[Mechanical Equipment Technology Edition] Daily Question 201801031: Common Faults in Refrigeration Systems (8.7)

The inlet pressure decreases due to leakage losses

An excessively high compression ratio leads to over-compression, increasing the internal energy of the refrigerant

1. This leads to a decrease in volumetric efficiency; when it drops to a certain level, the compressor’s gas delivery coefficient becomes 0, and the compressor can hardly draw in refrigerant vapor, thereby losing its cooling capacity. 2. It causes the actual compression process to deviate more from the ideal condition, resulting in increased actual power consumption of the refrigeration compressor, reduced efficiency, and a lower coefficient of performance. 3. This inevitably leads to an increase in the exhaust temperature of the refrigeration compressor, sometimes even exceeding the allowable limit for that temperature. At the same time, excessively high exhaust temperature can also affect the proper operation of the refrigeration cycle. 4. Generally, the pressure ratio n for compressors in use is ≤8; for piston-type refrigeration compressors using Freon refrigerants, this ratio is ≤10. For centrifugal refrigeration compressors, the maximum achievable pressure ratio per cycle is ≤4. For these reasons, at a condensation temperature of 35°C, the lowest evaporation temperatures that can be achieved with different refrigerants are as follows: R717 = -22°C, R12 = -33°C, R22 = -34°C, R134A = -37°C. In summary, in some refrigeration systems, Freon-based single-stage systems can operate at medium temperature levels, whereas ammonia-based systems require a two-stage system to achieve better performance. It is also for this reason that some customers choose Freon systems: in certain compressors with advanced technology, such as those from Bitzer, single-stage systems are used to handle low-temperature conditions, and they can also be utilized for high-temperature conditions as well (but only in companies with advanced technical capabilities). Additionally, using supplementary liquid spraying can also meet the requirements for lower temperatures (for certain types of refrigeration screw units). If the compression ratio is too low, it’s simply like using a powerful engine for a small task, resulting in wasted electrical energy.

1. This leads to a decrease in volumetric efficiency; when it drops to a certain level, the compressor’s gas delivery coefficient becomes 0, and the compressor can hardly draw in refrigerant vapor, thereby losing its cooling capacity. 2. It causes the actual compression process to deviate more from the ideal condition, resulting in increased actual power consumption of the refrigeration compressor, reduced efficiency, and a lower coefficient of performance. 3. This inevitably leads to an increase in the exhaust temperature of the refrigeration compressor, sometimes even exceeding the allowable limit for that temperature. At the same time, excessively high exhaust temperature can also affect the proper operation of the refrigeration cycle. 4. Generally, the pressure ratio n for compressors in use is ≤8; for piston-type refrigeration compressors using Freon refrigerants, this ratio is ≤10. For centrifugal refrigeration compressors, the maximum achievable pressure ratio per cycle is ≤4. For these reasons, at a condensation temperature of 35°C, the lowest evaporation temperatures that can be achieved with different refrigerants are as follows: R717 = -22°C, R12 = -33°C, R22 = -34°C, R134A = -37°C. In summary, in some refrigeration systems, Freon-based single-stage systems can operate at medium temperature levels, whereas ammonia-based systems require a two-stage system to achieve better performance. It is also for this reason that some customers choose Freon systems: in certain compressors with advanced technology, such as those from Bitzer, single-stage systems are used to handle low-temperature conditions, and they can also be utilized for high-temperature conditions as well (but only in companies with advanced technical capabilities).

