By: Stephen Horne
Rotary lobe and rotary screw blowers utilize positive displacement. This means they pressurize air by trapping a fixed amount and forcing (or displacing) it into a discharge pipe. Industrial applications include fluid aeration (wastewater treatment, bioreactors, and flotation), process air, pneumatic conveying, as well as fluidization.
Although all of these applications generally work within a low pressure range (up to 14.5 psi), they have very different operating cycles and needs. Fluid aeration applications generally have variable flow rates, but at constant pressure.
Others, like pneumatic conveying, require a near constant flow rate with high pressure fluctuations. Sometimes the blowers are required to idle, running without back pressure from the process side. This happens when there are no bulk goods in the line to move.
Naturally it’s important to decide which blower technology is best suited to the application. Technical requirements must be taken into account, such as a broad flow rate curve during pressure fluctuations. Ultimately, the choice may hinge on the amount of energy savings achievable from different alternatives. In determining energy savings, the “power bill” is determined solely by output (kW) x time (h) x rate ($/kWh).
The big variable here is time, which significantly impacts energy costs. Unless the cost per kWh is very high, the more efficient blower may need to run a lot more hours to justify the higher investment.
External (isochoric) versus internal (isentropic) compression
To determine which of these blowers would be more cost effective for a given application, it is important to first understand in greater detail how each functions.
Rotary lobe blowers:
Image 1 shows a cross-section of the rotors and cylinders, running parallel in the longitudinal direction and illustrates how the volume enclosed between the housing and the rotor blade remains constant. In thermodynamics, this is referred to as isochoric compression. The pressure does not build until the air molecules are pushed beyond the blower into the connected process line. In this way, with rotary lobe blowers, pressurization occurs externally. Moreover, if the process line is free of resistance (e.g. no bulk goods in a pneumatic conveying line), there is virtually no back pressure. In this regard, the rotary lobe blower can also be seen as adaptive: it only produces the amount of pressure needed.
With screw blowers (image 2), the tried-and-true technology of the single-stage screw compressor has been optimized for low pressures.
The rotor geometry is based on the screw. The inlet air is initially captured within the cavity between the two rotors where its volume is gradually decreased along the length of the rotors and then pushed out through the discharge port. The geometry of the rotors and housing (i.e. contour of the discharge port) determine how much air is proportionally compressed within the screw blower and how much pressure is built up internally. This internal pressurization can also be called isentropic compression.
Pushing an already compressed volume of gas against the system back pressure requires less energy than pushing the un-reduced volume created in isochoric compression (rotary lobe blower). The result is significantly lower electrical demand, and in many cases the screw blower delivers great ROI over a lobe blower. However, the better specific performance of the screw blower may not pay off if the running hours and/or cost per kWh don’t out-weigh the additional cost of the screw technology. You must do the math.
This blog post is adapted from our white paper, “The Proper Application of Rotary Lobe and Rotary Screw Blower Technologies”. Download the complete whitepaper here.