Heating a large volume of air, heat energy vs power

[jt783]

I want to heat a large volume of air in a well insulated box. I did some research but am unsure about how to calculate the time it would take as well as heat energy vs power. I want to raise the temperature from 20C-50C. Once it reached 50C, a PID will maintain the temperature with a damper.

mass of air = 50kg
cp @ 50c = 1007.4
DT = 30C
Q = 1511100 J?

If heat the box with an electric heater that supplies 2000W, do I just divide for time?

t = 1511100/2000 = 755.55s?

I am also confused on the units. using the Q=mcDT equation, heat is given to me in joules? However, for example, the heat specifications from a cpu are given in watts. What is the difference?

 

[moltenmetal]

Heat is in units of energy (joules). Power is in units of energy flow per unit time (joules per second, i.e. watts).

 

[prex]

You need to account for the heat loss to and through the walls of the container during the heat up. Also you'll likely need to move the air if natural convection isn't sufficient.
Q is an amount of heat or energy and is in joules, and the heat provided by a heater or a loss through the walls is expressed per unit time and is a power : if you can't clearly see the difference, I'm afraid you'll need to come back to a basic textbook.

prex
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[LittleInch]

So this is a box approx. 3.5m cube. = 50 kg air

If you stuck a 2kW fan heater in there to mix the air around then yes, 13 minutes to gain 30 C sounds about right. This works because the heating elements in the heater will be much hotter than 50C so the difference in temperature won't affect the heat transfer rate to any significant degree. It also assumes heat transfer out is negligible.

In reality there will be increased losses and the heat from the heater may get less efficient as temperature rises, but maybe 15 minutes??

As said above watts are Joules per second, but then you figured that out with your equation above to find time t.



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[chicopee]

Eventho. the box wall is well insulated, the wall itself will absorb heat as well as the floor, the ceiling and door. Sure the time element of 13 minutes seems reasonable, expect your time to increase perhaps three folds.

 

[LittleInch]

I agree, but does depend on what the box is made of and how thick. Could easily add significant heat amounts to the total heat required unless your insulation is on the inside?



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.