Rotary-Screw Compressor

Principles of Gas Compressors

 

The compression of air and other gasses is an important step in many industrial processes. Fundamentally, compression of a gas is the conversion of kinetic energy from some power source (electric motor, combustion engine, turbine, etc.) into potential energy stored within the working fluid as increased pressure. The stored energy may later be recovered by allowing the gas to expand and decompress.

 

Many devices have been developed to compress gases. For low to intermediate flow rates across a range of pressures, positive displacement compressors are the most common. These compressors operate by mechanically reducing the volume of a gas, thereby raising its pressure. Piston and diaphragm compressors utilize a linear action to achieve compression, and thereby produce an inherently pulsatile output. Positive displacement compressors which utilize a rotational action, such as rotary vane, rotary screw, and scroll compressors typically provide a more continuous output.

 

Among these different designs, a rotary screw compressor provides compressed air to the Martigny fueling station, while several air-driven reciprocating piston-type gas boosters are used to compress hydrogen.

 

Operation of a Rotary-Screw Compressor

Rotary screw compressors are positive-displacement devices which enable continuous delivery of relatively high volume flow rates, without excessive pulsation. The compressor housing contains two intermeshing helical ‘screws’, which are driven by an external motor and gear train. Tight tolerances between the two screws, and the screws and the compressor housing, create a dynamic seal as the screws rotate. As a result, each ‘valley’ of the screw creates a helical cavity.

 

At the intake side of the compressor, gas is allowed to fill the cavity via an intake port in the compressor housing. As the screw turns, the cavity rotates past the intake port, effectively sealing the volume of gas within the cavity. Further rotation of the screw causes the helical length of the cavity to be reduced (similar to the ‘shortening’ of the exposed thread of a screw as it is tightened). As the screw turns and the length and volume of the cavity decrease, the pressure within the cavity continues to build. Finally, at the exhaust side of the compressor, the cavity rotates past an exhaust port where the high-pressure gas is driven out of the compressor at the desired final pressure.

 

Variable-Frequency Drive

Unlike many other positive-displacement compressors, rotary screw compressors provide a continuous flow of compressed gas. As a result, the output flow rate of the compressor is directly proportional to speed of the drive motor. By using a variable-frequency drive, it is possible to modulate the speed of the compressor to achieve a wide range of flow rates. The efficiency of the compressor is thus greatly improved by only supplying the power needed to meet the immediate demand for compressed gas. Moreover, the compressor may be operated semi-continously, reducing the need for compressed air storage, and reducing the number the number of ‘start-stop’ events.

Air Treatment

Prior to compression, the intake air is filtered to remove particulates which could damage the compressor or any downstream consumers. After exiting the compressor, the air must be treated to ensure that it is clean and dry before being supplied to the hydrogen gas boosters or other air consumers. Any oil is removed by passing through an oil mist eliminator, which essentially uses a fine filter element to capture small droplets of oil. Following the oil separator, the compressed air passes through a refrigerated dryer to remove any water vapor. Inside the dryer, the air is cooled to just a few degrees above the freezing point of water, causing much of the humidity to condense. The air leaving the dryer thus has a precisely known dew point and relative humidity. The condensate is automatically discharged from the system, allowing for continuous operation.

Installed at Martigny Filling Station Demonstrator

A Kaeser BSD-75 series variable frequency drive rotary screw air compressor is used to supply clean, dry compressed air to the fueling station demonstrator. This compressor plays the critical role of supplying compressed air to drive the multiple stages of reciprocating gas boosters needed to compress the hydrogen from 10 bar (at the electrolyzer outlet) up to 900 bar (for filling vehicles to 750 bar). Waste heat is also recovered from the system and used to heat the building, and release hydrogen from the 2kg hydride H2 storage.