Forced Convection-Watt Cals 1

[thermoD]

Hi Guys, I am new to the forum and I am a fresh engineer as well, I came up with some calculations and want someone to verify whether I am on the right track or not.

I want to estimate the wattage required of an electric heater,the heater will be used to heat up air from 700F to 1100F, Air will be fed to this electric heater through a blower at 20,000 CFM..
I used Q= m Cp dT
m= 20000*.0283*1.2/60 = 11.32 Kg I assume Air density=1.2kg/m^3

Cp=1.0 KJ/Kg degC

dT= 400F = 222C
Q= 2513 KJ/s= 2513 KW

Does that mean if we use a 500KW generator" to operate this heater", at 50% capacity it would take 10 seconds to heat up 11.32Kg of air..i.e, would 500 KW generator be powerful enough for this situation?
Does the air speed have any effect at all?
Thanks in advance..

 

[magicme]

thermoD

the air is only heated while it's in contact with the heating element.
at 20,000 cuft/min = 333 cuft/sec * 10 seconds = 3330 cuft.

that's a cube about 15 ft on a side (IF it's a cube)..... and that does not include the volume of the heating coils.

is that about how big this heater is?

and is the heating element a labyrinth that the air has to tumble through for 10 seconds?





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there's no place like gnome.

 

[thermoD]

Magicme, The heater could be 1200 to 1300 Cuft, the heating elements will be coils, but you're right about my calculations if it would take 10 sec to heat up 3330 Cuft, I am actually reducing my CFM to 2000 not 20,000..

The point here is I want to heat up this 333 Cuft/Sec but the KW is killing me..(I am limited to the 500KW generator and asked to opeate it at 50% load)..
do you have any suggestions?

 

[dvd]

Can you use gas burners? They may prove to be more effective.

Otherwise, contact a heating element company to get their input, don't re-invent the resistance air heater. I like to work with Chromalox. You can find a little more information on air heating at:

http://www.chromalox.com/technical/pdf/en/gas.pdf

you should rummage around on their website to see the information they have available.

 

[magicme]

i agree with dvd .... get a reality check from the hardware people.

but, based on the numbers, i don't think you can get there from here...

assuming a volume of 1250cuft

time(sec) = 1250*60/CFM

mass = 1.2 * 1250 * .0283 = 42.5 kg (inside the box)

Q = 250 Kw = (mass / time) * Cp * DeltaT
= 42.5 * CFM * 1.0 * DeltaT / 1250 / 60

which says DeltaT = 441000. / CFM

also

CFM = 441000 / Delta A, so if you need DeltaT = 222, the max CFM would be... 2000, as you said in your previous message.

regards,

magicme




at 20,000 CFM, DeltaT = 22 C




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there's no place like gnome.

 

[MintJulep]

The density of air at your temperatures will be less than half of the 1.2 kg/m^3 you are using in the calculation

 

[thermoD]

Thank you all for these valuable info..If the density of the air will be less than half so that mean the KW is reduced at least by half..I didn't know how to get the density at Diff. pressure and temperatures..Is there tables or something to figure out that?

the bottom line is I need 1250KW (if the air density is .6), and if I can't get a generator to generate this energy, I should be looking at a chemical reaction process..or maybe I can use both electric and gas burning.

The gas burner is an option, but mgmt wants to seek the elecrtic option first, do you have any experience or information about the Arc furnace technology?

 

[dvd]

Make sure that you check-in with your boss from time to time. Sometimes projects are assigned to new people so that they can learn from their mistakes, and sometimes your boss or co-workers will only help you if you ask.

 

[MintJulep]

Do you think an arc furnace will somehow use less power to provide the same power?

If you need 1.2 MW and you only have 250 kW to work with this is what is called a fundamental problem.

1.2 MW is really pretty modest for a gas burner.

http://www.eclipsenet.com/products/products.aspx?pid=159487

 

[thermoD]

Thanks for the fast response..In my conception design, we need to heat up the air from 70F(ambient temperature) to 1200F at very high speed..To do that we need the fan to blow air to an electric heater or gas burner at low rpm (to allow time for heat transfer) and then recycle the hot air back to the heater..once the discharged hot air from the heater reaches 1200F, then the fan will blow air at least 2500 or 3000 rpm..this hot air will be used to heat up asphalt with high speed (50-60 mph), also, we will use a heat recovery system to reuse the return hot air into the heater(to save the KW)..initial estimate shows the return hot air will be at 700F or more , so we will do a lot of analysis for the air flow and pressure drop-that’s critical to this system)..
This heating system will be on mobile truck and the estimated elevation is between 30-36”..The fan will work steady at 2500rpm we just need it to work at low rpm during the warm-up cycles (should be 2-5 mins before reaching 1200F)..As for pressure, I am assuming low to medium pressure range..
No from my initial calculations, if I design the dischage holes or slots at the bottom of air chamber (see attached) in a way that would have the venturi effect, I found out a diameter of 1" will dischage air at 75mph..does anybody have any suggestion about what I should be careful with in this design? i.e, I am Worried about this high velocity and would that affect the temperature significantly and will I have negative pressure?
I don't need any precise numbers but just in general, any red flags here..
Thanks in advance

 

[dvd]

I think you are going to have a hard time with dirty air being sucked back through your electric heater arrangement and ductwork. I also think that you aren't going to have as much velocity after 36 inches of throw. A long single slot, or long multiple slots would be better, but you don't really state your objective other than high velocity. I think that recycling hot asphalt smoke is going to cause you grief. Gas burners seem like your best bet. If you could take advantage of the radiant heat from burner flames it would be good. With the proper burner arrangement, I'm thinking Linoflame from Maxon, you could possibly eliminate the fan and just have a combustion air blower operating with clean air for the burner. You really need to define what you are doing and contact a sales rep.