Several different technologies are usually suited, essentially, for each specific application. Rotary vane pumps, side-channel blowers, rotary lobe blowers, rotary screw blowers and last but not least, turbo/centrifugal blowers – traditionally, these have been the most prevalent for servicing low-pressure applications. Screw blowers, which developed out of oil-free compression rotary screw compressors optimised for the low-pressure range, have additionally been available for several years now. Consequently, ever since, many operators and planners have faced the choice of whether to use rotary lobe blowers – or the new screw blowers – for traditional low-pressure applications. Naturally, there is no one correct answer; the best solution should always be determined on a case-by-case basis.
The first factor to consider is the type of application. In the industrial sector, these range from aeration of fluids (wastewater treatment, bioreactors, flotation) to air supply for furnaces as well as pneumatic conveyance of bulk materials and fluidisation. Most low-pressure applications operate at a working pressure of one bar or less, yet often have dramatically different run and load times.
Some of these applications, most in the area of fluid aeration, require a variable flow rate yet a virtually constant pressure curve over time. Others, such as pneumatic conveyance of bulk goods, require a virtually constant flow rate despite dramatically fluctuating pressures – and even allow the blowers to idle temporarily, i.e. the blowers operate without facing counter-pressure from the process end. This is the case, for example, when no bulk goods are introduced into the conveyor.
Deciding on the most advantageous compression technology depends on process-specific requirements (such as a wide control range and constant characteristic curve over pressure for flow rate) as well as, ultimately, the achievable energy savings. The latter depends not only on the machine’s power consumption, but also and equally on its operating period.
The “electricity bill” derives from power (kW) x time (h) x rate ($/kWh). Put simply: to save on energy costs, one must have sufficient time – i.e., sufficient load hours – only then can the full energy-saving potential of more efficient machines be fully realised.
In contrast to dynamic compressors, such as turbo compressors, which transfer kinetic energy to the air by acceleration, which is then converted into pressure in the downstream diffusor, rotary lobe and screw blowers are both dual-shaft displacement compressors. “Dual-shaft” because the rotary screw block or cylinder contains two rotors, which turn toward each other, in opposite directions; and “displacement compressors” because they push the air forward and an existing volume is continuously reduced. In order to make a more informed decision about the superiority of rotary lobe or screw blowers for any given application, let’s examine how each works in more detail.
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