Reasons for frost formation in refrigeration system and several methods of defrosting

I. Why frost forms and the mechanism of frost formation:

First, it is clear that frost formation occurs in the evaporator of the refrigeration system, and it is an air-cooled evaporator: when the ambient temperature is relatively low (for example, below 0 degrees), the evaporation temperature measured by the evaporation is relatively low (for example, in a 0℃ environment, the evaporation temperature is close to -15℃ or even lower);

We can assume that the temperature of the copper tube in the middle of the evaporator is approximately equal to the evaporation temperature, so the temperature of the copper tube is around -15℃. At this time, if there is water vapor in the air, then the water vapor in the air encounters the low-temperature fins ( Copper tube), will inevitably condense into small ice crystals, and over time, the evaporator will frost.

Of course, the lower the temperature, the easier it is to frost. Generally, in the refrigeration industry test field, when the dry bulb/wet bulb = 2/1, it is a frosting condition, or when the dry bulb/wet bulb = 0/0, it is a severe frosting condition; at this time, the frost of the unit is the most obvious and the easiest;

Why is it easy to frost in this working condition?

Because the humidity in the air is the highest at this time, the water vapor content is the highest; under low temperature conditions, such as below -15℃, the relative humidity in the air is low, so the frost is not very serious.

II. Why do we need to defrost???

Frosting can mainly lead to the following problems:

1. The problem of reduced heat transfer level leads to increased system operation time, increased energy consumption, increased temperature in the cold room, and damaged food;

2. The problem of air flow obstruction leads to decreased performance of the air cooler. The air cooler may be damaged by ice accumulation.

In summary, frosting can lead to poor heat exchange of the evaporator, which directly reduces the cooling capacity of the cold machine.

How often do we need to defrost, and how long does it take each time?

1) This should take into account factors such as the type and humidity of the stored products, the frequency of people walking in the cold room, the number of times the door is opened every day, ventilation conditions, and air humidity.

2) Defrosting must be done as frequently as possible, and as early as possible, so as not to be too late

3) If the defrosting time is too short and the ice is not completely melted, more ice will be produced.

III. Several ways of defrosting:

Current mainstream defrosting methods:

1. Natural defrosting - hot air, water defrosting;

Natural defrosting - air (room temperature must be >40C)

2. Electric heating defrosting

Electric heating defrosting (expensive but effective)

3. Hot air bypass defrosting

Hot air from one end of the condenser

IV. Natural defrosting mechanism and process:

Including air defrosting and water defrosting

Air defrosting: Use the hot air of the evaporator (above 4℃℃), turn on the evaporator fan to circulate the hot air, remove the frost from the evaporator,

Defrosting process

Start defrosting - compressor stops - evaporation side fan continues to run - hot air flows through the evaporator - defrost - defrost is completed - compressor Compressor start

Water defrosting: add a water collection tank at the top of the evaporator, and open a round hole at the bottom of the tank; after the tap water enters the water collection tank, the copper tube round hole flows evenly from top to bottom through the evaporator fins. On the one hand, the frost is washed away by the gravity of the water, and on the other hand, the water temperature is relatively high, and the frost is turned into water and flows away together!

Defrosting process: defrosting mode - compressor and fan stop - water circuit solenoid valve opens - water enters the water collection tank to wash the evaporator - evaporator fan starts to absorb excess water droplets - defrosting ends - compressor starts

V. Electric defrosting process: start defrosting - compressor stops - condensing/evaporating fan stops - electric heating is powered on - start defrosting - set time arrives - defrosting ends - refrigeration system starts

VI. Hot gas defrosting mechanism and process

The hot gas used comes from the high-pressure gas at the high-pressure end of the refrigeration system.

In principle, the hot gas defrosting system can save energy

Hot gas defrosting is a more complex defrosting method, mainly used for large systems with more than 3 to 4 evaporators. When defrosting the air cooler, the evaporator can continue to work.

Hot gas defrosting requires more valves and a more complex control system.

The figure shows the simplest hot gas defrosting device: a hot gas bypass pipe is connected from the exhaust port, and the other end is connected to the evaporator inlet. This method cannot save energy. In order to protect the compressor, an air intake liquid reservoir is required.

The function of the intake pressure regulator is to prevent the intake pressure from being too high, thereby protecting the compressor.

Hot gas defrosting process: Turn on defrosting - defrosting solenoid valve opens - hot gas enters the evaporator through the bypass circuit - start defrosting - set time is reached - defrosting ends - defrosting solenoid valve closes - refrigeration system continues