heat transfer for molten wax

[schlebb]

We want to cool liquid paraffin wax from about 155F to about 125F. It will have a viscosity of around 15 cst.
We will be using a heat exchanger with a fan. I can't find any information on the heat transfer properties of wax so I can size the cooler. Normally, my calculations are for water or oil.

Can anyone help?

 

[Compositepro]

The heat transfer should be pretty similar to oil unless the paraffin freezes on the cooling surfaces. The flow rate of the paraffin as well as the cooling fluid (in your case air) and its temperature will be factors.

 

[25362]

The viscosity of wax near its congealing point may be much much higher than 15 cSt.

What type of wax are you referring to ?

 

[IRstuff]

Your paraffin melting point will limit your cooling rate, since your cooling air will need to be above the melting point to ensure that the paraffin stays liquid. Any solidification within the exchanger will fubar the process, since solid paraffing will be like an insulator.

TTFN

FAQ731-376

 

[moltenmetal]

Don't use an air cooler- you're asking for a freeze-up eventually. Simply turning off the fan or shutting louvers on a fan cooler will NOT drop the heat loss rate enough to prevent a freeze-up.

You'll very likely need a tempered heat transfer medium. Going this route is going to cost you more than a fan, but it'll be so much more reliable that you should just forget about the fan.

What are you doing with the wax after you've cooled it?

 

[schlebb]

Thanks. This really helps. I hadn't thought about the possibility of freezing at the boudaries of the heat exchanger at lower temperatures. We are trying to cool the wax just before it is dispensed into small containers (approximately 3 gallons). Currently, there is a long wait time for the wax to cool so that the open containers can be moved to complete the packaging process. The idea was to pre-cool so the dwell time wouldn't be so long.

 

[mielke]

moltenmetal, what exactly do you mean by a 'tempered heat transfer medium'?

 

[moltenmetal]

A "tempered' heat transfer system is one where the heat transfer rate is controlled by controlling the temperature of the heat transfer medium rather than by controlling its flow. By doing this, you can avoid making the heat transfer surfaces themselves too hot or too cold, avoiding problems like undesired solidification, condensation, denaturing etc. In your case you're worried about solidifying the wax leading to fouling and blockages.

Rather than removing heat directly to the air, which will always be colder than the solidification temperature of your wax, what you need is a recirculating heat transfer medium- water, glycol/water, heat transfer oil etc.- whose temperature you can control such that it is always above the solidification point of your wax.

The HT medium is pumped through your wax cooler (a shell and tube or plate and frame unit etc.), then through an air cooler or cooling water exchanger, then returned to its expansion tank. You control the temperature of the heat transfer medium to obtain the desired exit temperature from your wax cooler, typically by means of a bypass around the air cooler or CW exchanger. Unlike your wax, your HT medium will not solidify if it sits stationary in the air cooler and cools completely to ambient temperature.

The wax cooler is designed to have sufficient area to remove the required amount of heat when the HT medium inlet temperature is above the melting point of the wax. It'll be a bigger exchanger, but it won't foul as quickly.

This presumes of course that you're not trying to remove the heat of solidification of the wax itself by sub-cooling the wax, in which case good luck- you need to allow the wax to solidify in the final containers or else to deal with wax solidifying inside your exchanger. There may be tricky ways to accomplish this, but don't go there unless you can afford to get it wrong a few times before you get it right.

A start-up heater (in the expansion tank for the heat transfer medium for instance) is required to get the loop up to temperature, and to hold it there whenever the wax line is down for maintenance etc.

 

[darai]

Thanks. This really helps. I hadn't thought about the possibility of freezing at the boudaries of the heat exchanger at lower temperatures. We are trying to cool the wax just before it is dispensed into small containers (approximately 3 gallons). Currently, there is a long wait time for the wax to cool so that the open containers can be moved to complete the packaging process. The idea was to pre-cool so the dwell time wouldn't be so long.

Your problem is "dwell-time" after the fill. So what you really want to do is cool the filled containers, not the pre-filled paraffin wax ( Think about it, cooling the pre-fill will increase its viscosity and make it more un-economic to get it into the containers!

So, first suggestion that comes to mind is to turn your fan loose on the filled containers! Recommended.

Extreme case, turn chilled air on to them, but will your pocket book allow this? Not recommended!

 

[IRstuff]

Does it need to stay above 125F? If so, that also limits your ability to manipulate the temperature after the fill. However, I assume that 125F still results in a liquid, so you might able to do something with an immersion cooling system after the fill.

TTFN

FAQ731-376