IT-05

Rack Cooling (ASHRAE)

ASHRAE thermal guidelines · EIA-310-E · ANSI/BICSI 007-2024 · NEC 310.15(B)

01 · Heat load

Equipment load per rackⓘ

Number of racks

Building envelope and adjacent gainsⓘ

02 · Room

Room floor area

m²

Lighting loadⓘ

W/m²

Occupants (worst case)ⓘ

03 · Cooling design

ASHRAE thermal class

Target inlet air temperatureⓘ

°C

Permitted temperature rise across the rackⓘ

Cooling redundancy

Safety margin

Awaiting input11 fields left: Equipment load per rack · Number of racks · Building envelope and adjacent gains · Room floor area · Lighting load · Occupants (worst case) · ASHRAE thermal class · Target inlet air temperature · Permitted temperature rise across the rack · Cooling redundancy · Safety margin

Capacity and layout

The drawing appears when the inputs are complete

01
Size cooling and airflow for an equipment room from rack heat load, target inlet temperature and permitted rise, checked against the ASHRAE thermal class envelope.
02
Tip. The inlet temperature is what the equipment actually experiences, and it is what the ASHRAE envelope governs. A room held at the right average can still exceed the envelope at a rack inlet if exhaust air is allowed to recirculate around the side of the cabinet.
03
18 – 27 °C — ASHRAE A1–A2 inlet
04
3.412 — Watts to BTU/h
05
3517 W — One ton of cooling
06
11 K — Typical rise across a rack

ASHRAE thermal guidelines

Virtually all the electrical power an equipment rack consumes leaves it as heat, which makes thermal load a direct consequence of the power schedule rather than an estimate. What the cooling system has to achieve is not an average room temperature but an inlet temperature at each rack, inside the envelope the equipment is rated for.

Heat load

Equipment power converts to heat one for one. People and lighting add to it; room size does not — floor area only sets the heat density, which decides what kind of cooling is appropriate.

Use cases

1 W = 3.412 BTU/h
100 W sensible heat per seated occupant
Lighting counts at its full wattage × floor area
3517 W = one ton of refrigeration

Formula

Total W = (equipment + lighting + people × 100 + envelope) × (1 + margin)

ASHRAE thermal guidelines

Airflow and inlet envelope

Airflow follows from the heat to be removed and the temperature rise permitted across the rack. The inlet condition is what the equipment warranty actually depends on.

Use cases

Class A1–A2 inlet range 18–27 °C
Humidity typically 40–60 % RH
Front-to-rear airflow, sealed blanking panels
Add margin for growth — 1.25× is common practice

Formula

V̇ = Q ÷ (ρ × cp × ΔT)

Typical ΔT across a rack ≈ 11 K

Room dimensions do not add heat. They are used only to derive heat density in watts per square metre, which indicates whether comfort cooling, close-control cooling or in-row containment is the appropriate system type.