sliding-vane compressor; two-stage compressor; intracooling
Intercooling is a well-known practice in compression technology for reducing the discharge temperature and the power consumption of the process. Intracooling, a similar yet not identical concept, is the cooling of the compressed gas between two compression stages by way of spraying a liquid coolant in the gas flow without separating that liquid prior to the second compression stage. This liquid coolant can be the cooled lube oil. The present work reports the experimental experience on a first prototype of a small-scale two-stage sliding-vane compressor based on the concept. The prototype is design for a relatively low delivery pressure, 0.7-1.0 MPa. Moreover, it is characterized by an oil injection system comprising pressure-swirl nozzles placed on the end-plates of the compression stages and along the intracooling duct. This duct is equipped with eight nozzles: six of them perform a radial inward injection and are equally spaced on the tube length, while the other two are located at its ends for an axial injection, one cocurrent and the other countercurrent to the air flow direction. The experimental tests differ by the number and the position of the active nozzles along the duct. The outcomes indicate that intracooling does not yield operability issues and that the intracooling effectiveness increases with the number of active pressure-swirl nozzles, reaching a decrease in temperature along the duct of about 5°C. However, the configuration with the lowest mechanical specific power, by 4.4% with respect to a single-stage compressor, has only one nozzle active and spraying along the axial flow direction. The results suggest that the compromise among oil flow rate, number of active nozzles and their position, is the best solution to obtain the maximum efficiency for the overall system. In the future, an improved intracooling duct and a mid-size intracooled compressor for higher pressures will be manufactured and tested.