What is the normal value for the evaporator inlet temperature of an ammonia refrigeration system?
Thread Content
Our refrigeration system uses an ammonia compressor refrigeration system with an economizer. The circulating water temperature is about 35 degrees. According to the refrigeration principle, the 65-degree ammonia gas at the outlet of the ammonia compressor can only reach a minimum temperature of 35 degrees after the condenser exchanges heat with the circulating water. Then the liquid ammonia passes through the economizer and then passes through the throttle valve, evaporates and absorbs heat in the evaporator, and finally reaches the ammonia compressor inlet at 22 degrees. The problem is: Liquid ammonia evaporates and absorbs heat in the evaporator and also takes away the heat of the cooled medium. In this case, the evaporator inlet should be lower than 22 degrees. So how good is the refrigeration effect of the economizer?
Yes, the compressor inlet temperature is definitely higher than the evaporator evaporation temperature
The problem does not match the description. To give a simple answer, the constant pressure evaporation temperature point of ammonia in the evaporator corresponds to its pressure point, that is, the evaporation temperature is equal to the saturation temperature point corresponding to the pressure of ammonia in the evaporator. The ammonia compressor inlet temperature will be higher than the evaporation temperature because of the presence of superheat. The economizer only increases the circulation subcooling degree and increases part of the heat exchange.
Your logic is unclear. Are you wondering where the lowest temperature of the system is only 22°C? Where does the low temperature come from? Ammonia becomes high-pressure ammonia after compression, and becomes liquid ammonia at normal temperature after cooling. Liquid ammonia at normal temperature vaporizes and absorbs heat, and can reduce the temperature to about -50°C. What is this economizer? The liquid ammonia refrigeration station has a heat exchanger called a subcooler, which cools the liquid nitrogen from 30°C to about 5°C and sends it to the refrigeration system. It may be the same thing as your economizer.
You said, "Ammonia becomes high-pressure ammonia after compression, and becomes normal-temperature liquid ammonia after cooling. The normal-temperature liquid ammonia vaporizes and absorbs heat, and can reduce the temperature to about -50°C." -50°C refers to the temperature of the cooling medium. What I mean is that the normal-temperature liquid ammonia absorbs heat after being vaporized, and the generated ammonia reaches only 22.8°C at the compressor inlet. During this period, it is only subcooled by the economizer. So isn't the subcooling effect too good? If the economizer does not work, does the liquid ammonia reach normal temperature when it reaches the evaporator inlet?
What do you want to say? It seems so laborious. From your description, it seems that the ammonia coming out of the evaporator passes through the economizer and then enters the compressor. The newly formed liquid ammonia also passes through the economizer. The cold ammonia gas in the economizer cools down the liquid ammonia at room temperature and reheats itself to 22.8℃
The economizer will not be supercooled. The function of the economizer is basically to separate gas and liquid. This is just because the valve throttling before the economizer causes the pressure to drop, and part of the liquid evaporates into gas, making the remaining liquid temperature lower. At this pressure, the saturation temperature of ammonia is almost more than 22 degrees. Therefore, the temperature will not rise unless the liquid is completely evaporated. The heat absorption first turns the liquid into a gas. It doesn’t matter whether there is an economizer or not. The temperature of -50 degrees is only related to the evaporator pressure. As long as the valve is throttled to the saturation pressure corresponding to -50 degrees when you reach the evaporator, the temperature of the evaporator will be -50 degrees.