# Immersion Heater Question

by anolan23
Tags: heater, immersion
 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?
 HW Helper Thanks P: 5,359 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?
 P: 14 Well, heat can escape through the vessel walls. The vessel material is 316 stainless steel.
 Mentor P: 7,320 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.
 Sci Advisor HW Helper Thanks PF Gold P: 5,258 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
 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
 P: 14 I was told a watt density under 30W/in^2 shouldn't cause any problems with burnout
 Sci Advisor HW Helper Thanks PF Gold P: 5,258 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
 P: 14 This is Newton's Law of cooling...based on my work from my photo attachment is this the Qout?
P: 14
 Quote by Chestermiller 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
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P: 5,359
 Quote by anolan23 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.
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PF Gold
P: 5,258
 Quote by anolan23 Do you mean just Watts instead of W/sec? Units of Q are in Joules and then Qdot is J/sec
Yes.
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PF Gold
P: 5,258
 Quote by anolan23 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
 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?
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P: 5,359
 Quote by anolan23 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!