Outdoor 5G Cabinet Cooling: Dust, Pressure

An outdoor cabinet has three problems at once, and only one of them is heat

An outdoor radio cabinet has to reject the heat of its own electronics, survive the weather that reaches it, and stay serviceable on a site that nobody visits.

Heat is the problem that gets sized. The other two get decided, usually by omission, and they decide the answer before any fan is chosen: a sealed cabinet cannot be ventilated, a coastal site cannot tolerate a filter that stays wet, and a remote pole site cannot absorb a maintenance interval measured in months.

The heat load itself is larger than the equipment list. Take a 600 x 600 x 1 800 mm outdoor cabinet with 800 W of electronics, light grey paint, standing in full sun at a 45 °C site. The sunlit skin absorbs 576 W and the walls give back 257 W, so the air path has to remove about 1 120 W. Slightly over half of that load never came from the electronics at all.

The weather is not vague either. ETSI EN 300 019-2-4 defines what a non-weatherprotected telecom site does to equipment, and the definition is more severe than most cabinet drawings assume: 1 120 W/m2 of solar radiation, 15 mm/min of impacting rain, 100 % relative humidity at +30 °C with condensation, ten days of salt mist, 20 mg/(m2 h) of dust settling on every square metre, and a wind speed of 50 m/s.

The service problem is the one that changes the architecture. A cabinet that needs its filter changed every three months needs sixty visits over a fifteen year life, each one a vehicle and a technician at a site that may be a pole beside a road or a mast on a roof. Once those visits are costed, a closed air path with no filter begins to look cheaper than the open one, and the rest of this article is about the numbers that decide it.

The environmental class is already written down, and it already assumes the sun

A cabinet drawing that says “outdoor” and nothing else is not a specification. For telecom equipment there is a published class with published severities.

ETSI EN 300 019-1-4 classifies the environmental conditions and EN 300 019-2-4 specifies the tests that verify them. Class 4.1E is the extended class for a stationary, non-weatherprotected location - a pad or a pole or a mast, not a shelter - and it is the class most outdoor radio cabinets are actually deployed under, whether or not the procurement document names it.

The most useful line in that class is the high air temperature: +45 °C, or +60 °C when solar radiation is included. Note 2 in the standard says explicitly that the lower value applies when the equipment is protected against solar radiation or is ventilated, and that the higher value includes the heating effect of the sun. That is the standard telling the designer, in its own words, that a sunlit cabinet is a hotter place than the air around it.

The rest of the class is equally specific, and each line closes off an option. The 1 120 W/m2 solar figure is why surface finish belongs in the thermal calculation. The 15 mm/min rain figure is why a horizontal face needs a rain hood or a louvre that turns the water away. The ten day salt mist is a warning that a coastal site is a corrosion problem first. The 20 mg/(m2 h) dust figure and the 300 mg/m3 sand figure are why a filter has a loading curve rather than a fixed pressure drop.

Writing those numbers into the requirement costs nothing and saves a redesign. A specification that carries the class, the finish, the altitude and the corrosivity category can be quoted against; one that carries only an ambient temperature cannot.

The cooling load has three terms, and the sun is the one that gets forgotten

The load the air path has to remove is the equipment dissipation plus what the sun absorbs, less what the walls give back.

The calculation is short enough to do on one line, and doing it in that order is what keeps the sun from being left out:

The equipment term comes from the losses of the equipment at the load and the power condition it will actually run at, not from a nameplate. The solar term is the absorptivity of the finish multiplied by the sunlit area and by the irradiance. The wall term is the overall heat transfer coefficient of the skin multiplied by its area and by the difference between the site air and the air you want inside, and it is a loss - a negative number - only for as long as the inside is hotter than the outside.

For the worked cabinet, a design irradiance of 1 000 W/m2 is the usual screening value and 1 120 W/m2 is what the ETSI class assumes. Taking the lower figure so that the example is conservative in the other direction, the three terms come out as shown below. The result is 1 120 W, and about 576 W of it is the sun.

Two consequences follow. The first is that a cabinet specified without a solar term is undersized by roughly half. The second is that the wall term changes sign at night and in winter, when the inside is colder than the site air, so the same path that rejects heat in August has to be able to admit it in February.

