – heat pump supplies the thermal energy for the high performance floor heating of the sewage sludge dryer and allows for all year round sludge drying.
2.3 Digested sludge and filtrate from sludge dewatering
High temperature = high energy recovery. This simple formula applies if the HUBER RoWin wastewater heat exchanger is fed with WWTP digestor sludge or filtrate from a sludge dewatering system.
Temperatures of approx. 36 °C prevail in digestors but this high heat potential has previously remained unused. Due to its special design and automated self-cleaning the HUBER HUBER RoWin wastewater heat exchanger is able to guarantee a continuously high heat transfer also with these media.
The energy recovered can be used directly on site to pre-heat the raw sludge. Further energetically attractive is the fact that a feed pump is unnecessary due to the hydrostatic primary pressure of the digestor.
2.4 Industrial wastewater
As in the municipal sector, a considerable amount of energy is lost with the wastewater flow also in industrial companies. Very high temperatures are achieved especially in physical and chemical processes.
A lot of companies work 24 hrs nowadays so that a continuous volume flow is available. In most applications the recovered energy can directly be introduced into the industrial process so that line lengths can be kept short. Heat extraction from wastewater has the positive side effect that discharge requirements can be met.
by Alexander Steinherr