【Mechanical Equipment Technology Edition】Daily Question 201806011: Refrigeration Systems — Refrigeration Parameters (5)
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This post was last edited by YORK Industrial Refrigeration on 2018-6-13 at 08:56. This year’s 【Daily Question】is divided into three categories: screw refrigeration compressors, refrigeration systems, and oil circuit systems, aiming to provide a comprehensive understanding of refrigeration technology. (If you encounter something you don’t know or can’t answer, please actively look up information; as you do so, your skills will improve!) ) All participants in the comments on this post will receive 1-3 wealth points; those who give complete answers will get 10-15 points, with the validity period being two days. ============================== Refrigeration system — Refrigeration parameters (5): Which one, suction superheat or exhaust superheat, better reflects the condition of the ice machine? Why? The evaporators of refrigeration units are mainly divided into two types: flooded and dry-type. Dry evaporators have a suction superheat, while full-liquid evaporators have a suction superheat that is close to 0. The suction superheat can indicate certain conditions, and so can the exhaust superheat. For maintenance purposes, checking the exhaust superheat is a useful approach; by comparing it with the optimal exhaust superheat for that operating condition, it is possible to determine the condition of the equipment. ============================== High-quality promotions: Mechanical equipment——Videos on the repair and calibration of York compressors https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=1806833 Mechanical equipment——Maintenance procedures for York screw compressors https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=1806833 Mechanical equipment——Upgrading of York Quinton control centers https://bbs.hcbbs.com/thread-1804837-1-1.html Mechanical equipment——Major repairs of GEA Grasso screw compressors https://bbs.hcbbs.com/thread-1800467-1-1.html Mechanical equipment——Disassembly and maintenance of British HOWDEN screw compressors https://bbs.hcbbs.com/thread-1832529-1-1.html Mechanical equipment——Disassembly and maintenance of Japanese Maekawa MYCOM screw compressors https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=
The superheat of a refrigeration machine is generally divided into two types: exhaust superheat and suction superheat； The exhaust superheat is equal to the exhaust temperature minus the saturation temperature corresponding to the exhaust pressure; this superheat ensures that the exhaust gas from the compressor in the high-pressure chamber does not liquefy there, thereby preventing the refrigeration oil from being diluted ； Suction superheat is equal to the suction temperature minus the saturation temperature corresponding to the suction pressure; the presence of suction superheat ensures that no liquid refrigerant returns to the compressor, thereby preventing liquid slugging in the compressor. Inlet superheat is set to prevent liquid slugging and to ensure exhaust superheat. However, the suction superheat setting must ensure proper oil return and prevent the exhaust temperature from becoming too high. When the unit is in heating mode, if the condenser’s structure is poor, the high compression ratio can result in a very high exhaust temperature of the compressor, even if the superheat of the refrigerant being drawn in is low. Therefore, the suction superheat can more accurately reflect the condition of the chiller.
It should be the exhaust superheat. Exhaust superheat provides a better indication of the unit’s operating condition; the level of temperature directly reflects any issues arising from the unit’s internal operation – a high value suggests the presence of friction or an excessively high pressure ratio. If it is too low, wear of the machine body seal line can occur, and an excessive meshing gap leads to significant air leakage.
The suction superheat provides a clearer indication of the operating condition of the nozzle; a low superheat level can cause frosting on the nozzle (personal opinion)
It should be the exhaust superheat. Exhaust superheat provides a better indication of the unit’s operating condition; the level of temperature directly reflects any issues arising from the unit’s internal operation – a high value suggests the presence of friction or an excessively high pressure ratio. If it is too low, wear of the machine body seal line can occur; an excessive meshing gap leads to significant air leakage
1. The definitions of intake superheat and exhaust superheat have been answered previously, so they will not be repeated here. 2. It is better for compressors to use suction superheat to control the throttle valve. The reason is as follows: the suction superheat reflects the suction condition of the compressor, that is, it indicates the state of the refrigerant after it has absorbed heat through evaporation in the environment where cooling is required, and then leaves the evaporator. It is determined by the evaporation temperature and the set superheat; the operating conditions of the cooling system are what we require, and this value is fixed. The exhaust superheat, on the other hand, varies depending on the operating conditions of the compressor and the outdoor temperature, and is therefore an uncertain value. Therefore, it is difficult to control the throttle valve. Furthermore, controlling the suction superheat helps to prevent liquid slugging in the compressor. Liquid slugging is harmful to compressors of all types. A vantage of screw compressors is that they are not sensitive to wet strokes; however, it is generally best to avoid allowing liquid to enter the compressor, as liquid is incompressible. Inlet superheat can ensure normal exhaust superheat to a certain extent, unless there is severe fouling of the condenser. For the above reasons, the throttling valves in refrigeration systems are controlled using suction superheat; it can also be said that suction superheat provides a more accurate reflection of the condition of the chiller. This also reflects, from one perspective, the importance of inhalation superheat.
The exhaust temperature provides a better indication of the compressor’s operating condition. It not only reflects directly the state of the refrigerant at the compressor outlet, but also indirectly reflects the condition of the refrigerant at the evaporator outlet.
It should be the suction superheat. Superheat = suction temperature of the compressor – evaporation temperature of the refrigerant in the evaporator. A certain level of superheat is necessary to ensure that the refrigerant entering the compressor is free of liquid, thereby preventing wet strokes. Generally, at the evaporator outlet and the compressor discharge port, the actual temperature of the working fluid is higher than the saturation temperature corresponding to the actual pressure. This is where superheat (intake superheat and exhaust superheat) comes into play. If there is no superheat at all in the intake air, it may lead to liquid returning with the air flow, which can even cause wet-stroke liquid hammer damage to the compressor. To avoid this phenomenon, a certain suction superheat is required to ensure that only dry steam enters the compressor (given the properties of the refrigerant, the presence of superheat indicates complete evaporation of the liquid refrigerant). However, too high a superheat also has its disadvantages; a high level of superheat leads to an increase in the compressor’s exhaust temperature (exhaust superheat), which deteriorates the operating conditions of the compressor and reduces its lifespan. Therefore, the suction superheat should be kept within a certain range. The expansion valve senses the temperature difference between the return gas temperature and the actual evaporation pressure (which corresponds to the saturation temperature) through a temperature-sensing element located on the compressor return pipe or at the evaporator outlet; this temperature difference is the suction superheat. Based on this set value for superheat, the expansion valve adjusts its opening degree, which in turn controls the amount of liquid supplied to the evaporator, thereby enabling control over the suction superheat.