Continuous Heating (Spray or Deluge)

[favourablejnr]

Hello,

Before I begin, I am a manufacturing engineer. I am providing some assistance to a friend to redesign his operations. I am not proficient in thermodynamics, fluid mechanics or heat transfer (but I can re-read my old texts, etc.). In its most basic sense, I am attempting to redesign a batch processing system to a continuous processing system.

Here is the issue: I am trying to design a system that applies hot water (90 degC) to an aluminium object (c.c. 2kg with dimensions 500mm x 400mm x 50mm - this object is not solid) initially at ambient temperature so that the aluminium object is heated to 90 degC.

A simple solution is to submerge the aluminium object in a bath of heated water, then remove (current batch process). I need the heating process to be continuous and envisage the aluminium objects traversing a conveying system. Above the traversing objects is a system that either sprays or deluges the passing objects with the hot water.

Imagine a 4m long conveyor, travelling at 2 m/min on which the objects are placed. The objective is to infeed the ambient temperature objects and outfeed the objects at 90 degC. The 4m length is a preliminary guess based on initial experimentation that requires the objects to be submerged in hot water (90 degC) for 120 secs. I am interpreting the required submersion time as time required for the object to be in contact with the hot water.

Though 120 secs exceeds the calculable time for heating an aluminium object of this mass from ambient to 90 degc, it is relevant for this task due to other considerations (i.e the objects are covered in contaminants.)

I do not know how to determine the heat loss for the proposed water application system. I expect that it will be far less efficient than submersion. There is information available for spray cooling (which I have yet to review in any detail), but I have not readily found information about spray heating or the temperature effects of a deluge system.

There may be some parallels with fire protection systems designs.

I am expecting that the conveyor length will need to be greater than the simple calculation used to determine the length suggested above, due to the (expected) inefficiency of heat transfer from the hot water manifolds (maybe we could call them taps) compared to submersion.

If a manifold could be efficiently designed that provided flowing hot water (90 degC) over an area of 4000mm x 500mm, then I would be confident that each object is getting hot water applied to it for 120 secs. My preference is to minimise the manifold size (i.e. vastly less than 4000mm x 500mm).

The idea is that the water will be recycled (i.e. collected after application, cleaned, re-heated and returned to the manifold arrangement).

The water flow rate is also an open question, and may be intimately connected with the heat transfer question here.

Any pointers, guidance or suggestions will be appreciated.

Thanks, Alex

 

[IRstuff]

" I am interpreting the required submersion time as time required for the object to be in contact with the hot water."

That's not quite true, since anything other than complete immersion will result in the water temperature changing as it's being applied, and the part radiating and convecting all the while. Note also that 90C water will be vigorously evaporating from the item, further cooling the item.

But, what's the air temperature, and what's the mass of the item?

Can you not simply run the belt into a vat of 90C water and back out? That would seem to be the closest to what you're doing now, and is the approach most likely to have the fewest problems.

TTFN
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[MintJulep]

I agree with IRStuff.

If 120 seconds completely immersed is working for you now then design a conveying system with a 120 travel time through a bath.

Alternately, design a conveyor to travel through an oven and spray water on it. This will offset the bulk of the other problems the IRStuff properly noted.

A few tests will get you a more accurate answer with greater confidence than trying to figure this out analytically.

 

[LittleInch]

You can calculate the heat required from mass times delta T times heat capacity of Aluminium. From this you can get some idea of the volume of water required assuming 100% transfer of heat from the water to the object. If you think that the water will only decrease in temperature on average by a few degrees (say 5, but also try 2) then you'll get a first order number of what it is going to take in terms of volume of water sprayed onto each item to meet that total heat requirement. Clearly the first part is going to decrease more than the second due to the heating up of your object. I think this will help you see how much liquid you're going to need per item. you can then add some inefficiency for some water not impacting the item or vapourising, though I suspect you'll need an enclosed area which will quickly turn into a steam room.... Actually have you considered a steam room instead?

How you transfer the heat though is a different matter. Can you suspend this thing from a single point otherwise the bottom bit resting on the conveyor will take longer to heat up than the top bit getting deluged with water which is not the same as immersing it in your bath I suspect. A close set of spray nozzles or deluge nozzles would seem better, but not sure how complex your object shape is.

I suspect you will need to experiment with a single stationary object and pour 90C water over it continuously at different flow rates and time how long it takes to heat up...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way