5.1. Refrigeration of indirect methods

The indirect refrigeration systems most commonly used in the winery are plaque exchangers, tubular exchangers and spiral exchangers. The tubular exchangers consist of a central conduct, of small diameter through which the must or wine circulates, which is concentrically in the interior of a tube of greater diameter through which circulates the refrigerating fluid. The diameter of the inner tube is determined by the viscosity of the fluid flowing through it. Diameters greater than 75–90 mm are recommended for musts and crushed-grapes; for wine, diameters of 50 mm are optimal. The exchange surface and the number of tubes required by the thermal transfer equations will be seen below.

The plaque exchangers consist on fine rectangular surfaces through which the refrigerant and must circulate counterflow. The separation between the plaques does not exceed 10mm and have rough surfaces to create turbulence between the exchange fluids

In the spiral exchangers, the must/crushed-grapes enter at one end and run along a spiral path until it leaves the center axis. At the same time, the refrigerant fluid counterflows on the opposite side.

The cooling power generated by the refrigerant in the heat exchanger is defined by the general energy transfer equation (Eq. (15)) [8, 9, 24]:

$$\mathbf{dQ/dt} = \rho \times V \times \mathbf{d}T/\mathbf{dt} = F\_T \times \mathbf{U} \times \mathbf{S} \times (\Delta T)\_{\text{ml}} = W \times \mathbf{C}\_t \times (t\_1 - t\_2) = W' \times \lambda \tag{15}$$

where

dQ/dt is the dissipated energy per unit of time (kJ/h).

ρ is the must density (kg/m<sup>3</sup> ).

V is the tank volume (m<sup>3</sup> ).

dT/dt is the must temperature variation per unit of time (�C/h).

FT is the correction factor depending on the must and the refrigeration fluid. It is a measure of the thermal efficiency of the exchange.

U is the global coefficient of heat transfer (w/m2�C). Characteristics of each type of exchanger and the conditions of vinification and outdoor environment.

S is the exchange surface (m<sup>2</sup> ). (ΔT)ml is the logarithmic mean temperature difference between glycol water and must (Eq. (16))

$$(\Delta T)\_{ml} = \frac{(t\_{em} - t\_{sa}) - (\ t\_{sm} - t\_{\text{et}})}{\ln \frac{(t\_{em} - t\_{u})}{(t\_{simu} - t\_{u})}} \tag{16}$$

W is the mass flow of refrigerant (kg/h).

5. Refrigeration production techniques for cooling musts and crushed-

For the refrigeration of white wine must, until the temperature of the debourbage or cryomaceration and for the decrease of the temperature of red wine crushed-grapes to the one stablished in fermentation, indirect and direct systems of heat transfer may be used. Among the indirect systems of possible application in the winery, the refrigeration equipment with scraped surface evaporator or with evaporator of concentric tubes and the exchangers are described. As direct methods, CO2 solids and liquid CO2, known as carbonic snow, are used.

The indirect refrigeration systems most commonly used in the winery are plaque exchangers, tubular exchangers and spiral exchangers. The tubular exchangers consist of a central conduct, of small diameter through which the must or wine circulates, which is concentrically in the interior of a tube of greater diameter through which circulates the refrigerating fluid. The diameter of the inner tube is determined by the viscosity of the fluid flowing through it. Diameters greater than 75–90 mm are recommended for musts and crushed-grapes; for wine, diameters of 50 mm are optimal. The exchange surface and the number of tubes required by the thermal transfer equa-

The plaque exchangers consist on fine rectangular surfaces through which the refrigerant and must circulate counterflow. The separation between the plaques does not exceed 10mm and

In the spiral exchangers, the must/crushed-grapes enter at one end and run along a spiral path until it leaves the center axis. At the same time, the refrigerant fluid counterflows on the

The cooling power generated by the refrigerant in the heat exchanger is defined by the general

FT is the correction factor depending on the must and the refrigeration fluid. It is a measure of

U is the global coefficient of heat transfer (w/m2�C). Characteristics of each type of exchanger

dQ=dt ¼ ρ � V � dT=dt ¼ FT � U � S � ðΔTÞml ¼ W � Ce � ðt1– t2Þ ¼ W<sup>0</sup> � λ (15)

have rough surfaces to create turbulence between the exchange fluids

grapes in prefermentative operations

5.1. Refrigeration of indirect methods

energy transfer equation (Eq. (15)) [8, 9, 24]:

ρ is the must density (kg/m<sup>3</sup>

S is the exchange surface (m<sup>2</sup>

the thermal efficiency of the exchange.

V is the tank volume (m<sup>3</sup>

dQ/dt is the dissipated energy per unit of time (kJ/h).

).

).

dT/dt is the must temperature variation per unit of time (�C/h).

and the conditions of vinification and outdoor environment.

).

tions will be seen below.

opposite side.

84 Refrigeration

where

Ce is the specific heat of refrigerant fluid (kJ/kg�C).

tem is the initial temperature of must (�C).

tsm is the final temperature of must (�C).

W' is the vaporization rate of the refrigerant (kg/h).

λ is the latent heat of vaporization of the refrigerant (kJ/kg).

Geankoplis [9] and McCabe et al. [8] propose a formula (Eq. (17)) for the calculation of the thermal transfer coefficient U, the Donohue expression applied for low values of the Reynolds number, Re:

$$\begin{split} \frac{\mathrm{LUD}\_o}{k} &= 0.2 \left( \frac{D\_o \sqrt{G\_a} G\_m}{\mu} \right)^{0.6} \\ \left( \frac{\mathcal{C} \mu}{k} \right)^{0.33} \left( \frac{\mu}{\mu v} \right)^{0.14} \end{split} \tag{17}$$

U is the thermal exchange coefficient (w/m2�C).

k is the thermal conductivity of the must (w/m2�C).

Do is the outer diameter of the tubes (m).

Ga is the mass transfer rate of the glycol water (kg/m2 h) equal to ma/sb.

Gm is the mass transfer speed of must/crushed-grapes (kg/m2 h) equal to mm/Sc.

ma, mm is the mass per unit of time of glycol water and must in circulation (kg/h).

Sa, Sc is the internal and external contact surfaces as a function of the number of tubes (m2 ).

μ: is the must/crushed-grapes viscosity (cP).

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

μ<sup>w</sup> is the specific heat of glycol water (kJ/kg�C).

This complex expression is difficult to apply in practice by relying on factors not always known in oenology, so in a generally accepted form empirically obtained values of U are used.

For tubular exchangers used in musts, the value of U is set at 400–1300 w/m2�C for smooth tubes and at 1700–2400 with rough surface [16], from 500 to 900 w/m2�C [8], from 600 to 900 w/ m2�C [13], from 750 to 1200 w/m2�C [34], from 700 to 1100 w/m2�C [3, 35].

In plate exchangers U = 2900–4800 w/m2�C [10], at 3500–6500 w/m2�C [8], from 2400 to 2600 w/ m2�C [10, 13], of 2000 w/m2�C [3].

In spiral exchangers U = 1700 w/m2�C [10], of 2000–2100 w/m2�C [8], from 760 to 1060 w/ m2�C [10, 13, 36].
