First, the reason for frost on the evaporator
Because the relative humidity of the air is relatively high. When the refrigeration system is operating normally, the surface temperature of the evaporator is much lower than the dew point temperature of the air, and moisture in the air will precipitate and condense on the evaporator tube wall. When the tube wall temperature is lower than 0 ° C, water dew will condense into frost.
The effect of frost layer on refrigeration system
Because the air's ability to transfer heat to the working fluid in the evaporator is weakened, the liquid working fluid in the evaporator may not be able to absorb heat and vaporize completely, causing the working fluid to enter the compressor, causing liquid hammer and damaging the compressor.
The greater the relative humidity, the faster the thickness of the frost layer on the surface of the evaporator increases, and the easier the air circulation area between the fins will be blocked. As the evaporation temperature decreases, the frost changes from needle to dense, and the frost layer density increases.
Traditional defrosting methods
, Artificial defrosting (defrosting)
Working method: Use a broom to directly remove the frost layer on the cooling tube evaporator, or use a crescent-shaped defrost shovel and other special tools on the cooling tube evaporator to shovel the frost layer on the cooling tube evaporator. Perform eradication.
Applicable scope: smooth tube evaporator, mostly used in small cold storage and places with few tubes
Effect: Incomplete defrost
Advantages: The evaporator can be stopped without affecting the cooling in the cold room.
Disadvantages: high labor intensity
natural defrosting (stop defrost)
In cold storage with a temperature of not less than 5 ℃, natural defrosting can be used. During defrost, the compressor is stopped and the refrigeration of the evaporator is stopped for a period of time. During this period, the fan continues to run, and the surface frost is formed by the enthalpy of the air blowing through the evaporator.
, Water melting frost
Working mode: Use water spray device to spray water on the outer surface of the evaporator, so that the frost layer is melted by the heat of water and washed away
Scope of application: air cooler with drainage pipe
Effect: Good defrost effect
Advantages: short time, simple operation and easy management
Disadvantages: The oil in the evaporator pipe cannot be discharged, and the water consumption is large.
Strict technical measures are needed to prevent water from causing harm to cold storage
, Electric heating defrost
Working mode: Electric heating is used to provide defrosting heat. Electric heating elements are attached to the fins. In order to prevent the thawed frost from freezing before it is discharged from the warehouse, a ribbon heater must be wound on the water receiving tray and the drain pipe to melt it. After the frost water should be drained out of the warehouse in time
Scope of application: finned tube air cooler, small refrigeration unit or single warehouse
Effect: Good defrost effect
Advantages: easy to defrost, simple operation, easy to realize automatic control
Disadvantages: Consumption of electrical energy is large, temperature fluctuation in the cold room is large, it is necessary to stop the operation of the cold air blower and close the liquid supply valve during the defrost.
、 Defrost of hot working gas
Working mode: Use the sensible and latent heat of the superheated steam discharged from the compressor to heat and melt the frost layer on the outer surface of the evaporator
Scope of application: all types of evaporators, mostly used in large cold storage and ammonia compression refrigeration systems
Effect: Good defrost effect
Advantages: short time, low labor intensity, can discharge oil and dirt in the evaporator
Disadvantages: the operation is complicated, the energy loss is large, and the evaporator needs to stop working when the frost is thawed
, Four-way valve reversing reverse defrost
Four-way valve is switched, the original high and low pressure part of the refrigeration system is switched. After the four-way valve is actuated, the system is switched from the normal heating cycle mode to the defrost cycle. The refrigerant state at each point in the system is a dynamic change process. It is different from the dynamic change process that the refrigeration system starts under equilibrium. For the defrost cycle, after the four-way valve is activated, the original low pressure of the wind-side heat exchanger is connected to the high-pressure pipeline from the four-way valve to the compressor exhaust port, and the high-pressure system of the indoor heat exchanger is connected to the four-way valve. The low-pressure line of the compressor air inlet is connected, so after the defrost cycle starts, the high-pressure side and the low-pressure side of the system have their own pressure balance.
[Note] When the refrigeration device is started normally, the pressure on the high-pressure side of the system from the compressor exhaust port to the throttle valve inlet (including the condenser and high-pressure accumulator) is balanced, from the throttle valve outlet to the compressor suction port The pressure on the low side (including the evaporator and the gas-liquid separator) is also balanced. After the heat pump system is started, under the drive of the compressor, the pressure on the high-pressure side and the low-pressure side of the system changes in different directions.
Four-way valve reversing reverse defrost disadvantages
) The four-way valve is reversed, and the original high and low pressure parts of the refrigeration system are switched, which causes the phenomenon of "oil running" in the refrigeration system, reducing the reliability and service life of the system;
) When defrosting, the refrigerant must absorb heat from the heating system for defrosting, which causes the temperature of the hot water to fluctuate sharply, which affects the comfort of the air conditioning system;
) From the start of the defrost to the end of the defrost, the four-way valve needs to be operated twice, and the high and low pressure of the system are also switched twice and then the balance is re-established, which makes the total time of the system defrost longer.
4. New defrost method
Sensible heat defrost
Sensible heat defrost refers to the bypass circuit of the compressor's exhaust pipe to the front of the electronic expansion valve, which directs the high temperature and high pressure exhaust of the compressor directly to the front of the electronic expansion valve, and then passes through the isentropic throttling of the electronic expansion valve. The compressor exhaust is introduced into the air heat exchanger, and the frost layer on the outside of the air heat exchanger fin is removed by the compressor exhaust heat, and at the same time, the refrigerant is only exchanged in the air heat exchanger without sensible heat. Condensation.
(4) The four-way valve does not need to be reversed during defrosting. In this way, various problems caused by the four-way valve's reverse defrosting have been solved. The heat source for defrosting is the work done by the compressor and the heat storage capacity of the compressor casing. As the refrigerant continues to flow, the refrigerant further cools down. Will the refrigerant condense due to the release of latent heat? As can be seen from the pressure-enthalpy diagram, the condition that the refrigerant does not condense is that the saturation temperature corresponding to the refrigerant pressure in the air heat exchanger is lower than the frost temperature outside the tube. Because the refrigerant has no way to discharge its latent heat of condensation at this time, the refrigerant can only be cooled to the same temperature as the frost layer.