|A typical canning kettle that can hold seven quart jars typically holds 21 quarts of water
Repeat as many times as necessary.
Further, let's assume this is not your first rodeo. You didn't use 59 degree tap water, you filled buckets, or maybe your bathtub and let it warm up to the dew point...which just happens to be 69 degrees in Eaton Rapids at the time of this writing.
If the food going into the jars starts at 80F then seven quarts (14 pounds) requires 1400 BTU to bring it up to 180F, the minimum temperature needed to kill MOST spoilage organisms. The 28 pounds of water starting at 69F requires 4000 BTU to bring it to 212 F.
About 25% of the heat goes into heating the food (the payload) and 75% of the heat goes into heating the water (a necessary evil).
Assuming 10000 BTU/hr for the large burner on a typical home stove and a 50% heat loss rate, it will take sixty-five minutes to bring the kettle up to heat, not counting convective losses from the kettle.
|I know there are two kettle drawn, but assume there is a single kettle. Also assume it holds ten jars to make the numbering work out.
Then, it bypasses the canning kettle and preheats the incoming jars. That is, the heat that was removed from the jars that were cooling was used to heat the jars heading toward the kettle.
The system is designed so there is never a large temperature difference between the jars being heated/cooled and the water they are in. That minimizes the entropy in the system. The total heat used is the difference between the warm jars leaving the process and the cold jars entering, presuming the fluid is recycled.
Jars 1-through-12 are warming up. Jars 34-through-23 are cooling off.
The water stays in the tubs and does not move. The operator moves incoming from right-to-left and finished, cooling product from left-to-right.
The product that is cooling down is always warmer than the water in the tub. The product that is pre-heating is always cooler than the water in the tub.
The fact that you are recovering most of the heat that was invested in the jars means you don't need to burn the fuel or take the clock-time to burn the fuel.
The fact that the heated water is not tossed out means that water only needs to be heated from 69F once...except for what needs to be added to make up for evaporative losses.
There will never be enough time or labor.
There will never be enough fuel.
But I repeat myself. Every chunk of wood required that a person go into a swamp in the winter, cut down a tree, buck it into manageable bolts, drag it out of the swamp, cut to 16" lengths, split it, stack for drying, then move it to the site where it will be burned.
Being able to recycle the hot water decreases the fuel requirements (and the time to burn it) by 75% compared to the first system described above. Salvaging half of the heat from the jars coming out of the kettle and using it to pre-heat the jars going into the kettle would reduce the total BTU required per jar to 12% of the original system.
That is why efficiency matters. A 88% reduction in fuel and time-per-jar is a very big deal when nothing is easy.