Cycling compressors can cause temperature fluctuations within the cabinet. Accurately maintain cabinet and enclosure temperature with tight tolerance air conditioners.

Tight tolerance air conditioners typically have continuous-run compressors. Constant pressure refrigerant-metering valves provide a stable low-side pressure and supply-air temperature.

As cabinets and enclosures are more densely packed with electronics, waste heat from these devices becomes more problematic. Removing cabinet heat can be accomplished with enclosure air conditioners. However, some processes require more than heat removal - they demand accurate temperature maintenance.

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.

Enclosure coolers incorporate a heater assembly (left), heater control module (center) and return-air temperature sensor (right). Tight tolerance air conditioners may incorporate a heater assembly downstream of the evaporator coil.

Many of today's high tech applications cannot tolerate an air temperature swing of several degrees Celsius. Tight tolerance air conditioners typically incorporate continuous-run compressors to make cooling available without interruption. Constant pressure refrigerant-metering valves provide a stable low-side pressure and supply-air temperature. A temperature-controlled condenser fan may be used to regulate the high-side head pressure of the refrigeration system. Condenser performance is varied to match load conditions, which enables the metering device to operate with greater accuracy.

A condenser fan-speed controller and temperature sensor are two of the components used to regulate the condenser. The temperature-controlled condenser fan regulates the high-side head pressure of the constant-pressure refrigeration system.

Return-air temperature can be regulated with a proportionally controlled heater element placed downstream of the evaporator coil. The proportional controller uses a thermistor in the return airstream to provide temperature sensing. As the temperature of the return air approaches setpoint, the controller begins to apply power to the heater element. This bucking technique of heat with cold yields a typical air temperature control of I0.1¿C. Other advantages to this approach can include increased compressor life and precise control of enclosure humidity.

Fig1 Low-level liquid scintillation analyzers require an accurate temperature setpoint. Conventional air-conditioning caused unacceptable temperature swings in the counting chamber. By replacing the conventional air conditioner with a tight tolerance air conditioner, enclosure air temperature was stabilized.

Putting It to Use

Packard 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.

Designed for laboratory and pharmaceutical applications, liquid scintillation analyzers from Packard Instrument Co., Downers Grove, IL, incorporate tight tolerance air conditioners to maintain temperature control.

By replacing the conventional air conditioner with a tight tolerance air conditioner, enclosure air temperature was stabilized to within I0.1¿C of setpoint (figure 1). The improved sample temperature stability enabled Ontario Power to calibrate liquid scintillation analyzers measurements and verify results.

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.