Cooling System Operation - Cisco CRS-1 System Description

Cooling System
Figure 20: Air Filter
1 Air filter cover plate
Note
A lattice of wire exists on both sides of the air filter with an arrow that denotes airflow direction and a pair
of sheet metal straps on the downstream side of the filter assembly.

Cooling System Operation

The chassis cooling system uses multiple fan speeds to optimize cooling, acoustics, and power consumption.
There are five normal operating fan-speeds and one high-speed setting used when a fan tray has failed.
At initial power up, the routing system control software powers on the fans to 2,950 RPM, then changes based
on the average temperature. This provides airflow during system initialization and software boot, and ensures
that there is adequate cooling for the system in case the software hangs during boot. The fan control software
initializes after the routing system software boots, which can take three to five minutes. The fan control
software then adjusts the fan speeds appropriately.
During normal operation, the system averages the temperatures reported by inlet temperature sensors in the
card cage. To determine the appropriate fan speed for the current temperature, the fan control software compares
the averaged inlet temperature to a lookup table that lists the optimal fan speed for each temperature. The
software then sets the fan speed to the appropriate value for the current temperature. The temperature ranges
in the lookup table overlap to ensure a proper margin to avoid any type of fan speed oscillation occurring
between states.
Note
When there are no active alarms or failures, the fan control software checks temperature sensors every one
to two minutes.
Thermal Alarms
Local thermal sensors (on individual cards) monitor temperatures and generate a thermal alarm when the
system is not cooling properly. A temperature sensor might trip in response to elevated ambient air temperature,
a clogged air filter or other airflow blockage, or a combination of these causes. A fan failure causes a fault
message, but if no thermal sensors have tripped, the fan control remains unchanged.
2 Chassis air filter
Cisco CRS Carrier Routing System 8-Slot Line Card Chassis Enhanced Router System Description
Cooling System Operation
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