【 Weak current College 】 hot gas bypass defrosting technology in air cooled chiller unit (heat pump)---Power By 【 China power house network 】

Air cooled chiller (heat pump) unit heat bypass defrosting technology in large, long-term industrial operation shows that the unit's reliability and heating performance is greatly increased by the users become more and more widely recognized and welcomed. Hot gas bypass defrosting and widespread use of the four-way valve reverse cycle defrosting work characteristics are as follows: 1. defrost on heat pump performance with frosting time grows, the more the frosting will accumulate, the more likely to make the air channel completely blocked, causing the unit does not work properly. Therefore, air source heat pump unit in frosting condition runtime need timely defrosting, defrosting time will not be able to provide heat, the heat from the Interior of the draw, the impact of the unit heating effect, defrosting process loss and defrost. Four-way valve reverse cycle defrosting is currently widely used a defrost, defrost when the four-way valve action so that the unit by the heating cycle into a refrigeration cycle, at the fan stopped working, high-temperature exhaust gas to enter the wind side heat exchanger for defrosting. The defrost cycle defrosting process, the unit out of the water absorption of heat supply system to the wind side heat exchanger removal cream, unit heat generated is negative, negative calorie count and in the same time produces is approximately equal to the heat. So from the energy perspective, this loss of defrosting process is equivalent to two times the defrost time down, the unit of quantity of heat down 10%. Hot gas bypass defrosting method does not need to change the unit of the heating cycle, only needs defrosting of the heat exchanger by-pass for defrosting, assume that the figure of air heat exchanger defrosting, the required workflow is as follows:
Each group of finned coil has a hot gas bypass valve 2 and 3 main solenoid valve, defrosting time corresponding hot air by-pass valve open, master solenoid valve closes, the use of exhaustheat from reaching the aim of defrosting. After defrosting, hot gas bypass valve closed, open the master solenoid valve, normal operation of finned coil, for the next group of finned coil defrost. In the course of the entire defrosting, heating and defrost at the same time, its obvious loss of defrosting is less than four-way valve reverse cycle defrosting method. 2. four-way valve reverse cycle defrosting of reliability problem 1) compressor fluid compression issues four-way reversing valve, the unit by the heating cycle into a refrigeration cycle, the system of inspiratory pressure and exhaust pressure changes, on the compressor and other components to make a big impact. At the beginning of the defrost water side, due to the heat exchanger of pressure suddenly becomes small so that before defrosting for condenser with a large number of liquid refrigerant easily return compressors, compressor fluid in compression. 2) compressor oil loss in defrosting process, since the beginning of the wind side heat exchanger of a large number of frost exists, resulting in condensation pressure rise slowly, so that the system of high and low pressure difference is relatively small, smaller refrigerant mass flow, allowing the compressor suction pressure further reduced, to meet the defrost the refrigerant flow needs. General thermostatic expansion valve action pressure difference above in order to ensure that 150kPa has just the right amount of refrigerant flow through the valve for defrosting, the result is the compressor suction pressure started by the defrosting down to 150kPa following 300kPa. For reciprocating compressor, crankcase lubricant due within contain large amount of refrigerant, low pressure produced large quantities of foam, oil and refrigerant enters the compressor discharge from the exhaust pipe, the serious deterioration of the compressor lubrication, lead bearings and other moving parts missing oil damage. After defrosting, the system restart, defrosting process into the air side heat exchanger for oil and gas-liquid refrigerant mixture is pumped back into the compressor, resulting in the loss of oil and oil pressure safety protection device action. For screw compressors, compressor oil cavity in the exhaust Chamber, oil circulation is dependent on oil cavity and injection point (pressure slightly higher than the inlet pressure) pressure difference. When defrosting starts, the system of high and low pressure difference smaller led unit lubrication conditions deteriorated, at the same time, screw compressor reduces exhaust pressure suddenly can also cause behind the deterioration of the oil fuelling lubrication, these factors greatly reduced compressor operation reliability and service life, serious and directly led to the failure of compressor and so on. This requires in the four-way valves for forward to compressor after uninstalling downtime, reversing and then wait for the pressure balance, while starting compressor should start after the compressor in a minimum pressure of run time does not exceed 20 seconds. The defrosting process is longer, the heating time, caused the heating effect of the variation and the water temperature fluctuation in the increase, in order to shorten the time to improve the heating defrosting, performance control eliminates the need for reversing outages and the delay time, the compressor was reliability. 3) defrosting in the early stages of the process, there is a low voltage attenuation systems tend to decay to a low value, affecting the life of the compressor; four-way valves for frequent actions, making the reliability of variation. 4) defrosting end stage prone to high-voltage protection cause downtime. 3. hot gas bypass defrosting process reliability analysis when entering the defrosting procedures, the crew remains heating work cycle, the exhaust side of the compressor out of a hot gas defrost entering need air heat exchanger, so that high temperature gases in higher than 5 ¡æ occurring phase release of latent heat up defrost purposes (see above hot air by-pass defrosting state diagram). Defrosting of refrigerant liquid into another side of normal heating of heat exchanger to evaporation, thus, this part of the refrigerant is still played by heating, vaporization back into gas avoids the compressor for defrosting liquid may be caused by the back. If for some reason do not stop causing defrost, hot gas not directly back into the compressor, which is the normal access to the other side of the heat exchanger for heating, thermal expansion valves at this system will be based on the superheat changes automatically increase for fluid, so that the compressor suction maintained at appropriate superheat range. In the course of the entire defrosting, to avoid the refrigerating cycle and heating cycle of mutual transformations, compressor suction and exhaust State basic remains stable, in order to solve the four-way valve reverse cycle defrosting of reliability problems. 4, two defrosting method comparison
5. closing air-cooled heat pump units in winter heating runs, the outdoor side heat exchanger frosting properties directly affect the unit heating effect, at the same time defrost reasonable or not is not only related to the unit heating effect, but also the reliability of the unit. Extensive practical experience has shown that both screw or reciprocating air source heat pump unit, unreasonable defrosting will cause very high failure rates, or evenCompressor so serious accidents. Our company adopts heat bypass defrosting method, it can not only enhance the air-cooled heat pump heating performance and, more importantly, can significantly improve the reliability of defrosting unit.