Heat Transfer and Boiling

Question 1

What is heat transfer?

Answer 1

Energy (heat) flows from an area where there is a lot of it to one where there is less.

Question 2

What equation is used to calculate the energy required to heat a substance?

Answer 2

q = M x Cp x (T2 - T1)

Question 3

What does ‘M’ represent in the equation q = M x Cp x (T2 - T1)?

Answer 3

Mass of substance in kg.

Question 4

What does ‘Cp’ represent in the equation q = M x Cp x (T2 - T1)?

Answer 4

Specific heat capacity in kJ/kg °C or K.

Question 5

What does ‘T1’ represent in the equation q = M x Cp x (T2 - T1)?

Answer 5

Starting temperature in °C or K.

Question 6

What does ‘T2’ represent in the equation q = M x Cp x (T2 - T1)?

Answer 6

Target temperature in °C or K.

Question 7

How much energy is needed to heat 1000 hL of wort from 75°C to 100°C?

Answer 7

10,320,000 kJ or 10,320 MJ.

Question 8

What is the heating rate used to heat the kettle to boiling?

Answer 8

1°C/min.

Question 9

What is the calculated heat transfer rate (Q) for heating wort to boiling?

Answer 9

412.8 MJ/min.

Question 10

How do you convert Q from MJ/min to kW?

Answer 10

Q = (412.8 / 60) x 1000 = 6,880 kW.

Question 11

What is the specific heat of vaporization (hfg) for water?

Answer 11

2,257 kJ/kg.

Question 12

How much water is evaporated to achieve a 5% evaporation in 100,000 L?

Answer 12

5,000 L or 5,000 kg of water.

Question 13

What is the total heat energy needed to evaporate 5,000 kg of water?

Answer 13

11,285,000 kJ or 11,285 MJ.

Question 14

What is the calculated heat transfer rate (Q) for the boiling process?

Answer 14

188.08 MJ/min.

Question 15

What is the calculated power requirement (Q) in kW during boiling?

Answer 15

3,134.67 kW.

Question 16

What factors affect the rate of heat transfer (Q) during wort boiling?

Answer 16

• Surface area (A)
• Temperature difference (ΔT)
• Overall heat transfer coefficient (U)

Question 17

What are the units for Q in the heat transfer equation?

Answer 17

kW.

Question 18

What does the overall heat transfer coefficient (U) depend on?

Answer 18

• Film heat transfer coefficient of condensing steam (hs)
• Heat transfer coefficient of fouling on the steam side (hfs)
• Thickness of the tube (x)
• Thermal conductivity of the calandria tube material (k)
• Heat transfer coefficient of fouling on the wort side (hfl)
• Film heat transfer coefficient of the wort side (hl)

Question 19

How is the overall heat transfer coefficient (U) calculated?

Answer 19

1/U = 1/hs + 1/hfs + x/k + 1/hfl + 1/hl.

Question 20

What does a higher overall heat transfer coefficient (U) indicate?

Answer 20

More effective heat transfer.

Question 21

What is film boiling?

Answer 21

A condition where steam bubbles cling to the surface, reducing heat transfer.

Question 22

What should be avoided during wort boiling to ensure effective heat transfer?

Answer 22

Film boiling.

Question 23

What is forced convection?

Answer 23

Occurs at low temperature differences with high wort velocity across the heating surface.

Question 24

What is nucleate boiling?

Answer 24

Relies on steam bubble formation to improve heat transfer.

Question 25

What is the effect of fouling on the wort side during boiling?

Answer 25

Increases resistance to heat transfer.

Question 26

What is the primary heating medium used in wort boiling?

Answer 26

Steam.

Question 27

Why is stainless steel preferred over copper for brewing?

Answer 27

Copper causes stale beer flavor and forms permanent haze.

Question 28

What is two-phase flow?

Answer 28

A mixture of liquid (wort) and gas (vapour) in the calandria.

Question 29

What causes thermosyphon in the calandria?

Answer 29

The rapid rise of vapour bubbles due to lower density than wort.