Frosting in the cryogenic section is not a fault, but the normal operation: the water vapor in the incoming gas and some solvents condense on the low-temperature heat exchange surface, and the frost layer thickens with the operation time. With proper management, frosting is only one item in the operation subject. Let alone, the thickening of frost layer will cause the pressure difference to rise and the yield to decline, which will seriously block the heat exchange surface and force it to stop. Three things in frosting management: judging to what extent the frost layer should melt, which defrosting method to choose, and how to arrange the cycle. There are data to rely on for all three things, which do not depend on feelings or the experience differences of the personnel on duty.
The state of frost layer depends on three signals: pressure difference-the pressure difference between the inlet and outlet of the heat exchanger continues to rise with the thickening of the frost layer, and defrosting should be arranged when it reaches a certain multiple of the reference value, which is a reliable criterion. Yield-condensate volume decreased significantly at the same temperature level, indicating that frost layer has affected heat transfer; Temperature level-the running time of the refrigerator becomes longer and the evaporation temperature does not reach the set value, which also indicates deterioration of heat exchange. The pressure difference in the three signals is direct. It is suggested that the pressure difference should be given priority to, and the yield and temperature level should be supplemented to change the "melting" from feeling to number, and the shift change should also be explained clearly.
There are three commonly used defrosting methods: stop the machine to defrost naturally, and remove the frost by ambient temperature or incoming air temperature, which is simple and convenient, but the device should be withdrawn from operation, which is suitable for systems with complete spare channels or switchable branches; Hot gas defrosting, heating the heat exchange surface with compressor exhaust, melting fast, system exit time is short, and configuration is slightly complicated; Electric defrosting, electric heater defrosting, convenient control, increased power consumption. The choice depends on two points: the system is not allowed to stop-heat gas or electric defrosting is not allowed to stop; The composition of the frost-the frost mainly containing water vapor is good. The mixture mixed with the solvent condensate should consider the drainage path. Where and how to collect the melted liquid should be determined in the plan.
The defrosting cycle is not a fixed value, but is determined according to the pressure difference trend: in the season with low moisture content of incoming air, it melts from a few days to a week; In the rainy season and when the moisture content of incoming air rises, the cycle is obviously shortened. The differential pressure record is connected into a curve, and the corresponding relationship between the differential pressure rise rate and the defrosting interval is found out in two or three cycles. After the cycle is scheduled, it will be written into the operation regulations, indicating the terms of correction according to the pressure difference-the regulations will be given to the benchmark, and the data will be corrected, so that defrosting will not waste cold quantity in advance and block the channel with delay, and the personnel on duty will follow the regulations.
| way | Features | Applicable occasions |
|---|---|---|
| Natural Melting Cream | Simple and time-consuming | Standby channel switchable |
| hot air defrost | Fast speed and small disturbance | Long downtime is not allowed |
| electric defrosting | Convenient control of increased energy consumption | Common for small and medium-sized devices |
| trigger criterion | Pressure difference is the main yield temperature position auxiliary | Numbers do not judge by feeling |
| Cycle arrangement | According to differential pressure trend | Dynamic correction of moisture change |
| Drain path | Collection destination of defrosting fluid | The program phase is well set. |
No. When the frost layer is thin, heat transfer deterioration is limited, and early defrosting consumes cold and heat in vain, and increases system disturbance. According to the trigger value, the frost is thick enough to affect heat transfer and re-melting, and the economy is the best.
The frost is the one where the pressure difference rises quickly and falls back to the place after defrosting. After defrosting, the pressure difference is still high and scaling is the one that rises slowly for a long time. The two are treated differently: frost by melting, dirt by cleaning. The differential pressure curve is a good tool to distinguish between the two.
Not necessarily. In winter, when the air itself is dry, the frost load is light. High humidity in summer is the peak of frost. The cycle follows the incoming air and moisture, and is revised with the season. Don't follow the calendar.
This article by LuoyangWohong PetrochemicalEquipment Co., Ltd. arranges that the company provides design, manufacturing and operation and maintenance services for condensation recovery, membrane separation recovery and complete solvent recovery devices for chemical and pharmaceutical enterprises. If you need process evaluation or scheme configuration, please contact and exchange.
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