Performance Assessment of air coolers new tubes layout to achieve optimum design by using finite element model and experimental data: A case study

摘要

Air compressors are located in air generation package unit to prepare the air for instrument devices. In each compressor an air cooler is furnished to reduce the temperature of air and oil. Each air cooler consists of four tube bundles, one stage for oil cooling and three tube bundles for air cooling. According to inspection reports, the air cooling system (three stages of each air cooler) has high corrosion rate. This is because of the violent humidity in site area (Assalouye-lran) which is always more than 70% by year. Humid air decreases air cooler's efficiency by reducing the heat transfer rate and additionally in contact with carbon steel tubes produces corrosive particles. These particles reduce system efficiency and cause costly damage to compressor blades in downstream. The rate of metal lost in air cooler tubes is considerable so this will make the air cooler unavailable in the future. Several solutions were proposed and finally according to corrosion intensity it was decided to change air cooler materials from carbon steel to stainless steel A213 which prevents the corrosion. Considering thermal loading to be constant, thermal coefficient of new tubes is lower than carbon steel tubes. New expected heat transfer rate has been calculated based on two different methods and results showed that changing CS to SS would cause decrease in overall heat transfer rate about 15% to 20%. So it is necessary to redesign the air cooler and its accessories consequently. To prevent mentioned problem since the limited area was available for tubes, it was proposed to add an extra row of tubes. This would increase heat transfer surface which compensates reduction in thermal conductivity coefficient owing to change tube materials. By adding these new tubes there will be an increase in pressure drop which cause channelizing the air flow. In that case air does not flow uniformly over oil and air stage tubes and this result in unsteady temperature distribution which decrease system efficiency. So it is necessary to design an optimum layout for new added tubes to prevent mentioned problem. In this article first the root causes of corrosion in air cooler tubes are analyzed then the combination of effective factors in pressure drop including tube diameters, distances and their layout are investigated using finite element model and experimental data to achieve the best configuration and optimum layout for newly added tube bundles.