Most cabinet and enclosure air conditioners cycle the compressor on and off to maintain the desired air temperature. Even when the enclosure is well insulated, this method of operation can result in significant cyclic air-temperature variations. Cabinet air temperatures can fluctuate several degrees from the desired temperature. Vibration incurred as the compressor cycles also can be undesirable in certain applications.
The most common refrigerant metering device utilized with conventional air conditioners is a simple capillary tube. The diameter and length of a capillary tube is designed to meter low pressure refrigerant to a particular temperature. However, variations in load and ambient temperature can cause instability in this type of metering process. The consequence can be unstable supply-air temperature from the evaporator coil.
Additional complications can arise for applications that re-quire enclosure air temperatures approaching 32¿F (0¿C) because evaporator coil freezeup be-comes a concern. Any condensation freezes onto the evaporator coil and acts as an insulator, resulting in loss of thermal transfer from the evaporator coil to the air. The enclosure air temperature will begin to rise as the amount of ice in-creases. A defrost cycle will prevent freezeup, but it also can contribute to enclosure air temperature instability.
Putting It to UsePackard Instrument Co., Downers Grove, IL, manufactures liquid scintillation analyzers for spectral analysis of liquid samples in laboratory and pharmaceutical applications. Liquid specimens are lowered into a lead-lined counting chamber via an automated cassette device. A radioactive isotope placed in the test vial causes photons of light to be emitted from the sample. Photon detectors are used to secure the liquid's spectral signature from the light emissions.
Low-level liquid scintillation counting often requires chilling the counting chamber to a temperature below ambient to reduce background environmental radio-activity that could interfere with monitoring. An accurate temperature setpoint and stability are necessary to ensure accurate and reproducible results.
Until early 1999, Packard Instrument Co. used a conventional air-conditioning unit to cool its liquid scintillation analyzers (LSAs). One of its customers, Ontario Power, Whitby, ON, employs the analyzers in routine environmental monitoring services for corporate customers.
For routine environments, Ontario Power found the traditional air conditioner allowed the analyzer to operate effectively. For low activity samples, however, temperature swings in the counting chamber caused an unacceptable data variable.
Based on the good results Ontario Power achieved, Packard Instrument made the switch to tight tolerance air conditioners, which are now supplied on all of the company's Tri-Carb low-level liquid scintillation analyzers.