LISTA DE FIGURAS
1.1 Marco Teórico
1.1.4 Comportamiento a la flexión
At the maximum speed of the stirrer the temperature gap between the oil and the pan surface was still too large. This was due to a low heat transfer rate between the oil and
110 115 120 125 130 135 140 0 10 20 30 40 50 60 70 80 90 100 110 Te mperature ( ˚ C) Time (min) Oil Surface of pan 400 rpm 750 rpm 1000 rpm 1300 rpm 1600 rpm put pastry put pastry
48 Chapter 3: Model food and cooking system development
the pan. To determine this effect, the feature of heat transfer in this cooking pan system was drawn and illustrated in Figure 3.17. The diagram displays the cross section of the pan sheet and the temperature of oil (Toil), surface of pan (Tsi) and a boundary layer (x0) (Figure 3.17).
Figure 3.17 Consideration of resistances to heat transfer in pan cooking system
From the Figure 3.17, it can be seen that for a heat convection system a boundary layer normally occurs across the exchanger surface. A boundary layer is the layer of fluid in the immediate locality of a bounding surface where the velocity of the fluid becomes quiescient. A boundary layer interferes with heat transfer in a convection system. However if the boundary layer can be reduced then the heat transfer coefficient (h) can be increased (Equation 3.1). The heat transfer coefficient (h) has a relationship with the heat flux (
I
) (Equation 3.2).0
x k
h (3.1)
and I hA(Toil Tsi) (3.2) where
I
is heat transfer rate across the surface area of the pan (J·s-1 or W), h is heattransfer coefficient (W·m-2·
K-1), A is heat exchange surface area (m2), Toil, Tsiare the temperatures of the oil and surface of the pan (K), respectively, k is thermal
conductivity (W·m-1·
K-1) and x0 is a boundary layer of unmoving fluid (m).
Equation 3.2 shows that the the heat transfer rate (
I
) is a direct function of the heat transfer coefficeint (h) and heat exchanger surface area (A). According to Equation 3.2, increasing the heat exchange surface area (A) and the heat transfer coefficient (h) can reduce the temperature difference between the temperature of the oil and thetemperature of the pan’s surface. Increasing the heat exchange surface area (A) can be achived by adding fins. However, this method is costly and would only have a limited effect.
Chapter 3: Model food and cooking system development 49 Considering Equation 3.1, a high value of the heat transfer coefficeint (h) is caused by a high thermal conductivity (k) and a low thickness of the boundary layer (x0). The
thermal conductivity (k) is a specific property of the heating media (oil). Consequently by manipulating conductivity it is possible to increase the heat flux. A comparison of the properties of heat transfer oil and cooking oil is shown in Table 3.1. The requirement properties such as a high thermal conductivity (k), high specific heat (cp)
and low viscosity (η) were determined as a criteria for selection.
Table 3.1 shows that cooking oil has marginally better thermophysical qualities than the heat transfer oil in terms of higher thermal conductivity (k), higher volumetric density (ρ) and lower viscosity (η). However, the heat transfer oil offers a greater stability for avoiding rancidity and polymerisation over long term use. In addition, the chemical and physical properties of the heat transfer oil vary less over the temperature range of interest compared with the cooking oil. Therefore, the heat transfer oil was chosen as the heating media.
Table 3.1 Specification of heat transfer and cooking oil
Key properties Thermal
conductivity (k, W·m-1·K-1) Viscosity (η,cP) Heat capacity (cp,kJ·kg-1·K-1) Density (ρ, kg·m-3)
Heat transfer oil1 0.142 at 38˚C 43 at 40˚C 1.90 at 38˚C 0.85 at 38˚C 0.130 at 260˚C 6.4 at 100˚C 2.76 at 260˚C 0.71 at 260˚C Cooking oil2 0.16-0.22 30-40 at 40˚C 1.67 0.9
8 at 100˚C
Sources: 1http://www.chevronlubricants.com 2/12/2010: 5:09pm; 2(Abramovi & Klofutar, 1998 and Demirbas, 2008)
The cooking system was run to test the effect of the heat transfer oil on the heat transfer rate across the heat exchanger area. The temperature profile of the oil, pan, and pastry were recorded and are presented in Figure 3.18.
The result shows that the oil temperature was very close to the set point of 140˚C after 10 minutes of processing and showed a stable profile throughout to the end of the
50 Chapter 3: Model food and cooking system development
process (Figure 3.18). The temperature of all positions on the pan reached the steady state quickly (within 10 minutes) and they were close to the oil temperature at all points. It can be concluded that the heat transfer efficiency of this system was good for both the rate and uniformity of heat transfer. However the temperature difference between the oil and the pan was still large at around 4˚C.
A pastry sample was placed on the pan when the temperature was constant after 30 minutes. After placing the pastry on the pan, the temperature of the pastry quickly increased within 5 minutes, and came up to a temperature near that of the pan surface temperature but it took another 25 minutes to rise the last 5°C to match the surface temperature of the pan. The long time taken for the pastry to come up to the temperature of the pan means that non-isothermal conditions existed throughout the heating process, so increasing the temperature quicker is needed. The next step explored stirring options to achieve this result.
Figure 3.18 The temperature profile of oil and pan at different positions