Sensible Heat

As mentioned earlier, sensible heat is the heat you can feel or see (e.g. observing a temperature change on a temperature probe of a vessel that is being heated).

When the temperature of an object changes, sensible heat is being exchanged.

When the temperature of an object increases, sensible heat has been added. Conversely, when the temperature of an object decreases, sensible heat has been removed. The amount of sensible heat required to change the temperature of an object is dependent on a property of the object called the specific heat capacity (C). The specific heat capacity of an object is a measurement of the amount of heat required to change the temperature of one-unit mass by one degree.

The SI unit for heat capacity is J kg-1 K-1.

The table below shows some common brewing fluid specific heat capacities.

Table: Heat capacities of common brewing fluids

Fluids

Heat Capacity (J kg-1 K-1)

Water (20°C)

4,180

Wort (20°C)

4,000-4,100

Mash (20°C, 15°P)

3,730

Mash (20°C, 20°P)

3,600

Air (20°C)

1,005

 

The specific heat capacity of a substance can vary with temperature and is also dependent on the state of matter – solid, liquid, or gas.

Heat capacity values are available for gases at either constant pressure (Cp) or constant volume (Cv). This is due to the expanding and contracting nature of gases when subject to temperature, pressure, and/or volume changes.

To calculate sensible heat associated with a temperature change we will use the following equation:

Q space equals space M C increment T space

Where

Q is the required amount of heat, measured in J

M is the mass of the substance being heated, measured in kg

C is the specific heat capacity, measured in J kg-1 K-1

ΔT is the temperature defined as ΔT = (Tf - Ti), where Tf is the final temperature and Ti is the initial temperature measured in K or C.

As a reminder, the conversion between °C and K is linear. To convert °C to K, simply add 273.15. Therefore, the difference between two temperatures is the same if you are working in °C or K.

Let us look at an example you might find in a brewery.

We would like to know how much heat is required to bring our wort to 98°C, close to boiling. We have 7,860 kg wort at 78°C. We will use 4,000 J kg-1 K-1 as the heat capacity for wort.

We will use the previous equation for heat (Q) and enter the values.

Q space equals space M C increment T

Q space equals space 7 comma 860 space k g space cross times space 4000 space J space k g to the power of negative 1 end exponent space K to the power of negative 1 end exponent space cross times space left parenthesis 98 space minus space 78 to the power of o C right parenthesis space equals space 628 comma 800 comma 000 space J space equals space 628 comma 800 space k J

So, 628,800 kJ is the total amount of heat required to increase 7,860 kg wort from 78°C to 98°C.

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