Juniper E Series Hardware Manual page 45

Installing multiple systems in a single rack allows you to maximize your
available space.
You can install an ERX-14xx model, an ERX-7xx model, and an ERX-310
router together in the same rack (see Figure 18).
If you install an ERX-7xx model or ERX-310 router directly above an ERX-14xx
model in the same rack, you must install a plenum between the ERX-7xx
model or ERX-310 router and the ERX-14xx model so that the air can circulate
between the systems (see Figure 54). This plenum is available from Juniper
Networks.
If you install an ERX-14xx model above an ERX-7xx model or ERX-310 router,
there is no need to install a plenum between the units because the ERX-7xx
models and ERX-310 routers vent air out the side of the chassis.
WARNING: To maintain airflow requirements, a plenum (or another ERX-14xx
model) must be installed above an ERX-14xx model before any other piece of
equipment is installed above the router.
Install heavier systems, such as an ERX-14xx model, on the bottom of the
rack. Lighter systems, such as the ERX-310 router, should be mounted
higher in the rack.
Installing E-series Routers
Rack-Mounted Installation
29