The common phenomenon of cold storage is that the temperature of the cold storage cannot be lowered and the temperature of the cold storage is slow.
1. The cold storage has a large loss of cooling capacity due to poor thermal insulation or sealing performance.
Poor thermal insulation performance is due to insufficient thickness of thermal insulation layers such as pipelines and warehouse thermal insulation walls, and poor thermal insulation and thermal insulation effects. It is mainly caused by improper selection of thermal insulation layer thickness during design or poor quality of thermal insulation materials during construction. In addition, in the process of construction and use, the thermal insulation and moisture-proof performance of the thermal insulation material may be damaged, resulting in the thermal insulation layer being damp, deformed, or even eroded. . Another important reason for the large cooling loss is the poor sealing performance of the warehouse, and more hot air intrudes into the warehouse from the leak. Generally, if there is condensation on the sealing strip of the warehouse door or the sealing of the cold storage insulation wall, it means that the sealing 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 cooling capacity in the warehouse. Avoid opening the warehouse door as much as possible to prevent a large amount of hot air from entering the warehouse. Of course, when the warehouse is frequently stocked 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.
2. The frost on the surface of the evaporator is too thick or there is too much dust, and the heat transfer effect is reduced
Another important reason for the slow drop of the storage temperature is the low heat transfer efficiency of the evaporator, which is mainly caused by the thick frost layer or excessive dust accumulation on the surface of the evaporator. Since the surface temperature of the evaporator in the cold storage is mostly lower than 0 ℃, and the humidity in the warehouse is relatively high, the moisture in the air is easily frosted or even frozen on the surface of the evaporator, 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 needs to be defrosted regularly. Two relatively simple defrosting methods are introduced below: ① Defrosted by stop. That is, stop the operation of the compressor, open the door of the warehouse, let the temperature of the warehouse rise, and restart the compressor after the frost layer automatically melts. ②Defrosted by washing. After moving the goods out of the warehouse, directly rinse the surface of the evaporator pipe with tap water with a higher temperature to dissolve or fall off the frost layer. In addition to the poor heat transfer effect of the evaporator due to the thick frost, the heat transfer efficiency of the evaporator will also be significantly reduced due to the excessive dust accumulation on the surface of the evaporator because it has not been cleaned for a long time.
3. There is more air or refrigeration oil in the evaporator, and the heat transfer effect is reduced
Once more refrigeration oil is attached to the inner surface of the heat transfer tube of the evaporator, its heat transfer coefficient will decrease. Similarly, if there is more air in the heat transfer tube, the heat transfer area of the evaporator will be reduced, and its heat transfer will be reduced. Efficiency will also drop significantly, and the rate of drop in warehouse temperature will slow down. Therefore, in daily operation and maintenance, attention should be paid to timely removal of oil stains on the inner surface of the evaporator heat transfer tube and discharge of the air in the evaporator to improve the heat transfer efficiency of the evaporator.
4. 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 flow of refrigerant into the evaporator. When the throttle valve is opened too large, the refrigerant flow rate is too large, the evaporating pressure and evaporating temperature will 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 cooling capacity of the system is also reduced, and the temperature drop rate of the warehouse will also slow down. Generally, it can be judged whether the refrigerant flow rate of the throttle valve is appropriate by observing the evaporating pressure, evaporating temperature and the frosting condition of the suction pipe. The blockage of the throttle valve is an important factor affecting the flow of refrigerant. The main reasons for the blockage of the throttle valve are ice blockage and dirty blockage. Ice blockage is due to the poor drying effect of the dryer, and the refrigerant contains moisture. When it flows through the throttle valve, the temperature drops below 0 °C, and the moisture in the refrigerant freezes into ice and blocks the throttle valve hole; dirty blockage is Due to the accumulation of more dirt on the filter screen at the inlet of the throttle valve, the flow of refrigerant is not smooth, resulting in blockage.
5. The amount of refrigerant in the system is insufficient, and the cooling capacity is insufficient
There are two main reasons for insufficient refrigerant circulation. First, the refrigerant charge is insufficient. At this time, only a sufficient amount of 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 each pipeline and valve, and then fill in a sufficient amount of refrigerant after repairing the leaked parts.
