Keeping Your Cool: Heat, Vents, Fans and Airflow in an RF Shielded Test Enclosure
Overview
Every so often a customer calls us with the same story. They put their device in one of our
chambers, closed the door, ran their test — and an hour later the inside of the box was
noticeably warm, or the device throttled back, or a temperature-sensitive instrument
drifted. The chamber was doing exactly what it was designed to do: it sealed the device off
from the outside world. The trouble is that “the outside world” includes the cool air of your
lab.
An RF shielded test enclosure is, thermally speaking, a picnic cooler. It’s a sealed, gasketed
box — and to add insult to injury, the LS-30 absorbing foam that lines the walls to keep your
RF measurements clean is a very respectable thermal insulator. The same open-cell
polyurethane structure that soaks up microwaves also does a fine job of keeping heat from
reaching the aluminum walls. Whatever power your device dissipates has to go somewhere,
and in a shielded chamber, “somewhere” is almost entirely air moving through the
vents.
This paper explains the whole business: why chambers heat up, why the wattage printed on
your device’s label is usually the wrong number to design around, how the honeycomb
vents pass air without passing RF, how much airflow it takes to hold the temperature rise to
a given limit, and why two fans on one chamber don’t necessarily give you twice the
cooling. We’ll keep the math down to one honest rule of thumb in the main body; the full
derivation lives in the appendix for those who want to check our arithmetic.
As usual, we’ll use real JRE chambers as examples — the JRE 1714, the JRE 2525, and the
big JRE 3036, including its new six-vent high-airflow version, which moves enough air to
cool a fifty-phone device farm — with real fan numbers, including the derating nobody’s
spec sheet likes to talk about.
Why the box gets warm
Start with a law you can’t negotiate with: essentially every watt your device draws from its
power source ends up as heat inside the chamber. A 40-watt device is a 40-watt space
heater. (The sliver of energy that leaves as RF down a coax cable or as data down a filtered
line is a rounding error — even a “high power” 1-watt transmitter is only exporting 1 watt.)
In open air on your bench, that heat wanders off on its own — room air drifts past the
device, warm air rises away, and the whole room is the heatsink. Close the chamber door
and you’ve cut off that escape route. Now there are only three exits, and two of them are
lousy:
Exit 1: conduction through the walls. Heat can soak through to the aluminum shell and
radiate off the outside. But the LS-30 foam lining stands in the way — foam is mostly
trapped air, and trapped air is what every insulation product on earth is made of. Some
heat gets through (the foam isn’t magic, and the aluminum floor under the DUT conducts a
little), but it’s a slow, weak path. Don’t count on it for more than a handful of watts.
Exit 2: passive convection through the vents. Warm air is lighter than cool air, so if the
chamber has a low vent and a high vent, warm air drifts out the top and pulls cool air in the
bottom all by itself — a tiny chimney. This works, and it’s free, but the driving pressure is
feeble (we put a number on it in Appendix A.3 — it’s about a thousand times weaker than a
fan). Passive venting comfortably handles a device dissipating a few watts up to maybe 15–
25 watts, depending on the chamber’s vent placement.
Exit 3: forced air — a fan. This is the workhorse, and it’s what the rest of this paper is
about. A fan pushes lab air through one honeycomb vent, across your device, and out
another vent, carrying the heat with it. How much heat it carries is a simple function of how
much air actually moves — which, as we’ll see, is not the number printed on the fan.
Figure 1 shows the whole picture in cross-section: heat rising off the DUT, foam-insulated
walls, cool air entering the low vent and warm air leaving the high one.
Figure 1 — Where the heat goes: a shielded chamber in cross-section
One more definition before we go on, because it matters. When we talk about “temperature
rise,” we mean the rise of the air inside the chamber above your lab’s room temperature.
Your device then runs its own additional rise above that internal air temperature, just as it
would on the open bench. Cooling the chamber air well doesn’t make your device run cold
— it makes the device run the same amount above 30 °C chamber air that it would have
run above 25 °C room air. The chamber’s job is to keep that internal ambient from climbing.
The label lies (politely): nameplate watts vs. actual watts
Here’s the single most common sizing mistake we see, and it errs in the expensive
direction. A customer reads the label on their device — “Input: 12 VDC, 10 A” — multiplies, and
concludes they have 120 watts to remove. Sometimes they read the wall-adapter rating
instead, which is padded even further. Then they call us asking for airflow numbers that
would keep a toaster comfortable.
