Saturday, December 6, 2008

Rotary Compressors

Air compressors of various designs are used widely throughout DOE facilities in numerous applications. Compressed air has numerous uses throughout a facility including the operation of equipment and portable tools. Three types of designs include reciprocating, rotary, and centrifugal air compressors.

Rotary Compressors

The rotary compressor is adaptable to direct drive by induction motors or multicylinder gasoline or diesel engines. The units are compact, relatively inexpensive, and require a minimum of operating attention and maintenance. They occupy a fraction of the space and weight of a reciprocating machine of equivalent capacity. Rotary compressor units are classified into three general groups, slide vane-type, lobe-type, and liquid seal ring-type.

*The rotary slide vane-type, as illustrated in Figure 3, has longitudinal vanes, sliding radially in a slotted rotor mounted eccentrically in a cylinder. The centrifugal force carries the sliding vanes against the cylindrical case with the vanes forming a number of individual longitudinal cells in the eccentric annulus between the case and rotor. The suction port is located where the longitudinal cells are largest. The size of each cell is reduced by the eccentricity of the rotor as the vanes approach the discharge port, thus compressing the air.

*The rotary lobe-type, illustrated in Figure
features two mating lobe-type rotors mounted in a case. The lobes are gear driven at close clearance, but without metal-to-metal contact. The suction to the unit is located where the cavity made by the lobes is largest. As the lobes rotate, the cavity size is reduced, causing
compression of the vapor within. The compression continues until the discharge port is reached, at which point the vapor exits the compressor at a higher pressure.

*The rotary liquid seal ring-type,
illustrated in Figure 5, features a forward inclined, open impeller, in an oblong cavity filled with liquid. As the impeller
rotates, the centrifugal force causes the seal liquid to collect at the outer edge of
the oblong cavity. Due to the oblong configuration of the compressor case, large longitudinal cells are created and reduced to smaller ones. The suction port is positioned where the longitudinal cells are the largest, and for the discharge port, where they are smallest, thus causing the vapor within the cell to compress as the rotor rotates. The rotary liquid seal compressor is frequently used in specialized applications for the compression of extremely corrosive and exothermic gasses and is commonly used in commercial nuclear plants as a means of establishing initial condenser vacuum.

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I remember talking with the late Bob Pease about the state of the art in digital techniques for solving complex problems. He politely let me babble for a few minutes and then laughed, “Yep, I solved that same problem 10 years ago with two op-amps”. I wanted to crawl under something, but his office was completely full of every magazine he had ever received… but that’s another story. He was correct – sometimes a straight forward analog solution can not only be the most elegant, but also the most efficient. Sometimes you need the power of a DSP processor when systems are non-linear or the signal processing is not realizable in the analog domain. However sometimes simple analog circuitry can solve the problem. Don’t forget your roots.

This is one of my favorite topics in engineering systems design (they don’t call me the “Energy Zarr” without reason). In fact, I often rant about waste in solving a problem with brute force. Now… with that said, sometimes a hammer is more effective when dealing with a nail, but in general, what goes in, must come out… and most of what comes out is heat. Take the quintessential LCD display like the 60” version sitting in your living room. That beauty has white LEDs for a back-light so it must be “green” right? Well, did you know that up to 80% of the light emitted by those LEDs is absorbed by the color filters on the LCD glass? It might be “thin” but it is definitely not efficient with the back-light energy. Technologies such as OLED or Sequential Frame LCD (SFLCD) do not use filters. OLEDs are self emitting and draw zero power when off. SFLCD technology still uses a back-light, but they are RGB LEDs. Each color frame (red, green, blue) is switched at such a high speed that the eye integrates the image into the proper colors. Each pixel is now larger and brighter with less power. How much less? Try 80 watts for an SFLCD TV versus 350 watts for the traditional LCD. Energy currently is a limited resource, so innovate where you can to save it.