6. The compressor efficiency is low, and the cooling capacity cannot meet the warehouse load requirements
Due to the long-term operation of the compressor, the cylinder liner and piston ring and other components are seriously 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 pressures of the compressor. If the refrigeration capacity of the compressor decreases, the commonly used method is to replace the cylinder liner and piston ring of the compressor. If the replacement still does not work, other factors should be considered, or even dismantling and overhauling, and troubleshooting.
Problems in configuring combined cold storage equipment
Combined cold storage is a kind of fast and simple refrigeration equipment developed in recent years. It is professionally produced and prefabricated in various building components and heat insulation panels by specialized factories, and then assembled on the construction site. At present, most of the small combined cold storages can be disassembled for a second time. Briefly introduce the categories of active cold storage:
Outdoor movable cold storage: Large, medium and small combined cold storages are generally designed for outdoor type. Different from indoor type, outdoor type needs to be covered with outer protective structure and foundation. The outer structure is wind, rain and sun protection.
Indoor mobile cold storage: a small combined cold storage built in an existing building.
This paper analyzes the reasonable configuration of the evaporator and refrigeration unit of the combined cold storage equipment.
1. The configuration of the evaporator evaporation area of the combined cold storage equipment is unreasonable:
There is a big gap between the evaporation area of the evaporator in the combined cold storage and the actual technical requirements of the refrigeration process. According to the field observation of some combined cold storages, the evaporation area of the evaporator is only about 75% of the one that should be configured. We know that for the configuration of the evaporator in the combined cold storage, various heat loads should be calculated according to its design temperature requirements, and the evaporation area of the evaporator should be determined, and then configured according to the refrigeration process requirements. If the evaporator is not properly configured according to the design requirements and the configuration area of the evaporator is blindly reduced, the refrigeration coefficient per unit area of the evaporator of the combined cold storage will be greatly reduced, the cooling load will increase, and the energy efficiency ratio will be significantly reduced, resulting in a combined cold storage. The internal temperature drops slowly, the working coefficient of the refrigerator is rising, and the operating cost increases. Therefore, when designing and selecting the evaporator of the combined cold storage, the area of the evaporator should be selected according to the optimal heat transfer temperature difference. .
2. The configuration of the refrigeration unit of the active cold storage equipment is unreasonable:
The refrigeration units configured on the combined cold storage produced by some manufacturers are not reasonably configured according to the total cooling load calculated by the library design and the thickness of the thermal insulation layer of the combined cold storage envelope, but are equipped with additional refrigeration units. The number of units can be used to meet the requirements of rapid cooling in the warehouse. Taking the BZL-3×4 type movable cold storage as an example, the storage is 4 meters long, 3 meters wide, 2.7 meters high, and has a net volume of 28.723 cubic meters. It is equipped with two 2F6.3 series refrigeration units and two independent groups. Serpentine light tube evaporator, each unit and an independent evaporator form a complete refrigeration system for refrigeration operation. According to the estimation and analysis of the machine load of the cold storage, it can be seen that the machine load of the active cold storage is about 140 (W/m3), and the actual total load is 4021.22 (W) (3458.25 kcal). According to the above data, one 2F6. 3 series refrigeration units (standard refrigerating capacity 4000kcal/h) can also meet the refrigeration process requirements of the active cold storage (up to -15℃～-18℃), therefore, it is superfluous to add one more refrigeration unit to the warehouse, and It will also increase the maintenance cost of the unit.
As we all know, in the refrigeration system, the evaporating temperature and the evaporating pressure have a functional relationship with each other. They are related to several conditions such as the temperature of the object to be cooled, the cooling capacity, the heat exchange area (mainly the area of the evaporator) and the capacity of the compressor. If one of these conditions changes, the evaporating temperature and evaporating pressure of the refrigeration system will change accordingly. In the BZL-3×4 type 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 incompatible with the suction capacity of the compressor. (The evaporation volume Vo is much smaller than the suction volume Vh of the compressor), that is, V0 < Vh, the evaporation pressure Po and the evaporation temperature to drop sharply, the heat transfer temperature difference Δt increases, and it is difficult to cool down in the active cold storage. If it is low, the performance index of the compressor will be reduced, and the economic index will be deteriorated.