temperature calculation inside purged enclousure 3

[George1979]

My background isn't strictly engineering so I'm having some difficulties in calculating the temperature inside of a purged box - I would really appreciate your help.
Quick description: Enclosure of dimensions 3x3m has around 30m of 1" piping which is around 250 degC hot. It will be purged with room temperature air at around 20 l/min. What I would like to calculate is the temp inside of the box (ideally being able to incorporate different thicknesses and R-values for the insulation - since I still don't know what material will be used - does 1 1/2" with R=12 sound reasonable?). I purposely omitted few elements like pump or tank -described simplified calculation would be sufficient for me.

Thank you very much for your help in advance.

Regards,
George

 

[IRstuff]

(3m)^3/20L/min = 22.5 hr, so air flow is essentially nonexistent. So, what is the purpose of the purge; is it to cool the box? If so:

> The interior air will be EXTREMELY hot, which is a problem for maintenance
> The hot interior is going to kill the reliability of any components in the box
> Since the flow is is low, you need to use the lowest convection coefficient for air, which is around 2.5 W/m^2-K, but the air temperature will be high, so the temperature difference will be low.

The overall problem is in a few pieces:
> Transfer of heat from the fluid to the pipe
> Transfer of heat through the pipe wall
> Transfer of heat from the pipe wall to the air
> Transfer of heat from the air to the enclosure walls
> Transfer of heat through the walls
> Transfer of heat from the walls to the outside air

Some of the "through" heat transfers might not be required, if the walls are thin, or the wall is highly conductive.

The basic approach is to set up a bunch of serially related temperatures, starting with the fluid temp, pipe temp, inside air temp, etc. You plop all the equations into a numerical solver and have at it. All of the equations will have the form of h*A*del_T = heat_flow. The heat flow must be identical to satisfy continuity.

Since the temperature is so high, radiation should be included, but that's an added complication.

TTFN

FAQ731-376

 

[George1979]

Thank you very much for your post - it's true I didn't think that carefully enough. I just took the flowrate value as an example - this could be easily modified and increased - could you help me establish what should that flowrate be to bring the temp to a sensible level?
Another question would be shouldn't the insulation on the pipes be a helping factor(using some sort of material with extremely low thermal conductivity - lets say aerogel)?

Thank you once more for your help.

Regards,
George

 

[Bribyk]

Do you know the flowrate, fluid (and phases) and inlet and outlet temperatures of the pipes?

If so, the sensible heat being added to the space, assuming no phase change of the fluid, is:

Qin = mdot * C * deltaT , where:
Qin is the heat being added to the space per time (power),
mdot is the mass flow rate
C is the heat capacity of the fluid, and,
deltaT is the difference between the inlet and outlet temperatures.

Then, all you have to do is rearrange the same equation for the purge air to find mass flow rate by making the Qin's the same. Your deltaT in that case will be the desired temperature inside the box minus the temperature of the air being drawn/forced in.

To be thorough, you should add in the power consumption of any pumps and/or equipment inside the box. Also, you will have a bit of a safety factor because the heat lost by conduction/radiation through the walls is not accounted for.

It is probably going to be easier to approach this problem using the "global" approach outlined above, than to go through all of the heat transfer calcs.

If the required air flow rate is too high, use the maximum feasible value (may be determined by worker comfort with the wind created) to determine the maximum heat addition allowable and then figure out how much insulation will be required on the piping to achieve this.

Also, if you want to retain this heat (i.e. it's not a waste product and can't feasibly be used for other purposes) you should insulate the piping to reduce the operating cost of heat lost.

 

[IRstuff]

OK, so you want the fluid to stay hot, then? Then, yes, hypothetically, 1-in thick aerogel would result in about 160 W of worst-case heat loss from the total area of the piping. --> 30 m * 3in*pi * 0.1 W/m^2-K * 220K = 160W

Volumetric air heat capacity is roughly 1.3J/L*K. Assuming that you don't want excruciatingly hot exhaust air, so say no more than 15ºC rise, with 160 W of waste heat, you'd need a minimum of 500 L/min to maintain the air temperature. --> 160 W / [(1.28 J/L-K) * 15K] = 500L/min

Very rough numbers, since it assumes perfect mixing in the air.

TTFN

FAQ731-376

 

[George1979]

Thanks again guys - I wasn't really clear with my description I suppose. Yes I wanted the fluid to stay hot and at the same time I wanted the temperature inside the enclosure to stay as cool as possible by pumping air(to achieve good operating conditions for some of the equipment). I don't need to worry about the extracted air temperature at all. I'm slowly getting there - much obliged.

 

[IRstuff]

You personally might not worry about the exhaust temp, but unless the box is on the Moon, or something like that, some human being can come into contact with it, and there's a bunch of safety related things that any such box must meet.

The outer surface needs to be less than about 55ºC or so to prevent contact burns, etc.

Many types of hardware lose 50% of their useful life for every 10ºC increase in temperature rise.

TTFN

FAQ731-376

 

[chicopee]

The whole problem with your original post is terminology or semantics and the ability to describe precisely the problems and the ojective.
When you mean purge, this is not really what you want to do as in purging to remove contaminants. What you want to do is to ventilate the enclosure at 20 l/min of makeup air. You have not described the temperature of the make up air which you need in order to determine some confortable temperature level within the enclosure.
The two dimensions of the enclosure did not provide an idea as to its volume. We could only guess that it was 30 x 30 x 30. Furthermore, if you go metric on the units stay that way unlike the pipe size description.
The problem is relatively easy to solve but with your original post it is impossible to do. I hope that you communicate a lot better at work.

 

[George1979]

True - shouldn't post on the Saturday evening - plus English is not my first language - so sorry for that. The box is a cube 3x3x3, pipe has a 19mm OD and inlet air temperature is a room temperature 21 degC - in addition the flow can be adjusted to smth higher than 20l/min.
As far as the exhaust air it will get extracted - so it should be safe and during operation the enclosure will remain fairly closed - so no one should get burned.
Hopefully that should be everything.

 

[chicopee]

Sorry George, I did not realize that english was not my first language, neither was it mine.

 

[IRstuff]

Obviously, your reliability requirements will drive the air flow rate up, but you still need to keep the outer surfaces of the box under 55ºC, so the interior air can't be hotter than around 60ºC-65ºC. So, that's about double the delta temperature, so the flow would only go down by a factor of 2.

TTFN

FAQ731-376