Holroyd transfers helical rotor technology into a new generation of high performance screw superchargers.
The market for screw type superchargers is developing fast, even for conventional road vehicles, due to the greater efficiency of the helical rotor design. Efficiency in superchargers is all-important because it is a key factor in how a supercharged engine system will perform. It is determined by the power the compressive process will consume, and by the temperature of the air delivered to the engine. Cool air is better for power simply because of its greater density; however, the process of compression heats up the air involved, so the emphasis is on keeping this effect to the absolute minimum. The best way to achieve this is to compress the air to the pressure required as efficiently as possible. This is exactly what screw type superchargers do. In contrast to lobe-type superchargers, which are not compressive in their action, and suffer from heating, flow reversals and excessive turbulence, screw superchargers provide a positive displacement action with high adiabatic efficiency in compression of 75 - 80%. This efficiency gain, compared to an equivalent lobe-type device (60% maximum), has been found to correspond to a reduction of air delivery temperature of 30o-40o C and a saving in power consumption of 25-30% over a range of test conditions.
Holroyd's involvement in the latest generation of superchargers is a direct result of the Company's expertise in helical rotor production. Holroyd not only manufactures rotors for use in compressors, air conditioners and transcontinental gas pipelines, but also produces the machines that make the rotors, and exports them worldwide.
Key to Holroyd's success is the Ex series of high speed milling machines, which offer the high power, high stock removal rates and onboard measuring capabilities, which are fundamental to providing the flexibility to produce new types of rotor profiles in a matter of hours; thus reducing development times.
Holroyd's Sales Director for Machine Tools and Rotors, explains just how the features of the Ex range of machines are key to achieving reduced development times, and also high levels of throughput. "The Ex range is designed to enable complete integration with automated parts handling systems. It offers the user high levels of onboard intelligence, effectively overcoming the reliance on skilled personnel by providing powerful, menu-driven, touch screen programming for unprecedented ease-of-use. To fully exploit this feature, the machines are equipped with chucking and workholding options that complement quick change tooling for accelerated set-up, and also high power spindles offering unbeatable levels of rigidity: a combination that ensures outstanding metal removal rates".
Screw Superchargers - How They Work.
The screw supercharger consists of two helical rotors, male and female, which intermesh very closely without ever making contact. The helix action is of a continuous movement from the entry port towards a closed face adjacent to which is the delivery port. Air is drawn in normally at the open end of the rotors and is trapped as the rotors mesh together; it is gradually compressed between the rotor lobes and the closed end face as it is propelled along, then expelled from the delivery port at the required pressure.
An important advantage of this process is that the area of the flutes acting on the charge diminishes as compression takes place; therefore the power requirement remains more or less constant throughout the process. This is a significant advantage over conventional types of superchargers where increasing effort is required, due to the entire lobe area being exposed to higher pressure as delivery takes place.
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