Continuous setting must be used for produce loads, as they need continuous air flow to handle the heat of product respiration. Continuous air flow allows for more consistent temperature throughout the trailer for the duration of transport. All reefer units have a ‘Set Point’. For example, if you have a set point of 33°F, the temperature reading displayed by the set-point will stay constant on a smart reefer download. On continuous setting, the reefer has a set defrost cycle (definition: Defrost cycle – the process of removing ice or frost buildup from a coil). The intervals are factory set for every 6 hours, but the unit will auto defrost based on a temperature difference across the coil. The defrost cycle can be changed to compensate for produce that has higher heat respiration, for higher ambient temperature (definition: Ambient temperature - the certain temperature within an enclosed space), and for door openings. For example, if it is a humid day and the driver is doing a lot of door openings, you may need to have a more frequent number of defrost cycles. Although the reefer has this defrost cycle on continuous setting, it does not change the product temperature due to the fact that most reefers have a very high heating capacity (definition: Heating capacity – The amount of heat energy needed to raise the temperature of a given mass of a substance by one degree Celsius). The temperature in the air of the reefer will heat at a faster rate than the temperature of the product that is in the trailer. The time of a defrost cycle is not is significant enough to affect the product temperature.
Below is an example of a temperature recorder printout for a reefer in continuous mode (note spike at end of 41°F is at unloading point:
Cycle-Sentry setting should only be used for frozen loads, i.e. loads that are intended to ship colder than 24°F. Frozen loads can handle more significant temperature variances without causing damage to the product. On cycle-sentry, the load also has a ‘Set Point.’ For example, if you have a set point of 22°F, the set-point will also stay constant on the download, as it does on continuous setting. However, when the ambient temperature in the reefer drops to between .5 to 1 °F lower than the ‘Set Point’ (in this case that would be 21.5°F), it turns off. It then waits until temperature warms up 5°F past the set point (in this case, 27°F), and then turns itself back on.
Below is an example of a temperature recorder printout for a reefer in cycle-sentry mode:
Produce loads should not be set on cycle-sentry, and the only benefit of operating on this setting is significant fuel savings. Because the temperature of a load on cycle-sentry can vary dramatically during the time of transport, this can cause freezing on top boxes or condensation in packaging.
4 Steps to a Successful Shipment. Sensitech Cold Chain Visibility. Powerpoint Presentation. Received on 11/08/2009, Accessed for document on 01/06/2010.