
2. INSTALLATION
B–63463EN/01
6
Heat generated in units and parts installed in a cabinet raises the
temperature of the air inside the cabinet, compared with the external air.
Since the generated heat is radiated from the surface of the cabinet, the
temperature of the air in the cabinet and the outside air balance at certain
heat levels. If the amount of heat generated is constant, the larger the
surface area of the cabinet, the less the internal temperature rises. The
thermal design of the cabinet refers to calculating the heat generated in the
cabinet, evaluating the surface area of the cabinet, and enlarging that
surface area by installing heat exchangers in the cabinet, if necessary.
Such a design method is described in the following subsections.
The cooling capacity of a cabinet made of sheet metal is generally 6 W/°C
per 1m
2
surface area, that is, when the 6W heat source is contained in a
cabinet having a surface area of 1 m
2
, the temperature of the air in the
cabinet rises by 1°C. In this case the surface area of the cabinet refers to
the area useful in cooling , that is, the area obtained by subtracting the area
of the cabinet touching the floor from the total surface area of the cabinet.
There are two preconditions : The air in the cabinet must be circuited by
the fun, and the temperature of the air in the cabinet must be almost
constant. The relationship between the temperature rise inside the cabinet
and the temperature of the external air is represented in the following
expression:
Internal heat loss P [W] x
6[W/m
2
⋅°C] × surface area S[m
2
]×rise in temperature[°C]
A cabinet having a surface area of 4m
2
has a cooling capacity of 24W/°C.
For example, to limit the internal temperature increase to 13°C under
these conditions, the internal heat must not exceed 312W. If the actual
internal heat is 360W, however, the temperature in the cabinet rises by
15°C or more. If the allowable ambient temperature range for the unit is
exceeded, it is necessary to improve the cooling capacity of the cabinet
by using heat exchangers.
2.4
THERMAL DESIGN
OF THE CABINET
2.4.1
Temperature Rise
within the Cabinet