Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

  • Congratulations IDS on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

BTU usage on a Semi-Trailer?

Status
Not open for further replies.

PurplePenguin

Computer
Jul 22, 2004
5
The job I work at has some Semi-Truck trailers that can be deployed and used as temporary office/computer rooms in Disaster/Rental Situations.

What I am tasked with doing is finding out how much of our current A/C units is being used just to cool our trailers that way I can take the BTUh left to figure out how much equipment/people we can put inside.

I just picked our largest trailer to start with because it is the newest and we have the most information about it (we acquired these during a acquisition of another company and a lot of the paperwork was lost).

I've looked at our current equipment and on this trailer we have 3 each.. Carrier 40QKE048-3 & 38QR036C-5 which equates to over 9-Tons of cooling.

My problem is finding out how much cooling is used to keep our trailer cool once it is at a stable temperature ~70F. The outside temperature is going to be changing because these can be deployed anywhere in the country.

I was wondering if there is some formula that works by taking the room/trailer down to 70F and then shutting down the A/C units for a hour and measuring the temperature and some how get BTUh.

I'd be more then happy to get as much information as you might need to help me figure this out because I need to get this done by week end.

I do need any formulas used because this is going to be applied to 7 trailers each having different sizes and characteristics.
 
Replies continue below

Recommended for you

It sounds like you are looking for the "UA" factor, the overall heat transfer through the envelope.

The rigorous way to determine this experimentally requires a controlled climate chamber.

Put a known amount of heat input on the inside. Hold the outside temperature constant. Wait for equilibrium.

BTU/hr = UA x delta T x some conversion constants

If you are in a location where the outdoor temperature stays reasonably constant, you could do this outdoors overnight.
 
Would this have to be done overnight or could it be done in a matter of hours..

I'm here in College Station, Tx ~ in the middle of Houston, Auston, & D/FW.

Could you define "delta T" and the conversion constrants?

I could easily put a known amount of heat in the inside of the trailer but if it's 90+ outside right now would it be easier to have the heat on the outside?

Or... Take the trailer to outside temperature.. and measure the heat rise above ambient after a given amount of time?

I need a complete formula so I can give this a shot..

Alittle extra information...

This is basically a glass box with a plywood roof. 0 (zero) insulation other then a wind break.
 
PP,

To be accurate you need to reach a steady-state equi;ibrium condition.

Delta T is the temperature difference between inside and outside.

Conversion constants are to convert your measured heat input to BTU/hr. If you are using electric heater this will likely be from watts, so 3.41.

The heat input to the system must be known (measured) and constant. Can't rely on mother nature to provide either of those conditions.

If your geometry is really simple it should be easy to calculate. See ASHRAE fundamentals, or most any engineering handbook for method.
 
Status
Not open for further replies.

Part and Inventory Search

Sponsor