BTU/hr calculation

[denso1985]

I have a small container that I am looking to purchase a heater for that I can adjust the temp. Can someone help me with calculation for required BTU/hr? Details below.

Surface Area: 9.388 ft2
Room Temp: 70F
Desired temp in container: up to 750F
Well insulated.


Mcmaster Carr says this but it doesn't make sense (unit wise) to me. It would be saying ft2*F=BTU/hr????

Step 1: Determine the surface area of your floor, walls, and ceiling in square feet:
(2 x length x width)+(2 x length x height)+(2 x width x height)
Step 2: Estimate your heat loss factor by choosing the description that best fits your building:
Very well sealed and insulated = .25
Well sealed, but not insulated = .75
Not well sealed or insulated = 1.25
Step 3: Decide how much you want the temperature to rise in ° F:
If you don't currently have interior heat this would be the difference between the outside temperature and your desired temperature. If adding to existing heat this would be the difference between your current temperature and your desired temperature.
Step 4: Multiply the results from steps 1-3 for your estimated Btu/hr. requirement. You may need to consider more than one heater to meet your total requirement.

 

[BronYrAur]

The McMastar Carr factor is why your units are off. They are reporting overall heat transfer coefficients or "U-values." The units are BTU/HR/SQ/F.

So the equation works out to BTU/HR

It looks like you will need 1,596 BTU/Hr

 

[pmover]

based on the desired temp stated (750°F), you will likely need a heater & container made of materials that meet/exceed that temp in normal operating conditions. furthermore, the geometry of the container is not clear (cylinder or rectangle - although you reference rectangular). frankly, the reference material (mcmaster carr) is definitely questionable in this case in determining the amount of energy required.

perhaps a little further explanation of the situation will result is better responses.

good luck!
-pmover

 

[denso1985]

ok, that makes much more sense. They are just using: Q=hA(T2-T1) and h is .25 in my case. Although, I hate the estimation.

What I have is a rectangular box with a smaller 2.5 in. diameter tube attached (about 3 ft. long). The air will be blown into the rectangular box through the tube and out to the room. I'm not sure what material I will use yet. My desired temp. will most likely not be that high and I think will be too costly to make it to those standards. We will most likely blow only warm air though there to get a rough estimate of what is happening. (maybe 120F).

 

[BronYrAur]

If you are continuously blowing air into the box and it is escaping out into the room, then your load is probably more a function of heating the entering air rather than heat loss through the box wall. The BTU/Hr required to heat your air is (CFM)*(1.08)*(Delta-T), where Delta-T is your operating temp minus the entering air (room temp). I would preheat this air to the desired temperature prior to introducing it into the box, rather than trying to heat the box itself. This will work if the airflow is adaquate --- now define adaquate??? I don't know, but if the airflow is large enough to change the air in the box several times per minute, then I would bet you are OK. What is the volume of the box (cubic feet) and what airflow (cfm) are you thinking about?

 

[denso1985]

maximum of 105 cfm airflow
box volume is about 2880in3

what is the 1.08 in your equation?

 

[IRstuff]

What exactly are you trying to accomplish?
How fast is the air moving?

Your original posting implied something totally different than your last post is apparently saying.

TTFN

 

[BronYrAur]

The 1.08 is air density times specific heat. Typ values are density of 0.075 lb/ft^3 and specific heat of 0.24 Btu/lb/degF. You must also multiply by 60 min/hr to get the units to work out.

It looks like you have 1.67 Ft^3 of volume and you are moving 105 CFM. That gives you 63 air changes per minute. I would say that regardless of your box material or insulation, if you introduce air at the desired temperature, you will be able to hold that temperature and conduct your tests.

Now to IRstuff's point, I still don't know what you are trying to accomplish either.

 

[denso1985]

sorry if im not being really clear here. what im trying to do is evolving as i go along.

basically what the system is trying to do is simulate the exhaust of a car. The box is the exhaust manifold and the pipe is the exhaust pipe. Heat affects our results (im not sure how much) so thats why i am looking into a heater. my initial thought was "lets get real temps" in the pipe and now I don't want the high costs of trying to acheive those results so i want maybe 120F. I would like to use plexiglass for the box now which can handle temps up to 250F I believe.

 

[BronYrAur]

This may sound crazy, but what about a hair dryer or one of those bathroom hand dryers? I don't know their CFM, but I bet they would give you enough heat.

 

[IRstuff]

Simulate for what purpose or measurement? "Well insulated" is not a descriptor that I would apply to an exhaust system.

There are commercial heat guns used for shrinking shrink tube that should get plenty hot:

http://www.uline.com/ProductDetail.asp?model=H-491

Are you simulating the pulsations, the gas content?

TTFN

 

[pmover]

denso1985,

IRStuff has requested clarification.

fyi, when unclear, vague, and/or ambiguous data is provided, the results are just the same. as you can note in the responses, they vary.

i'm considering in making a notification to have this posting removed; and request that you start anew (that is, be a little more explicit to what the situation is). in fact, i will make the notification.

please make future postings a little more clearer and concise so as to receive concise and clear responses.

meanwhile, good luck!
-pmover