Compression ratio = condensing pressure / evaporating pressure = exhaust pressure / intake pressure. An excessively high compression ratio can easily lead to an increased load on the compressor; when the load is high, the efficiency of operation decreases, and electricity consumption rises as well. 　　As the compression ratio of the compressor increases, the internal temperature rises, which not only affects the properties of the refrigerant but may also lead to a decrease in the viscosity of the lubricating oil. With reduced lubrication effectiveness, the lubricating oil is unable to perform its proper function within the compressor, thereby increasing the likelihood of wear and damage to the compressor. The main reasons for an increased compression ratio are as follows: 1. Low suction pressure: A low suction pressure indicates a shortage of Freon, which can be caused by two factors: 1) insufficient Freon, or 2) blockages. When there is less Freon, the motor isn’t cooled properly; the friction that occurs during the motor’s operation also generates heat! There is less Freon available; without sufficient Freon to cool them, the overall temperature of the engine head rises naturally. 2. Excessive exhaust pressure: ① Insufficient flow rate of cooling water or high water temperature; ② Presence of non-condensable gases in the system. This prevents the compressed gas from condensing in the condenser, resulting in a high condensation temperature. This leads to high condensation pressure. Solution: Drain the refrigerant and evacuate again. ③There is an excess of refrigerant; the excessive amount of refrigerant occupies the heat exchange area, resulting in a high condensation temperature and consequently high condensation pressure. It is manifested as high exhaust pressure and a relatively high operating current. ④Scaling on the condenser reduces the heat exchange area and also affects the flow rate of the cooling water. The symptoms are a large pressure difference between the inlet and outlet of the unit as well as an increased temperature difference; when touching the condenser, it can be felt that the temperatures at the top and bottom are both high. ⑤Insufficient flow of chilled brine is indicated by a decrease in the pressure difference between the inlet and outlet of the unit, as well as an increase in the temperature difference. The cause of this phenomenon is either too low a water pressure in the system or the presence of air. Solution: Install a vent valve at the upper part of the pipeline to allow exhaust or increase the water flow rate.

A high condensation temperature (for example, a high temperature of the circulating water) and a low evaporation pressure (requiring a lower supply water temperature) can both lead to an increase in the pressure ratio.

An excessively high compression ratio leads to over-compression, increasing the internal energy of the refrigerant

Answer: 1. Possible issues related to the refrigerant； Choose the appropriate refrigerant to control the compression ratio of the compressor. 2. Caused by blockages in pipelines and valves ； The filter can be replaced, enhancing its ability to remove impurities that may arise in industrial refrigeration systems as well as those present in refrigeration lubricants, thereby preventing blockages in pipes and valves. 3. Insufficient refrigerant leads to high suction pressure, which in turn causes problems such as a high compression ratio and high exhaust temperature. In other words, the compression ratio, as well as the exhaust pressure and exhaust temperature, can be reduced by increasing the intake pressure. 4. The condenser does not provide good cooling performance.

1. This leads to a decrease in volumetric efficiency; when it drops to a certain level, the compressor’s gas delivery coefficient becomes 0, and the compressor can hardly draw in refrigerant vapor, thereby losing its cooling capacity. 2. It causes the actual compression process to deviate more from the ideal condition, resulting in increased actual power consumption of the refrigeration compressor, reduced efficiency, and a lower coefficient of performance. 3. This inevitably leads to an increase in the exhaust temperature of the refrigeration compressor, sometimes even exceeding the allowable limit for that temperature. At the same time, excessively high exhaust temperature can also affect the proper operation of the refrigeration cycle. 4. Generally, the pressure ratio n for compressors in use is ≤8; for piston-type refrigeration compressors using Freon refrigerants, this ratio is ≤10. For centrifugal refrigeration compressors, the maximum achievable pressure ratio per cycle is ≤4. For these reasons, at a condensation temperature of 35°C, the lowest evaporation temperatures that can be achieved with different refrigerants are as follows: R717 = -22°C, R12 = -33°C, R22 = -34°C, R134A = -37°C. In summary, in some refrigeration systems, Freon-based single-stage systems can operate at medium temperature levels, whereas ammonia-based systems require a two-stage system to achieve better performance. It is also for this reason that some customers choose Freon systems: in certain compressors with advanced technology, such as those from Bitzer, single-stage systems are used to handle low-temperature conditions, and they can also be utilized for high-temperature conditions as well (but only in companies with advanced technical capabilities). Additionally, using supplementary liquid spraying can also meet the requirements for lower temperatures (for certain types of refrigeration screw units). If the compression ratio is too low, it’s simply like using a powerful engine for a small task, resulting in wasted electrical energy.

Under normal circumstances, the compression ratio of a screw compressor should be adjustable using sliders and valve spools, with different positions corresponding to different compression ratios.