Freshly harvested berries measuring

Freshly harvested berries measuring 2 cm in diameter are chilled from 18ºC to 7ºC in a
chiller at
2ºC, with a surface heat transfer coefficient of 16 W m
2 K
1
. They are then
loaded in 250 kg batches into containers and held for 12 h in a cold store operating at

2ºC, prior to further processing. The cold store holds an average of 2.5 t of food and
measures 3 m high by 10 m  10 m. The walls and roof are insulated with 300 mm of
polyurethane foam, and the floor is constructed from 450 mm of concrete. The ambient
air temperature averages 12ºC and the soil temperature 9ºC. An operator spends an
average of 45 min day
1 moving the containers in the store and switches on four 100 W
lights when in the store. Each container weighs 50 kg. Calculate the time required to cool
the berries in the chiller and determine whether a 5 kW refrigeration plant would be
suitable for the cold store. (Additional data: the thermal conductivity of the berries is
0.127 W m
1 K
1
, the thermal conductivity of the insulation is 0.026 W m
1 K
1
, the
thermal conductivity of the concrete is 0.87 W m
1 K
1 (Table 1.5), the specific heat of
the berries is 3778 J kg
1 K
1
, the specific heat of the container is 480 J kg
1
K
1
, the
density of berries is 1050 kg m
3
, the heat produced by the operator is 240 W, and the
average heat of respiration of berries is 0.275 J kg
1 s

1
.)

Kết quả (Anh) 1:

          º º   º            .                º                                    º   º                                                ( :                          ( )                                   .)