Design points of sewage heat exchanger

With the emphasis on environmental protection, sewage source heat pump systems are more and more used in life, but the water quality of sewage is poor, so the use of sewage source heat pumps to exchange heat with sewage is often an indirect method. Therefore, the current common practice is not to let the sewage directly enter the heat pump unit, but to exchange the heat to the intermediate water through the sewage heat exchanger, and then the intermediate water to exchange the heat to the heat pump unit. Since the flow and heat transfer of sewage is very different from that of clean water, the previous design experience and parameters cannot be applied, and designers are faced with many new problems. However, in practice, many designers lack understanding of the particularity of sewage heat transfer, and directly apply the design parameters and methods for clean water, which is very dangerous for the quality of the project.

1. Characteristics and parameters of sewage heat exchanger 1) The viscosity of sewage in the heat exchanger is high, and the heat exchange surface is seriously polluted, resulting in the resistance of the sewage side being greater than that of the clean water side, and the heat transfer coefficient is smaller than that of the clean water side.

2. Due to the danger of blockage and pollution, it is impossible to adopt bellows, inner fins, inserts and other facilities to enhance heat exchange like clean water.

3. Limited by conditions, the sewage heat exchanger can only exchange heat with a small temperature difference. For example, in the severe cold region of the north, the temperature of the sewage inlet in winter is only about 10 °C, and due to the limitation of the evaporation temperature of the heat pump unit, the temperature of the intermediate water cannot be too low, so the maximum temperature drop of the sewage can only be 3 to 4 °C, and the two-stage heat exchange for each stage. The temperature difference is less than 3 °C.

4. Due to the small heat transfer temperature difference and small heat transfer coefficient, the cooling of sewage requires a long process, usually about 30 m. This requires a new design of the internal structure of the shell-and-tube heat exchanger, including the arrangement of the tube side, the structural design of the shell space, etc., to ensure the whole process of countercurrent heat exchange and ideal heat exchange everywhere inside the heat exchanger. Thermal temperature difference.

5. To ensure that there is an ideal heat transfer temperature difference everywhere in the heat exchanger, it is only from the perspective of structural design to realize the full counterflow of the two fluids. There are two methods: one is to install longitudinal partitions in the shell space, and the other is to connect multiple heat exchangers in series. Generally speaking, the design of heat exchanger should seek to use less consumables, have smaller volume, and consume less water pump electricity to obtain the expected heat exchange effect. Although the above requirements are contradictory, people already have mature design experience for general heat exchangers. It is not easy for the sewage heat exchanger to meet the above requirements. It must be calculated accurately and carefully in order to avoid too many consumables, too large volume or too high pump consumption, and to find the best at least one among the indicators. balance point. The parameters related to the three major indicators of consumables, volume, and pump consumption not only include structural parameters, such as pipe diameter, pipe length, pipe spacing, number of pipe passes, etc., but also operating parameters, such as flow rate, flow rate, pressure drop, and input at each node. Outlet temperature, heat exchange, etc.