How are frost and ice formed?
1. Frost forms below 0℃.
2. Turn into water above 0℃.
3. The water flows down and vaporizes. During the defrost process, if the water on the fins has not completely vaporized, the unit enters the next round of heating process, and the unit begins to frost again. And the frost turns into ice. What remains in the air is frost, and the water remaining on the fins forms ice when it is below 0°C.
Once the water on the fins freezes before it has time to evaporate, it cannot be removed. At this time, the high and low pressures of the unit are gone, which will cause the greatest damage to the compressor. It would be even worse if it is a screw compressor, because the screw compressor supplies oil under pressure. Once the high and low pressures are lost, it will be like heart ischemia, with serious consequences.
At 5℃, the air energy The efficiency of heat pump water heaters is relatively low. In addition, defrosting consumes 20% of the energy. If there is frost that cannot be melted, the life of the unit will be in danger and the energy efficiency ratio will be greatly reduced.
Air energy It is inevitable for heat pump water heaters to produce frost in winter. When we use air conditioners, we will find frost and defrost on the outdoor unit, while the indoor unit mainly relies on electric auxiliary heating to heat the indoor temperature. Similarly, if the heat pump water heater is frosted, it must be defrosted. The configuration of the heat exchanger design system can delay frost formation and reduce the impact of frost on the performance of the air source heat pump water heater. It can make the unit frost slower, and the impact of frost on the performance of the unit will also be slower.
It is impossible for frost to form under dry conditions, but it is possible to form frost under zero-temperature conditions. When the evaporator temperature drops to the air temperature of the air passing through the evaporator, the water molecules in the air remain on the fins. The zero-degree operating condition is beneficial to the heat exchange of the unit, with sensible heat and latent heat. The dry operating condition is sensible heat exchange, and the zero-degree operating condition is latent heat exchange. If the surface temperature of the evaporator is lower than 0 in the zero-degree operating condition, Frost will form at ℃. When there is slight frost on the surface of the evaporator, the effect on the heat exchanger is improved. It can enhance heat dissipation and the surface of the evaporator appears burr-like. However, as the frost layer thickens, the resistance to air circulation increases. It will inevitably hinder the circulation of air. The evaporator absorbs heat through the air. When the air flow decreases, the heat absorption also decreases. At this time, the performance of the unit begins to decrease.
So how to overcome this difficulty in the design of air-energy heat pump water heaters?
Through Howwatt’s many years of practice, it has been found that 1. The fan speed can be increased to increase the exhaust volume. The frosting process is that the steel pipe is frosted first, then the evaporator fins are frosted, and then the inside of the evaporator is frosted. frost. 2. The heat exchange area increases, and the temperature difference between the evaporation temperature and the ambient temperature is relatively small, making it less likely to frost. 3. Use a small circulation system to rest the unit. The defrosting time is shortened, while ensuring that the hot water storage tank has hot water of sufficient temperature and improving the energy efficiency ratio. This will definitely increase the cost. This is different from the heat exchanger on the outside of the air-conditioning heat pump hot and cold water unit. The air-conditioning heat pump hot and cold water unit provides heating in winter and cooling in summer. The outdoor heat exchanger of the unit is designed with the condenser in mind, so the condensation load determines the evaporation area.
You can also increase the spacing and surface treatment of the fins. The smaller the distance between the fins and the more compact they are, the greater the fan resistance will be, and the heat exchange effect and space utilization will be very good. However, the effect is not good in winter, and the air flow channel is easily blocked when frost forms. The smaller the fin spacing, the faster it will clog. (The fins of cold storage are at least 6mm or more) The fins of general air source heat pump water heaters are about 2mm, and some are even 1.8mm. Therefore, increasing the fin spacing can delay frost formation. So how to choose the fin distance? This This is a question worth asking,
How to defrost it?
1. The four-way valve turns to turn cooling into heating. If there is frost on the evaporator, the system works in reverse. The evaporator becomes a condenser, allowing the heated water to melt away the frost. This is a commonly used method. This method is also used for air conditioner defrost.
2. Hot gas bypass defrost, that is, converting the heat on the evaporator to defrost.
Four-way valve defrost is better than hot gas bypass valve defrost. The four-way valve defrost converts the generated hot water to defrost. Strong defrosting force can make the frost thicker and then remove it. The hot gas bypass defrost has less impact on the system, but is weaker than the four-way valve defrost. It can allow the hot gas bypass defrost to be performed more often, the frost layer will be thinner, and the defrost time will be longer.
No matter which defrost method is adopted, the defrost control method is very important. If the hot air bypass method is used to build a four-way valve defrost system, the frost may not be removed, so each method has advantages and disadvantages.
The methods of defrosting are different. The way to control defrost is destined to be different.
The simplest and most primitive way to control defrost is scheduled defrost. In about 40 minutes, the unit will enter the defrost stage. It will work as usual regardless of whether there is frost or not. This kind of defrost control is simple and cost-effective. Low. .
Timing defrost plus temperature defrost, defrost starts when the temperature and time reach the set values. This requires when to defrost, at which temperature to defrost, and under what circumstances to exit defrost? It won't work early, it won't work late, and it won't work if it's not cleaned thoroughly. Because the longer the defrosting time is, the shorter the heating time is, and multiple defrosts will cause the unit’s condensation temperature to rise, which is also detrimental to the unit.
Timing defrost plus temperature defrost plus humidity defrost, defrost will start when the temperature, time and humidity all reach the set values. And humidity is a must. This method has relatively high reliability, but the technical coordination and cost are also relatively high.
When using defrost, you should also pay attention to the relationship between the unit capacity and the water tank capacity. Generally speaking, the frosting time of an air source heat pump water heater is about 40 minutes. If the water is well prepared, even if the unit is frosted, you will not be afraid. Therefore, it is particularly emphasized here that the small circulation system is of great benefit to defrost. It can also save most of the energy consumption and improve the COP value.
So for a large cycle or a small horse-drawn cart style with a small unit working hard against a large water tank, everyone knows the consequences. The operation mode, control mode and configuration mode of the unit are all important factors that directly affect defrost and equipment life.
Reliability and efficiency are contradictory. High reliability requires high efficiency, which is more difficult to design. This requires dealing with unfavorable operating conditions so that the unit can handle properly under high temperature and low temperature conditions, which is entirely possible. But it is very difficult to use R22 refrigerant to make hot water at 55°C. When the condensation temperature is 57-58°C, the evaporation temperature exceeds the limit. In 2010, R22 will be produced in limited quantities until it is eliminated. Therefore, the widespread use of new refrigerants is imperative.