1 Heat pump hot water unit is generally composed of compressor, condenser, filter, expansion valve, evaporator and other components.
The implementation of the ODS phase-out action, the Montreal Protocol is playing a significant and important role in the recovery of the atmospheric ozone layer and global environmental protection. At the 19th Conference of the Parties to the Montreal Protocol, held in Montreal, Canada in September 2007, the international community further reached a decision on accelerated phase-out of hydrochlorofluorocarbons (HCFCs). Compared with the previous regulations, the Montreal Protocol has advanced the overall phase-out target for hydrochlorofluorocarbons (HCFCs) by approximately 10 years. This latest phase-out schedule will accelerate the replacement of R22 in the world. The European countries have basically banned HCFC. The United States also bans the use of R22 in new production refrigeration equipment. China has joined the WTO organization, China's refrigeration and air-conditioning enterprises will also face tremendous pressure from these developed countries' green products, and must follow the pace of developed countries in the issue of alternative work. Therefore, the alternative task of R22 has been very urgent.
2 heat pump hot water unit with R22 as working fluid
At present, most of the units in the Chinese market use R22 working fluid, and the products are relatively mature. Now I have a circulating heating air source unit with R22 as the working medium, according to the national standard GB / T21362?
2008? The nominal operating conditions of heat pump water heaters for commercial or industrial use and similar purposes are analyzed from two aspects: thermodynamic theoretical analysis and experimental testing.
2. 1 theoretical design calculation
According to the nominal working condition, the unit's condensation temperature T c = 56, the unit's evaporation temperature T v = 10, the degree of subcooling T sc = 10, the suction superheat T sh = 5. According to the low temperature condition, the unit's condensation temperature T c = 56, the unit's evaporation temperature T v = - 2 , undercooling T sc = 10, suction superheat T sh = 5. The theoretical cycle of each state point parameters and COP, pressure ratio calculation.
The unit with R22 working medium is limited by the compressor, and it can't produce high temperature hot water of 65 or more. Most R22 compressors on the market have a safe working condensing temperature of only 65. When the unit is running at high condensing pressure When it is down, its exhaust temperature will be very high, which will lead to carbonization of the compressor oil and affect the service life of the compressor.
2. 2 prototype test
For example, a unit with a nominal heat of 45 kW and a nominal power consumption of 11. 5 kW is used. The unit is R22 and the charge of R22 is 20 kg. According to the national standard GB / T 21362? The test method for the primary heat pump water heater was tested in 2008. 529. The ratio of measured COP to theoretical COP under nominal conditions is 0. 681. The ratio of measured COP to theoretical COP under low temperature conditions is 0. 529.
2. 3 defrosting control of the prototype
When the unit is running in winter, it is inevitable to face the problem of frosting in winter. The unit must solve the problem of defrosting. The main purpose is to improve the structural design of the fin heat exchanger and delay the frosting speed. Effective defrost after frosting. The unit adds 3 rows of supercooled pipe sections at the bottom of the fin heat exchanger to slow down the frosting speed. At the same time, the unit adopts the reverse operation mode, and the four-way valve is reversed to switch the operation mode and directly compress the compressor. The high temperature and high pressure refrigerant vapor is passed to the heat of the fin heat exchanger tube to achieve fast and effective defrosting purposes. Therefore, the design pressure of the fin heat exchanger needs to withstand the high pressure of the compressor discharge.
3 Comparison of thermal properties of refrigerant R744 and R22
(1) R22 is colorless and odorless, non-combustible, non-explosive, low in toxicity, non-corrosive to metal, safe to use. The standard evaporation temperature is -40. 8 and the solidification temperature is -160, which has good thermal properties and thermophysical properties. It has moderate pressure in normal temperature and ordinary low temperature range, large volume capacity per unit volume, although heat transfer performance is not as good as ammonia, but stronger than some Freon working substances (such as R134a), poor water solubility, limited solubility in lubricating oil.
(2) At room temperature, R744 is a colorless and odorless gas with very stable chemical properties, which is neither flammable nor combustion-supporting. The relative molecular mass is 44. 01, the critical pressure is 7. 3773M Pa, the critical temperature is 304. 1 K, the triple point temperature is 216. 6 K, the triple point pressure is 0. 51795MPa, under standard atmospheric pressure, The sublimation temperature is 195.0 K, and the sublimation heat is 573.27 kJ / kg, which has a large cooling capacity per unit volume.
