DVN 5000
Environmental Guidelines
16
24-10520-22 Rev. –
This document contains confidential and proprietary information of Johnson Controls, Inc.
© 2009 Johnson Controls, Inc.
Simulations and real-world testing show that moving air through a cabinet from
bottom to top results in the lowest internal cabinet temperatures.
During normal operation and in the event of fan failure, downward airflows are not
recommended, since they create
mixed convection
(a mixture of forced air and
convection).
Some equipment use internal fans that draw air in through the rear (or sides) and
exhaust out the sides (or rear), which re-circulates the cabinet air. Care should be
taken as to the placement of this type of equipment so the natural convective rise of
heat is not disturbed. For example, do not place this type of rack tightly against a
wall.
Hot air rises. The hotter it gets, the more CFM flow occurs by natural convection.
The friction of all vents gets in the way of the flow; more open area, in the form of
slots or perforations, is always better.
Planning Airflow: Passive Convection
At normal room temperature, a rack can dissipate 300 to 500 watts (only 1 DVN per
rack) of heat through natural convection (no fan assist). This requires adequate vent
openings at the bottom and top of the unit (none in the middle for effective
chimney
flow), and an unimpeded airflow inside the rack.
When using passive convection, limit the number of DVN 5000 units in a
rack to 1. Installing additional DVNs or DVRs in the same rack with
passive convection cooling will cause the DVN to overheat, resulting in
drive failures.
The main advantage of natural convection is its intrinsic reliability. Air movement in
a properly configured cabinet is generated by
thermal gradients
. Proper
configuration most importantly includes optimization of component placement.
Hotter equipment located lower in the rack provides a better natural airflow.
Calculating airflows in a passive convection enclosure is complicated. The slow
speed of airflow makes it nearly impossible to measure, and smoke tests show air
can enter and exit from the same vent.
Equipment that passively vents (without fans) sometimes has intake vents on the
bottom or vents on the top, so take care not to block these with equipment stacked
directly on top of each other; otherwise, it is acceptable to stack equipment directly
on top of each other.
Many times installers simply put vents between each piece of equipment without
regard to the re-circulation of hot air. This can
short-circuit
the airflow because the
vents are placed too close to fans or heat sources.
Good airflow strategies break the temperature stratification areas, which cause
hot-spots
. In some cases you may have to re-arrange the vents and equipment.