Hebron ND Roller Mill
Packaging “Silver Queen” flour
LM&E: Machinery
Corn Meal
Although the mill was built to produce 70 barrels (1bbl weighed 196 lbs or 14 British stones) of flour a day, corn meal was also produced in a smaller amount. Corn was dumped into a hopper on the first floor and it emptied into a corn crusher in the basement. The corn crusher broke the kernels into smaller pieces that emptied into the foot of an elevator. It carried the crushed corn back up to the first floor where it emptied into a three roll -2 break roller mill. The top and middle roller would break the corn into coarse pieces, then be ground into finer pieces by the middle and lower rollers to produce cornmeal.
Flour
The process of converting a bucket of wheat to a sack of flour involves a number of steps, and there is a machine for each step. Since materials were fed into the tops of the machines and came out at the bottom, there had to be a way to get the materials from the bottom of one machine to the top of the next machine in the process. The usual way to do that was to have a mill building with three (or four) floors with the roller mills, the heaviest machines, on the first floor. Elevators would move grain from the roller mills to the second and third floors where other machines would clean and sift the material. Chutes from each machine directed the material down to the next machine in the process or to an elevator that would bring the material up to the next machine.
Mill Infrastructure
The 1905 and 1910 Sanborn Fire maps list the machines that were installed on each floor of the mill. The maps are the only source of information about how the mill was configured.
First floor
4 stands of double rollers: The rollers were precisely machined with patterns of grooves that were designed for a specific operation. The rollers of the first stand were breakers and they rotated towards each other with the upper roller spinning 2.5 to 3 times as fast as the lower roller. The sharp grooves of the faster roller would aggressively force the wheat kernels down to the slower roller which crushed and tore the kernels apart. If the both rollers turned at the same speed, the grain would be crushed rather than be torn apart and friction would heat the flour, degrading gluten formation when kneading bread. Elevators took the grist from the first two breaker rollers to the bran duster and purifier.
The other three stands were for reduction and had rollers with finer machined patterns set closer together in order to produce finer particles of flour. The faster roller ran about one and a quarter times as fast as the other so the grinding process was not as aggressive as the breaker. Just as the flakes of breakfast cereal corn flakes are all the same thickness but different sizes and shapes, the particles coming from a reduction roller have the same thickness but a range of sizes. Grist from the first three reduction rollers went to the Salem sifters which separated the coarser particles and routed them to the three other reduction rollers and reel sifters. Reel sifters separate the particles by size with the finest going to the flour bin, some going to the next reduction roller, and some going back to the same reduction roller.
2 hopper scales: During the milling process, flour and bran were routed to separate bins or hoppers. Scales were fitted to each hopper to determine the weight of material in the hopper.
1 Packer: The packer allowed the flour or bran in the hoppers to fill the barrel or cotton sack with the proper weight of flour or bran.
Second floor
2 separators: When the grain comes from the threshing machine in the field, there is often chaff, dirt, mud, pebbles, weed seeds, bugs, and broken or moldy kernels. A separator removes those impurities before the grain is milled. The separator had a fan and several sloped vibrating screens, each with different mesh size. Grain was introduced into the top of the separator and as it fell onto the first screen which had the smallest mesh size, the fan would blow away the dust, chaff, and other light particles. As the screen vibrated, the smaller seeds fell through the screen while the larger seeds moved to the next screen where the next smaller seeds fell out. The process continued until the seeds reached the screen that would allow only the wheat kernels to fall through.
1 bran duster: The purpose of a bran duster is to separate the flour and endosperm particles that adhere to the bran. The coarsely ground grain coming from the first break roller enters the bran duster where rotating paddles dislodge the flour and other particles from the bran. These are separated and the bran goes to the bran bin for packing while the flour and other particles known as middlings go to the second break roller.
1 purifier: Middlings are a mixture of different size particles of flour, endosperm, germ, and bran remnants and the purifier separates these particles. The purifier is like a big vibrating box with a number of sieves with different size pores to sort the flour and middlings by size. As the middlings are vibrated in the box, the heavier particles sink to the bottom of the box. The particles fall through the sieve whose pores are the size of the particle and are routed to the second break roller (the sieve sequence in the box is smallest to largest). Germ was the largest size particle and would not go to another roller. Since the bran particles are light and fluffy, they float to the top of the material in the vibrating box and are swept away by a current of air. It was important to remove the germ because if it remained with the flour, the oils in the germ would become rancid and spoil the flour.
1 suction fan: The suction fan is part of a mill's dust collection system. Ducts run from the fan to the roller mills and sifters to pull fresh air through the machines, then push the dusty air to the dust collector where the dust is filtered and the filtered air is returned to the mill.
Third Floor
3 Reels: A reel is a type of bolter or sifter that consists of a rotating drum covered by several cloth filters, typically silk gauze, with a certain mesh or fineness. The reel was tilted and vibrated as it rotated so that as the material from a roller entered at the high end of the reel, the finest particles of flour would pass through the first filter and drop to the flour bin while the coarser middlings would drop down to the next sequential roller on the first floor. The size of the coarser particles depended on the particular roller that fed the reel, the reduction rollers at each successive step were set closer together to produce finer particles.
1 Tubular Dust collector: Dust is a major problem for flour mills – it can cause breathing problems for the workers and can form an explosive cloud in air. The amount of dusty air that a dust collector can filter depends on the total surface area of the filter. A tubular dust collector is essentially a big box with a large number of spouts coming out of it with a fabric bag is attached to each one. Since there is a large number of bags, the effective filter area is quite large. Just like a vacuum cleaner, when the bag is full, it is replaced with a clean one.
1 Scourer: After the major impurities like chaff, weed seeds, stones, and dirt have been removed by the separator, there are wheat hairs, the outer covering of the grain, and worm-eaten grains still mixed with the wheat. The scourer consists of a rough sieve drum with beaters inside the drum to rub the grains together, thus polishing them and dislodging the impurities which fall through the sieve.
2 Salem sifters: A third Salem Sifter was listed on the 1910 Sanborn Fire map and would have been a part of the mill expansion from the initial 50 barrels to 75 barrels capacity. Salem Sifters were a type of “long sieve sifter”, typically at least 20' long. It was a long, vibrating rectangular wooden box with screens to separate the coarser from the finer particles coming from the break rollers. The finer particles were primarily endosperm and would be sent to the next milling process while the coarser particles would be sent to a different process.