(only a schematic diagram, not corresponding to any product in the article)
1. Cream suppression technology
Humidity is a key factor affecting the formation of cream. Therefore, cream suppression technology through solid or liquid dehumidification has been fully developed. As for solid dehumidifiers, they mainly include silicone , silicate and activated carbon; while liquid dehumidifiers mainly include lithium chloride , lithium bromide , calcium chloride and ethylene glycol . The liquid dehumidifier can be directly sprayed onto the air inlet or the surface of outdoor heat exchangers. Dehumidification not only reduces the humidity of the air, but also increases the air temperature due to the adsorption or absorption process.
However, the main disadvantage of solid/liquid dehumidification and cream suppression technology is the need for regeneration. Both solid and liquid dehumidifiers need to be regenerated for continuous operation, which limits its application in air source heat pumps. Among them, compared with solid dehumidifiers, the regeneration temperature of liquid dehumidifiers is significantly lower.
Another important frost suppression technology is the surface treatment technology that changes the surface characteristics of outdoor heat exchangers. shows as shown in the figure below. According to the contact angle, the material surface can be divided into hydrophilic, hydrophobic and superhydrophobic. The hydrophilic surface inhibits the frosting process by interfering with ice crystal formation and water molecule fixation.
has a more sparse distribution of condensate droplets on the hydrophobic surface, which can delay the freezing of droplets and delay frost. The surface of superhydrophobic can "pop up" tiny water droplets before the frost is formed, which can more effectively inhibit frost. Surface treatment technology is efficient, cheap and environmentally friendly, but the only problem that needs to be solved is the long-term effectiveness of the surface coating.
Contact angles of hydrophilic, hydrophobic and superhydrophobic surfaces
In addition, relevant studies have also proposed methods of ultrasonic vibration, air jet, external AC or DC electric field and external magnetic field to prevent or delay frost. However, since these technologies require expensive equipment and large energy consumption, their application in actual engineering is largely limited.
2, defrost method
Compared with frost suppression technology, defrost technology mainly attempts to promptly and effectively remove the frost layer on the surface of the heat exchanger. Generally speaking, there are five basic defrosting methods shown in the figure below, including:
(1) compressor shutdown defrost;
(2) electric defrost;
(3) hot water spray defrost;
(4) hot air bypass defrost and
(5) reverse cycle defrost.
Among them, reverse cycle defrost and hot air bypass defrost are the most common defrost methods for air source heat pumps.
In the compressor shutdown defrost mode, the compressor is shut down and the outdoor fan continues to operate, so that the outdoor air continues to pass through the outdoor heat exchanger for defrost. This method can only work when the outdoor temperature is higher than 0°C and has a long defrost time, but it is inexpensive. In addition, electric heaters can be installed on outdoor heat exchangers to accelerate melting of frost. It is worth noting that since electricity is a high-quality energy source, this limits the application of electric defrost in household air source heat pumps. For hot water spray defrosting, it is necessary to spray hot water on the outdoor heat exchanger and turn off the fan of the outdoor unit at the same time. Hot water defrosting methods are only suitable for specific occasions, not all air source heat pumps.
reverse cycle defrost is the most traditional air source heat pump defrost method. During the defrosting process, the four-way valve is reversed, the outdoor heat exchanger is used as the condenser , and the indoor heat exchanger is used as the evaporator , and the high-temperature refrigerant flows into the outdoor coil to melt the frost. At the same time, in order to avoid discomfort in the defrosting process, the fan of the indoor unit will also stop running.
For hot air bypass defrosting, part of the refrigerant discharged from the compressor will enter the outdoor heat exchanger to melt the frost, and the other part will enter the indoor heat exchanger to heat. This is a commonly used method for industrial air source heat pump units. This method avoids the reverse impact and "oil run" problems that occur during reverse cycle defrost, and can maintain good comfort in the room during the defrost.
As mentioned earlier, reverse cycle defrost and hot gas bypass defrost are the most widely used and basic defrost methods. On this basis, there are also some developments to improve countercycle defrost and hot gas bypass defrost.
utilizes heat storage to be a typical new defrost method based on the development of reverse cycle defrost. In this technology, phase change material stores heat dissipation or excess heating of the compressor housing during the heat pump heating stage, and it is used as a heat source for defrost. This method can effectively shorten the defrost time, improve indoor comfort during the defrost process, and reduce the risk of wet compression of the compressor. In addition, on the basis of hot air bypass defrosting, a sensible heat defrosting method is also proposed. This method discharges part of the refrigerant from the compressor first throttling through the expansion valve , and then enters the outdoor heat exchanger to defrosting. It is suitable for large-capacity air source heat pump units.
3, defrost control strategy
In addition to defrost technology, the control methods for defrost start and termination are another key factor affecting the defrost performance of air source heat pumps. "Frost-free defrost, frost-free without removal" will lead to poor defrost effects, which are very common in actual products.
In order to obtain a suitable defrost start time, existing research proposes time control method, time-temperature control method, time-temperature difference control method and superheat control method. time control method is the most widely used control method at present, it automatically defrosts with fixed time periods. In contrast, the time-temperature control rule also requires measuring the surface temperature of the outdoor heat exchanger, and the time-temperature difference control rule requires obtaining the temperature difference between the outdoor air and the surface of the evaporator. However, time and temperature are not the only factors that cause frost, and ambient temperature, relative humidity and partial load rate can also affect heating performance and frost formation.
smarter defrost methods focus on "demand defrost". These intelligent defrost start methods include:
(1) using holographic interference technology to measure the thickness of the frost layer;
(2) using infrared thermometer to measure the surface temperature of the frost layer;
(3) measuring the stability of refrigerant flow;
(4) using optocoupler , optoelectronic system or fiber sensor to measure the thickness of the frost layer;
(5) neural network to simulate the frost volume;
(6) analyzing the surface temperature of the fin to obtain effective mass flow score;
(7) measuring the outdoor unit air passage pressure drop or outdoor unit air fan current. The technologies mentioned above are not yet mature and need further testing and development.
At present, the control method for defrost termination is mainly based on the temperature of the fin surface of the outdoor heat exchanger tube, the refrigerant pressure drop and the defrost time. finned tube surface temperature is the most commonly used method to judge. When its surface temperature reaches the set termination temperature, the air source heat pump will exit the defrost and enter the heating mode. The set termination temperature depends on the outdoor working conditions and different requirements. For the multi-circuit air source heat pump unit of outdoor unit, detecting the temperature of the outlet surface of the coil is the most commonly used method.
Copyright Statement: This article is excerpted from Architecture and Heating , edited and compiled by HETA editor. The views of the article do not represent the position of this official account. Please indicate the source when reprinting.
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