Solving the Problem of In Transit Warming

May 01, 2004

With daytime temperatures approaching 90°F and higher in the Southern part of the country, we are beginning to see pulp temperatures above 40°F on arrival of fresh strawberries shipped from California to Eastern markets.

The California strawberry trade typically does an outstanding job of pre cooling strawberries to 33°F. While we acknowledge that the Tectrol bag increases average pulp temperatures by 1-2°F, this still does not account for the amount of warming we are seeing in bagged and un-bagged product. So why do we see warmer arrival temperatures during the summer months, especially when the transit trailers reefer system appears to be working just fine?


The answer is multi-faceted, but follows one basic concept. The trailer sidewalls absorb heat (radiant, solar, etc.). At some point, the amount of heat load that is passed through the trailer insulation sidewall is so great, that the rate of refrigerated air circulating inside the trailer is insufficient to sweep away all of this heat. If the circulating air does not remove heat, then the heat warms the loaded product. We believe this has become a more pronounced problem with the migration to 53’ trailers.


Clearly we must allow the trailer refrigeration system to work effectively, circulating air at a rate sufficient to sweep incoming heat. While the basics are well known, practicing the basics is made complex by the nuances of loaded produce, equipment availability, equipment condition, freight pricing and so on. Following are the basics to maximize air velocity and reefer performance:


1) Well-maintained and calibrated reefer unit.

2) Well-maintained and properly designed bulkhead.

3) Air chute properly attached, free from damage.

4) Trailer walls, floor, doors, and door seals are repaired and in good condition.

5) Loaded product is palletized and braced away from sidewalls.

6) Loaded product does not touch the air chute or exceed maximum height limits.

7) Product must be properly pre-cooled. Reefer set point must be correct.


Unfortunately, we can still see gradual in transit warming even when the above best practices are in force. Even with good equipment and proper loading practices, the refrigerated air velocity of circulating air inside a trailer is “overloaded” during some parts of the day during the summer months.


What is the solution?


The key is to move a higher velocity of refrigerated air inside the trailer when the heat load is greatest (usually corresponding to the afternoon and evening times). Both Carrier and Thermo king have recently introduced reefer units that allow drivers to program the refrigeration unit to run in “high speed fan mode” during high heat load periods. This is a very important innovation. When the heat load on a trailer increases, the reefer unit senses this and automatically increases the velocity at which refrigerated air is moved around the product, thus sweeping away excess heat.


As these units are purchased and introduced into truck fleets, the opportunity to flat line produce temperatures at the correct temperature during the entire transit will be excellent. The down side is these units are new on the market, and likely will cost more to purchase as well as run. We must also remember that even the best equipment design can be defeated by poor equipment maintenance and loading practices.


What about now?


Until these types of units are commonly available, the only defense that minimizes transit warming is to use well-maintained equipment and load product properly, especially away from sidewalls. Drivers who know the operating design of their equipment and have confirmed loading patterns and initial pulp temperatures can also set their units at lower temperatures. Rather then setting units at 36-38°F during summer months, it is reasonable to reduce set points to the 32-34°F range for products such as strawberries.