Regulation systems are becoming more advanced and fast-paced development offers a variety of new solutions. In addition, all compressors are equipped with some form of monitoring equipment to protect the compressor and prevent production downtime. Let us take a closer look at the technology that makes this possible, starting with transducers.
What are transducers?
The transducer is used to sense the current condition of the installation. Information from the transducers is processed by the monitoring system, which gives a signal to an actuator, for example. A transducer for measuring the pressure or temperature often consists of a sensor and a measurement converter. The sensor senses the quantity to be measured. The measurement converter converts the sensor's output signal to an appropriate electrical signal that can be processed by the control system.
How do we measure temperature in compressors?
A resistance thermometer is normally used to measure the temperature. It features a metal resistor as a transducer whose resistance increases with as temperature increases. The change in resistance is measured and converted to a signal of 4–20 mA. Pt 100 is the most common resistance thermometer. Nominal resistance at 0°C is 100 Ω.
A thermistor is a semiconductor whose resistance changes with the temperature. It can be used as a temperature controller, for example, on an electric motor. PTC, Positive Temperature Coefficient, is the most common type. The PTC has an insignificant change in resistance with increased temperature up to a reference point, where the resistance increases with a jump. The PTC is connected to a controller, which senses this "resistance jump" and gives a signal to stop the motor, for example.
How do we measure pressure in compressors?
A pressure sensing body, for example, a diaphragm, is used to measure pressure. The mechanical signal from the diaphragm is then converted to an electrical signal, 4–20 mA or 0–5 V. The conversion from a mechanical to an electrical signal can take place in different measurement systems. In a capacitive system, pressure is transferred to a diaphragm.
The position of the measurement diaphragm is sensed by a capacitor plate and is converted in a measurement converter to a direct voltage or direct current that is proportional to the pressure. The resistive measurement system consists of a strain gauge connected in a bridge connection and attached to the diaphragm. When the diaphragm is exposed to pressure, a low voltage (mV) is received. This is then amplified to a suitable level. The piezo-electric system is based on specific crystals (e.g. quartz) that generate electric charges on their surfaces. The charges are proportional to the force (pressure) on the surface.
Monitoring equipment is adapted to the type of compressor. This necessarily involves a large range of equipment to suit all types of compressors. A small piston compressor is only equipped with a conventional overload cut-out for the motor, while a large screw compressor can feature a number of cut-outs/transducers for overloading, temperature and pressure, etc. On smaller, more basic machines, the control equipment switches off the compressor and the machine is unable to restart when a cut-out gives an alarm value. A warning lamp can, in some cases, indicate the cause of the alarm. Compressor operations can be followed on a control panel for more advanced compressors, for example, by directly reading the pressure, temperature and status. If a transducer value approaches an alarm limit, the monitoring equipment will issue a warning.
Measures can then be taken before the compressor is switched off. If the compressor is shut down by an alarm, compressor restart is blocked until the fault has been rectified or the compressor is reset manually. Troubleshooting is significantly facilitated on compressors equipped with a memory where data on temperatures, pressure and operating status are logged. The capacity of the memory may cover the last 24 hours. This feature allows trends over the last day to be analyzed and logical troubleshooting to be used to quickly identify the reason for the downtime.
Remote monitoring of a compressor installation
In various compressor installations, there may be a need to monitor and control compressor operations from a remote location. On smaller installations, it is fairly easy to connect an alarm, operating indicator etc. from the compressor. It is also usually possible to perform remote starting and stopping. On larger installations, where significant financial investment is at stake, central monitoring is often desirable. It should consist of equipment that provides a continuous overview of the system, and which also provides access to individual machines in order to control details such as the intercooler pressure, oil temperature etc.
The monitoring system should also have a memory in order to produce a log of what has happened over the last 24 hours. The log is used to plot trend curves, which serve to easily identify values that tend to deviate from the default. The curves can form the basis for continuing operations or planning a system stop. The system frequently presents compressor installation status reports at different levels, from a total overview to detailed status for individual machines.