The purpose of the thermal design of the cabinet is to limit the difference
in temperature between the air in the cabinet and the outside air to 10°C
or less when the temperature in the cabinet increases.
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
surface area, that is, when the 6W heat source is contained in a
cabinet having a surface area of 1 m
, 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 following expression must then be satisfied to limit the
difference in temperature between the air in the cabinet and the outside air
to 10°C or less when the temperature in the cabinet rises:
Internal heat loss P [W]
⋅°C] × surface area S[m
]×10[°C] of rise in temperature
For example, a cabinet having a surface area of 4m
has a cooling capacity
of 24W/°C. To limit the internal temperature increase to 10°C under these
conditions, the internal heat must not exceed 240W. If the actual internal
heat is 320W, however, the temperature in the cabinet rises by 13°C or
more. When this happens, the cooling capacity of the cabinet must be
improved using the heat exchanger described next.
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. Section 3.7 explains five heat exchangers supplied by
FANUC. Select one of these according to the application.
If cooling fin A is used for the cabinet, the total cooling capacity of a
cabinet having a surface area of 4 m
in the example above is improved
as follows :
⋅°C × 4m
+ 9.1W/°C= 33.1W/°C
The calculated value verifies that even if the internal heat is 320 W, the
temperature rise can be limited to less than 10°C.
See Section 3.6 for installing the heat exchanger.
THERMAL DESIGN OF