3.1. Air chilling

Air chillers use forced convection to circulate cold air at high speed (4 m/s); thus, the thickness of the boundary film is reduced, and the heat transfer rate is increased.

Air chilling of foods is performed in chilling tunnels or chilling rooms; usually, the chilling tunnels operate continuously, while the batch chilling is used for chilling rooms [4].

In a refrigeration tunnel, air circulates longitudinally, transversally, or vertically (Figures 1 and 2). Two-phase chilling can also be used in order to speed up the process and limit the evaporative weight losses [4]. Figure 3 presents the schematic of the two-phase chilling tunnel. In the first

Figure 1. Air chilling tunnel, with longitudinal circulation of the air (vertical cross-section). 1, evaporator; 2, fan; 3, false ceiling; 4, product; 5, baffle.

Figure 2. Air chilling tunnel, with vertical circulation of the air (vertical cross-section). 1, 3, evaporators; 2, auxiliary fan.

Figure 3. Two-phase chilling tunnel (horizontal cross-section). 1, evaporator; 2, conveyer; I, II, cooling sections.

chilling phase, the air temperature is around �10�C, and its speed is approximately 1 m/s; the products, hanged on the conveyer, travel through section (I) of the tunnel in about 4–5 h. The surface of the products is chilled rapidly in this first section. In the second section of the tunnel (II), air temperature is about 0�C, and its speed is 0.3 m/s; the duration of the chilling process is 10–15 h, until the product reaches a relatively uniform temperature in its entire mass [4]. This system allows the diminishing of the evaporative weight loss compared with the one-phase chilling systems because the surface of the product is quickly cooled in the first phase, and a lower temperature difference between the product and the cooling medium is achieved in the second phase.

Chilling rooms have a lower capacity than chilling tunnels; because of the lower air speed (0.3 m/s), the duration of the chilling process increases. Figure 4 presents some examples of chilling rooms: the air discharge ducts are placed in the upper side of the room, while the air intake ports are placed in the lower part.
