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Refrigeration Encyclopedia-Why doesn't the cold storage refrigerate and how to solve it?

2018-09-25 10:50

1, cold storage evaporator surface frosting is too thick, too much dust, so that the heat transfer effect decreased

An important reason for the slow decline in cold storage temperature is the low heat transfer efficiency of the evaporator, which is mainly due to the excessive frost layer or dust on the surface of the evaporator. Because the surface temperature of the evaporator of the cold storage is mostly lower than 0 ℃, and the humidity of the warehouse is relatively high, the moisture in the air is easy to frost on the surface of the evaporator and cause icing, which affects the heat transfer effect of the evaporator. In order to prevent the surface frost layer of the evaporator from being too thick, it is necessary to defrost the surface of the evaporator regularly. Introduce two simple defrosting methods: ① stop defrosting. That is, stop the compressor, open the warehouse door, let the temperature rise, wait until the frost layer melts, and then restart the compressor. ② Frost. Move the food or goods in the warehouse out of the cold storage, and then directly rinse the surface of the evaporator pipe with higher temperature water to dissolve and fall off the frost layer. The heat transfer effect of the evaporator is not good. In addition to the frost that is too thick, the surface of the evaporator has not been cleaned for a long time and the dust is too thick, which obviously reduces the heat transfer efficiency.




2, cold storage due to poor insulation or sealing performance, resulting in large loss of cold capacity

The poor insulation performance is due to the insufficient thickness of the insulation layer of the pipeline, warehouse insulation wall, etc., and the poor insulation and insulation effect. It is mainly caused by the improper selection of the insulation layer thickness during the design or the poor quality of the insulation material during the construction. In addition, in the process of construction and use, the thermal insulation and moisture-proof performance of thermal insulation materials may be damaged, resulting in moisture, deformation and even erosion of the thermal insulation layer. Its thermal insulation capacity decreases, the loss of anti-cooling capacity increases, and the decrease of reservoir temperature slows down significantly. Another important reason for the large cold loss is the poor sealing performance of the warehouse, and more hot air intrudes into the warehouse from the air leakage. Generally, if condensation occurs at the sealing strip of the warehouse door or the seal of the thermal insulation wall of the cold storage, it indicates that the seal is not tight. In addition, frequent opening and closing of the warehouse door or more people entering the warehouse together will also increase the loss of cold capacity in the warehouse. Try to avoid opening the library door to prevent a large amount of hot air from entering the warehouse. Of course, when the stock in the warehouse is frequent or the stock is too large, the heat load increases sharply, and it generally takes a long time to cool down to the specified temperature.

3. The throttle valve is improperly adjusted or blocked, and the refrigerant flow is too large or too small

Improper adjustment or blockage of the throttle valve will directly affect the refrigerant flow into the evaporator. When the throttle valve is opened too large, the refrigerant flow is too large, the evaporation pressure and evaporation temperature also increase, and the temperature drop rate of the warehouse will slow down; at the same time, when the throttle valve is opened too small or blocked, the refrigerant The flow rate is also reduced, the refrigerating capacity of the system is also reduced, and the temperature drop rate of the warehouse will also slow down. Generally, the throttle valve refrigerant flow rate can be judged by observing the evaporation pressure, evaporation temperature and the frosting of the suction pipe. Throttle blockage is an important factor affecting refrigerant flow, and the main causes of throttle blockage are ice blockage and dirty blockage. The ice blockage is due to the poor drying effect of the dryer. The refrigerant contains moisture. When it flows through the throttle valve, the temperature drops below 0°C, and the moisture in the refrigerant forms ice and blocks the throttle valve hole; the dirty blockage is due to The throttle valve inlet filter has accumulated a lot of dirt, and the refrigerant does not flow smoothly, forming a blockage.

4. There is more air or refrigerant oil in the evaporator, and the heat transfer effect decreases. Once more refrigerant oil is attached to the surface of the evaporator heat transfer tube, the heat transfer coefficient will decrease. Similarly, if there is more air in the heat transfer tube, the heat transfer area of the evaporator will decrease, and the heat transfer efficiency will also decrease significantly, and the temperature drop rate of the warehouse will slow down. Therefore, in the daily operation and maintenance, attention should be paid to timely remove the oil on the surface of the evaporator heat transfer tube and discharge the air in the evaporator to improve the heat transfer efficiency of the evaporator.

5. The amount of refrigerant in the system is insufficient and the cooling capacity is insufficient

There are two main reasons for the insufficient refrigerant circulation. One is the insufficient refrigerant charge. At this time, only enough refrigerant needs to be added. Another reason is that there are many refrigerant leaks in the system. In this case, you should first find the leak point, focus on checking the connections of the pipes and valves, and then fill in a sufficient amount of refrigerant after repairing the leak.

