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The Energy Conversion Efficiency Laboratory is currently running four Atlas Copco air compressors. The compressors are piped with 1" PVC to a ventilation duct. The piping is terminated with noise filters supplied by Wenniger Co. The piping includes an orifice plate installed at a union 1 from the compressor exit air hand valve. The purpose of the orifice is to simulate a load on the c ompressor. The orifice plate was sized to allow the compressor to stay running at a constant load without unloading. The compressors are being run, and the operating conditions being monitored include power consumption, oil temperature, exit air tempera ture and pressure, ambient air temperature and pressure, sound pressure, and ambient air relative humidity. The operating conditions are monitored with a data acquisition unit that is connected to a PC. The monitored conditions are analyzed and graphed using Sigma PlotÒ software.
Brief Description of Air Compressors
The Atlas Copco compressor model GA 11 is a stationary, single stage, oil-injected screw compressor driven by a Siemans 460V 3-phase 11 kW electric motor. The compressors are equipped with electronic regulators designed to control and pr otect the compressors and monitor components subject to service. The loading and unloading pressures can be programmed. The regulator monitors the rate of decreasing pressure when unloading. When the expected unloading period exceeds a programmed value , the regulator will stop the compressor. If the expected unloading period is below this programmed value, the regulator will keep the compressor running to prevent short cycling. The regulator unloads the compressor for 30 seconds after manually stopp ing the unit.
The electronic regulator also monitors temperature at the inlet and outlet of the compressor. If the pre-programmed limits are reached the regulator will shut down the compressor. The compressor will also be stopped in case of overload of the compres sor motor. This will be indicated on the display to inform the operator of the reason for the shut down.
The electronic regulator also continuously monitors critical components subject to service (oil, oil filter, air receiver, and air filter). The hours logged on each component is compared to pre-programmed limits. Exceeding these limits causes a messa ge on the display to warn the operator to replace the indicated component. Similarly, the regulator also tracks running hours, loading hours, and compressor starts and stops.
Description of Sensors and Data Acquisition
A watt transducer supplied by Ohio Semitronics supplies an output of 0 10 VDC, which corresponds to 0 60 kW. To achieve more accuracy from the watt transducer the supply voltage wire was wrapped around the current transformers two times. This would give an output reading of 0 10 VDC that corresponds to 0 30 kW. The watt transducer can handle up to 100 amps and 300 VAC on each line. The accuracy is ± 0.2% of reading or ± 0.04% of full scale. These accuracies include the combined effects of voltage, current, load, and power factor. The response time to read 99% of a step change in power consumption is less than 400 ms.
The watt transducer, model GW5-008D, contains transformers for each leg of power. The transformers step down the in-coming current. This current is then wired to the transducer. The transducer is also wired to each leg of the in-coming 460V supply. The transducer requires 120VAC power. By monitoring the current and voltage, the transducer supplies a DC output proportional to the kW power consumption.
Omega type T thermocouples are mounted in the oil fill plug of the air receiver and on the moisture trap at the air exit. Type T thermocouples consist of copper/copper-nickel alloy combination. The temperature range and accuracy is 76 to 212F 1.8F. The thermocouple leads are fed into the data acquisition unit. The time required to reach 63.2% of an instantaneous temperature change is between 4 and 5 seconds. These thermocouples can be connected directly into the data acquisition unit. The thermocouples are secured in place by compression fittings that are machined into the oil fill plug and an access cap on the moisture trap. The compression fittings make an airtight seal around the 6" ungrounded 0.125" dia. 304 stainless steel sheath.
The pressure transducer supplied and mounted on the moisture trap is being used to monitor the exit pressure. The pressure transducer is a Kavlico Corp. model P165-5162. It has an input pressure range of 0 to 246.569 psig. Its output voltages are 0. 500 VDC at 0 psig and 4.500VDC at 246.569 psig with a maximum allowable deviation at 68F of 1.25% of full scale. The response time is 15 ms maximum to read 63 % of full-sca le pressure with a step change of pressure on the unit. The output readings are monitored from the electronic regulator and fed into the data acquisition unit
Ambient Condition Sensors
The ambient conditions being monitored are air temperature, pressure, and relative humidity. The ambient air sensors are supplied with DC voltage by a dual output lab bench DC power supply, model HP E3620A. The correct DC voltage sho uld be known and set on power supply before powering any sensor.
A Humitterâ 50Y, manufactured by Vaisala Inc. is used to sense relative humidity and ambient air temperature. The Humitterâ requires a power supply of 7 - 28 VDC. The humidity sensor, INTERCAPâ (part no. 15778) has an output of 0 1 VDC corresponding to a range of 0 100% RH. The accuracy is ± 2% RH at 10% RH and ± 3.5% RH at 90% RH. It takes approximately 15 seconds to respond to 90% of a 1-% change in RH. The temperature sensor used in the Humitterâ 50Y is a 1000-ohm platinum resistor (Pt 1000). It has an output of 0 1 VDC that corresponds to 40 - 60° C. Its accuracy is ±0.5°C at -10°C and ±0.8° C at 60° C. Its response time is dependent on air movement.
A SETRA, model number 276 barometric pressure sensor is used to measure ambient air pressure. This sensor requires 9 14 VDC input and supplies an output of 0.1 5.1 VDC that corresponds to 800 1 100 milli-bar. The accuracy is ± 75 mbar and requires less than 5 ms to respond to a 75-mbar change in pressure.
Data Acquisition Unit
The data acquisition unit is a Hewlett Packardâ HP 34970A. This unit allows for direct measurement of thermocouples, resistive temperature devices (RTDs), thermistors, dc and ac voltages, resistances, dc and ac currents, frequency, and period.
The unit has three module slots in the rear to accept the data acquisition or switching modules. A HP 34902A 16-channel reed multiplexer will be used to accept the output signals of the instrumentation used to monitor the operating conditions and ele ctrical power consumption. This multiplexer has 16 channels of 300 V- 2 W switching capability, a built in thermocouple reference junction, and a maximum input current of 50 mA. This module is not capable of monitoring ac or dc current. Each channel is fully isolated, yet the module can be configured for 4 wire resistive measurements. The wires carrying the monitoring signals are connected to the screw terminals inside the 16- channel multiplexer.
Following will be a brief description of the front panel controls followed by a description of the current data acquisition setup.
Thermal Imaging Camera
The imaging radiometer, model 760 IR, is manufactured by Inframetrics.
This IR camera is designed for real-time analysis of static or dynamic thermal patterns. It
is a completely self-contained thermal imaging, archival, and analyti
cal system. The base unit contains a integral color LCD and a 3.5" floppy diskette
drive, so the unit is capable of down loading and retrieving monitored images to and from a
diskette. The typical minimum detectable temperature difference is 30
The IR camera could be used to monitor slight changes in temperature across different parameters of the air compressor unit. Pictures of the air compressor could be taken at certain times during different runs to monitor any changes that might occur. The IR camera could also be useful in detecting any faulty wiring. Taking IR pictures of the watt transducer, the compressors electronic regulator, or the disconnects could reveal "hot-spots" in the wiring. "Hot-spots" would indica te an increase in resistance, which in turn would indicate a bad connection and a possible hazard.