"My reefer ain't working!"
No, this doesn't mean somebody isn't getting their usual buzz from their giant joint. As LordBrawl mentioned here, reefer is layman's slang for refrigeration unit. However, I felt a lot more can be said and learned about reefers, so... here I go.
There are two major manufactures of truck refrigeration units (hereafter referred to as the more preferable reefer), ThermoKing and Carrier. ThermoKing is, as the name implies, the crème de la crème, and is preferred by drivers and repairmen persons alike. Each brand comes with its own unique features and configurations, and because there are different sized trucks, there are different sized reefers, but they all operate on basically the same premise. They are all simply an additional diesel engine attached to the "box" of the truck (not just an ordinary cargo box though, an insulated box), used to power an A/C unit, located inside the box, which provides various levels of cooling to the box. On smaller trucks, like a pickup, and in straight-bed trucks (like those big U-Hauls, the box and the cab are all on one frame) the A/C unit uses forced air (ie, a big fan)from the unit to cool the entire box. Because of its relatively small size, this does provide adequate cooling for even a 10 skid straight-bed. Larger trucks (read: big rigs, tractor trailers), are slightly more complicated. Even with a huge fan, the back of the truck would still not receive proper cooling. Thus, an air channel is used to allow better cooling to the rear. This may sound fancy, but honestly, it's not. It's more or less a heavy-ply plastic sheet riveted at various points along the ceiling of the box, which helps distribute air more evenly along the length of the box.
Many newer, more modern reefer units have a unit control panel located inside the cab of the truck. This allows the driver to continuously monitor the status and temperature of his cargo. All the units I have seen thus far have been LED based panels, 2-3 inches high and 5-6 inches wide, equipped with various buttons to control the temperature of the box, and fan speed, and of course, the all important on/off button (If it doesn't work, reboot!) Older models, or in spaced limited cabs, the control units will be placed outside the cab of the truck, or worse yet, the unit will be controlled with toggle switches and buttons located directly on the unit. Both of these options are useless, in my eyes, because they require the driver to stop the truck to monitor and adjust any settings. And, because all delivery men are lazy, this is not something one likes to do very often. Also, many units can be hooked up to an electric supply. This is most often used when the truck is left loaded overnight. While you can leave it run on diesel power during this time, it's generally more economical to use electricity. Naturally, like most things nowadays, there is no standard for this, and many units by the same brand require different plugs and different power input levels.
So, how cold can a reefer get? The unit on the truck I drive can drop the temperature inside my box to -20º F / -29º C.
Frozen dairy products, like ice cream, are usually shipped at -10º F / -23º C.
Frozen pre-processed meats are generally stored and shipped at -15º F / -26º C, although there are various reasons why this isn't always so.
Meat, such as the shoulder of a cow or freshly ground beef, can be shipped and delivered at 20 to 40º F / -7 to 5º C. Basically, if it's meat, and it hasn't been frozen yet, mild refrigeration is all that is needed.
Ice, which is my specialty... doesn't really matter. You can keep the temperature at 32º F / 0º C and you'll be fine, or even let it hover around 34 to39º F/ 1 to 4º C on a moderately cold day. During the summer time, however, to help minimize the effects of the thaw/freeze cycle and thus being stuck with a truckload of ice bricks instead of ice bags, a steady temperature of 15 to 20º F / -10 to -7º C is necessary. After all, nobody wants to deliver a brick, but more importantly, nobody wants to take one home and stick it in the freezer.
Vegetables and some fruits are shipped in a refrigerated state. Since you probably don't want to take home two frozen tomatoes, these products are generally shipped at about the temperature your refrigerator at home runs, around 40º F / 5º C.
Speaking from my miserable experience, there is one major advantage to having a job driving a reefer'ed truck around. Namely, you never have to worry about the status of your lovely salad or jug full of water. If it's "in the back," it's gonna be cold.
Of course, with the good comes the bad, and reefers tend to be very loud, especially when they sit right over your head (like mine does).
Bananas seem to have a liking for being inside a reefer'ed environment. They come out tasting better than ever after 1-2 hours of 15º temperature.
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