【Mechanical Equipment Technology Edition】Daily Question 20191126: Basics of Refrigeration – Refrigeration Pressure-Humidity Diagrams – Refrigeration Cycles (6)
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This post was last edited by Zhang Xinlin from Binglun Environment on 2019-11-28 at 23:50. Refrigeration Basics – Pressure-Enthalpy Diagrams – Refrigeration Cycles (6). I. Overview: Building upon what has been covered previously. First, let’s understand subcooling and superheating. Starting with superheating, it occurs at the suction side of our refrigeration compressors. Question: Talk about your understanding of superheat – what is the role of superheat? What is superheat? Answer: Superheat primarily prevents liquid from entering the compressor, and it also helps to increase the cooling capacity. Superheat = Compressor suction temperature – Saturated temperature corresponding to the suction pressure (Figure 1) (Figure 2) ============================== Premium promotion: Daily questions on motorized equipment – refrigeration applications, their value and functions: https://bbs.hcbbs.com/thread-2848974-1-1.html Internal training materials for manufacturers of screw refrigeration compressors: https://bbs.hcbbs.com/thread-2850090-1-1.html
In terms of the properties of water and water vapor, superheat refers to the degree to which the temperature of the steam is higher than the saturation temperature at that pressure
Superheat = Compressor suction temperature – 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 exhaust, 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. The effect of suction superheat: if there is no superheat at all in the suction air, it may lead to liquid entrainment in the return gas, which can even cause wet-stroke liquid impact and 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 gas 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 regulates the amount of liquid supplied to the evaporator, thereby allowing control over the suction superheat.
Under saturated pressure conditions, if the saturated vapor is continued to be heated until its temperature exceeds the saturation temperature, this state is called superheating； The vapor in this state is called superheated vapor, the temperature is called the superheat temperature, and the difference between the superheat temperature and the saturation temperature is called the degree of superheat. Effect of superheat: An appropriate amount of superheat is beneficial for the proper operation of the expansion valve, as it prevents liquid from returning with the gas flow and avoids damage to the compressor caused by wet-stroke vapor shock.