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Immersion Heater Question

by anolan23
Tags: heater, immersion
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anolan23
#1
Jun23-14, 04:03 PM
P: 14
I have a closed 4" diameter cylindrical vessel with Volume=400in^3 filled with air at room temperature. A 3500W immersion heater with Watt Density=23.9W/in^2 will be used to heat the air. Can the air get up to 600 degrees Fahrenheit? How can I find out what temperature the air will get to?
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NascentOxygen
#2
Jun24-14, 01:19 AM
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Hi, and welcome to the Physics Forums.

You have a closed volume to which you are constantly adding heat energy? is there any way for heat to be lost from this system?
anolan23
#3
Jun24-14, 07:56 AM
P: 14
Well, heat can escape through the vessel walls. The vessel material is 316 stainless steel.

Integral
#4
Jun24-14, 09:26 AM
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Immersion Heater Question

Emerson heaters are generally designed to work in water. You may burn out your heater if you try to use it in air.

Computing final temperature of anything is non trivial given complete information. You have not given near enough info to get even a good guess.
Chestermiller
#5
Jun24-14, 09:43 AM
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To estimate the rate of heat loss from the cylinder, you need to know the heat transfer coefficient on the outside of the cylinder, from the cylinder surface to the room air. This depends on the orientation of the cylinder and also on whether there are air currents in the room. You can do better if you insulate the cylinder.

Chet
anolan23
#6
Jun24-14, 10:20 AM
P: 14
From this video:

https://www.youtube.com/watch?v=rKpodie_0ko

I was able to get a convection heat transfer coefficient of 15 W/m^2*K.

What can I do? I've attached a photo of some of my work.
Attached Thumbnails
photo.JPG  
anolan23
#7
Jun24-14, 10:21 AM
P: 14
I was told a watt density under 30W/in^2 shouldn't cause any problems with burnout
Chestermiller
#8
Jun24-14, 01:12 PM
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The equation to work with is:

hA(T-Tair)=Q

where h is the convective heat transfer coefficient, T is the final temperature, A is the surface area of the cylinder, Tair is the room air temperature, and Q is the rate of heating supplied by the immersion heater (W/sec). This doesn't say anything about how long it will take for the system to reach that temperature. Make sure everything is in consistent units.

Chet
anolan23
#9
Jun24-14, 03:35 PM
P: 14
This is Newton's Law of cooling...based on my work from my photo attachment is this the Qout?
anolan23
#10
Jun24-14, 03:57 PM
P: 14
Quote Quote by Chestermiller View Post
and Q is the rate of heating supplied by the immersion heater (W/sec).
Chet
Do you mean just Watts instead of W/sec? Units of Q are in Joules and then Qdot is J/sec
NascentOxygen
#11
Jun24-14, 07:06 PM
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Quote Quote by anolan23 View Post
I was told a watt density under 30W/in^2 shouldn't cause any problems with burnout
If the heater is valuable, you might consider first testing it with reduced voltages to the heater element, afterwards inspecting for signs of heat intolerance. A VARIAC would be handy for this, but make sure you are not supplying reduced voltage to any control circuitry associated with the heater.
Chestermiller
#12
Jun24-14, 08:04 PM
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Quote Quote by anolan23 View Post
Do you mean just Watts instead of W/sec? Units of Q are in Joules and then Qdot is J/sec
Yes.
Chestermiller
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Jun24-14, 08:06 PM
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Quote Quote by anolan23 View Post
This is Newton's Law of cooling...based on my work from my photo attachment is this the Qout?
At steady state, the rate that the heater supplies heat is equal to the rate that heat is lost to the surrounding air.

Chet
anolan23
#14
Jun24-14, 10:18 PM
P: 14
I want to heat the air to 600F so there has to be some sort of energy accumulation. Wouldn't this mean the heat rate lost to air would be less than heat rate supplied by the heater? Therefore, not a steady state problem?
NascentOxygen
#15
Jun24-14, 10:39 PM
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Quote Quote by anolan23 View Post
I want to heat the air to 600F so there has to be some sort of energy accumulation. Wouldn't this mean the heat rate lost to air would be less than heat rate supplied by the heater? Therefore, not a steady state problem?
Sure! But it does mean it will be a steady state once the contents reach 600F!
Chestermiller
#16
Jun24-14, 10:41 PM
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Quote Quote by anolan23 View Post
I want to heat the air to 600F so there has to be some sort of energy accumulation. Wouldn't this mean the heat rate lost to air would be less than heat rate supplied by the heater? Therefore, not a steady state problem?
It depends. As the temperature rises, the rate of heat loss to the room increases until the system eventually reaches steady state. Do you want the 600F to be the final steady state temperature, or do you want to reach 600 F, and then have the possibility to get even higher if you continue supplying heat. The first thing to do is to see if the system can even reach 600F, or whether the steady state temperature would be below 600F, and you could never reach 600 F. So, the first thing to do is to determine what the steady state temperature would be.

Chet
anolan23
#17
Jun25-14, 08:44 AM
P: 14
Aha! thanks for all your help
Jobrag
#18
Jun26-14, 11:04 AM
P: 478
Remember that the immersion heater in a water tank will,probably be sitting at a max of 180 F (ish), if the element is in air at greater temperatures the wiring will reach the same temperature as the element so make sure your insulation and fittings are good for high temperatures, and or provide a way of cooling your wiring. I suspect that the element will burn out well before you reach your required temperature, but it's probably a good idea to check the data sheets for max operating temp. Please have your electrical safety devices checked prior to running this, it might be a good idea to check the max working temp of any fixtures and fittings you are using. BTW what are you trying to make/demonstrate?


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