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Article Excerpt
INTRODUCTION
Refrigerant pressures are critical measurements for performance monitoring, control, diagnostics, and optimization of vapor compression cycle equipment. They are typically used as inputs in determining evaporating and condensing temperatures, liquid line subcooling, and suction line superheat. These quantities are used for equipment monitoring and within diagnostic algorithms (Rossi and Braun 1997; Li and Braun 2003; Cui and Wang 2005; Reddy 2007; Li and Braun 2007a). They can also be used in combination with compressor maps to predict refrigerant mass flow and power consumption and thus system efficiency and capacity (Li and Braun 2007b; Reddy 2007) for performance monitoring and fault impact evaluation, and they can be used to derive decoupling features (Li and Braun 2007b; Li and Braun 2008) that provide an indication of fault levels.
Pressure measurements are relatively expensive, and their use has been an impediment to the development of cost-effective embedded diagnostic systems. Accurate pressure sensors are much more expensive than temperature sensors (Li and Braun 2007c). Furthermore, the cost of installing pressure sensors in the field can be expensive. Ideally, connections for permanent installation of pressure sensors should be brazed to the suction and discharge piping for the compressor. For field installations, this requires that refrigerant be evacuated and recharged, which is an expensive procedure. If pressure sensors are permanently connected to available threaded service ports on the compressor, then it is likely that refrigerant will leak over time (Li and Braun 2006).
The objective of the work described in this paper is to remove physical pressure sensors and estimate the compressor discharge line pressure, condensing pressure, liquid line pressure, evaporating pressure, and suction line pressure using low-cost temperature sensors. The performance should be robust against variations in driving conditions and all kinds of faults.
To this end, the starting point is that evaporator and condenser pressures can be estimated using the knowledge that refrigerant is a two-phase mixture somewhere in both the condenser and evaporator as long as the system is not severely undercharged (i.e., 80% or less than its nominal charge). If the saturation temperature can be accurately and reliably measured, then saturation pressure can be estimated using refrigerant property correlations. The evaporating and condensing temperatures can be estimated by using thermocouples that are soldered to the surface of return bends at suitable locations and then insulated from the environment. However, there are several technical issues that need to be considered, including 1) identifying suitable locations for mounting thermocouples that will ensure that refrigerant conditions are saturated for a wide range of driving conditions and in the presence of all kinds of faults, 2) accounting for the pressure drop between locations where temperatures are measured and the points where virtual pressure measurements are needed, and 3) evaluating the overall accuracy and impact on diagnostic performance of employing virtual pressure measurements. The section in this paper titled "Development of Virtual Pressure Sensors" describes the technical development of virtual pressure sensors and addresses sensor locations and pressure drop estimations. The section titled "Laboratory Evaluations of Virtual Pressure Sensors" provides an extensive evaluation in terms of accuracy in estimating pressures and quantities derived from pressure and other measurements, including liquid line subcooling, suction superheat, compressor power consumption, and refrigerant flow rate. The evaluation uses laboratory data for a number of different systems tested over a large range of operating conditions. In the section titled "Diagnosing Multiple Simultaneous Faults Using Virtual Pressure Sensors," the virtual sensors are used as a part of a decoupling-based fault detection and diagnosis (FDD) technique (Li and Braun 2007a, 2007b, 2008) to diagnose multiple simultaneous faults and the impact of the virtual pressure sensors on the FDD performance is evaluated extensively.
DEVELOPMENT OF VIRTUAL PRESSURE SENSORS
Overview of General Approach
Figure 1 illustrates a typical vapor-compression system. The system includes four major components: compressor, condenser, expansion device, and evaporator. There is also piping between components, including a discharge line between the compressor and condenser, a liquid line connecting the condenser to the expansion device, and a suction line between the evaporator and compressor. The expansion device is usually located in close proximity to the evaporator with small feeder tubes that distribute refrigerant to individual evaporator flow circuits.
[FIGURE 1 OMITTED]
Two types of condenser structures are illustrated in Figures 2 and 3. The Type I structure uses two separate circuiting arrangements to handle the desuperheating, condensing, and sub-cooling of the refrigerant, whereas the Type II uses three circuiting arrangements. The Type I condenser in Figure 2 has [n.sub.cond,1] parallel desuperheating and condensing circuits with [m.sub.cond,1] passes within each circuit, then combines into [n.sub.cond,2] subcooling circuits with [m.sub.cond,2] passes within each circuit. This type of condenser is commonly used for residential and small commercial rooftop unit systems. The Type II condenser in Figure 3 has [n.sub.cond,1] parallel desuperheating circuits with [m.sub.cond,1] passes on each circuit, then combines into [n.sub.cond,2] condensing circuits with [m.sub.cond,2] passes on each circuit, and finally combines into [n.sub.cond,3] subcooling circuits with [m.sub.cond,3] passes on each circuit. Type II condenser arrangements...
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