That nameplate figure is the maximum the device is permitted to draw — a safety and
regulatory number, sized for the worst instant of the device’s life: every radio transmitting,
every peripheral powered, the battery fast-charging, the processor flat out, all at once, plus
margin. It is almost never what the device draws sitting in your chamber running your test.
Real devices have a duty cycle. A radio transmits in bursts and listens in between. A
processor sprints and idles. A charger tapers off. Figure 4 shows the shape of the thing: a
device with a 120 W nameplate, spiking briefly at startup, pulsing up to 65 W during
transmit bursts, and idling near 22 W — for an honest average dissipation around 40 W. And
it’s the average that heats the chamber. Air has enough thermal mass that a few-second
burst just doesn’t move the needle; the chamber temperature settles according to the long-
term average power, not the peaks. Designing your cooling for 120 W when the truth is 40
W means buying three times the airflow you need — or, worse, concluding a chamber
“can’t be cooled” when it can.
Figure 4 — What the label says vs. what the device actually does
So don’t read the label — measure the draw. It’s cheap and quick:
• DC-powered device: run it from a bench supply through the chamber’s filtered DC
interface and read volts × amps right off the supply’s display while your actual test
runs. Watch it for a few minutes and eyeball the average.
• AC-powered device: put a $25 plug-in power meter (the classic “Kill A Watt” style)
between the wall and the device’s supply, run the real test, and read the watts. Most
of these meters will happily show you an averaged figure.
• Battery-powered device under charge: the heat inside the chamber is the
charger’s input power minus the energy actually banked in the battery — but for
sizing purposes, just use the measured wall draw during charge. It’s conservative and
it’s easy.
Measure during the test you’ll actually run, because the duty cycle is the test. A throughput
soak test dissipates a lot more than a receive-sensitivity test on the very same device.
A real example of how far apart the two numbers can sit. A customer building a mobile
device farm — fifty phones on charge in one chamber — budgeted 30 W per phone, the
fast-charge maximum: 1,500 W. But a phone doesn’t fast-charge for twelve hours; it bulk-
charges hard for twenty or thirty minutes and then falls off a cliff, settling near 8 W once
the pack is topped off. The honest sustained load for the whole farm came out around 425
W — about a quarter of the nameplate arithmetic. The 1,500 W figure wasn’t useless: it’s
the right number for sizing the power feed and for a worst-case ceiling check. It was just
the wrong number to design the sustained cooling around.
One heat source people forget while they’re adding up device labels: the power
conversion gear itself. USB hubs, PoE injectors, and power supplies mounted inside the
chamber all burn their own conversion losses as heat — figure roughly 10% of the power
passing through them. A hub whose datasheet says “0.12 W” is quoting its idle draw; when
that hub is pushing a kilowatt out to phones, it’s dissipating an extra hundred watts of its
own. If the supply lives inside the chamber, its losses are your problem.
The honeycomb vent: how air gets through a wall that RF can’t
At first glance a vent in a shielded wall sounds like a contradiction — didn’t we spend two
other papers (this one and this one) preaching that every opening leaks?
The trick is the shape of the opening. Our vents are built from a thick panel of small
hexagonal metal cells — a honeycomb. Each little cell is a short piece of waveguide
operating below cutoff: a metal tube so narrow that radio waves below its cutoff
frequency physically cannot propagate through it — the wave dies off exponentially inside
the tube instead of traveling. Make the tube a few times longer than it is wide and the
attenuation is enormous. Air molecules, of course, couldn’t care less about waveguide
cutoff and stroll right through.
The result is a panel that flows air freely while holding better than 100 dB of shielding at
1 GHz and better than 95 dB at 10 GHz. And here’s a property worth tucking away:
below cutoff, that attenuation actually gets stronger as you go down in frequency. Keep that
in your pocket — it becomes important when we talk about mounting fans.
The honeycomb does exact one toll for its services: flow restriction. Pushing air through
hundreds of narrow cells takes pressure, and that’s the root of the fan derating we’ll get to
shortly.
The one formula that explains it all
Here it is — the entire thermal design of a ventilated chamber in one line:
CFM ≈ 1.76 × watts ÷ allowable rise in °C (or 3.16 × watts ÷ rise in °F)
CFM is cubic feet per minute of air actually moving through the chamber, watts is the
measured average dissipation inside, and the rise is how far above room temperature you’ll
allow the chamber air to climb. The formula is nothing exotic — it falls straight out of the
heat capacity of air, and Appendix A.1 derives it in four lines. Air just isn’t a very dense
fluid, so it takes a fair volume of it to cart away a watt.
