Introduction

For quite some time now, I've been planning on building a unitank for my brewery. After desiging a traditional Tall unitank, I purchased the new book by George and Laurie Fix called "An Analysis of Brewing Techniques". I was happy to find that the new book had information about unitanks and design considerations that effect fermentation and the finaly product.

One of the main geometric considerations described in the book is that a squat unitank gave more consistent results than the traditional tall design. With this in mind, I decided to examine some of the costs and problems associated with design of a squat fermenter instead of a tall fermenter. For my study, I used a 24 gallons total volume as a requirement.

Below is a representation of what commerical versions of these two vessels ight look like in the 3 bbl size range. Note that the one on the right is computer generated and does not actually exist.

Design and calculations

In order to compare the two vessels, I calculated the size of the cylinder and cone for each vessel and then did volume calculations. The tall vessel has a radius of 7" in the cylinder because stainless sheet conservation.

For example:

24 gallons total volume

¹ both cones use a 60 degree internal angle
² Squat fermenter designed using space conservation. Unterstein, K. 1994. Brauwelt 12:4

In many cylindroconnical fermenters, temperature control is achieved by pumping chilled glycol around a jacket covering the fermenter. Regulation of the temperature occurs either by turning the pump to the glycol on and off or by a valve which regulates its flow.

In homebrewery unitanks where the volume is less than 1 barrel, the glycol jacket does not completely cover the vessel. Instead, these fermenters have a small chill band placed around the fermenters' cylinder. For example:

The reason that a chill band is used instead of covering the whole vessel with a jacket is probably to reduce the cost of fabrication. It is assumed that since most to the wort is in the cylinder portion of the fermenter, the small amount of unchilled wort in the cone will be insignificant.

In the squat fermenter design however, there is 2.5 times as much wort in the cone as in the tall fermenter. If you take in account that these fermenters are not design to be filled to the top, you have an even greater percentage of beer in the cone. For example, assuming you fill the fermenter allowing for 20% head space, you now have 1/3 of the wort in the cone in the squat design. It may therefore be necessary to chill the cone as well as the cylinder above in order to avoid large temperature gradients.

Possible consequences

Assuming that the squat fermenter will require a jacket around the entire fermenter, there are several consequences. These may include:

1) Increased cost of chilling. A larger volume of glycol will be required in the squat fermenter design. Many small fermenters are not insulated with a barrier between the glycol jacket and the outside atmosphere. Since a larger jacket leads to greater surface area to contact the outside air, you can expect larger loses of glycol temperature due to lack of insulation. This may even necessitate insulating the fermenter at additional cost.

2) Increased cost of fabricating the fermenter. This is an obvious problem. There will be very definite cost associated with building a second cone around the bottom to house the glycol and additional costs in getting the glycol to circulate around the jacket uniformly.

3) More problems with cold spots due the the relative glycol chilling to volume of wort at the bottom of the cone. At the tip of the cone, the amount of wort becomes very small. If the volume of glycol in the outside jacket is determined by a constant distance from the inside shell there may be a problem with too much cooling at the base. This may also be acerbated by the fact that the cool wort will want to settle in the cone due to thermodynamic currents. (This may be avoided by making an outside cone with greater than a 60 degree angle so that the distance from the inside jacket is less as you measure close to the tip of the cone but its going to be hard to fabricate.)

Comments

If I made any of my comments unclear, feel free to email me and I'll try to make it more clear. None of my ramblings are meant to be taken for fact. This is article is meant as a way of discussing unitank design and your comments are very much desired and welcome.

Equations
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