How to factor time of exposure into cooling 1

[whotmewory]

Can anyone advise how to factor in time of contact / exposure between two surfaces when estimating BTU rating for an AC unit to chill glue very fast?

I want to pass a glued bag between cooling blocks to chill the [hot fluid] glue from 245-250 deg. F down to 220 deg F. Here's the rub: my design constraints and speed requirements demand the glue be chilled with only 2-1/2 to 3 seconds of exposure to the chilling blocks.

I have been treating this like a single pass, parallel flow heat exchanger problem, but I cannot provide a known exit temperature of our cooling medium [air conditioned air] passing through manifold cooling blocks.

Any guidance from folks more "at home" with heat exchange" would be immensely helpful.

Cheers from Chris in NC NASCAR Country

 

[dalcazar]

I want to make clear that I'm not sure if I understand what you're asking from what I read...

your temperature reduction isn't that bad since you only want to cool by 15 degrees F.

First some questions to make sure we give you the right answer. What kind of bag is it? what material is it made out of? is it fat or flat? and are you trying to cool it with cold air?

 

[whotmewory]

Hi dalcazr!

The bag is polyethylene film over an inner weave (total thickness .035 inches)

The barrier separating the glued seal from the chilled air includes:
1- .070 inches of polyethylene bag (2 folds x .035 inches)
2- .006 inches SS (SS compression belt)
3- And 1/8-inch [air manifold wall] steel.

My thermal coefficient for these combined layers comes out to 29.31 BTU / (Hr Ft F). These layers are on both ides of the bag passing through.

I have confirmed that - in the 2-1/2 to 3 second pass through the "chilled compression" section of the run - our customers want the seal's temperature down to 200-degrees Fahrenheit at exit from the machine. Given the natural cooling that takes place i am estimating the "supplemental" cooling will need to bring the temp down the additional 30-50 degrees I mentioned initially.

Chris in NC NASCAR Country

 

[dalcazar]

ok, you're dealing with forced convection here, and since you have a limitless supply of air at a certain temperature you can basically treat it as a heat sink, the temp of the air exiting the system will be irrelevant.

you need to calculate how much heat needs to be dissipated over the area that the glue covers

How much heat will be dissipated is governed by the forced convection formula: heat flow Q is equal to the exposed Area A times the temperature differential time the convection heat transfer coeficient h

Q = h A (Tw-Ta)

where Tw is the temp. of the material and Ta is the temp. of the air. Ta will remain constant at all times if you have strong enough flow, so what determines your heat flow is the magnitude of the temperature differential and nothing more.

then you need to calculate the energy differential between the initial and final states of the glue, this differential is how much heat needs to be removed or your total heat flow over the 3 second period. this will be your Q, from there you can calculate what value of Ta will be needed to cool the glue since everything else is constant, including your contact time. and from your value of Ta you can calculate the BTUs of the cooling system.

I don't have access to any good thermodynamics or heat transfer books so I can't give you much else at the moment including the values of h.

are you limited to using cold air? I would use a fine mist of chilled water along with cold air to increase your heat transfer since air is a rather poor conductor of heat, it would help cool things faster in case you have a target temperature for the air that is just too cold.

 

[whotmewory]

Dalcar:

I would sure like the freedom to use a water mist here too, but I dismissed it at first because of "challenges" to the perishable product in the [poly-weave] bags. But as you [re]mention this option, considering the bags will be sealed (open bags will be about 6 or 8 feet upline), you have peaked my interest in reconsidering this.

For not having any tthermo books, you seem to have this right on the tip of your tongue. Thanks for your thorough and very helpful reply.

Chris in NC NASCAR Country

 

[dalcazar]

are you running the line thru an enclosed box to cool it? I don't see any problems at all with the use of a chilled water mist, a baffle or divider with a blower in the right position will eliminate any chance of the product contacting with water (hell even a cheap old piece of clear plastic tarp some 4 ft up the line will do it)

I just thought that you should be careful with how cold you go with the air, you don't want to make snow onto the bags.

you might get more heat transfered by using very cold dry air instead of chilled water at only 2 degrees C or something close to that. So your problem lies in figuring out if you get more heat transfer from a bigger delta T under forced convection, or from conduction into a smaller delta T into chilled water.

Another thing to mention here that I hadn't thought about before is that water does not wet Polyethylene so your contact area with the droplets will be limited and heat transfer thru conduction will decrease accordingly. another reason to check into what I mentioned before.

Come to think of it, making snow onto the bags might help more because ice crystals won't slide off the bag and they'll continue to cool the glue as they melt along down the line. I just don't know if you'd get your temperature drop over the 3 seconds.

just a few more ideas, and I'm glad to be of help

 

[whotmewory]

I KNOW, WE'RE EXPECTING TO "QUENCH" WITH ONE OF THE POOREST QUENCH MEDIUMS THERE IS.

 

[IRstuff]

I'm surprised there's this much involved. My personal experience with hot glue is that it usually cools TOO FAST. But that may just be me.

It's not clear from what you have described whether you are actually blowing air on the workpieces. If possible, you could place holes in the cooling blocks and pump air through the blocks at the workpiece. The narrowness of the gap that you are probably contemplating usually eliminates passive convection and the only heat transfer mechanism would be strict conduction of the air. Forcibly blowing cool air would seem to be a better option.

TTFN

 

[XELR8]

Try these options for improved cooling:

1) Decrease glue application temperature from 245F to say 230F before applying or determine the minimum application temp required to apply the glue. 245F may be conservative.
2) Use less glue over the same area.
3) Call your glue supplier and work with them on a solution.
4) Apply the glue through a different nozzle for slower application.

 

[sterl]

Is the air jet applied direct to the Polyfilm or is the Polyfilm in contact with the Block for 3 to 4 seconds and the air cooling the Block? Those are (2) ver different problems....

The first step is to establish the heat removed in that 3-4 second period. The mass of glue, times your estimated temperature change required gives you that number. If the Cooling Cycle represents 3 to 4 seconds out of 10, or 3-4 seconds out of 5: determines how fast you have to move heat overall, and the design capacity of your "big" cooler. It also determines if the Thermal Mass Ratio of the Glue and Bag, vs that of the jaws, is of signficance...

To make sure you can move one dose of heat in the targeted 3-4 seconds is the heat transfer topic...if air needs to be the contact cooling medium: Run compressed air through a refrigerated precooler and expand it from pressure right at your blocks...while it will take a little time to start up every shift, the Stirling effect will make +20 deg F with supercooled water along for the ride, and if this is direct injection, the jets will toss frost on the outside of your film as well.

Short of secoondary gases and barring Phase Change that's about the best you will do with a full loss, direct injection system.

Course, if you want to run liquid nitrogen...

 

[mechprocess]

This is still a transient problem even though we are talking about an isothermal cooling medium.


MCdT/dQ = UA(T - t)

The cooling time is calculated by solving the first order DE above.

dT/(T-t) = UA/MC

M = Mass of glue
C = Specific heat of glue
T = Temperature of glue
Q = time in seconds
U = combined radiation and natural convection coeff
A = Effective surface area
t = Temperature of cooling medium