5.2. Direct refrigeration systems

In direct refrigeration system, two fluids that exchange energy are physically in contact. In oenology, the inert gases N2 and CO2 are used as refrigerating fluids, since when vaporizing at low temperatures they absorb heat from the must/crushed-grapes.

Of the two inert gases mentioned above, CO2 in solid and liquid forms is the most used in oenology, due to its low sublimation temperature at atmospheric pressure –72.4�C, added contribution of inert atmosphere of conservation, possibility of partial freezing of the must with the consequent sugar concentration, ease of handling and no residues.

The cooling capacity provided by CO2 in solid form is defined by the following expression (Eq. (18)) [4, 8–10, 13]:

$$dQ = \ \rho \times V \times \mathbb{C}\_{\epsilon} \times (T\_1 - T\_2) = M \times (\lambda\_{\text{FS}} + \lambda\_{VP}) \tag{18}$$

If ice is produced due to the freezing of the water from the must to this expression (Eq. (19)), we must add the energy due to the sublimation heat of CO2:

$$Q\_{\rm H\_2O} = m \times \lambda\_{\rm H\_2O}, \text{ resultado} : dQ = m \times \lambda\_{\rm H\_2O} \, + \, \rho \times V \times \mathbb{C}\_{\varepsilon} \times (T\_1 - T\_2) = M \times (\lambda\_{\rm FS} + \lambda\_{\rm VP}) \tag{19}$$

where

m is the formed ice mass (kg).

λH2O is the latent heat of water fusion (0.108 kJ/kg).

ρ: is the must/crushed-grapes density (�C).

V is the must/crushed-grapes volume (m<sup>3</sup> ).

Ce is the specific heat of must/crushed-grapes (kJ/kg �C).

T<sup>1</sup> is the initial temperature of must/crushed-grapes (�C).

T<sup>2</sup> is the final temperature of must/crushed-grapes (�C).

M is the mass of CO2 in solid form required for refrigeration (kg).

λFS is the latent heat of fusion of solid CO2 (573 kJ/kg).

λVP is the latent heat of vaporization of liquid CO2 (393 kJ/kg).

To form 1 kg of ice in the must mass, it is necessary to add 0.58 kg of CO2 to the freezing temperature of the must (–2.5 to 3.0�C). In the refrigeration of musts, wines and crushedgrapes, the formation of ice is unadvisable to increase the risk of subsequent physical and chemical instability, so that for the purpose of calculating the cooling power, generally the energetic term QSB is not taken into account.

According to Boulton et al. [10, 13], in order to reduce the temperature of a white wine must (5�C), it is necessary to use 4.42 kg of solid CO2 for each hectoliter of must. Geankoplis [9] stablishes that due to the energy losses by the deposit walls, this quantity must increase to 4.6 kg/hl.

The use of CO2 in liquid form supposes a decrease of the dissipated energy of the deposit in relation to the solid ice, since in the energy transfer equation the latent heat of fusion of CO2 does not come into play, resulting (Eq. (20)):

$$\mathbf{d}Q = \mathbf{m} \times \lambda\_{\text{H}\_2\text{O}} + \rho \times V \times \mathbb{C}\_e \times (T\_1 - T\_2) = M \times \lambda\_{VP} \tag{20}$$

As stated above, it is not advisable to form ice from the water of constitution of the must or crushed-grapes, whereby the above expression is modified as follows (Eq. (21)):

$$\mathbf{d}Q = \rho \times V \times \mathbb{C}\_{\epsilon} \times (T\_1 - T\_2) = M \times \lambda\_{VP} \tag{21}$$

ρ: is the must/crushed-grapes density (kg/m3 ).

V is the must/crushed-grapes volume (m<sup>3</sup> ).

Ce is the must/crushed-grapes specific heat (kJ/kg �C).

T<sup>1</sup> is the initial temperature of must/crushedgrapes (�C).

T<sup>2</sup> is the final temperature of must/crushed-grapes (�C).

M is the mass of CO2 in solid form required for refrigeration (kg).

λVP is the latent heat of vaporization of liquid CO2 (393 kJ/kg).