The colour of the paint is a thermal specification

Solar absorptivity is a property of the finish, and it multiplies the whole solar term. It is one of the few thermal decisions that costs nothing to change.

Published absorptivity values for cabinet finishes run from about 0.25 for white to over 0.90 for matt black. On the worked cabinet, with 1.44 m2 of sunlit skin, that range moves the solar term from 360 W to 1 296 W. Everything downstream - the net load, the airflow, the fan size, the noise - moves with it.

Light grey, around 0.40, is the usual industrial finish and it is a reasonable middle. White is measurably better and is worth specifying where the site is hot and dusty, because the dust that settles on it is itself dark. Dark grey and black should be avoided on a sunlit site unless the load is small enough that the extra airflow is irrelevant.

A sun shield is the other lever. A canopy standing about 100 mm off the top shades the roof, which is the face with the highest irradiance and the worst convection, and the gap also acts as a chimney. Shading only the top takes the solar term from 576 W to 432 W in this example, which is a 13 % reduction in the airflow the duty needs. Combining a light finish with a top shield brings the same duty to 278 m3/h.

None of these numbers replace a measurement, but they show the scale of the decision. A colour and a canopy change the airflow requirement by more than a factor of two across the range, and they cost less than the fan.

Four cooling arrangements, and one rule that chooses between them

The first question is not how many watts the cabinet makes. It is whether the outside air is ever hotter than the temperature the electronics are allowed to reach.

If the site air is always cooler than the target internal temperature, an open path will work and filtered ventilation is the cheapest way to build it. If the site air is sometimes hotter, an open path cannot hold the target at all, because air cannot cool anything below its own temperature. That single comparison eliminates two of the four arrangements before any capacity is discussed.

The second question is whether the site air is clean. A heat exchanger keeps the outdoor air out while still using it as the heat sink, so it solves the dust, salt and humidity problem without a refrigeration circuit, and it has the same low energy use and the same low failure rate as a fan because the only moving parts are the fans on the two sides of the plate stack. Its limit is the temperature difference it is given: it cannot cool below ambient, and the smaller the working difference, the larger the unit.

The third question is whether the cabinet has to go below ambient. Battery compartments, high density radio units in a hot humid climate, and anything that has to hold a fixed internal set point regardless of the weather all need a refrigeration circuit, and that brings condensate, a compressor, a much higher energy draw and a correspondingly higher failure rate. A thermoelectric unit sits between the two: sealed and simple, but only economic at low capacity.

On the worked cabinet the answer is uncomfortable for the quiet options. Removing 1 120 W across the 10 K between a 45 °C site and a 55 °C internal limit needs 112 W/K. Published air-to-air exchangers in the Pfannenberg PAS range are published at 20, 65 and 100 W/K, so the largest one in that range reaches 56 °C rather than the 55 °C target. At a 50 °C site with the same 55 °C limit the working difference falls to 5 K and the duty doubles to 224 W/K, which no plate exchanger of that class will meet. That is why high density outdoor cabinets end up on cooling units even when the site is clean.

The airflow number: what the cabinet needs is not what the fan is rated at

Cooling is a mass flow, and the number on a fan datasheet is a volume flow measured on a free-air bench. Both have to be converted before they can be compared.

The cooling equation sets the mass flow: heat equals mass flow multiplied by specific heat capacity multiplied by the air temperature rise you are willing to accept. With a 10 K rise allowed and a specific heat capacity of 1 005 J/(kg K), the 1 120 W duty needs 0.111 kg of air per second. Converting that mass to a volume needs the density at the inlet condition - at 45 °C and a barometric pressure of about 95.6 kPa the density is 1.047 kg/m3 - and 0.111 kg/s at that density is 383 m3/h, or 225 CFM.

That is the number the installed air path has to deliver, and it is not the number to put in a purchase order. A filter fan is published at its free-air flow; once the filter, the grille, the louvre and the exhaust are in place, the operating point moves along the fan curve. In Rittal's own published data for a TopTherm filter fan unit, the same unit delivers 485 m3/h against a 700 m3/h free-air figure once an outlet filter is fitted, a ratio of 0.69. Dividing 383 by 0.69 gives 555 m3/h free air to specify, and that is before the filter has loaded.

