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Development, evaluation, and demonstration of a virtual refrigerant charge sensor.

Article Excerpt
INTRODUCTION

A number of studies conducted by various independent investigators (Proctor and Downey 1995; Cowan 2004) have concluded that more than 50% of the packaged air-conditioning systems in the field were improperly charged due to improper commissioning, service, or leakage. Proper refrigerant charge is essential for a system to operate efficiently and safely. A study sponsored by the American Council for an Energy-Efficient Economy (ACEEE) concluded that improper charging of air conditioners and poor maintenance could increase energy use in homes by 20% and waste 17,600 terawatt-hours of energy nationwide every year (Neme et al. 1999). There is a direct link between carbon dioxide production (global warming) and energy efficiency and between leakage of chlorofluorocarbon/hydrochlorofluourocarbon refrigerants and ozone depletion.

A number of investigators have developed methods for detecting low or high refrigerant charges (e.g., Rossi and Braun [1997]; Li and Braun [2007]). However, these methods do not provide an indication of the level of charge. Charge level is very useful information in evaluating whether service is justified and in aiding a technician in adding or removing the appropriate amount of charge during service. The typical approach used to verify refrigerant charge in the field involves the use of either superheat at the evaporator outlet when the expansion device is a fixed orifice or capillary tube and subcooling at the condenser outlet for systems that use variable-area expansion devices. Manufacturers typically provide specifications for superheat or subcooling. However, these specifications are typically not applicable over a wide range of operating conditions (e.g., low or high ambient and high or low mixed-air wet-bulb temperatures) and when faults are present (e.g., low indoor airflow). In addition, the current charge verification protocols utilize compressor suction and discharge pressures to determine refrigerant saturation temperatures that are used to determine evaporator superheat and condenser subcooling. However, the measurement of pressures requires the installation of gauges or transducers that can lead to refrigerant leakage. As a result of these limitations, the current protocols for checking refrigerant charge may be doing more harm than good in many situations.

In order to accurately determine charge level with current practice, technicians would need to evacuate the system and weigh the removed charge. The correct amount of charge would then be added to the system using a scale. This method is very time consuming and costly.

This paper presents a method for obtaining accurate estimates of refrigerant charge level using noninvasive temperature measurements obtained while the system is operating. This method could be used as part of a permanently installed control or monitoring system to indicate charge level and/or to automatically detect and diagnose low or high levels of refrigerant charge. It could also be used as a stand-alone tool by technicians to determine existing charge and during the process of adjusting the refrigerant charge.

The accuracy of the virtual refrigerant charge sensor method is evaluated using laboratory data for a number of different systems and over a wide range of operating conditions with and without the presence of other faults. Seven different systems are considered, including a window unit, residential split systems, and light commercial packaged systems employing either fixed-orifice expansion devices or variable-area expansion devices and R-22 or R-410a as the refrigerant. The virtual refrigerant charge sensor is shown to have good performance in terms of accuracy and robustness. It has the potential to be easily implemented and installed in terms of both hardware and software.

DEVELOPMENT

Refrigerant Charge Indicator

By mass, most of the refrigerant exists as liquid (saturated or subcooled) for normally charged units. In addition, the majority of the refrigerant charge (up to 70%) typically accumulates as liquid in the high-pressure side of the system, including the condenser and liquid line (piping and filter/drier). If there is condenser subcooling, then the liquid line is completely filled with liquid and its liquid volume does not change with refrigerant charge. In addition, the volume of liquid in the two-phase section is relatively constant, because the refrigerant quality varies between and 1 in a nearly linear manner. Therefore, changes in high-side refrigerant charge mostly occur in the sub-cooled section of the condenser. Furthermore, the volume of liquid within the subcooled section of the condenser is nearly proportional to the amount of subcooling at the exit, [T.sub.sc], and the liquid density is nearly constant throughout the high-side of the system. With this discussion in mind, the high-side refrigerant charge is related to subcooling using

[m.sub.hs] = [k.sub.sc][T.sub.sc] + [m.sub.hs,0], (1)

where [m.sub.hs,0] is the high-side refrigerant mass for the case of saturated liquid leaving the condenser, [k.sub.sc] is a constant that depends on the condenser geometry, and [m.sub.hs,0] is assumed to be a constant, independent...

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