Enthalpy

January 7, 2018

Icicles-at-Waterfall.jpg

A few days ago, in the midst of the recent cold, I went up to give the pigs their afternoon whey. I pulled open the valve on one of the tanks and heard the swoosh of moving liquid. It continued for a few seconds and then it just stopped. Nothing was coming out of the filling pipe, even though I was certain I had drained it that morning. In my experience moving liquid usually thaws out anything it’s touching, so what could have made it freeze so fast that it could clog a pipe?

Over the past week the battle has continued, with the whey going from liquid to solid impossibly fast. Slush starts forming the moment it hits a tank or bucket, and it is becoming a struggle to keep valves from freezing shut. The grimest prospect, which the warm forecast for later this week will hopefully prevent, is of whole tanks completely siezed up, leaving me with no easy method to keep the pigs watered (or wheyed, as the case may be).

So why is the whey behaving differently than ever before? My best guess is the degradation of bales around the tanks - hay is a good insulator so long as it remains intact, but it’s not so effective once it’s composted to dirt, as has most of the stuff that previously held off the worst of the cold. This has led to the whey cooling faster than in past winters, which brings me to an experiment you can do at home.

Put a container of room temperature water in your freezer with a good thermometer placed inside it. Take temperature readings every few minutes and plot them on a graph. It should look something like this:

watergraph.jpg

That flat area, where the water reaches freezing temperature and then actually transitions from a liquid to a solid state, represents a huge amount of energy, known as the enthalpy or latent heat of fusion. While the temperature is dropping the environment is absorbing energy from the water as heat. When it plateaus, the environment is absorbing the energy released by the process of molecules crystallizing.

In winters past ice has formed in the tanks, but never to the extent of this year. My guess is that there has previously been a large core of liquid whey that has remained a few degrees above the flat part of the above graph. Right now there’s still plenty of liquid in the tanks, but it is much closer to freezing. In fact, given how quickly slush forms atop it when its put in the pigs’ trough, and given the particular manner in which whey freezes - forming a slurry long before it becomes solid - I suspect the whole tank is actually a suspension of ice crystals.

When this hits something colder, like air or length of pipe that is -15, it can freeze almost instantly, so fast that even movement and pressure aren’t enough to keep it liquid. An open pipe can slam shut, and a smoothly operating valve can instantly lock up.

The good news is that the whey is north of 100 when it arrives at the farm, and we should get a fresh load in the next couple days, which will thaw the tanks completely. Still, I’ll be breathing easier once the ambient temperature starts consistently climbing out of the low single digits.

Photo Credit - Garth Brown

Graph taken from www.fao.org

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