Rotary lobe or screw blowers? Take a close look!

The search for solutions for low-pressure applications often comes down to a choice between different suitable technologies. In the dual-shaft rotary compressor family, rotary lobe blowers long held sway until the recent arrival of rotary screw compressors optimised for the low pressure segment. Since then, the question is which type is more suitable, when and why?

To start with: There is no hard and fast rule. However, there are several useful factors that will help you to arrive at a decision. The potential uses for blowers are numerous. Industrial applications extend from the aeration of fluids (wastewater treatment, bioreactors, flotation) and air injection for furnaces to the pneumatic conveying of bulk materials and fluidisation.

As a rule, these applications are in low pressure ranges and at pressure differentials of up to 1 bar. However, they are characterised by very diverse loads and running times. Some of them, especially in connection with fluid aeration, require the blower to deliver a highly variable volume flow at constant pressure. Other applications, such as the pneumatic conveying of bulk materials, show wide pressure fluctuations over time combined with a stable volume flow. Sometimes blowers are placed in idle mode, which means that they keep running with no process-side pressure from the downstream distribution network. This happens, for example, when no bulk material is being fed into the transport line while the blower is running.

When selecting the most suitable compressor technology for your application, the first question is whether a variable volume flow is needed and, if so, within what range. After that it must be determined how the required operating pressure will vary over time. You should have these two facts at hand not only to determine in general which compressor type is best, but also to predict the potential energy savings. In that respect, some compressors that may at first appear to be the most efficient option may in fact rank lower when running at partial capacity or for only a few hours a day.

Functional differences

But let’s start by looking at the thermodynamic and design differences between rotary lobe blowers and screw compressors to arrive at recommendations for their ideal applications.

A look at the geometry of the rotors reveals the decisive distinction between these two compressor technologies. In rotary lobe blowers, the male and female rotors have identical cross sections, for example with a three-lobe design, positioned longitudinally. The volume of air captured between the cylinder and lobes remains constant as the rotors turn, i.e. during the compression process. In thermodynamics this is referred to as an isochoric process. The actual pressure is built up downstream from the compressor in the process line. It results from air molecules being constantly pushed against the resistance in the line. If there is no resistance in the line – for example, in pneumatic conveying, there is no bulk material in the line – the feed-in of air causes practically no back pressure. Rotary lobe blowers can therefore be called “adaptive”, as they always generate only as much pressure as is needed to overcome the current level of resistance in the process.

Screw blowers, by contrast, have intermeshing screw-shaped rotors. The inlet air is trapped in the grooves of the rotors. It is squeezed and decreased in volume as the rotor turns before being discharged through the outlet port. Consequently, a decrease in volume and corresponding increase in pressure take place in the compressor itself. The ratio of the inlet and outlet volume is determined by the geometry of the rotor and the outlet port. The compression process corresponds to the principle of isentropic compression. It takes less displacement work to expel an already reduced volume of air against the pressure in the process line. This saves energy, especially at higher pressure levels.

However, it is important to ensure that the required operating pressure for the process is close to the pressure generated by the rotary compressor through its internal compression in the compression chambers. Discharge pressure in excess of the required operating pressure is referred to as over-pressure. It means that more compression than necessary is performed. As a result, the potential energy savings are lower. It is also important to remember that screw compressors, even when there is no downstream pressure, for example when there is no bulk material in the line, still create pressure due to the internal generation of pressure in the chambers. As a result, they use more energy than rotary lobe blowers in this operating mode.

The following two examples present specific data to highlight the difference.

Example

Case A: Constant pressure during entire operating time, e.g. when drawing air into basins for wastewater treatment with constant depth. No idle mode, i.e. no blower operation without back pressure.

Case B: Alternating operation against back pressure and in idle mode, i.e. unit sometimes operates with no pressure. This case is often seen, for example, in the pneumatic transport of bulk materials.

Comparison:

Inlet pressure 1.0 bar, volume flow 15 m³/min, operating time: 8,000 hours per year, varying load and idle ratios.

At a pressure differential of 500 mbar, i.e. 1.5 bar operating pressure (absolute); pressure ratio: 1.5:

At a pressure differential of 1000 mbar, i.e. 2.0 bar operating pressure (absolute); pressure ratio: 2.0:

The above examples clearly show that the potential savings through the use of screw blowers increase with higher pressure differentials and longer load hours.

Summary

Screw and rotary lobe blowers operate at maximum efficiency only when they are used appropriately. It is therefore essential to make a detailed analysis of the required pressure and volume flow before purchasing a system. Ideally, this should take the form of a projection based on real-world assumptions or, for existing systems, by tracking pressure over time. Otherwise there is a risk of basing energy cost projections and the choice of compressor on rare peak loads instead of the average operating pressure. The same applies for the required volume flow over time. The goal is to find the best blower for the common operating ranges to avoid optimising energy use for a highly unusual operating status that is only reached on a few days per year. The blower must be designed to deliver the maximum pressure and volume flow required while operating at peak efficiency under the conditions that occur most frequently in reality.

Compressor manufacturers can use various analysis systems on request to track operating pressures and other operating data to perform computer-based simulations to compare energy consumption.