Applying Rotary Lobe and Rotary Screw Blower Technologies

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.

Image 1

Screw blowers:
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.

Image 2

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.

How WWTPs Can Get Greener Grass on the Other Side of the IIoT

By: Stephen Horne

The next big thing is here and it’s the Industrial Internet of Things (IIoT). With pundits waxing philosophical on this big breakthrough, it’s hard to cut through the noise and understand what the IIoT actually is and how it applies to individual plants. I offer the following as a definition. The IIoT means collecting, interpreting, and applying data to actively improve processes or operations. To understand how the IIoT applies to a wastewater treatment plant, look no further than your front yard. Continue reading “How WWTPs Can Get Greener Grass on the Other Side of the IIoT”

It’s Alive…With Inefficiency

By: Stephen Horne

Have you got a Frankenstein blower package lurking in your system? Without a doubt, it comes alive when you flip the switch, but do you really know how efficient the package is? If you’ve got a monster of an electricity bill, you are not alone. Here’s how package integration can help keep that inefficiency monster at bay. Continue reading “It’s Alive…With Inefficiency”

Go with the Flow

By: Kaeser Compressors, Inc.

We recently published a blog entry on rotary blowers for vacuum applications and the benefits they can provide in providing better hold down, reduced footprint, and energy and maintenance savings. Here is a case study example from one of our customers who made the switch to blowers. Continue reading “Go with the Flow”

Rotary Blowers for Vacuum Applications

By: Stephen Horne

Most router tables that handle sheet goods require vacuum pumps for product hold down. The router table manufacturers often supply vacuum pumps for their tables that range from 15 – 50 hp. It is common that the capacity of the vacuum pump is less than the flow required to maintain the vacuum. When the work piece slips, the assumption is that a pump that can achieve higher vacuum is needed to correct the problem. What is really needed is a vacuum pump with a higher flow capacity. This is especially true with spoiler board applications. Continue reading “Rotary Blowers for Vacuum Applications”