The filter is the reason the number is not a fixed one. Its pressure drop rises as it holds dust, so the air path has to be sized at the loaded state, and the clean state then delivers more cooling than the design needs. Sizing at the clean state is the most common way an otherwise correct airflow calculation turns into an overheating cabinet in its second summer.

A published telecom example shows the level of detail that survives a review. One radio maker's outdoor fan unit is documented at more than 1.44 m3/min and 70 Pa, over a 36 to 60 V range, at 10.8 W, from -20 to +70 °C, with the cooling specified to continue with one fan still working. Every one of those numbers belongs to that unit and that cabinet. None of them is transferable, but all of them are the kind of thing that has to be written down.

An IP65 fan does not make an IP65 cabinet

The IP code describes what was tested. On an assembled outdoor cabinet, what was tested is the assembly, and every cut-out, gland and gasket is part of it.

IEC 60529 defines a two position code. The first position rates protection against solid objects and dust from 0 to 6, where 5 means dust protected and 6 means dust tight. The second position rates protection against water from 0 to 9, where 4 is splashing water, 5 is a jet from a 6.3 mm nozzle, 6 is a powerful jet from a 12.5 mm nozzle, and 7 and 8 are immersion. An X in either position means that characteristic was not declared, not that protection is absent.

Two things about the code catch people out. The first is that jets and immersion are different tests, so a component rated for immersion has not necessarily passed a jet test, and a cabinet that must survive both needs both positions named. The second is that the code says nothing about corrosion or ultraviolet ageing: a stainless cabinet and a painted one can carry the same IP number and have completely different lives on a coastal site.

For a cabinet, the rating is a property of the assembled object. The gasket has to be compressed as designed and not aged out, the cable entries have to use glands of the same class, the fan cut-out has to be a sealed interface rather than a hole with a fan over it, the drain or breather has to be a designed device rather than a gap, and the filter housing has to close against the skin. Change any of those and the rating has to be re-established, on the assembly, not on a datasheet.

The practical reading for an outdoor cabinet is this. IP55 with filtered ventilation is the common minimum, and it is a real rating as long as the filter frame is part of the seal. If the requirement is to keep the site air out of the electronics compartment, the number to write is IP65 or IP66 on the complete cabinet, and the arrangement that delivers it is a closed path. Writing IP65 and then fitting a filtered fan is the single most common contradiction in outdoor cabinet specifications.

On a coastal site, the salt question decides the arrangement

Salt reaches a cabinet as airborne mist and as a wet film, and it attacks the parts that a thermal calculation never mentions.

ISO 9223 classifies the corrosivity of an atmosphere from C1, which is a dry indoor space, to CX, which is an offshore platform. A site inside about 2 km of the coast with high humidity is C5, and one within 2 km with moderate humidity is C4. The class is a property of the location, and it can be established before the cabinet is ordered.

The test that verifies resistance to it is the cyclic salt mist test in IEC 60068-2-52, which alternates sodium chloride spray, damp heat and drying rather than running one continuous spray. The method scales with the class: method 4 for a moderate site is two cycles, method 5 for a C4 site is four cycles, and method 6 for a C5 site is eight cycles, the cycle length being about seven days. Offshore conditions are covered by methods 7 and 8. For comparison, the ETSI requirement for a non-weatherprotected telecom site is ten days of salt mist at 35 °C in 5 % sodium chloride - an entry level figure, not a coastal one.

What salt does to a ventilated cabinet is not subtle. It corrodes the filter housing and the grille, it attacks the fan bearing and the connector plating, and a filter that stays wet while holding salt is effectively a corrosion cell that is regularly refilled. The maintenance interval also shortens, because a salt-loaded filter blocks faster than a dust-loaded one on the same site.

The consequence for the arrangement is direct. On C4 and above, a closed air path stops being an option and becomes the default, and the parts that remain exposed - the enclosure skin, the fixing hardware, the cable glands and the fans on the outdoor side of the exchanger - have to be specified for the class: coated or stainless, with a salt mist test result attached to the component that carries it.

Condensation is the failure that no datasheet covers

A sealed cabinet still contains air, and that air still holds water. The temperature cycle is what turns it into liquid.

By day the cabinet is warm inside and the internal air holds moisture. At night the skin radiates to the sky and cools faster than the air behind it, so the metal falls below the dew point of the air it is in contact with and water condenses on the inside face and drips onto whatever is below. The same motion happens in reverse at start-up on a cold morning, when a heater or a warm board meets a cold surface.

