The How of refrigerant through the evaporator coil is
typically controlled by a valve, such as an electronic expan
sion valve or EXV.
Refrigeration systems, including particularly refrigeration
transport systems, requires operation under a Wide variety of
ambient temperatures and operating loads. The applicants
have found that, in order to operate properly under loW box
temperature conditions, it Was desirable to use particular
commercially available pressure transducers for sensing the
evaporator refrigerant in order to obtain high accuracy in
reading the evaporator outlet pressure under loW box tem
perature conditions. Unfortunately, this transducer does not
result in accurate readings outside a refrigerant pressure
range of —14.7 psig to about 100 psig.
Therefore, the present invention discloses a system and
process for controlling an electronic exchange valve for an
transport refrigeration system evaporator in the absence of a
evaporator outlet pressure transducer reading. The process
includes monitoring for the absence of reliable evaporator
outlet pressure readings, monitoring the supply air
temperature, and supplying the supply air temperature to a
processor Within a controller for use in an algorithm so as to
approximate the saturation reference temperature and super
heat levels to derive the necessary control signals for the
EXV.