Our standard design target is a 10 °C rise. That’s comfortable for nearly all electronics — a
25 °C lab gives you 35 °C chamber air, no worse than a warm summer day — and it keeps
the airflow requirement civilized. If your payload is touchier (precision oscillators, calibrated
instruments), design for 5 °C and expect to need double the airflow. If your DUT is rated for
a hot environment anyway, 15 °C buys you slack in the other direction.
A few landmark points, at a 10 °C rise:
Average dissipation – and the Delivered airflow needed:
25 W ~4.5 CFM
50 W ~9 CFM
100 W ~18 CFM
175 W ~31 CFM
250 W ~44 CFM
500 W ~88 CFM
Figure 2 plots the full picture — required airflow vs. watts for 5, 10, and 15 °C rises — with
our actual fan options marked on it so you can see at a glance what a given fan setup can
carry.
Figure 2 — Airflow needed to hold a given temperature rise
Fans: rated CFM vs. delivered CFM
Now for the fine print that catches nearly everyone.
Every fan CFM figure in our catalog — and in everybody else’s — is a free-air rating: the
flow the fan moves with nothing in its way, blowing from open room into open room. Bolt
that same fan onto a honeycomb vent feeding a foam-lined box that exhausts through
another honeycomb, and the restriction pushes back. The fan slides down its performance
curve, trading flow for pressure. As a practical rule, figure on delivering about half
the free-air rating through a chamber’s vent path. Sometimes you’ll do a bit better; plan
on half and be pleasantly surprised.
Here’s the JRE fan lineup with both numbers side by side (delivered @ 10 °C uses the rule-
of-thumb formula):
Two more rules about combining fans, and these are the ones that bite. A borrowed
electrical analogy makes both of them stick: airflow is like current, static pressure is
like voltage.
Rule 1 — Fans in series (push-pull) add pressure, not flow. Put one fan pushing air in
the intake vent and a second pulling it out the exhaust, and you have two fans on one flow
path — sources in series. Their pressures add, but the flow is bounded by the path:
two 31 CFM fans in push-pull deliver roughly 31 CFM of through-flow, at roughly double the
pressure. That extra pressure genuinely helps — it’s what muscles air through the
honeycomb restriction and keeps the delivered flow up near the rating — but it does not
double your airflow. In steady state every bit of air that comes in must go back out; it’s all
one air stream. If your cooling budget was built on the idea that fans add up, recheck it
before anything else.
Rule 2 — Parallel paths do add flow. Fans ganged side by side, facing the same way,
are current sources in parallel: flows add at the same pressure. Two independent intake-to-
exhaust paths — which requires four vents — move twice the air of one, and three paths
(six vents) move three times. That’s the whole difference between chambers in the next
section.
Figure 3 draws both cases side by side.
Figure 3 — Fans in series vs. fans in parallel
And one shopping tip that surprises people: axial fans come in two very different families —
high flow at low static pressure, and lower flow at high static pressure. A
honeycomb vent is a restriction, and a chamber packed with equipment is another one, so a
high-pressure fan will outperform a glamorous high-CFM low-pressure fan through that path
by a wide margin, even though the low-pressure one looks better on paper. Our super-high-
flow 120 mm unit is the working example: it’s a 38 mm-thick, 4,500 RPM ball-bearing
industrial fan rated to nearly an inch of water column of static pressure — built specifically
to push against backpressure, so it holds much closer to its rated flow in a real restricted
path. A thin, whisper-quiet fan with the same headline CFM would stall against the
honeycomb and deliver a fraction of it. When comparing fans, read the pressure-flow curve,
not the headline number.
The price of pressure is noise. That industrial fan runs about 62 dBA — and six of them
on one chamber lands around 70 dBA at the box. That’s vacuum-cleaner territory: fine in a
lab or server room, miserable beside somebody’s desk. High-pressure, high-RPM fans are
inherently loud, so decide where the chamber will live before choosing the fan. Stepping
down one fan grade often trades a few degrees of rise for a dramatically quieter room — a
real trade-off worth making deliberately rather than discovering on install day.
Worked examples: three chambers, three stories
Model numbers, for reference: the first two digits are the approximate outside width in
inches, the second two the depth — a 1714 is about 17” × 14”. Full details on the selection
guide.
