CO2 reciprocating compressor renovation

Carbon dioxide reciprocating compressors are commonly used in the urea process of the chemical industry, the food industry, and gas bottling for welding protection. Because there are many carbon dioxide compressors that have been personally designed and modified, there are two issues that I am willing to share with users and peers

The first point is about the compression stage of carbon dioxide compressors used in the urea industry. The urea industry uses carbon dioxide compressors with exhaust pressures of around 160 kilograms and 220 kilograms. Most designs are designed for five level compression, but a small number of products are designed for four level compression. An article published in the journal "Compressor Technology" mentioned this point, stating that the advantage of the four stage compression scheme is that it does not have a series of cylinders (early carbon dioxide compressors generally used four column M-type), has one less stage container, and has a simple structure. Based on the experience of renovating carbon dioxide compressor projects around Chengdu, the four stage compression scheme is completely unreasonable because the actual gas volume is too far from the design gas volume. After the carbon dioxide compressor of a certain company was changed from four stage compression to five stage compression, the first three stages of the cylinder were not modified, only the original four stage cylinder was changed to four or five stage cylinders with a balance section, and the gas volume increased by 30% directly. From the perspective of theoretical calculations of compressors, using four stage compression and five stage compression will not result in such a significant difference in air volume. However, practical engineering experience has proven that four stage compression is seriously unreasonable

Second: The compressibility coefficient of carbon dioxide. When the exhaust pressure is relatively low, follow the manual to check the compressibility coefficient of carbon dioxide for thermodynamic calculation, and the calculation result is normal. When the exhaust pressure is high, especially above 100 kilograms, designing strictly according to the compressibility coefficient calculation in the manual will result in the actual air volume of the compressor being lower than the theoretical air volume. A certain company in Chongqing has two carbon dioxide compressors undergoing balance stage removal renovation. Before the renovation, the gas volume was 78 cubic meters, and their fourth and fifth stages were equipped with balance stages. After the renovation, the balance section was eliminated and the four or five stages were changed to inverted stages. Compared with another 85 cubic meter compressor (also without a balance section), there was no difference in air volume. Comparison of parameters between two compressors, with the same speed and stroke, similar structure, but different cylinder diameters. The first three stages of the 85 square compressor have larger cylinders than the 78 square compressor, but do not inflate much. The root cause is that during the design of the 85 square compressor, a certain parameter was unreasonable due to the compressibility coefficient of carbon dioxide.

So theory still needs to be tested and revised by practice in order to be perfected.