Raising the bar for cooling in compressors

Oil is used in air compressors to lubricate the moving parts and act as a sealant to prevent air leakage. By retaining heat during the compression process, it also serves as a coolant. In the common practice of intercooling, where the temperature of the compressed gas between the low and high pressure stages of compression is reduced using a heat exchanger, this oil is first separated from the air. Typically, water or air acts as the coolant. 

In our latest research, we tested a similar concept of ‘intracooling’, where the oil is left in the flow after the first stage, to lubricate the next compression stage and cool the compressed air, before it is separated from the compressed gas at the end.

Instead of using a dedicated cooling system between the two stages and the auxiliary power needed for it, cooled lubricant oil is sprayed as small droplets into the gas flow within an intracooling/ connection duct, transferring the thermal power from the gas to the liquid and not to the environment.

Proving the theory

A prototype of a sliding-vane air compressor was created and equipped with the instruments required to measure the temperature and pressure of air and lube-oil in all key points of the circuit, as well as the injected oil flow rate and the electrical power. Thirteen different configurations of the pressure swirl nozzles which inject the oil in the intracooling duct were tested, all with the same pressure and temperature injection conditions. Data from the prototype was used in conjunction with numerical models to carry out simulations of conventional single-stage, intercooled and intracooled compressors.

Efficiency gains

Comparing the results with a conventional single-stage compressor, a two-stage compressor with intracooling is shown to deliver energy savings of up to 10 per cent. Remarkably, 2.3 per cent less input power is needed with an intracooled system compared to an intercooled one due to a better compression process. This is because at the low-pressure stage, the intracooled compressor can inject the oil at a higher pressure, leading to a finer atomisation of the oil for better heat transfer. In intercooling, the pressure at the end of the first compression stage has to be increased because of the pressure drop on the cooling fan. This means that more power is needed to compress the air at the second stage. In intracooling, no pressure drop occurs into the connection duct, so the pressures are matched between the two stages. What’s more, this innovative method reduces temperatures by up to 10 °C across the duct between the two stages.

So, what do these findings mean for the efficiency of sliding-vane rotary compressors? Because an intracooled compressor delivers a better compression process compared to the intercooled one at the low-pressure stage, less power is ultimately used. The possibility of extending the operating range of pressure is also enabled and there are fewer pieces of equipment to operate and maintain. 

Further development is needed to enhance the design even further and the high-pressure stage is an area where even more energy savings can be made. Mattei is also in the process of developing a smart system to control nozzle activation, with the effectiveness of the intracooling process heavily dependent on the number of active nozzles and their positions, which consequently affect the oil flow rate. There is still room to improve the energy efficiency performance of this novel system, so watch this space.