# What's the round tank suspended beneath the trailor on some tractor-trailors for? **URL:** \ **Category:** Factual Questions\ **Created:** [June 4, 2005, 2:01am UTC](https://boards.straightdope.com/t/whats-the-round-tank-suspended-beneath-the-trailor-on-some-tractor-trailors-for/306822 "2005-06-04T02:01:16Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![minor7flat5](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/minor7flat5/32/258_2.png) [@minor7flat5](https://boards.straightdope.com/u/minor7flat5)\ **Post date:** [June 4, 2005, 2:01am UTC](https://boards.straightdope.com/t/whats-the-round-tank-suspended-beneath-the-trailor-on-some-tractor-trailors-for/306822/1 "2005-06-04T02:01:16Z")
Short and to the point: While in traffic today, I spotted a tractor-trailer by my side that had a silver tank mounted athwartships directly behind the landing gear of the trailer. There was a small gauge in the center of the end. It was fairly big, almost as big as a 55 gallon drum, but it certainly wasn’t the truck’s fuel tank (featured prominently below the driver’s door). ---
**Author:** ![danceswithcats](https://avatars.discourse-cdn.com/v4/letter/d/9e8a1a/32.png) [@danceswithcats](https://boards.straightdope.com/u/danceswithcats)\ **Post date:** [June 4, 2005, 2:03am UTC](https://boards.straightdope.com/t/whats-the-round-tank-suspended-beneath-the-trailor-on-some-tractor-trailors-for/306822/2 "2005-06-04T02:03:30Z")
Probably a fuel tank for the reefer unit located on the trailer head end. ---
**Author:** ![minor7flat5](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/minor7flat5/32/258_2.png) [@minor7flat5](https://boards.straightdope.com/u/minor7flat5)\ **Post date:** [June 4, 2005, 2:27am UTC](https://boards.straightdope.com/t/whats-the-round-tank-suspended-beneath-the-trailor-on-some-tractor-trailors-for/306822/3 "2005-06-04T02:27:05Z")
I did consider that, and I verified that the truck did have a reefer unit. Just looking for verification – the tank seemed pretty darned big to be for that, and I guess I have always figured that those units would be run off of the truck’s power (perhaps a naive assumption). The gauge looked like a pressure gauge, set into the center of the tank’s end. ---
**Author:** ![danceswithcats](https://avatars.discourse-cdn.com/v4/letter/d/9e8a1a/32.png) [@danceswithcats](https://boards.straightdope.com/u/danceswithcats)\ **Post date:** [June 4, 2005, 2:31am UTC](https://boards.straightdope.com/t/whats-the-round-tank-suspended-beneath-the-trailor-on-some-tractor-trailors-for/306822/4 "2005-06-04T02:31:55Z")
No. The reefer is an independent unit. It stays/keeps cool on it’s own. ---
**Author:** ![Nanoda](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Nanoda](https://boards.straightdope.com/u/Nanoda)\ **Post date:** [June 4, 2005, 4:06am UTC](https://boards.straightdope.com/t/whats-the-round-tank-suspended-beneath-the-trailor-on-some-tractor-trailors-for/306822/5 "2005-06-04T04:06:59Z")
Well now _I_ need to know what a ‘reefer unit’ is. ---
**Author:** ![danceswithcats](https://avatars.discourse-cdn.com/v4/letter/d/9e8a1a/32.png) [@danceswithcats](https://boards.straightdope.com/u/danceswithcats)\ **Post date:** [June 4, 2005, 4:39am UTC](https://boards.straightdope.com/t/whats-the-round-tank-suspended-beneath-the-trailor-on-some-tractor-trailors-for/306822/6 "2005-06-04T04:39:48Z")
Reefer is slang for refrigeration as in an insulated trailer with an integral chiller to keep perishables in good order while in transit. ---
**Author:** ![ParentalAdvisory](https://avatars.discourse-cdn.com/v4/letter/p/53a042/32.png) [@ParentalAdvisory](https://boards.straightdope.com/u/ParentalAdvisory)\ **Post date:** [June 4, 2005, 4:55am UTC](https://boards.straightdope.com/t/whats-the-round-tank-suspended-beneath-the-trailor-on-some-tractor-trailors-for/306822/7 "2005-06-04T04:55:21Z")
It sounds as though it could be a compressed air tank. ---
**Author:** ![whatsittoya](https://avatars.discourse-cdn.com/v4/letter/w/13edae/32.png) [@whatsittoya](https://boards.straightdope.com/u/whatsittoya)\ **Post date:** [June 4, 2005, 5:27am UTC](https://boards.straightdope.com/t/whats-the-round-tank-suspended-beneath-the-trailor-on-some-tractor-trailors-for/306822/8 "2005-06-04T05:27:13Z")
Its a reefer. the reefers I have pulled had tanks that held about 50 gallons of diesel fuel and could run the reefer for about a week without refilling (if it wasn’t august and I wasn’t carrying ice cream) The reefer is powered by a engine about the same size as you would find in a VW rabbit. and can cool down to -20 degrees F. for hauling ice cream and such or can heat up to about 85 or so in the winter if there is a need. There are larger and smaller tanks avaliable depending on need. For instance on reefers that were piggy backed (loaded on trains for part of the trip) my company had 100 gallon tanks to insure they wouldn’t run out of fuel as they made their way across the country. OTOH a reefer that never leaves town could get by on 25 gallons or less allowing more cargo to be loaded before weight restrictions kick in.