Some Problems about Plate Heat Exchanger Used in Refrigeration Equipment

Hofmann plate heat exchanger

At present, plate heat exchangers have been applied in small refrigeration equipment (cooling water), and their application will be further expanded to other areas. It is mainly due to the fact that plate heat exchangers having excellent heat transfer performance, small in size, light weight, and continuously improved safety of plate heat exchangers. Overall, even though the actual application effect is good, there are also some problems.

Because plate heat exchangers have very strong heat exchange capacity (the coefficient of plate heat transfer is several times than conventional heat exchangers, they have gained popularity among researchers and users. But, the pressure-resistant property of plate heat exchangers is bad, sealed performance is poor, which restricted the application of plate heat exchangers in engineering. Previously, they are mainly used in the cleaner working mediums; working pressure is not too high, so needed less demanding on preventing leakage. After having leakage, they will have no great influence between the environment and work medium between. They are often applied in civil hot water exchange systems and steam hot water exchanger systems.

Today, heat exchangers applied to refrigeration equipments, mainly are small equipments, and lots of them are imported welded plate heat exchangers. As for the single plate heat exchanger used in the condenser and evaporator of a large water chiller, is feasible in theory, but there are still not having relevant reports. That is, people still have to think twice for making further popularization and application of plate heat exchangers on refrigeration industry. Some related problems remain to be further solved.

Now analyze a set of refrigeration equipments which are in using.

The device is adopted from two sets of 7.5 MANEUROP air-cooled chillers worked in parallel, making cold water to cool in the heat preservation tank produced fresh beer and in the heat preservation tank. Add antifreeze in cold water to control the freezing point - around 6 ℃, the control points of cold water temperature is set in imported hole of plate evaporator, with temperature around 2 ~ 4 ℃.

The main problem of this set of equipments is freezing and plugging of the plate evaporator. The system is running normally under higher temperature conditions, but under the condition of low temperature (the inflow temperature 2 ℃ or so) are prone to freeze and plug at the time of stopping. At the moment of freezing and plugging to plate evaporator, the working condition is going to deterioration, and in a short time the whole plate evaporator can be frozen inside.

Freezing and plugging are fatal for plate heat exchangers, because the plate heat exchanger is more delicate equipment; the thickness of plates is small, which can't bear the impact of the external force. When the freezing and plugging occurred, ice bulge will directly make the internal parts of heat exchangers deformed or leaked, so bring bad influences on the operation and production of the refrigeration equipment.

Problem analysis:

1. The refrigeration system does not match, and evaporator is too small; or due to the long-term operation, there are scaling and filth blockages in the evaporator, which can cause the heat transfer capability of plate-type evaporator decreased. So in the process of actual operation, the evaporation temperature is low (-10℃).

1). The evaporation temperature is below freezing point of cold water, which increases the possibility of freezing and plugging of plate-type evaporator.

2). The heat transfer temperature difference of evaporator is big, which did not give full play to the advantages of plate-type evaporator, unfavorable to improve the refrigeration efficiency.

2. The freezing point of cold water is higher. When the evaporator is running at the low temperature (inflow temperature is 2 ℃), the outlet water temperature is 3 ℃ higher than the freezing point. This is not to say that the actual operation is not allowed, but it increases the possibility of freezing and plugging. So need to control temperature more accurately. Moreover, cold water viscosity is great, liquidity is poor, and the unit flow area of plate-type evaporator is very small. People can reduce the freezing point to improve the outlet water temperature, and to increase cold water flow.

3. The control device is imperfect. The starting and stopping of cold water pump has not been interlocked with the operation of the refrigeration system, and also has not made testing control for the cold water flow of the evaporator and the pressure drop. The refrigeration system has a low pressure controller, but only used to control zero-pressure parking of the compressor to prevent bearing high pressure that have not been used the equipment for a long time, which have not protected operation in low voltage. Once the pump stopped, the freezing and plugging will happened.

4. Improper maintenance

1). The water temperature controlling are not repaired for a long-term. The displayed value is 1.5 ℃ lower than the actual value, and the instrument can't reflect the reality temperature of the inlet cold water in time. In the actual operation process, it will cause the temperature is near to freezing point but not stop.

2). Though the temperature control devices are equipped on the plate evaporator, the freezing and plugging have already occurred and anti-freezing and plugging devices are still not in action. Because the cold outlet water temperature and freezing point are very close, it is not advisable to adjust it to the best control point.

5. lacking system refrigerant can also cause the occurrence of freezing and plugging different from conventional evaporators. The reason is related to the structure of plate evaporator. Plate heat exchanger is overlaid by numerous very narrow unit channels, the flow of cold water or refrigerant within each unit is very small, and the plate is very thin, so the heat exchange ability is very strong. When the system is lack of refrigerant, which can cause the uneven distribution of refrigerant in every unit channel, the evaporating pressure is very low, and the limited multiple units can be blocked due to violent heat exchanger, then make adjacent units blocked, which make a chain reaction, until the whole evaporator is frozen completely.