Transport refrigeration system with improved temperature and humidity control

In a transport refrigeration system having a refrigeration circuit including a compressor having discharge and suction ports, a condenser, an evaporator, a liquid line between the condenser and evaporator, expansion means in the liquid line, a suction line between the evaporator and suction port of the compressor, a hot gas line, and valve means disposed to selectively connect the hot gas line from the discharge port of the compressor to either the condenser or the evaporator to initiate cooling and hot gas heating cycles, respectively, and further including air delivery means for the evaporator which draws air from a served space for conditioning by the evaporator, and for returning the conditioned air to the served space, and a prime mover for the compressor, the improvement comprising:

• a thermostat having a temperature sensor disposed to sense the temperature of the air returning to the evaporator from the served space,
• said thermostat providing a digital signal responsive to the difference between the sensed temperature of the air and a desired set point temperature,
• a modulation valve in the suction line,
• and modulation control responsive to the digital signal provided by said thermostat for continuously controlling said modulation valve in a predetermined temperature range above and below the desired set point temperature, during both cooling and hot gas heating cycles,
• whereby the temperature of the served space when containing a load of fresh produce may be maintained close to set point primarily by the cooling cycle, with minimal temperature drop across the evaporator, minimizing moisture removal from the conditioned air and maintaining a high relative humidity in the served space, and the set point may be set close to freezing without danger of damaging the load.