Forced Convection-Watt Cals
Forced Convection-Watt Cals
(OP)
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..
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..





RE: Forced Convection-Watt Cals
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.
RE: Forced Convection-Watt Cals
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?
RE: Forced Convection-Watt Cals
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.
RE: Forced Convection-Watt Cals
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.
RE: Forced Convection-Watt Cals
RE: Forced Convection-Watt Cals
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?
RE: Forced Convection-Watt Cals
RE: Forced Convection-Watt Cals
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://ww
RE: Forced Convection-Watt Cals
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
RE: Forced Convection-Watt Cals