Fibre heat-exchangers thus represent a good means for drawing heat from e.g. waste water. However, if used this way, the outer surface the hollow fibres must be periodically cleaned so as to prevent any gradual decrease in heat exchange. Owing to low temperatures and the slow flow-rate of liquids, waste water is associated particularly with fouling that is microbial in origin (microorganisms).
The introductory image shows heat-exchange bundles of hollow plastic fibres with flanges for connecting to heat-carrying materials.
There is a variety of ways in which the surface of the hollow fibres can be cleaned. The chemical method focuses on prevention: hollow fibres are coated with a special non-adhesive layer which reduces the catching of deposits. According to mechanical methods, heat-exchangers can be cleaned using a stream of liquid (by rinsing), a stream of air (blowing off contamination), or by using various mechanical tools.
The scientific team was presented with the problem of cleaning a particular structural design of heat-exchanger, where the vessel in which is placed a heat-exchanger comprising hollow fibres (and in which the heat exchange is realized) has the waste water inlet in the upper section, and the outlet pipe in the lower section, the latter equipped with an overflow positioned almost at the same level as the waste water entry to the vessel. The heat-exchanger was designed to be low-maintenance and therefore disassembly was not an option. Cleaning the outer surface of the hollow fibres was previously done by rinsing with pressurized water. One disadvantage of this otherwise effective system of cleaning is the need to generate pressure and the resulting energy consumption. The task of the technical solution was to design a more economic yet effective system to clean fibre exchangers.
Principle of the solution
In the normal operation of the heat-exchanger, the stop valve 4) is opened and waste water only runs through the reserve tank 5) to vessel 1) of the exchanger. The two-way valve 8) is set so that waste water flows over the overflow 9); thus the vessel 1) is filled.
The new solution for cleaning of fibres uses negative pressure produced in the piping system. When the stop valve 4) is closed and the two-way valve 8) is diverting waste water outside the overflow through the bypass 10), negative pressure formed in vessel 1) sucks atmospheric air back through valve 6) and air bubbles passing through the hollow fibre bundle intensively clean their outer surface. When vessel 1) is emptied, both valves 4) and 8) are re-set to their initial position and heat exchange continues. The valves can be also remote-controlled. If a reserve tank is installed into the system, cleaning can be carried out in the second stage. When the reserve tank 5) is filled during the first stage of cleaning, the stop valve 4) opens, the two-way valve 8) remains open to bypass 10) and the flow of waste water rinses the exchanger. The effect of the rinse is increased using a spraying device which distributes incoming water to the whole of the fibre bundle.
Almost the entire cleaning process is realized only mechanically, without any energy output, and this saving is the greatest contribution of the above solution to current state-of-the-art technology. The structural solution can form a part of the system for utilizing waste water heat. Such equipment can be installed in waste water treatment plants or directly as part of large plants, such as factories that use large volumes of water in their product processing. The solution to this technical problem is protected as a Czech utility model.
