The book "Brewing - Science and Practice" from the CRC Press has almost a whole chapter on Milling. In part, it says:
Roll (or roller) mills are commonly used in breweries. The rollers work in pairs. The malt grains are delivered to the first pair of rolls by a feed roll that determines the feed rate and is intended to deliver each corn `end-on' to the working rolls. Each corn is drawn between the rolls and is crushed, sheared (if the rolls are rotating at different speeds) and cut if the rolls are fluted (grooved). The theoretical capacity of a roll mill, Q (m^3/h), is given as Q=60*s*N*L*10^9, where s=rotational speed (rpm); N=the gap between the rollers or `nip'(mm); L the length of the working surfaces of the rolls (mm). In fact the practical working capacity is 10±30% of the theoretical capacity (Sugden et al., 1999). Working rates of different mills (in kg/h/mmroll length) are given as two-roll mills, 1.5±2.5; four-roll mills, 2±6 and six-roll mills, 1.5±10.
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The peripheral roll speeds in brewery mills are often 2.4±4 metres/sec (8±13 ft./sec.). As a particle moves between the rollers so it deforms and, if it is brittle, its structure fails and it breaks up (Sugden et al., 1999). Rolls may move at different speeds, for example the faster may rotate at 1.25 the speed of the slower. Consequently a particle passing between them will be torn by shear as well as being crushed. As well as that the grooves, or `fluting' milled in spirals on the surfaces of the rollers can be arranged not only to increase the coefficient of friction between the particles and the rolls, but also to cut the particles (Kunze, 1996).
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5.4 Dry Roller Milling
A large number of types of roller mills have been, or are, in use (Kunze, 1996; Narziss, 1992; Sugden et al., 1999). `Dry' mills have some characteristics in common. The feed roll delivers the malt to the first pair of crushing rolls, across their full width, at a controlled rate. The rolls are designed to deliver the corns `end on' to favour their being crushed along their length with the minimumdegree of husk breakage. The rolls are often about 250mm(9.84 in.) in diameter (in wet mills they are often larger). As noted the rolls are usually fluted and may run at different speeds. Both may be driven but sometimes only one is powered, the `follower' being dragged by the friction between the grist and the moving, powered roll. Mill rolls may operate at 250±500rpm. The roll length increases with machine capacity, to a maximum of about 1500mm (59.06 in.).
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The simplest mills are single-pass, two-roll dry mills. These are relatively slow
working and are inflexible, being suitable only for well-modified malts, special malts or
rice. They are used only in small units, such as pub breweries. Gaps used are 0.6±1.0mm
(0.024±0.039 in.) and working capacities are 1.5±2.5 kg/h/mm roll length (a working rate
of 1 kg/h/mm roll length is almost exactly 60 lb/h/in. roll length). It seems that three-roll
mills (with or without screens) are no longer in use. The arrangement of the three rolls
was like the grouping found in five-roll mills (see below; Sugden et al., 1999).
Whew!
I'm about to finish a 9.75" diameter roller, I hope. I'm mounting the bearings now.
