Why does a holding plate work?

"Any sufficiently advanced technology is indistinguishable from magic."

- Arthur C. Clarke, 1961 (Clarke's third law)

A holding plate is a magical device that keeps your refrigerator or freezer cold, even when the refrigerator compressor is not running. How does that work?

Here's the advanced technology behind the magic: The holding plate is a tank, filled with a water-propylene glycol mixture. The refrigeration system's evaporator coils (the source of the cold) are immersed in the liquid in the tank.

When the compressor is running, the coils get very cold, and begin to freeze water out of the glycol solution, eventually creating a kind of "slushy" inside the tank.

But wait... why doesn't the tank burst when the water freezes? The answer to that is the phase diagram for water/propylene glycol mixtures.

(Aside: why propylene glycol? Propylene glycol and ethylene glycol - which you probably know as automotive anti-freeze - are very closely related chemically. But, while ethylene glycol is a deadly poison, propylene glycol is a food additive. Both taste sweet, BTW.)

A sketch of the water/propylene glycol phase diagram is below:

The way to look at this diagram is like this.  The horizontal axis is delineated in % glycol, meaning that it runs from 100% water on the left, to 100% glycol on the right.  The vertical axis is temperature.
And there are two horizontal curves here:

• An upper one which has a dip in it.  This line marks the point, at any given composition, where freezing begins.  Above the line, everything is liquid; below it, a solid phase has begun to form.  (The lowest part of the dip is at a composition of 60% propylene glycol, called the eutectic point.)
• A flat one at -60° C.  Below this line, everything is solid.  Above it, there is some liquid present (except at the 60% glycol eutectic, where the upper and lower lines meet).

Now notice that at any composition away from 60%, the line, representing the melting/freezing point of the mixture, rises. Basically, this says that the best you can do in preventing freezing with propylene glycol and water is -60°C, at 60% glycol. This is a case where while some is good, but more is not better. (Not surprisingly, the water/ethylene glycol phase diagram is similar - putting too much antifreeze into your car won't help things...)

Next, notice that under the freezing point curve to the left of the 60% eutectic point, there is an area labeled "H₂O(s)+lq". This means that for compositions and temperatures that fit here, what is present is a mixture of solid H₂O (ice) and liquid. If you were to take a mixture of water and propylene glycol with less than 60% glycol and slowly lower the temperature (moving straight down on the chart), pure water ice crystals would form in the mixture. As water is removed from the solution by freezing, the concentration of glycol in the remaining liquid phase rises, and the subsequent temperature needed to freeze more water out of solution continues to fall... until the concentration of the remaining mixture is 60% glycol, at which time the remaining liquid all freezes solid at once.

(For completeness, to the right of the eutectic point, the unmarked area between the curves should say, "Glycol(s)+lq".  That is, for compositions greater than 60% glycol, what freezes out of the mixture is propylene glycol, not ice.  But still it is a slush.)

This shows why your holding plates don't burst. Until you get down to a temperature of -60°C, there will always be a slushy mixture of ice and water/glycol solution present.

So how much propylene glycol should you use?  I spoke at length with a Technautics engineer at a boat show about this.  He told me that Technautics uses 10% glycol in holding plates destined for refrigerators, and 15% in plates for freezers. So that's what I put in mine.

There.  Now you have the technology - magic is no longer required.