Q: At what size of a lauter tun should adjustable rakes be used? I am loooking at a 30bbl system and there is very little information as to the cost
A: Basically there are three types of mashing and separation systems used in brewing:
Traditional British styled infusion mash tuns with slotted false bottoms, deep mash beds and no raking. These systems rely on low protein, well modified malts (low beta glucan and high friability) which are milled on the coarse side. The mash-in is gentle and a single stand temperature, ie no addition of heat and hence no need to stir or agitate the mash. This helps preserve the filterability of the mash bed which may be up to three feet deep. No rakes are needed if this is done correctly and mashing extract efficiencies of 85-90% can be achieved based on the coarse grind as is basis. This system does not work well with less modified malts or materials like malted rye or wheat which have no husk. In my experience the grist composition of these materials cannot exceed 20% for reliable run-offs in an infusion mash tun without the addition of rice hulls.
Stirred mash tun with lauter tuns are the traditional European continental style of mashing and separation. This system was based on lower modified malts (higher beta glucan and lower friability) which sometimes required harsh treatment such as more intense milling and heating steps sometimes including boiling (decoction) to help break down the endosperm cell walls in order to extract and convert starch to sugars. The mash-in is thoroughly hydrated and the mash is stirred as it experiences temperature increases to optimize malt enzymes and break down the endosperm. The treatment impairs the filterability of the mash so that it would not run off in a deep bed as it would in the infusion mash system mentioned above. The mash must be pumped to a lauter tun which have the slotted false bottom and a set of adjustable rakes where is will form a relatively thin bed, about 12 inches deep, and may have to be cut a few times during run off. However this system can handle materials with less filterability like wheat malt in large amounts.
The third form is an adaptation of the mash and lauter tun where the malt is ground very finely by a hammer mill, mashed in a step temperature profile and then pumped to a mash filter which can run off the wort relatively quickly and with maximum extract efficiency. This system is best suited for larger operations running multiple brews per day.
As to your question, an infusion mash tun can be outfitted with an adjustable or stationary rake in sort of a hybrid infusion mash-lauter tun. This will help a deep bed to produce a run-off for first worts and second worts which will increase the extract efficiencies of the brewhouse by as much as 5-10% over no rakes at all. the range depends on the quality of the milling and operation of the mash tun to take first and second worts efficiently. It will work better than an infusion tun with no rakes in producing run-offs from materials like wheat and rye malts but not as well as a thin bed lauter tun for high percentage grist formulations and will not give the stirring necessary for temperature ramps or decoctions. In other words it may help in standard run-offs but will not have the flexibility that a separate stirred mash tun allows.
There is no minimum lauter tun size for rakes, pilot or nano-systems as small as 20 liters can be outfitted with lauter tun rakes, and I have seen systems in brewpubs of 5 hectoliters and above using them. The argument could be made that a brewhouse with a stirred mash tun and lauter tun or the hybrid infusion tun with rakes will have more flexibility to brew different styles and recipes than a simple infusion mash tun. The lauter tun will be able to handle less modified and less filterable materials like European pilsner malt, rye and wheat malt so that recipes involving large amounts of those materials are possible. The question becomes what do you want to brew and how much do you want to spend on the capital required to install a lauter tun or outfit an infusion mash tun with rakes?