In most cases, screw compressors are direct driven by electric motors operating at speeds up to 3,600 rpm. The compressor drive trains commonly are mounted on top of the horizontal oil separators. The oil separators serve as the oil reservoirs from which the oil is drawn for bearing lubrication and sealing of the rotors. The oil also is used to cool the compressed gas inside the compressor. Some compressors require full-time lube oil pumps during operation while others utilize differential pressure between suction and discharge to pump oil into the compressor.
Screw compressors are positive-displacement machines in which compression is achieved by two meshing rotors. The compressor has an infinitely variable capacity control, achieved by a hydraulically operated slide valve that typically is located in the bottom of the compressor casing. Generally, the slide valve allows a compressor to operate from full capacity down to 10 percent. For operation below the minimum capacity, the compressor requires false loading, which usually is achieved by recycling discharge gas back to the compressor suction.
For safe and efficient compressor operation, it is required to monitor compressor- operating parameters such as compressor suction, discharge and oil pressures and temperatures, oil levels in the oil separator, motor driver current and oil pump operation, and to properly react to changes to these conditions.
Controlling It AllDuring the 1970s and 1980s, compressor manufacturers and packagers would furnish electromechanical panels to control the operation of the compressors. The electromechanical panels had inherent limitations because they utilized hard-wired pressure and temperature switches and relays for control purposes.
In the 1980s, microprocessor-based panels became popular, developed by manufacturers for each particular brand of compressors. The microprocessor panels constituted a large improvement. Microprocessors use temperature probes and pressure transducers to monitor compressor-operating parameters. They also allow controlling the operation of the compressor slide valve; the electromechanical panels require a separate PID controller for that purpose.
The late 1990s brought the introduction of a PC-based control panel for the screw compressor packages. Just like in any computer, the heart of the system is a PC-based motherboard with a microprocessor. One in particular uses a 300 MHz microprocessor with 32 MB RAM memory. Unlike the microprocessor panels, the PC-based panel is provided with a large, colorful, easy-to-use display. The PC-based panels allow the user to calibrate all probes and transducers supplied with the machine as well as set the operating parameters and shutdown conditions. Some panels will trend and record compressor operations as well as alarm and shutdown history. If desired, the panel (or panels, depending on the application) can be provided with additional control modules that allow for control of the plant condensers and evaporators.
Technology Will Drive The ChangeComputer industry trends include advancements in hardware and software. Processor speeds and memory size have increased dramatically as consumers demand more speed and functionality. Software developers offer sophisticated packages for many applications. As in all of the technology industry, screw compressor control systems also have evolved. Many compressors, however, are still controlled with technology that is equivalent to early versions of DOS or 8086 microprocessor on your computer.
In this high rate of evolution in the industry, old technology is disregarded and new technology takes up the stocking shelves of the suppliers. Although the compressor package does not require much more technology to operate properly, replacement parts for the existing control panels may be difficult to find. It becomes a necessity for the users of the screw compressors to keep up with the evolution of control panels and control devices.
PC-based control panels offer advanced features, and they can be retrofitted on existing older compressor packages. Check the features of your control panel to see if it has the proper safeties such as compressor "stop load" and "force unload" on high compressor discharge pressure or "stop load" and "force unload" on high compressor motor amps, and so on. New panels offer efficiency features such as lead-lag sequencing (when not being controlled by a central control system for the refrigeration) that allow the control panels to communicate with each other and manage the loading and unloading of the screws when operating multiple compressors on the same system. This feature makes the compressors work together by keeping the largest machines loaded (at the maximum efficiency of the compressor) while allowing a smaller machine to float with the load.
Those who follow the "If it's not broke, don't fix it!" approach may encounter trouble when it comes to compressor control panels. Your old panel is functioning today, but if it failed, how long could you afford to be without it? How fast and at what cost can you get the parts to get your compressor back online? Owners of older compressor packages with early versions of microprocessors should check to see if the panel is still supported by the manufacturer. Also, check to see how long the parts and programs will be available. Is the original system manufacturer still in business?
PC-based panels are available in retrofit kits for installation on older machines that still operate with electromechanical or outdated microprocessor panels. Most users notice an immediate improvement in plant operating efficiency, reduced troubleshooting and maintenance time as well as eased plant startup once all capabilities of the system are utilized. The kits include the control panel and also offer replacement pressure and temperature transducers, slide valve potentiometers and other accessories, if necessary. PCE