An IP rating does not prevent this. It keeps new water out; it does nothing about the water that is already in the air, and it does nothing about vapour diffusion through gaskets and cable entries over years. That is why condensation is a thermal and control problem rather than an ingress problem.

A cooling unit makes it worse when it is set too cold. Pfannenberg state that their units are preset to 35 °C because that value balances useful service life against condensate build-up. The lower the internal set point, the more condensate is produced, and the worst case is not steady operation - it is a hot humid afternoon when the door is opened for a service visit and the whole site atmosphere meets a cold evaporator.

The measures that work are modest and specific: set the internal target no lower than the components actually require; add a hygrostat-controlled heater for the cold end so that the internal air stays above the dew point; provide a condensate drain or an evaporator tray and route it out of the cabinet; and use a breather or drain device where the housing allows one. The measure that does not work is adding a filtered fan as a backup to a cabinet that already has a cooling unit, because that breaches the seal and imports humid air directly onto the cold surfaces.

Redundancy, alarms and the cost of one maintenance visit

A single failure state has to be written down and demonstrated. Counting fans is not the same thing as providing redundancy.

Two fans in a cabinet are not twice the cooling. In parallel they add flow only where the system curve is flat, and the combined curve has to be read against the installed resistance rather than the free-air rating of each unit. If the cabinet is specified to keep cooling with one fan stopped, the remaining fan has to hold the internal limit against the loaded system curve on its own, and that is a duty point to be measured, not inferred from the fan count.

The alarm matters as much as the count. A fan that has stopped turning reports through its tachometer output, and a control that watches speed will catch it. A fan that is turning but moving no air - a failed blade, a blocked filter, a unit installed the wrong way round - reports nothing, because the speed signal is exactly what it should be. Catching that case needs airflow or differential pressure, or a temperature comparison across the electronics that trips before the components exceed their limit.

The maintenance plan belongs in the thermal specification. State the filter change interval with the dust loading assumption behind it, the access needed to do the job, and whether the cabinet has to be opened. A front-access filter change that keeps the door closed is worth more on a pole site than a small gain in efficiency. State also the air temperature the fan itself sits in, because its bearing life is set by that temperature rather than by the site ambient, and on a sealed cabinet with a cooling unit the fan may be sitting in the coolest air in the assembly while the outdoor-side fan sits in the hottest.

The economics come back to the visit count. Sixty filter changes over fifteen years is sixty truck rolls, and on a remote site that number usually decides the architecture before the efficiency does.

What MEGA Tech can supply, and what we need from you

We are a custom cooling fan manufacturer. We select and build axial DC and EC cooling fans, and backward-curved centrifugal units where the pressure a filter, a long air path or a densely packed cabinet demands more than an axial fan can hold at the flow you need. Selection is made against a stated duty - the airflow and the static pressure at that airflow, in the air temperature the fan will actually see - rather than against a part number. We supply a production-representative sample so that the validation happens in your cabinet or on your test rig, not on a free-air bench.

To quote a fan for an outdoor cabinet we need the airflow and the static pressure at that airflow, the air temperature at the fan inlet, and the arrangement it sits in - open filtered path or closed path with an exchanger. We also need the supply voltage and its tolerance, the control, feedback and alarm interface you want, the connector and harness arrangement, the fan cut-out and the space available behind it, the noise limit and the point at which it is measured, the environment including dust, salt, humidity and solar exposure, the expected life with its assumptions, and the IP and corrosion requirement that applies to the fan itself.

Our scope ends at the cooling fan and its interface. We do not supply the enclosure, the filter or its housing, the heat exchanger or the cooling unit, the cabinet controller or its firmware, or the IP, salt mist or safety certification of the finished assembly. The decision on how much margin the electronics need belongs to the equipment designer, and the certification of the complete cabinet belongs to whoever builds it. Saying so is what makes a quotation usable, because it makes clear which party is answering for which number.

We will not tell you that a candidate is equivalent because the frame size and the voltage match. Two 120 mm fans can differ by a large factor in the pressure they hold at the flow an outdoor cabinet presents, and they can differ just as much in how they behave at the low end of the voltage range on a cold morning. The comparison has to be made at the same flow, the same pressure, the same air temperature and the same duty cycle, and then confirmed on a sample.

