During the operation of a refrigeration unit, a layer of ice will form on the evaporator coils depending on the temperature set, the temperature of the cargo, the amount of fresh air ventilation and the cargo humidity.
The unit periodically enters a phase where heat is produced by a series of electrical bars, allowing defrosting to take place. At such times, all fans are turned off automatically in order to prevent heat from entering the cargo compartment.
However, the return air temperature sensor is so closely located to the refrigeration machinery that the temperature record will inevitably register some of this rise. The record will therefore display periodic temperature increases in keeping with the defrost periods. It must be stressed that these increases, which are conspicuous on paper chart recorders, have no immediate effect on the actual temperature of the cargo and are not an indication of an unstable refrigeration unit.
Electronic loggers usually indicate the timing and duration of defrost periods in addition to temperatures.
If, as described in the previous section, a cargo is loaded into a container in a "Warm" or Hot condition exceeding the specified temperature, the refrigeration unit will automatically work to bring the cargo temperature down towards the correct level.
This unintentional strain on the unit may result in a heavier accretion of ice on the evaporator coils, leading to an increase in the defrost patterns recorded.
Recorder charts do not identify refrigeration unit defects, but do give useful indications of correct operation.
Data logger records may give detailed information about system faults in addition to Set point, delivery and return air temperatures.
Container temperature recording systems do not usually record actual cargo temperature, only air temperatures, but cargo temperature may be recorded by shippers loggers within the stow.
The unit periodically enters a phase where heat is produced by a series of electrical bars, allowing defrosting to take place. At such times, all fans are turned off automatically in order to prevent heat from entering the cargo compartment.
However, the return air temperature sensor is so closely located to the refrigeration machinery that the temperature record will inevitably register some of this rise. The record will therefore display periodic temperature increases in keeping with the defrost periods. It must be stressed that these increases, which are conspicuous on paper chart recorders, have no immediate effect on the actual temperature of the cargo and are not an indication of an unstable refrigeration unit.
Electronic loggers usually indicate the timing and duration of defrost periods in addition to temperatures.
If, as described in the previous section, a cargo is loaded into a container in a "Warm" or Hot condition exceeding the specified temperature, the refrigeration unit will automatically work to bring the cargo temperature down towards the correct level.
This unintentional strain on the unit may result in a heavier accretion of ice on the evaporator coils, leading to an increase in the defrost patterns recorded.
Recorder charts do not identify refrigeration unit defects, but do give useful indications of correct operation.
Data logger records may give detailed information about system faults in addition to Set point, delivery and return air temperatures.
Container temperature recording systems do not usually record actual cargo temperature, only air temperatures, but cargo temperature may be recorded by shippers loggers within the stow.
