1. Application of plate heat exchangers in commercial air conditioners
(I) Application in refrigeration cycle In the refrigeration system of commercial air conditioners, plate heat exchangers are mainly used as condensers and evaporators. When used as a condenser, the gaseous refrigerant is cooled and condensed into liquid in the plate heat exchanger. For example, in the central air conditioning system of a large shopping mall, the high-temperature and high-pressure refrigerant gas discharged from the compressor enters the plate condenser, and through heat exchange with the cooling medium (usually air or water), the heat is taken away and the state of the refrigerant changes, thus completing a key link in the refrigeration cycle. When used as an evaporator, the liquid refrigerant evaporates and absorbs heat in the plate heat exchanger, reducing the temperature of the cooled medium (such as air). Taking the air conditioning system of a hotel as an example, the refrigerant absorbs heat from the indoor air in the plate evaporator to achieve cooling of the indoor air.
(II) Application in heating cycle In the heating process of heat pump type commercial air conditioners, plate heat exchangers also play an important role. It can be used as a condenser to release heat. For example, when some shopping malls in the north use heat pump air conditioning systems for heating in winter, the heat of the refrigerant is transferred to the indoor air through the plate heat exchanger to increase the indoor temperature. At the same time, in the reverse cooling stage of the heating cycle (for defrosting and other functions), the plate heat exchanger can work as an evaporator.
(III) Applications in improving energy efficiency Since the plate heat exchanger has a high heat transfer efficiency, it can make the heat exchange between the refrigerant and the cooling/heating medium more complete. This helps to improve the energy efficiency ratio (EER or COP) of the entire commercial air conditioning system. For example, compared with traditional shell and tube heat exchangers, plate heat exchangers can increase the energy efficiency of the air conditioning system by about 10%-30%, reduce energy consumption, and reduce operating costs.
II. Technical requirements for plate heat exchangers in commercial air conditioners
(I) Heat transfer performance requires high heat transfer coefficient: The plate heat exchanger should have a high heat transfer coefficient to ensure efficient heat transfer under a small temperature difference. Generally, the heat transfer coefficient is required to be between 2000 and 8000W/(m²・K), and the specific value varies depending on the refrigerant and working conditions. This is because a high heat transfer coefficient can reduce the heat exchange area of the heat exchanger, thereby reducing the size and cost of the equipment. Good heat exchange efficiency: The logarithmic mean temperature difference correction factor (F) of the plate heat exchanger should be as close to 1 as possible. For example, under the design conditions, the F value is greater than 0.9, which means that the actual average temperature difference is very close to the theoretical logarithmic mean temperature difference, which can ensure the high efficiency of the heat exchange process and reduce energy loss.
(II) Pressure resistance performance requires the ability to withstand high pressure: During the operation of commercial air-conditioning systems, the pressure of the refrigerant will change. The plate heat exchanger needs to be able to withstand higher pressures, and the general design pressure should not be less than 3.0MPa to ensure safety under various working conditions (such as starting, stopping, load changes, etc.). Especially for air-conditioning systems using high-pressure refrigerants such as R410A, higher pressure resistance is essential. Pressure drop control: While ensuring sufficient pressure resistance, it is also necessary to control the pressure drop of the refrigerant and the medium in the plate heat exchanger. It is usually required that the pressure drop on the refrigerant side does not exceed 0.05MPa, and the pressure drop on the water side (if water is used as the cooling or heating medium) does not exceed 0.07MPa. Smaller pressure drop helps reduce the power consumption of the compressor and improve the operating efficiency of the system.
(III) Material requirements Corrosion resistance: Due to the different chemical properties of the refrigerant and the cooling/heating medium, the material of the plate heat exchanger needs to have good corrosion resistance. For example, for a system with water as the cooling medium, the plate material of the heat exchanger is usually made of stainless steel (such as 316L) because it can resist corrosion by corrosive components such as chloride ions in water. For some special refrigerant-medium combinations, special coatings or alloy materials may also be required to enhance corrosion resistance. Good thermal conductivity: The thermal conductivity of the material directly affects the heat exchange efficiency of the heat exchanger. The thermal conductivity of the plate material is generally required to be between 10-200W/(m・K). For example, copper and copper alloys are commonly used materials with good thermal conductivity, but considering factors such as cost and corrosion resistance, composite materials are sometimes used to ensure a certain thermal conductivity and meet other performance requirements.
(IV) Sealing performance requirements to prevent leakage: The sealing performance of the plate heat exchanger is crucial, because refrigerant leakage will not only affect the performance of the air-conditioning system, but also cause harm to the environment and human health. It is generally required that the leakage rate of the plate heat exchanger should be less than 1×10⁻⁶m³/(s・m) (leakage per meter of sealing length under standard conditions) at the design pressure and temperature. In order to ensure a good seal, the material of the sealing gasket needs to have good compatibility with the refrigerant and the medium, and be able to maintain elasticity and sealing performance during long-term use. Temperature resistance and aging resistance: The sealing gasket needs to be able to withstand temperature changes during the operation of commercial air conditioners. It is usually required to be able to work normally in the temperature range of -20℃ to 150℃, and will not age, harden or lose elasticity in long-term high temperature and chemical environments. For example, nitrile rubber (NBR) sealing gaskets are suitable for general refrigerants and temperature ranges, while high-performance sealing materials such as fluororubber (FKM) may be required for high-temperature environments.
(V) Compactness and ease of maintenance require compact structural design: In commercial air-conditioning systems, space is often limited. The plate heat exchanger should have a compact structure, and its volume heat transfer coefficient (heat transfer per unit volume) is generally required to be above 3000-10000W/(m³・K) to achieve a larger heat transfer in a limited space. At the same time, the compact structure also helps to reduce the refrigerant charge and the overall weight of the system. Easy to clean and maintain: After long-term use, the surface of the plate heat exchanger may be scaled or blocked by impurities, affecting the heat exchange efficiency. Therefore, it should be easy to disassemble and clean, such as using a detachable plate structure, which is convenient for users to regularly inspect, clean and maintain the inside of the heat exchanger to ensure its long-term stable operating performance.