• Send the duty as airflow plus the static pressure at that airflow, with the air temperature at the fan inlet.
• Say whether the air path is open filtered or closed, because it changes the pressure the fan has to hold.
• State the loaded system resistance as well as the clean one, so the fan is selected at the worst point.
• State the IP and corrosion requirement for the fan, with the site's corrosivity class if it is coastal.
• State the noise limit and the distance and direction at which it is measured.
• Expect a sample validated in your assembly; a free-air figure is not a validation.

Engineer FAQ: outdoor 5G cabinet cooling

The arrangement that survives is the one that still holds the internal limit after the sun has been counted, the filter has loaded and the worst fault has happened.

Q: How much heat does an outdoor 5G cabinet actually make? A: It depends on what is in it, and the range is wide. A cabinet with a remote radio unit, a baseband unit and a power system has to be quoted from its own equipment list. The figure to be careful with is any rule of thumb that skips the solar term, because on a small cabinet the sun can contribute as much as the electronics.

Q: Is a 48 V DC fan automatically right for a 5G cabinet? A: No. The voltage tells you that the fan suits the rail; it says nothing about the airflow it delivers against the loaded resistance of this cabinet, the temperature it has to survive, its protection class, or whether the control and alarm interface matches the site controller.

Q: Does an IP-rated fan make the cabinet IP-rated? A: No. The cabinet rating is established on the assembled cabinet, with its gaskets, glands, louvres, filter housing and fan cut-out in their production form. A fan's own rating is one input to that, not a substitute for it.

Q: Should the fan be inside the electronics compartment? A: Only if the site air is allowed to reach the electronics. Where it is not - a coastal site, a dusty site, or a site where the ambient exceeds the internal target - the fan belongs on the outdoor side of a closed path, and the electronics compartment gets either a circulation fan or no fan at all.

Q: The site is at 1 500 m as well. What changes? A: Two things and neither is the volumetric flow. The air is thinner, so the mass flow for the same volume falls and the cooling with it, and the electrical clearance requirement rises with altitude. Both are checked separately, and the cooling calculation is redone at the site density.

Q: How much margin should the cooling have? A: Enough to cover the difference between the assumed and the measured solar and equipment load, and the filter at the end of its interval. A margin of the order of 10 to 30 % on the total load is common practice, and it has to be applied to a load that already includes the sun rather than to the equipment list alone.

Q: Can I add a filter fan as a backup to a cabinet that has a cooling unit? A: No. It breaks the seal and imports humid site air onto the coldest surface in the assembly, which is exactly where condensate forms. If more cooling is needed, the unit is the thing to change.

Q: What should a sample validate for an outdoor cabinet? A: The production-representative fan in the real or representative air path, at the highest air temperature, across the voltage range, at the loaded filter condition, with airflow, pressure, current, speed, component temperature, noise and alarm response recorded. If the cabinet is specified to cool on one fan, the sample test should include that state as well.

Questions for a design review

These are the questions that decide whether an outdoor cabinet cooling design will survive a design review and its first hot summer, and they are worth asking before the fan is ordered rather than after the cabinet has been built.

• Which environmental class has been assumed, and is it written on the drawing as a class rather than as an ambient temperature?
• Does the cooling load include a solar term, and is the absorptivity of the finish the one that will actually be applied?
• Has the sunlit area been taken as the worst credible combination of faces, including the roof?
• Is the internal target temperature the temperature the electronics need, rather than a round number that was easier to specify?
• Does the site air ever exceed the internal target, and if so, which arrangement handles that hour of the day?
• Is the fan requirement written as airflow plus the static pressure at that airflow, at the air temperature at the fan inlet?
• Has the airflow been sized at the loaded filter condition and at the hottest credible site temperature, not at the clean state?
• Is the IP rating specified on the assembled cabinet, with the fan cut-out, filter housing, glands and louvres included?
• Has the site corrosivity class been established, and do the exposed parts carry a salt mist test result for it?
• Is the condensate path designed - set point, heater control, drain - rather than left to the weather?
• Is the single failure state defined, and does the alarm sense loss of airflow rather than only loss of rotation?