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A smart mixed-air temperature sensor.

Article Excerpt
INTRODUCTION

Economizer systems typically use mixed-air temperature (MAT) measurements for feedback control of the outdoor and return-air dampers. In addition, diagnostic methods for both economizer systems and air-conditioning equipment require accurate measurements of MAT. However, packaged air-conditioning equipment for small commercial applications typically have small chambers for mixing outdoor and return air and can have very nonuniform temperature and velocity distributions at the inlet to the evaporator. As a result, there can be significant bias errors associated with employing single-point and averaging sensors (Avery 2002; Carling and Isakson 1999; Robinson 1999). Furthermore, the mixing process can change significantly as the position of the dampers changes with economizer operation. An array of at least four temperature sensors mounted symmetrically about the duct centerline may be necessary to achieve good accuracy for MAT (Wichman 2007).

The requirement for employing averaging sensors adds significant cost and may not ensure accuracy. Recently, a method was developed (Lee and Dexter 2005; Tan and Dexter 2005, 2006) that corrects for the bias error associated with employing a single-point sensor. The method employs "fuzzy sensor fusion," whereby the results of single-point measurement are fused with temperature distribution predictions from a CFD model for the mixing process. Although it was demonstrated that this method can provide improved estimates of the MAT, it is not obvious how the method can be generalized without requiring that CFD simulations be performed for any specific geometry of mixing box and sensor placement that may be encountered.

This paper develops and evaluates a method for estimating the bias error associated with a single-point measurement. The method only requires single-point temperature measurements for mixed, return, outdoor, and supply air that are already typically available for diagnostics and economizer control. The combination of a single-point measurement and bias prediction could be integrated into a smart-MAT sensor to allow self calibration and dramatically improved accuracy. The proposed method was evaluated using data obtained for an air-side economizer and mixing chamber integrated with a typical small commercial rooftop air conditioner.

SYSTEM DESCRIPTION, MEASUREMENTS, AND MIXING PERFORMANCE

Experiments were performed on a five-ton rooftop air conditioner equipped with an economizer. The rooftop unit was set up inside environmental chambers to simulate indoor and outdoor conditions. A diagram of the experimental setup is shown in Figure 1. Figure 2 shows a picture of the mixing chamber and dampers depicting the directions for airflow and a filter that is just upstream of the evaporator coil. The evaporator coil is located in very close proximity to the outdoor and return-air dampers, and the air intakes are not symmetrical, leading to very poor mixing.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The economizer damper was configured to be manually controlled using a 2-10 V variable DC voltage source to the motor. The motor had a DC voltage output indicating damper position using the same 2-10 V scale as the input, where a 2 V input returns the damper minimum position of about 5% outdoor air and the 10 V input returns a damper position of about 75% outdoor air. Due to the flimsy construction of the return-air damper, significant leakage passed the return damper when it was closed at the 10 V motor input, preventing the economizer system from achieving an outdoor-air fraction (OAF) greater than 75%. For the purpose of characterizing damper position, a normalized damper-control signal ([[gamma].sub.D]) was determined as

[[gamma].sub.D] = [[DS - 2V]/[8V]], (1)

where DS is the damper control signal.

Four different measurement grids were set up in the unit with a total of 33 type T thermocouples, each having a rated accuracy of [+ or -]1.8[degrees]R ([+ or -]1K). Nine thermocouples were arranged on grids at both the outdoor and return-air inlets. Fifteen thermocouples were placed symmetrically in five rows and...

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