Refrigerant charge reduction in low-temperature transport refrigerator with the eutectic plate evaporator

• The low-charge refrigerating system should operate without subcooling in the condenser.
• The required refrigerant charge is influenced by the maximum operating pressure (MOP) control system.
• The lowest charge can be achieved with electronically controlled expansion valve and parallel evaporator setup.
• The alternative control system is thermostatic expansion valve with MOP function and outlet pressure downstream regulator.

In the article a problem of refrigerant charge reduction of the transport refrigerating system with eutectic plate evaporator is analysed. Such system generates cold during the night, with the vehicle in the depot, and during the day it operates without access to electricity supply. Due to weight limitations the system is equipped with single-stage compressor, which is normally operated at very low evaporation temperatures (−43 ÷ −45 °C during the crystallization of eutectic mixture) and high compression ratios. At the initial stage of pull-down the temperature of eutectic plates is high and therefore maximum operation pressure (MOP) control is required. Other important features with regard to charge minimization are big thermal inertia and relatively big inner volume of the evaporator.During the research it was found that the refrigerant charge is strongly influenced by a system used for MOP control. The other charge influencing factors were also analysed. Two low charge refrigerators with different evaporator setup and MOP control systems were developed and tested. The refrigerant charge was reduced from 4.1 kg to 2.4 and 2.5 kg of R507 without a negative impact on performance. The possibility of further charge reduction is also discussed. The problem is analysed on the example of eutectic system, but the conclusions apply also to all low-temperature single stage systems with some means of MOP control.