Does the foam on top of boiling maple sap affect the rate of evaporation?

Question

This is a serious question from someone engaged in evaporating large quantities of water to turn sap into syrup at this time of year.

Probably some background will help. When sap boils vigorously it creates quite a bit of foam, which will overflow the evaporator (incidentally filling the building with a pleasing maple caramel smell as it burns on the side of the evaporator). When the foam gets too high we touch it with a bit of lard and the foam level drops (surface tension - I know). However, it is tempting for me to give a good swipe so that the foam almost disappears (instead of just dropping). The old-timers however contend that I should just reduce the foam to the point where it isn't overflowing any more. They say that it will take longer to boil away the water if I eliminate the foam.

I fail to see how the foam will improve evaporation (although it seems to me that it might slow it down).

Edit: by request ( @georg ) , a link to the evaporator in question https://sites.google.com/site/lindsayssugarbush/_/rsrc/1240515239201/Home/2005-03-30--12-25-21.jpg

Answer 1

It could go either way. If you are heating the liquid to the boiling point, then the foam will not limit boiling (unless it raises the pressure), but will limit convection/advection of air near the surface. Note that latent heat of water vapor is not the only method of heat loss from your pot. If air advects/convects over the surface, you are also heating air molecules. Also some heat is being lost by the surface via thermal radiation (probably roughly a kilowatt per meter squared). So the bubbles provide insulation, so that the heat loses other than into latent heat of water vapor are reduced..

But, if it is not actually boiling, but the temperature is controlled to be some value below boiling, then it loses water via evaporation, and that requires fluid to flow to and away from the surface, and the foam would seriously inhibit that.

Answer 2

I am too lazy to compute anything, but consider what is the foam?

The foam is a bubble of steam covered with a layer of water and sugar solution. If there is no foam the area of the pot exposed to air or evaporation is two dimensional, with maybe some big bubbles from the boil. Each little bubble in the multitude contributes more to the area exposed to the air where the surface cover water can evaporate, burst and release trapped steam, making way for the next bubble. It is a fractal problem,but the dimensions of the available evaporation surface are larger than 2 by far when there is foam.

Lets put it another way. The foam raises a film of hot water to contact with the air, without obstructing the trapped steam, so when a foam bubble breaks there is water out of the pot that would have been in the pot if there were no foam, the extra surface of the foam increases the evaporation.

So the oldtimers are probably correct.This argument depends on the surface of the foam bubbles being a solution with water.Of course an experiment should be carried out with the specific liquid to decide on this.