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How does the Heat Pump defrost

  • By:Heyi
  • Date:2021/11/17

Why do air source heat pumps freeze?

As we all know, the heat pump absorbs heat energy from the air, and the component of the air source heat pump system that absorbs the heat energy from the air is the refrigerant evaporator. The temperature of the refrigerant flowing in the evaporator tubes is very low and the outside air temperature is higher compared to the refrigerant flowing. So the refrigerant can absorb energy from the outside air. During the refrigerant evaporation process, the warmer air from the outside will condense on the surface of the evaporator. When the temperature drops to around 0°C, this accumulated dew forms a layer of frost.

 

What is the harm to the heat pump after frost/icing?

1: Block the channel between the auxiliary pieces and increase the air flow resistance

2: Increase the thermal resistance of the heat exchanger and reduce the heat transfer capacity

3: Frequent defrosting, endless defrosting. The defrosting process is an air-conditioning operation process, which not only cannot make hot water, but also needs to consume the energy efficiency of the original hot water, and the discharged frozen water is flushed back into the thermal insulation water frost. cause the water temperature to drop further

4: The evaporation temperature decreases, the energy efficiency ratio decreases, and the operation performance of the heat pump deteriorates until it can not work normally

5: Because the unit cannot work normally, it directly causes economic losses to customers

 

During the low-temperature heating process of the heat pump, the evaporator will inevitably frost, and the thickness of the frost layer will gradually increase with the passage of time, and the gradual frost formation will reduce the energy exchange and the heating capacity of the heat pump.

 

Therefore, in order to maintain the heat exchange capacity of the air source heat pump water heater and improve the heat exchange efficiency of the heat exchanger, the heat pump should have a reliable defrosting method.

 

Frost in the heat pump water heater?

First: normal frost formation. When the outdoor temperature is lower than 0 ℃ in winter, the heating operation time is long, and the surface of the entire heat exchanger of the outdoor unit is evenly frosted, which is a normal phenomenon. Reason: When the temperature of huanrqi2 is lower than the dew point temperature of the ambient air, condensation water will be generated on the surface of the heat sink on the entire heat exchanger. When the ambient air temperature is lower than 0 ℃, the condensation water will condense into thin frost. Of course, serious frost will affect the heating effect of the machine. General heat pump products have automatic frost function to ensure the normal operation of the unit

Second: abnormal frosting, which is the main reason for the failure of the heat pump operation. Roughly speaking, it includes three phenomena:

1: When the outdoor temperature is greater than 0°C, the condensation water on the surface of the heat sink on the entire heat exchanger will condense into thin frost soon after the machine is turned on, and soon the frost will become thicker and thicker. The heating effect of the indoor mechanism is poor, and the phenomenon of frequent defrosting occurs. The fault is generally caused by the surface of the heat sink on the outdoor heat exchanger being dirty and blocked, the fault of the outdoor fan system or the inlet and outlet of the outdoor heat exchanger are caused by 2 obstructions

2: The outdoor ambient temperature is greater than 0 °C, and soon after the startup, the bottom of the outdoor heat exchanger (starting at the inlet of the heat exchanger at the outlet of the capillary) is heavily frosted, and most of the heat exchangers have no condensation, and the frost will be delayed over time. , the frost is extended from top to bottom; the indoor fan is always running at a low speed of cold air; the air conditioner is frequently defrosted. The fault is generally a lack of refrigerant in the system.

Solution: first detect the system leak point repair, add refrigerant

3: The outdoor ambient temperature is greater than 0 °C, and it will be turned on soon. The frost on the upper half of the outdoor heat exchanger (the outlet of the heat exchanger and the air return pipe) is very thick, and the frost on the heat exchanger extends from top to bottom over time (there is a direction from the outlet of the heat exchanger to the inlet of the heat exchanger). The frost is spreading; and the heating effect is poor; the air conditioner is frequently defrosted. The fault is usually too much refrigerant in the system. Failure often occurs after maintenance and refrigerant addition

Solution: put the refrigerant to normal operation.

 

How to defrost an air source heat pump?

1. Natural defrosting

This is one of the simplest defrosting methods for the air source heat pump family. When the system needs defrosting, the system stops running for a period of time, and uses the heat of the surrounding environment to melt the frost layer on the surface of the heat exchanger. The crew will start working again

Natural defrosting is relatively simple, and does not require extra energy to defrost, but the disadvantage is that the longer the defrosting time, the greater the heat loss in the space, and its heating efficiency will decrease. , this defrosting method is not applicable

 

2. Water cream

This defrosting method is also relatively simple. It only needs to use a water spray device to spray water on the outer surface of the evaporator, so that the frost layer is melted and washed away by the heat of the water. This defrosting method is relatively waste water, and the scope of use is limited to air coolers with drainage pipes. In the colder places in the northern region, the remaining water after the unit is flushed needs to be removed in time after the water spray is completed, otherwise the water will be frozen on it, which will accelerate the rate of frost formation

 

3. Hot gas defrosting

The method of hot gas defrosting is to pass the exhaust port of the compressor into the evaporator, and use the heat of the exhaust port to melt the frost on the outer wall. In some cases excess heat is used, in more cases the hot gas is condensed in the evaporator, using its sensible and latent heat to defrost

The advantage of this defrosting method is that the frost layer melts from the inside to the outside. After the defrosting is completed, the hot air can be sent to the space immediately, but there are also problems of long defrosting time and high defrosting frequency.

 

4. Electric defrosting

The way of electric heating defrosting is to install electric heating wire outdoors and use electric heating to defrost. It is mainly used for finned tube air coolers, and the electric heating element is attached to the fins. During defrosting, the compressor stops working

The advantages of electric heating defrosting are: simple system operation and complete defrosting, which are mainly suitable for small equipment. However, what is missing is that part of the heat from electric heating and defrosting will be dissipated into the atmosphere, increasing the energy consumption problem, and the service life of the heating wire is also limited, which will also cause certain safety hazards.

Defrost is also an important technical issue for air source heat pump units, so when choosing to install an air source heat pump, be sure to find a well-known big brand manufacturer. In this way, both system maintenance and defrosting issues can be guaranteed. It operates efficiently while saving heat pump maintenance costs

 

5. Reverse cycle method

This method requires a device four-way valve. After the four-way valve is activated, the flow direction of the refrigerant is changed, so that the system switches from the air source heating mode to the cooling mode within a certain period of time. The high temperature gas discharged from the compressor directly enters the heat exchanger (evaporator) for defrosting

 

6. Hot gas bypass method

Using this method, the refrigerant flow does not change during defrosting. The heat pump will maintain the heating state, and the high-temperature gas discharged from the compressor will directly enter the box heater to defrost through the bypass pipe. The heat source for defrosting is the electrical energy consumed by the compressor and the heat storage capacity of the compressor casing

 

7. Energy storage defrosting method

Using this method, first the phase change material will store energy. When the evaporator freezes to a certain level and needs to be defrosted, the energy stored in the phase change material will be released to defrost.

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