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Fault detection and diagnostics for commercial coolers and freezers.

Article Excerpt
INTRODUCTION

In general, HVAC&R systems are not well maintained (Proctor and Downey 1995; Cowan 2004; Li and Braun 2006) because of the relatively high cost of service and low cost of energy. Recently, there has been a growing interest in the development of automated fault detection and diagnostics (FDD) for HVAC&R equipment. For vapor-compression cooling equipment, most of the methods presented in the literature (Grimmelius et al. 1995; Stylianou and Lau 1996; Rossi and Braun 1997) utilize differences between measurements and model predictions (residuals) of state variables to perform FDD. Although these methods have good performance for individual faults (Breuker and Braun 1998; Li and Braun 2003), they do not handle multiple-simultaneous faults. In addition, these methods require measurements over a wide range of conditions for training reference models, the development of which can be time consuming and cost prohibitive.

Recently, a diagnostic method was developed that handles multiple-simultaneous faults (Li and Braun 2007a) through the use of decoupling features (Li and Braun 2007b). Decoupling features are parameters that are uniquely influenced by individual faults and are insensitive to variations in ambient conditions. For example, air mass flow rate through the condenser is a feature that is strongly influenced by the level of fouling and condenser fan problems but is nearly independent of other faults that can occur for an air-conditioning system that incorporates fixed-speed fans.

In developing decoupling features, it is important to utilize low-cost sensors, such as temperature sensors (see Table 4 for a summary of the temperature measurements required to perform the diagnostics presented in this paper). These low-cost measurements are used in simple models as virtual sensors to infer other system measurements. For example, as described by Li and Braun (2007b), condenser airflow can be estimated using an energy balance with air-side and refrigerant-side measurements. For this energy balance, the refrigerant flow is estimated using a compressor map as a virtual sensor. Furthermore, virtual evaporating and condensing pressure sensors utilize surface-mounted temperature measurements at locations where saturated conditions exist and property relations to estimate saturation pressures.

The virtual and physical measurements are used to determine the decoupling features for diagnostic purposes. When a decoupling feature deviates significantly from its normal value, a fault is indicated (e.g., low condenser airflow for fouling or fan problems).

The decoupling-based FDD method was originally developed for air-conditioning (AC) systems. Although the vapor-compression equipment used for commercial refrigerators and freezers is very similar to that used for air conditioning, operation occurs over a different range of temperatures and the systems utilize different refrigerants. In addition, commercial refrigeration equipment typically utilizes liquid-line receivers that are not generally employed for air conditioners.

In this paper, the decoupling-based FDD method was applied to equipment used in small-scale walk-in coolers and freezers. Faults were artificially introduced in the laboratory, and the performance of the diagnostic method was evaluated.

WALK-IN COOLER AND FREEZER EXPERIMENTS

Walk-in cooler and freezer units were tested within psychrometric chambers to allow control of the condenser air inlet conditions. Figure 1 shows a cooler unit and walk-in cabinet. The walk-in cooler and freezer experiments utilized the same refrigerated space but employed different refrigeration equipment. The cooler system utilized R-22 as the refrigerant, whereas the freezer unit employed R-404A. Both systems were equipped with a thermal expansion valve (TXV) and a liquid-line receiver. The receiver provides a volume for liquid refrigerant to collect after it exits the condenser. This volume keeps the refrigerant exiting the condenser in a saturated liquid state during operation, unless the unit is overcharged to the point where the receiver is completely filled with liquid refrigerant. When the receiver is full, then additional charge added to the system will back up in the condenser and lead to subcooling of the refrigerant exiting the condenser under steady operating conditions. The liquid-line receiver is shown in Figure 2.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Figure 3 shows a schematic of the refrigerant cycle depicting the methods used for simulating faults and the measurements taken. Table 1 provides the list of faults along with a description of the fault simulation approaches. Refrigerant undercharge, refrigerant overcharge, liquid-line restriction, compressor valve leakage, condenser fouling, and evaporator fouling were considered. A leaky compressor valve allows high-pressure refrigerant to flow back to the low-pressure side of the system, which lowers the volumetric efficiency of the compressor and lowers the mass flow rate of the system. To simulate a leaky compressor valve, a bypass line with a flow control valve was added around the compressor that can be opened to allow refrigerant to flow from the high-pressure side of the system to the low-pressure side. Heat exchanger fouling in vapor-compression equipment can be characterized as a decrease in airflow across the coils. Previous work (Pak et al. 2005; Yang et al. 2007) has demonstrated that the primary effect of air-side heat exchanger fouling is an increased pressure drop leading to a reduced airflow rate. Fouling was simulated for both the condenser and evaporator by connecting fan speed controllers to control the airflow rates across the heat exchangers. During operation, a vapor-compression system can experience clogging of the filter dryer, which restricts the flow through the liquid line. In a system with a fixed-orifice expansion device, a liquid-line restriction can lead to a reduced mass flow rate. The walk-in cooler tested uses a TXV, which can compensate for the pressure drop incurred for moderate restrictions and keep the refrigerant flow rate relatively constant. For severe restrictions, the TXV will saturate, opening fully, and act like a fixed orifice. A flow control valve was added to the liquid line, which was partially closed to simulate this fault.

[FIGURE 3 OMITTED]

Table 1. Fault Simulation Methods Fault Type Simulation Method Compressor valve leakage Partially open bypass valve Condenser fouling Slow compressor fan using installed speed controller Evaporator fouling Slow evaporator fan using installed speed controller Liquid-line restriction Partially close liquid-line restriction valve Refrigerant undercharge Purposely undercharge the system Refrigerant overcharge Purposely overcharge the system

The instrumentation depicted in Figure 3 includes condenser inlet and outlet air temperatures ([T.sub.cai] and [T.sub.cao]) and evaporator inlet and outlet air temperatures ([T.sub.eai] and [T.sub.eao]) that were measured using thermocouple grids. Refrigerant pressures and temperatures were measured at the inlets and outlets of all components to accurately determine the refrigerant states. To understand where the two-phase regions in both the condenser and evaporator are located, eight thermocouples were soldered to the return bends of both the condenser ([T.sub.cond, 1-8]) and the evaporator ([T.sub.evap, 1-8]) and insulated from the surrounding air to obtain an indication of the refrigerant temperatures inside the tubes. Compressor power and refrigerant mass flow rate were also measured in order to determine system cooling capacity and efficiency.

Table 2 provides a description of the test matrix employed for the cooler. For the cooler, each individual fault was simulated at three different ambient conditions ([T.sub.cai]). Five fault levels were tested and characterized by percent of nominal cooling capacity. The nominal cooling capacity was determined...

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