DESCRIPTION OF THE PRIOR ART AND
SUMMARY OF THE INVENTION
A transport refrigeration system used to control enclosed
areas, such as the box used on trucks, trailers, containers, or
similar intermodal units, functions by absorbing heat from
the enclosed area and releasing heat outside of the box into
the environment. To accomplish this, a typical transport
refrigeration unit requires a highly pressuriZed refrigerant is
introduced into a loW pressure environment such as an
evaporator coil. The refrigerant absorbs heat from the box
return air across the evaporator coil and boils and evapo
rates. This refrigerant vapor is removed from the evaporator,
and is compressed to a higher temperature and higher
pressure through the use of a compressor, and is then turned
back into liquid refrigerant by passing through a condenser.
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.
Accordingly, one object of the present invention is to
provide a microprocessor control for the calculation of
superheat levels to control an EXV.
It is a further object of the invention to provide a micro
processor control for approximating superheat levels in the
absence of a reliable evaporator outlet pressure reading.
It is another object of the present invention to provide a
process for utiliZing box supply air temperature to calculate
actual superheat levels for an evaporator so as to control an
EXV.
These and other objects, features, and advantages of the
present invention Will become more apparent in light of the
folloWing detailed description of a best mode embodiment
thereof, and as illustrated in the accompanying draWings.