Evaporator and condenser used in "Hongxing" sewage source heat pump unit, manufacturer: Guangzhou United.

The selection of heat exchange tubes directly affects the cost and service life of the sewage source heat pump unit. The evaporator and condenser of the sewage source heat pump unit are usually used in the environment containing chloride ion corrosion. The corrosion resistance of the heat exchange tube is particularly important. The heat exchange tubes are made of various duplex stainless steels (such as 2205, 2507, etc.), copper-nickel alloys (B10, B30), etc.

Their respective characteristics are as follows:

B10 has better resistance to chloride ion corrosion, and the corrosion rate of B10 is lower than 0.03mm/a. With the increase of time, the average annual corrosion rate is getting lower and lower. And the corrosion resistance does not change much within 50 ℃. However, B10 is not resistant to erosion corrosion, and its corrosion resistance is greatly reduced in sandy water environment. The allowable seawater flow rate of B10 can be referred to the British ship engine code. Therefore, when choosing B10 as the material of the heat exchanger, the following points should be paid attention to:

(1) The flow rate of the B10 nickel-copper pipe should be controlled, and should not exceed 1.5m/s, especially the influence of impurities such as mud and sand should be considered.

(2) B10 nickel-copper tube and fittings should be connected by manual argon arc welding.

(3) In order to make the B10 nickel-copper tube form a complete protective film corrosion product film in seawater and ensure the expected service life of the B10 nickel-copper tube, the oil on the pipe end must be removed before butt welding between the B10 nickel-copper tube and the fittings. One is to ensure the welding quality, and the other is to avoid the formation of local carbon film in the tube.

B30 is also an excellent seawater heat exchanger material. Under the same corrosion conditions, its corrosion rate is about 1/3-1/2 of B10; B30 also has the problem of not being resistant to erosion corrosion, and its precautions are the same as B10; The upper limit of the scouring rate is higher than that of B10. In general design, the water velocity of B30 should not exceed 3m/s.

Both 2205 and 2507 have good corrosion resistance. When the chloride ion concentration is 1100mg/lL, 2507 will not have pitting corrosion and crevice corrosion. Due to the large relationship between corrosion and temperature, 2205 has the risk of crevice corrosion when the chloride ion concentration is 1100mg/lL and the temperature is higher than 32℃. Therefore, if 2205 is used as the material of the heat exchanger, measures should be taken to eliminate the gap between the pipe fittings and the intergranular gap (for example, do not use threaded connections in the project, and use non-metallic gaskets between the flanges to prevent the generation of intergranular gaps).