JRE 1714 — small box, surprisingly good lungs
The 1714 (15.75” × 12.75” × 9” inside) carries four vents: two low on the front, two high
on the back. That geometry is no accident. With no fans at all, it forms a natural chimney —
cool air in low at the front, warm air out high at the back — and passive convection alone
will keep a 10–20 W payload happy.
Add fans and the four vents pay off again: two low intakes and two high exhausts make two
parallel flow paths. With standard 31 CFM fans, that’s roughly 31 CFM delivered (two
paths × ~15 CFM each) — call it 175 W of cooling at a 10 °C rise. Step up to the high-flow
84 CFM fans on both paths and delivered flow climbs to ~84 CFM — about 475 W. That’s a
remarkable amount of dissipation for a box you can carry under one arm, and it means the
little 1714, thanks to its four vents, out-ventilates the much larger two-vent 2525.
Example: a Wi-Fi access point with a “54 W PoE” label, measured at 30 W average during a
throughput soak. Passive vents alone would run warmer than we’d like; one standard fan
(~15 CFM delivered → ~85 W capacity) covers it nearly three times over. Done.
JRE 2525 — big box, small lungs
The 2525 is our largest portable chamber (23.5” × 23.5” × 18” inside), and we’ll say it
plainly: it’s the weakest ventilator in the line. It has two vents — one low on one side, one
high on the other. Two vents means exactly one flow path, and one flow path means one
fan’s worth of delivered air, no matter how many fans you bolt on (Rule 1 above).
The ceiling for a 2525 is therefore the biggest fan that fits the vent: the 84 CFM high-flow
80 mm unit, delivering ~42 CFM through the path — about 240 W at a 10 °C rise. There
is no larger fan and there is no third vent. Two vents is two vents.
Example — a real case from our inbox. A research lab housed a set of sensitive digitizer
scopes in a 2525, fitted with two standard ~31 CFM fans in push-pull, and asked us for 150
CFM. Two things needed untangling. First, their two fans were delivering ~15 CFM, not 60 —
series fans don’t add (they’d assumed they did). Second, 150 CFM at a 10 °C rise
corresponds to about 850 W of dissipation — and when we asked for the measured draw
rather than the sum of the nameplates, the requirement came down substantially.
Upgrading the exhaust position to the 84 CFM high-flow fan tripled their delivered airflow in
the same chamber. The lesson runs both directions: check what your fans actually deliver,and check what your equipment actually dissipates, before concluding a chamber can’t do
the job.
JRE 3036 — when you genuinely need big airflow
If the arithmetic demands more than ~240 W of heat removal, the answer is the
3036. The standard build carries four 80 mm vents — two intakes, two exhausts, two
parallel paths. With high-flow 84 CFM fans that’s roughly 84 CFM delivered, or about 475
W at a 10 °C rise in a reasonably open chamber. (Pack the chamber wall-to-wall with racks
and equipment and the airflow pays a second toll beyond the honeycomb — figure less.)
And for the truly big loads, there’s now a six-vent version. Recently made a
standard build option — no custom engineering charges — the high-airflow 3036 carries six
120 mm vents: three low across the front, three high across the back. That’s three
parallel push-pull paths, placed so cool air enters low at the front where equipment sits,
sweeps the floor, and exits high at the rear — working with the rising heat instead of
fighting it. With the three 120 mm fan grades, the six-vent 3036 delivers (derated by half,
as always):
Fan choice (× 6) Delivered through-flow Heat removed @ 10 °C rise
Standard: 50 CFM ~75 CFM ~425 W
High-flow: 100 CFM ~150 CFM ~850 W
Super-high-flow: 205 CFM ~307 CFM ~1,700 W+
That top row is kilowatt-and-a-half territory — full device-farm scale — from a chamber that
still holds its full shielding spec, because every one of those six openings is a proper
waveguide-below-cutoff honeycomb, fabricated and final-tested by us.
Example — a real device-farm build. A customer sized a fifty-phone test farm for a 3036:
phones on fast charge, four big USB hubs, two Wi-Fi access points, and a switch. Nameplate
arithmetic said ~1,650 W; adding the hubs’ ~10% conversion losses put the true worst-
case ceiling near 1,800 W. But the sustained twelve-hour load — once the phones’ bulk
charge tapered to trickle — measured out around 425 W. On the six-vent 3036 with super-
high-flow fans (~307 CFM delivered), that’s about a 2 °C rise at the real load and only
~10 °C even at the full ceiling: in a 25 °C lab, roughly 27 °C inside during normal operation
and the mid-30s at absolute worst case. Lithium batteries don’t even clear their throat at
those temperatures. The same customer had previously run a similar farm in a chamber
with standard 80 mm venting — a ~35 °C-plus rise at that load — and cooked a cage of
phones into bloated batteries. Same physics, opposite outcomes: the difference was
nothing but delivered CFM. (Bloating cells are a fire hazard, not a nuisance — if your
payload is batteries, size the cooling with margin, not just adequacy.)
