
3. INSTALLATION
B–64113EN/01
18
The internal air temperature of the cabinet increases when the units and
parts installed in the cabinet generate heat. 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.
The air in the cabinet must be circulated by the fan to prevent an extreme
uneven temperature distribution.
For example, the following expression must be satisfied to limit the
difference in temperature between the air in the operator’s panel cabinet,
which accommodates the control unit, and the outside air to 13°C or less
even when the temperature in the cabinet rises.
Internal heat loss P [W] x
6[W/m
2
⋅°C] × surface area S[m
2
]×13[°C] of rise in temperature
(A cooling capacity of 6 W/°C assumes the cabinet is so large that
agitation with the fan motor does not make the temperature distribution
uniform. For a small cabinet like the operator’s panel, a cooling capacity
of 8 W/°C, indicated in Subsection 3.4.4, may be used.)
For example, a cabinet having a surface area of 4m
2
has a cooling capacity
of 24W/°C. 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. When this happens, the cooling capacity of the cabinet must be
improved using the heat exchanger.
For the power magnetic cabinet containing an I/O unit for Series 0i, the
internal temperature rise must be suppressed to 10°C or less, instead of
13°C.
If the temperature rise cannot be limited to 10°C by the cooling capacity
of the cabinet, a heat exchanger must be added. The heat exchanger
forcibly applies the air from both the inside and outside of the cabinet to
the cooling fin to obtain effective cooling. The heat exchanger enlarges
the surface area.
3.4
THERMAL DESIGN
OF THE CABINET
3.4.1
Temperature Rise
within the Cabinet
3.4.2
Cooling by Heat
Exchanger