Fault Detection and Diagnostics for Rooftop Air Conditioners

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Figure A1-9 shows the refrigerant mass flow rate curves at a fixed pressure drop.
It can be seen that:
1. The mass flow rate of type I & II valves is higher than that of a type III valve with
the same operation range at the same opening superheat except that the valves are
fully open or close. That is, for the valves with the same operation range, type I & II
valves require smaller superheat to get the same capacity than type III valve.
2. The mass flow of type III keeps increasing linearly with the opening superheat until
the maximum opening superheat arrives, while that of type I & II keeps increasing
nonlinearly and the increasing rate decreases smoothly down to zero when the valve
is fully open. That is, for type I & II valves to have the same reserve capacity as a
type III valve more superheat is required, so type I & II valves would be expected
to have smaller reserve capacity (around 10%) than that of type III (up to 40%) in
order to avoid abnormally high superheat at high capacity operation. However, a
possible advantage is that type I & II valves may help reduce the problem of the
TXV alternately overfeeding and underfeeding the evaporator, which is usually
termed hunting or cycling.
Mass flow rate
Type I & II
Type III
T
sh , static
T
sh , ratingopening
T
sh , max opening
Superheat T
sh
Figure A1-9 Mass flow rate curves at a fixed pressure drop
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