Sidebar: big watts size the power entry, too. The same measured wattage that sizes
your airflow also sizes your filtered power feed. Our PEM-1 filtered power entry module is
rated 10 A / 1,200 W at 120 V — so an 1,800 W ceiling is out of reach for one module, and
even two would run near 70% of rating for twelve-hour stretches. A filter running cool is a
filter that lasts: for that farm we specified three PEM-1s, each on its own wall circuit, each
loafing along under half its rating — with the side benefit that a tripped breaker takes down
a third of the farm instead of half. If your load is up in the kilowatts, plan the power entry
and the cooling from the same measured number.
The lineup at a glance (10 °C rise, honeycomb derating included)
But doesn’t a fan wreck the shielding?
This is the good question, and it has a clean two-part answer.
Radiated noise through the vent: not a problem. The honeycomb’s attenuation is a
property of the honeycomb’s own geometry — it does not care what’s sitting on the other
side of it. It provides its 100 dB whether there’s a fan in front of it, behind it, or nothing at
all. Better still, remember that pocketed fact from earlier: below cutoff, the attenuation gets
stronger at lower frequencies — and a brushless fan motor’s commutation noise lives in the
kHz-to-low-MHz range, exactly where the honeycomb is at its very best. Radiated fan noise
coming through the mesh is a non-issue.
Conducted noise on the wiring: this is the real hazard. A fan is a conducted noise
source before it’s a radiated one, and its power leads are conductors. If a fan lead
penetrates the chamber wall, shares a ground path with anything running inside, or simply
lies alongside your signal cabling on the way in, the honeycomb does nothing for you —
you’ve built the classic unfiltered-conductor leak our filtered-I/O paper warns about. This is
precisely why JRE fans mount entirely outside the shielded boundary and run from an
independent filtered external supply that never touches the I/O plate and never
touches your DUT’s power rails. Nothing new penetrates the wall, and the fan’s electrical
life stays outside the fence.
If you mount a third-party fan, follow the same rules: fan and all wiring stay completely
outside the shield; nothing new penetrates the wall; and if you must drill mounting holes,
fill every one with a metallic fastener — stainless preferred, seated tight — so the hole is
metal, not air.
Practical habits for a cool chamber
• Measure, don’t read. Actual average watts, during the actual test. The label is a
legal maximum, not a thermal fact.
• Intake low, exhaust high. Warm air wants to rise; arrange the flow so gravity helps
instead of fights.
• Don’t smother the vents. A DUT, foam block, or cable bundle parked against the
inside of a vent chokes the path. Leave breathing room on both faces, outside too.
• Give the air a route past the DUT. Airflow only removes heat from what it touches.
Position the device in the stream between intake and exhaust, not in a dead corner.
• Halve the fan rating in your head. Free-air CFM is an optimistic stranger;
delivered CFM is your friend.
• Don’t stack fans on one path expecting double flow. Series buys pressure,
parallel buys flow.
• Check it with a thermometer — but respect the shield. Simplest of all: an
infrared thermometer or thermal viewer. Pop the door at the end of a run and shoot
the DUT and interior surfaces — those hold their heat for well over a minute, so a
quick read is honest even though the air escapes immediately. For a record over time,
a $10 min/max or data-logging thermometer sitting inside the chamber logs while you
run and tells the story when the door opens. Two tempting shortcuts to avoid: a
wireless thermometer won’t work — if the chamber is doing its job, that signal isn’t
going anywhere — and pinching a probe wire through the door gasket breaks the very
seal you’re testing inside of. If you truly need a live readout with the door shut, run a
thermistor or thermocouple through a filtered terminal interface like any other low-
frequency line, and it crosses the wall legitimately.
• Duty-cycle your test, not just your device. If the chamber trends warm during an
all-day soak, a brief door-open between runs resets the clock.
