At present, the plate heat exchanger is used in small refrigeration (chilled water) equipment, and its application is bound to further expand. This is mainly due to the excellent heat transfer performance, small size, light weight, and the continuous improvement of the safety and reliability of the plate heat exchanger. In general, the actual application of good results. However, there are also some problems.
Because the plate heat exchanger has a strong heat exchange capacity (its heat transfer coefficient is several times the conventional heat exchanger, the unit volume of heat transfer area is large), and small size, light weight. Therefore, it has been favored by researchers and users. However, the plate heat exchanger is not good pressure resistance, sealing performance is poor, restricting the application of plate heat exchanger in the project.
Previously, the plate heat exchanger is mainly used in cleaner working media, working pressure is not too high, the leakage requirements are not too harsh, the leakage will not have a greater impact on the environment and work between the media equipment, such as the application of hot water delivery system and steam hot water exchange system in civil.
At present, refrigeration equipment using plate heat exchanger, mainly some small equipment, mainly imported brazed plate heat exchanger. As for the condenser and evaporator in large chiller units using a separate plate heat exchanger, theoretically feasible, but have not seen the relevant reports. In other words, people on the plate heat exchanger in the refrigeration industry to further promote the application of certain concerns, its safety can be * and its related issues need to be further resolved.
Now a set of refrigeration equipment in use as an example for analysis
The equipment uses two 7.5 horsepower Mayo air-cooled units working in parallel to produce cold water to the production of fresh beer holding tank, holding tank cooling, adding antifreeze in the cold water to control the freezing point at -6 ℃ or so, the cold water temperature control point is located in the plate evaporator inlet, the control temperature of 2 ~ 4 ℃.
The main problem of this set of equipment is the plate evaporator freeze plugging, the system operates normally under high temperature conditions, but under low temperature conditions (the inlet water temperature is about 2℃, when the unit is about to shut down) freeze plugging is easy to occur. When the plate evaporator freeze plugging occurs, the working condition deteriorates sharply, and the whole plate evaporator can be frozen inside in a very short time.
Freeze blockage of the plate heat exchanger is fatal, because the plate heat exchanger is a relatively delicate equipment, the thickness of the heat exchanger piece is very small, can not withstand the impact of external forces, when the freeze blockage occurs, ice crystal expansion will directly cause internal deformation or leakage of the heat exchanger. To the operation of refrigeration equipment and production has a great impact.
Problem analysis
First, the refrigeration system does not match, the evaporator is small; or due to the long-term operation of the unit, the evaporator internal scale, dirty plugging caused by the plate evaporator heat exchange capacity reduced. This causes the evaporation temperature to be low (-10℃) in the actual operation process.
1、The evaporation temperature is lower than the freezing point of cold water, which increases the possibility of freezing and blocking of the plate evaporator.
2, evaporator heat transfer temperature difference, did not give full play to the advantages of the plate evaporator itself, is not conducive to the improvement of refrigeration efficiency. When the cold water inlet temperature is 2 ℃ (evaporator inlet and outlet water temperature difference is 5 ℃), the evaporator outlet water temperature is -3 ℃, the heat transfer temperature difference is 9.3 ℃. Because the plate evaporator has a very high heat transfer coefficient, its heat transfer temperature difference should be at least smaller than the conventional heat exchanger, for example, choose about 2 ℃.
Second, the chilled water freezing point is high. When the evaporator in the low temperature point (inlet water temperature 2 ℃) operation, the outlet water temperature is only 3 ℃ higher than the freezing point. This is not to say that the actual operation is not allowed, but this, after all, increases the possibility of ice plugging, the need for more accurate control of the temperature. In addition, in the freezing point near the cold water viscosity, poor mobility, and plate evaporator unit circulation cross-section is very small, more suitable for the use of good mobility of the working medium. Therefore, if feasible, should be taken to reduce the freezing point, improve the cold water temperature, increase the flow of cold water and other measures.
Third, the control device is not perfect. The start and stop of the chilled water pump is not interlocked with the operation of the refrigeration system, and there is no detection and control of the evaporator's chilled water flow and pressure drop. Although the refrigeration system has a low pressure controller, but only used to control the compressor zero pressure parking (to prevent long-term equipment downtime plate evaporator under high pressure) and no low-pressure operation protection. Once the pump is stopped or evaporator internal dirty blockage caused by the reduction of water flow will cause ice blockage occurs.
Fourth, improper maintenance.
1, the inlet water temperature control long-term disrepair, the display value is lower than the actual value of about 1.5 ℃, and instrument inertia can not reflect the real temperature of the cold water inlet water. In the actual operation process, it will cause the chilled water except the number of temperature close to the freezing point and the unit still does not stop.
2, although the plate evaporator is equipped with anti-freeze plugging temperature control device, but often the ice plugging has occurred while the anti-freeze plugging device is still no action, because the chilled water temperature and the freezing point is very close to the water, it is not easy to adjust it to the best control point.
