Question: Various issues with refrigerant compressors
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I’m a newcomer here, and I have some questions for the experts. We produce liquefied natural gas, and we have one high-pressure compressor and one low-pressure compressor. At the outlet of the low-pressure compressor, there is air cooling followed by a gas-liquid separator; the same applies to the high-pressure compressor as well. I have the following questions: 1. How do the fuel supply temperature and exhaust temperature affect the refrigerant? 2. In the gas-liquid separators at the outlets of the high-pressure and low-pressure compressors, is the liquid present there the refrigerant? What effects can occur if the liquid level is extremely low? How can this liquid level be controlled? Is it necessary to add more refrigerant?
It depends on the components of the refrigerant; different components have different effects.
We use nitrogen, methane, n-butane, propane, and ethylene. Please help me analyze this, expert
1. The oil supply temperature is generally around 45-60°C, mainly to ensure that the oil and refrigerant in the compressor can be properly separated； 2. The exhaust temperature has no direct effect on the refrigerant. The exhaust temperature is affected by the intake temperature and the condensation temperature; an increase in either the intake temperature or the condensation temperature leads to an increase in the exhaust temperature. 3. Effect of the condensation temperature at the condenser on the refrigerant: (1) As the condensation temperature increases, the enthalpy value of the refrigerant leaving the condenser rises, resulting in a decrease in the cooling capacity per unit mass of refrigerant. This leads to a reduction in the cooling power at the cold end of the cryogenic tank, preventing the temperature of natural gas from dropping and causing an imbalance between the hot and cold ends. The compressor's power consumption will increase ； (2) Conversely, as the condensation temperature decreases, the enthalpy of the refrigerant leaving the condenser drops, resulting in a higher cooling capacity. This leads to reduced compressor power consumption and thus better cooling efficiency. (3) The condensation temperature is generally controlled at 40–50°C. If the condensation temperature is too low, n-butane and ethylene will condense and accumulate in the upper part of the cryogenic tank, resulting in a decrease in the amount of n-butane and ethylene in the lower part of the tank. As a result, the temperature in the upper part of the cryogenic tank drops to -70°C, while the temperature at the bottom remains around -140°C, failing to reach -165°C. When the liquefaction temperature of natural gas is too high, the temperature of the product does not meet the required standards. The gas-liquid separator at the outlet of the compressor is used to separate the refrigerant into gas and liquid phases. The gas phase consists mainly of methane, nitrogen, and ethylene, while the liquid phase is primarily composed of n-butane. A low liquid level indicates a shortage of heavier components in the refrigerant system, that is, a low amount of n-butane. This results in higher temperatures at the upper part of the cold box as well as at its bottom, leading to a large temperature difference between the inlet and outlet of the cold box and poor heat exchange efficiency. When the liquid level in a gas-liquid separator is low, the first step is to conduct an analysis of the components in the cooling unit to determine whether they match those specified in the design. If they do match, it means there is enough n-butane available; by lowering the outlet condensation temperature, the liquid level in the gas-liquid separator will rise. If there is a significant difference between the components in the cooling unit and those specified in the design, it indicates that there is insufficient n-butane in the refrigerant system. In such cases, n-butane needs to be added to compensate for the lack of cooling capacity in the system.
Thank you so much! I appreciate the detailed explanation from the expert; I’ve learned a lot