Summary
A shielded chamber isolates your device from RF — and, as a side effect, from the lab’s cool
air. The foam that keeps your measurements honest also insulates the walls, so nearly all
the heat must leave as warm air through the honeycomb vents, which pass air freely while
blocking RF by better than 100 dB. The sizing rule fits on a sticky note: CFM ≈ 1.76 ×
watts ÷ °C of rise, using measured average watts — not the nameplate, which reflects a
worst-case duty cycle your test probably never reaches (a farm of fast-charging phones is a
quarter of its label once the packs top off — and don’t forget the ~10% the hubs and
supplies burn on their own). Derate every fan’s free-air rating by about half for the
honeycomb restriction; remember that push-pull fans on one path add pressure but not
flow, while parallel vent paths genuinely multiply it; favor high-static-pressure fans against
the honeycomb, with eyes open about their noise; and keep all fan wiring outside the
shield. Do that arithmetic honestly and a 1714 cools 175 W with standard fans and 475 W
with high-flow, a 2525 cools 240 W, a standard 3036 cools 475 W — and the six-vent high-
airflow 3036 carries over 1,700 W, enough to keep a fifty-phone farm within a couple of
degrees of room temperature.
Questions about a hot payload, or a configuration that doesn’t fit the standard recipes?
Contact us — sizing this correctly the first time is a ten-minute phone call. More application
notes live on our resources page.
Appendix: The math behind the airflow
A.1 Where 1.76 comes from
Moving air carries heat according to:
P = ṁ · c_p · ΔT
where P is the heat carried away (watts), ṁ is the mass flow of air (kg/s), c_p is air’s specific
heat (≈ 1005 J/kg·°C), and ΔT is how much the air warms between intake and exhaust —
which, in steady state, is the chamber’s temperature rise.
Mass flow is density × volume flow: ṁ = ρ · V̇, with ρ ≈ 1.2 kg/m³ for sea-level air near
room temperature. Solving for volume flow:
V̇ = P / (ρ · c_p · ΔT) = P / (1206 · ΔT) [m³/s]
Converting to cubic feet per minute (1 m³/s = 2119 CFM):
CFM = 2119 · P / (1206 · ΔT) ≈ 1.76 · P / ΔT(°C)
For Fahrenheit, ΔT°F = 1.8 · ΔT°C, giving CFM ≈ 3.16 · P / ΔT(°F). That’s the whole
derivation — the constant is just the heat capacity of air dressed up in mixed units.
A.2 Altitude and temperature corrections
The 1.76 assumes sea-level air density. Thin air carries less heat per cubic foot, so at
altitude, scale the required CFM up by the density ratio: about +4% per 1000 ft. A lab in
Denver (5280 ft) needs roughly 20% more airflow for the same watts and rise. Hot intake
air matters the same way, but the effect is small (≈ 3% per 10 °C of intake temperature) —
the bigger issue with a hot lab is simply that “room + 10 °C” starts from a worse place.
A.3 Why passive convection is feeble: the stack effect
With a low vent and a high vent separated by height h, warm chamber air creates a natural
draft pressure:
ΔP = ρ · g · h · (ΔT / T)
For a 2525-class chamber — h ≈ 0.35 m, ΔT = 10 °C, T ≈ 295 K — that’s ΔP ≈ 1.2 × 9.81 ×
0.35 × (10/295) ≈ 0.14 pascals. A typical 80 mm fan develops static pressure in the range
of 20–40 Pa: a couple of hundred times more. Squeezing flow through two restrictive
honeycomb panels on 0.14 Pa yields only a few CFM — which, by the formula in A.1, carries
a couple dozen watts at most with a generous rise. That’s why passive venting is fine for
small payloads and hopeless for big ones.
A.4 Fan curves, system curves, and where “half” comes from
A fan’s datasheet curve runs from free-air flow (maximum CFM, zero pressure) down to
stalled (zero CFM, maximum static pressure). The chamber — two honeycomb panels, the
foam-lined interior, whatever clutter the air must dodge — presents a system curve:
pressure drop rising roughly with the square of flow. The fan operates where the two curves
cross. For typical axial fans against a two-honeycomb path, that crossing lands near 40–
60% of free-air flow, which is where our plan-on-half rule comes from. It also explains the
fan-selection tip in the main text: a fan whose curve stays flat (holds pressure) as flow rises
keeps its operating point high against a restriction, while a fan with a steep, droopy curve
— however grand its free-air number — collapses to a trickle. And it explains why a second
series fan helps some: adding the two pressure curves shifts the crossing point up the
system curve — more flow, but nowhere near double.