R744 has ODP = 0, GWP = 1, has excellent economy, and has no recycling problem; good safety and chemical stability, CO2 is safe, non-toxic, non-flammable, and suitable for various lubricating oils and common mechanical parts and materials. It does not decompose to generate harmful gases even at high temperatures; it has thermodynamic properties compatible with refrigeration cycle and equipment. The latent heat of vaporization of CO2 is relatively large, and the refrigeration capacity per unit volume is quite high; it has good transport and heat transfer properties, and is excellent in CO2. Flow and heat transfer characteristics significantly reduce the size of the compressor and system, making the entire system very compact.
Physical properties of R22 and R744 working fluids.
4 heat pump hot water unit with R744 as working fluid
R744 is one of the most promising natural working fluids in the heat pump system. The temperature slip at the gas cooler end of the CO2 transcritical system can be well matched with the variable temperature heat source, and its application in the heat pump water heater has advantages that other heating methods cannot match.
4. 1 theoretical thermodynamic design calculation
According to the national standard GB/T 21362? The nominal heating condition of the heat pump water heater in the 2008 standard. According to the nominal working conditions, the unit's gas cooler outlet temperature T c = 16, the unit's evaporation temperature T v = 10, the suction superheat T sh = 5, the exhaust pressure P d = 9. 5MPa, according to the low temperature conditions, The unit's gas cooler outlet temperature T c = 16, the unit's evaporation temperature T v = - 2 , the suction superheat T sh = 5, the exhaust pressure P d = 9. 5M Pa, then the theoretical cycle of R744, each The parameters of the state point and the calculation of the COP and the pressure ratio.
From the theoretical analysis, the following conclusions can be drawn. The transcritical R744 cycle, due to its temperature-slip characteristics under heat pump operation, can exchange heat with water well, its heating effect is remarkable, and its pressure is relatively small, compression The power consumption of the machine is low. R744 has a high unit volume cooling capacity, that is, the same cooling capacity of the unit, the R744 charge is much less than R22, the unit piping design, can reduce the pipe diameter, make the unit compact, save a lot Material; however, the operating pressure of the R744 is very high, the wall thickness needs to be increased, and each component that can withstand high pressure needs to be redesigned.
4. 2 test results test
Take a unit with a capacity of 40 kW in the name of a Japanese company and a nominal power consumption of 10 kW. The unit is R744 and the R744 is 9 kg (far less than the unit with R22). ). The unit is still in accordance with GB / T 21362? The standard operating conditions of 2008 and the test method of a heated heat pump hot water unit were tested. test results.
The ratio of measured COP to theoretical COP under nominal conditions is 0. 628. The ratio of measured COP to theoretical COP under low temperature conditions is 0. 596. And the unit can achieve a high water temperature above 65.
4. 3 defrost control
The pressure of the unit with R744 is very high, and the defrosting method is different from the defrosting method of the unit with R22. The design pressure of the fin heat exchanger is much smaller than the exhaust pressure. Therefore, The reverse defrosting mode cannot be used. The exhaust pressure of 12M Pa directly enters the fin heat exchanger, which will bring inevitable damage to the heat exchanger. If the design pressure of the fin heat exchanger is increased to Can withstand the pressure of 12M Pa, then the wall thickness of the heat exchange tube needs to be greatly improved. On the one hand, the wall thickness plus assembly increases the thermal resistance, reduces the heat transfer coefficient, and needs to increase the heat exchange area to ensure the heat exchange amount and increase the cost. The expansion process of the fin heat exchanger brings great problems. The existing mold cannot meet the expansion tube of the copper pipe with a large wall thickness, and the mold needs to be redesigned. Therefore, the R744 defrosting control is through a bypass decompression method, through a bypass line controlled by a solenoid valve and having a capillary tube. When the defrosting, the solenoid valve is opened, and the pressure is reduced by the capillary to the heat exchange. Below the design pressure of the device, the R744 working medium at a higher temperature is used to enter the fin heat exchanger for defrosting.
5 Conclusion
By comparing the physical properties of R774 and R22, and the comparison of theoretical cycles, and based on the test results of the unit prototype of similar heat, it can be found that the heat pump hot water unit COP with R744 is the heat pump heat with R22 working medium. The water unit is about 5.6% higher, and the heat pump hot water unit with R744 is able to achieve a high water temperature of 65 or more. R744 has a good reverse heat transfer effect with water, and its transcritical cycle can be well applied to heat pump hot water unit products. As an environmentally friendly work, R744 can be a good alternative to heat pump hot water unit products.
Ball Mill Grinding Ball,Grinding Ball For Mining,Forging Steel Balls,Forged Grinding Ball
SHANDONG SHENGYE GRINDING BALL CO., LTD , https://www.shengysteelball.com