6, the compressor efficiency is low, the cooling capacity can not meet the warehouse load requirements

Due to the long-term operation of the compressor, the cylinder liner and piston ring and other components are severely worn, and the matching clearance increases, the sealing performance will decrease accordingly, the gas transmission coefficient of the compressor will also decrease, and the cooling capacity will decrease. When the cooling capacity is less than the heat load of the warehouse, the temperature of the warehouse will drop slowly. The refrigeration capacity of the compressor can be roughly judged by observing the suction and discharge pressure of the compressor. If the refrigeration capacity of the compressor is reduced, the common method is to replace the cylinder liner and piston ring of the compressor. If the replacement is still ineffective, other factors should be considered, even the machine should be disassembled for maintenance and troubleshooting.

The problem of configuring combined cold storage equipment

Combined cold storage is developed in recent years, a rapid and simple assembly of refrigeration equipment, is in a variety of building components and insulation board, by a special factory for professional production of prefabricated, and then assembled at the construction site. At present, most of the small combined cold storage, can be two times of disassembly. A brief introduction to the activities of the cold storage category:

Outdoor cold storage: large and medium-sized combined cold storage are generally outdoor design, and indoor type is different from the outdoor type need to cover the outer envelope structure and foundation. The peripheral structure can be windproof, rainproof and sunscreen.

Indoor cold storage: small modular cold storage built in existing buildings.

This paper analyzes the reasonable configuration of the evaporator and refrigeration unit of the combined cold storage equipment.

1. activities cold storage equipment refrigeration unit configuration is not reasonable:

Some manufacturers of combined cold storage on the configuration of the refrigeration unit, not according to the library design and calculation of the total refrigeration load and combined cold storage enclosure structure of the insulation layer thickness and other aspects to carry out reasonable configuration, but to increase the number of refrigeration units to meet the requirements of rapid cooling in the library. Take BZL-3 × 4 type movable cold storage as an example. The storage is 4 meters long, 3 meters wide and 2.7 meters high, with a net volume of 28.723 cubic meters. It is equipped with 2 2F6.3 series refrigeration units and 2 groups of independent serpentine smooth tube evaporators. Each unit and its independent evaporator form a complete refrigeration system for refrigeration operation. According to the estimation and analysis of the machine load of the cold storage, the machine load of the active cold storage is about 140(W/m3), and the actual total load is 4021.22(W) (3458.25kcal). According to the above data, the selection of a 2F6.3 series refrigeration unit (standard refrigeration capacity 4000kcal/h) for the active cold storage can also meet the refrigeration process requirements of the active cold storage (up to -15 ℃ ~-18 ℃), in the library with 1 more refrigeration unit has been redundant, and will also increase the maintenance costs of the unit.

The configuration of the evaporator evaporation area of the 2. combined cold storage equipment is unreasonable:

The evaporation area of the evaporator in the combined cold storage is far from the actual technical requirements of the refrigeration process. According to the field observation of some combined cold storage, the evaporation area of the evaporator should be configured only about 75%. We know that the configuration of the evaporator in the combined cold storage should be calculated according to the design temperature requirements of the heat load, determine the evaporation area of the evaporator, and then configure it according to the requirements of the refrigeration process. If the evaporator is not reasonably configured according to the design requirements and the configuration area of the evaporator is blindly reduced, the refrigeration coefficient per unit area of the combined cold storage evaporator will be greatly reduced, the refrigeration load will be increased, and the energy efficiency ratio will be significantly reduced, resulting in various unfavorable factors such as slow temperature drop in the combined cold storage, rising working coefficient of the refrigerator, and increasing operating cost. Therefore, when designing and selecting the evaporator of the combined cold storage, the area of the evaporator should be selected according to the best heat transfer temperature difference.

As we all know, in the refrigeration system of the cold storage project, the evaporation temperature and the evaporation pressure are functions of each other. They are related to the temperature and cooling capacity of the cooled object, the heat exchange area (mainly refers to the evaporator area) and the capacity of the compressor. If one of the conditions changes, the evaporation temperature and evaporation pressure of the refrigeration system will change accordingly. In the BZL-3 × 4 combined cold storage, the evaporation area has not changed, but the capacity of the refrigerator has doubled, which makes the evaporation capacity of the evaporator of the movable cold storage not suitable for the suction capacity of the compressor (the evaporation capacity Vo is much smaller than the suction capacity Vh of the compressor), I .e. V0 〈 Vh, the evaporation pressure Po and the evaporation temperature to drop sharply, and the heat transfer temperature difference △t increases, cold storage activities but because of the difficulty of cooling, due to the evaporation temperature is too low, the performance of the compressor is reduced, the economic indicators worse.