Lesson 4 - Wort Boiling Technology and Process Control

Fouling is like insulation. It prevents the transfer of heat. For boiling to be effective and consistent we need to minimise fouling.

Factors that reduce fouling include:

• Soft water (prevents precipitated minerals building up a fouling layer on the heating surface but given the amount of calcium we need to add is never an option!).
• Hop vegetable matter (whole hops and particles from pellets help prevent fouling, while extract can stick to heating surfaces).
• Wort gravity (lower gravity = less dissolved material = less fouling).
• Lower ΔT (high ΔT can cause fouling).
• Mixing of any additions of liquid adjuncts (effective mixing prevents high concentrations of sugar being in contact with heating surfaces).
• Heat transfer mode (nucleate boiling produces less fouling than film boiling).

We can use our heat transfer equation to determine the effect of fouling on heat transfer in a boil.

We can consider this question. In our first boil after CIP the heat transfer coefficient is 1.57 kW/m²K but after 5 brews it has dropped to 0.85 kW/m²K. Our calandria has a heating surface area of 200 m². If we maintain a ΔT of 15°C what is the reduction in Q as a result of the fouling?

First brew:

Q = UAΔT

Q = 1.57 x 200 x 15 = 4,710 kW

After 5 brews:

Q = 0.85 x 200 x 15 = 2,550 kW

If we remember back to our 1000 hL brew with a 5% total evaporation and we use the h_fg of 2,257 kJ per kg of water evaporated, we need a total of 11,285,000 kJ. 1 kW is 1 kJ/s so we can work out how long we need to boil the wort for in both brews.

First brew:

After 5 brews:

Boiling at a slower rate means the changes that we need to occur in boiling do not occur as effectively and the wort produced in brew 1 and brew 5 are likely to have different properties. And of course, the number of brews we can do in a day is also greatly reduced. The only way to speed up the boil on brew 5 is to increase the heat input which will increase ΔT and speed up the rate of fouling even further!

The key message is wort boiling systems must be designed and operated to avoid fouling.

Now to examine the effect of area (A)

Point of reflection: One kettle, with a brew volume of 1000 hL, has an internal calandria with a heating surface area of 80 m². Another kettle (also 1000 hL brew volume) has an external calandria with a heating surface area of 200 m²

The overall heat transfer coefficient for both systems is 1.57 kW/m²K.

If both achieve a heat transfer rate of 4630 kW, what is the difference in temperature between the steam and wort for each kettle? What is the likely effect of the temperature differences on the wort?  

Give yourself 5-10 mins to think about this and then click ‘Next’ to continue with this lesson.