How do they keep goods that shouldn’t be frozen – like groceries, etc. – from freezing when they’re being trucked through below-freezing weather? Are the trucks heated?
I used to work in distribution systems for a major New England supermarket chain. Part of your question has me puzzled as well now that I think about it.
Perishable goods are shipped in a truck with a reefer unit in it. The trailer is well insulated and the reefer can maintain temperature that is both warmer and colder than the ambient outside temperature. These trailers will not allow the cargo to get too cold.
Ordinary 18 wheeler trailers don’t have a reefer unit. I have never though of it before but I assume that the mass of the trailer plus 40,000 pounds of cargo have enough thermal mass to resist getting that cold. It won’t matter much for most cargo but I suppose canned goods and things might not do well being inadvertently frozen en route.
That brings me to one of my favorite stories. My wife’s family owns a large cheese and dairy importing/distribution facility. One day, a driver called in sick and they needed a driver badly. They called a temp driver company and they sent a guy who was whacked out of his mind but they still needed to get things delivered. It so happens that a refrigerated truck broke down that day but it was a cold day and they figured that things should be Ok using a regular truck for local runs. The guy took a truck to unload a Whole Foods store. The receiver asked “Where is your reefer?”. The driver looked around sheepishly and said “Ah man, I’m all out of reefer.”
Trucking companies have a number of ways to deal with transporting freezable goods. It depends on the lengh of the route. When I worked for Home Depot it was important things like paint didn’t freeze. Most the time the truck was on the road for less then 12 hours. They would heat the trailer and cargo before sending it on its way. Basicaly they aimed a torpedo heater in while they loaded the truck.
For longer periods of transport companies used reefer trailers to heat the trailers.
Not everything made it to us in good condition. Plant companies were very good at losing whole trucks of tropicals to freezing weather. Based on their profit margins compared to a loss they just seemed willing to take the risk. Also if my recievers didn’t pay attention they would recieve dead or dying plants and would had to take the loss. The shipping companies for such goods are always very addiment since it was signed for in good condition it’s not their problem.
Long ago, when I was a teamster, and loaded general merchandise, anything marked do not freeze was packed in the center of the truck. A couple of feet of boxes around the stuff was a lot of insulation, especially since most of that space was dead air.
Of course, general merchandise does spend more time in the terminal than it does on the truck, most of the time. The terminal ain’t warm, you understand, but it seldom gets below fifty or so.
Tris
They don’t do enough.
Thanks for the info, all. I didn’t really think they heated the trucks, but couldn’t figure out what the alternative was.
Shagnasty – “reefer” – is that short for something? Refrigerator, perhaps? Except it sounds more like a heat pump (not that I’ve ever been able to wrap my mind around the concept of “heat pump” – how do you pump “heat” out of cold air? – but that’s another thread).
Run an air conditioner in reverse and it becomes a heater (stand ouside a window air conditior to feel this effect). Think of it as a climate control device, not just a heater or cooling device.
Many vans are vented, they have one, or two, adjustable vents in the front and rear of the trailers walls. Natural convection, aided by the movement of the truck, cools the interior. I’ve hauld lots of onions and melons in my van. Onions are tricky as they generate heat and can spoil quickly. On the other hand feezing them is also a danger. I used to p/u onions in Boise every December and haul them to San Diego, or L.A. I would close the vents until I got well down into northern Cal. and then open them for the rest of the trip. I also had to schedule my sleep breaks so that the loads didn’t get to hot or cold. I would load melons in the Imperial valley of southern Cal. and drive into the mtns. of northern Cal. where, by that time, it was late evening, before I slept. It was basically the opposite coming south w/ the onions.
Modern refrigerated units are, in fact, heat pumps and can maintain temps. by heating or cooling.
Refrigeration doesn’t create cooling, it removes heat, so it fairly easy to design a unit that can be reversed.
Short and simple answer: by making the cold air (actually the refrigerant) warmer through a reversable process (compression), cooling it while it’s in the “warm” state, then allowing it to re-expand to its original state, except now it has less heat energy and is colder. Depending on which end of the process is used, you can either heat or cool.