Lesson 8 - Wort Separation Final SAQ

***Using the data from the table below, and by reference to Darcy’s Law, explain why the mash filter is the fastest wort separation device.

Table: Design and operational parameters for the mash tun, lauter tun, mash filter

Response:

The table shows us that the mash tun (mt) only achieves five brews per day, compared to 10 brews per day with a lauter tun (lt), and 13 brews per day with a mash filter (mf). The mash tun cycle time includes 88 min for the mashing process. If this was deducted, then the true comparison of wort separation time would be 200 min, or 8.4 brews per day for the mash tun.

To understand this difference in brews per day, we can use Darcy’s Law which provides us with the variables affecting strong worts flow rate.

K is the grain bed permeability.

A is the cross-sectional area of the grain bed.

DP is the differential pressure across the grain bed.

μ is the wort viscosity.

L is the depth of the grain bed.

Create a table of the Darcy’s Law variables which are given in the question, and calculate the relative strong worts flow rate of the three devices.

Table: Using Darcy’s Law to compare strong worts relative flow rate

Darcy’s Law tells us that if cross-sectional area A increases, then the flow rate will increase. If cross-sectional area increases, then the bed depth L will automatically decrease. This will also cause wort flow to increase.

The mash filter has a very high surface area equivalent to a 20 m diameter tun, compared to the 8 m diameter lauter tun and the 5 m diameter mash tun. So the respective grain bed depths are 0.05 m for the mash filter, 0.5 m for the lauter tun, and 1.0 m for the mash tun. Darcy’s Law tells us that flow is inversely proportional to bed depth. If relative wort flow was only due to bed depth, then the relative flow of the devices would be:

mash filter – 20 (1/0.05)

lauter tun – 2 (1/0.5)

mash tun ‒ 1 (1/1)

In reality, the strong worts flow is: mash filter ‒ 4, lauter tun ‒ 2.3, mash tun ‒ 1. The mash filter runs 5x slower than bed depth predicts.

Darcy’s Law tells us that wort flow will increase with increasing grain bed permeability. So, If we assume that bed permeability is directly related to the % of the coarse husk fraction of the grist, then the relative flow will be:

mash filter ‒ 1 (5%)

lauter tun ‒ 5 (25%)

mash tun ‒ 8 (40%)

So, the reduced grain bed permeability of the finely ground hammer milled grist significantly mitigates the advantage of the higher surface area. Using the relative permeabilities from the table confirms this is the case.

Darcy’s Law tells us that increasing viscosity will reduce flow rate. Higher wort viscosity occurs with higher wort gravity and lower wort temperature. The viscosities from the table are:

mash filter ‒ 1.65 mPa.s

lauter tun ‒ 1.25 mPa.s

mash tun ‒ 1.85 mPa.s

The difference in viscosity is not so marked as bed depth and grain bed permeability. The mash tun wort viscosity is raised in part due to the fact that strong wort separation is at saccharification temperature (around 65˚C). The other two devices receive their mash from a mash conversion vessel, which is at about 76˚C. 

Darcy’s Law  also shows us that higher DP will improve wort flow. The mash filter can use an inlet pressure of 0.5 bar (DP 0.3 bar) whereas the lauter tun and mash tun have a DP limit of 0.1 bar to avoid excessive vacuum under the false floor. So the relative DP is: mash filter 3, lauter tun 1, mash tun 1.

The mash filter is the faster unit. Its flow is diminished by using a low grain bed permeability, but this is done to increase extract recovery by some 3%. Its relatively strong worts are higher in viscosity ,which will also diminish flow. However, all of this is more than compensated for by the higher increase in flow provided by the very high surface area and very shallow bed depth, as well as the ability to operate at a higher DP.

During strong worts separation, the mash filter can operate at twice the flow rate of the lauter tun, and four times the flow of the mash tun.