Fault Detection and Diagnostics for Rooftop Air Conditioners

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APPENDIX 1 PHYSICAL MODELS OF EXPANSION DEVICE
An expansion device is a relatively simple component, whose role is to reduce the
pressure and regulate the refrigerant flow to the low side evaporator in accordance with
load demands. As mentioned in Section 1, a model of the expansion device can be used to
estimate its upstream pressure. There are two kinds of expansion devices used in vapor
compression system: fixed-area and adjustable throat-area expansion valve.
A1.1 Fixed-Area Expansion Device Models
The fixed-area expansion devices are typically used on certain small air
conditioners and refrigeration systems where operating conditions permit moderately
constant evaporator loading and constant condenser pressures. According to their lengthto-diameter
ratios,
L/ D , fixed-area expansion devices fall into one of three categories:
an orifice with L / D < 3 , a short-tube with L / D = 3 ~ 35 , and a capillary tube
with L / D > 20 (ASHRAE 1998). Among them, capillary tubes are used in home
refrigerators and room air-conditioners of small capacity, while short-tubes are widely
used in packaged residential and small commercial air-conditioners and heat-pumps of
relatively large capacity.
A1.1.1 Orifice Models
Although orifices are seldom used as refrigerant expansion devices nowadays,
their operation principles are the basis for the operation of adjustable throat-area control
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