Data centers and air humidity: psychrometrics for IT infrastructure

Servers and network equipment are not indifferent to humidity. Too low a humidity causes electrostatic discharge, too high causes corrosion and condensation. The allowable range is narrow, and exceeding it can wreck hardware worth millions. ASHRAE Technical Committee 9.9 defines the allowable conditions in its Thermal Guidelines in four classes; the h-x diagram is the tool for verifying that a proposed system holds this corridor under all operating states.

The risk of low humidity is electrostatic discharge (ESD)

Below 30% RH, air becomes an excellent insulator. The movement of people, the handling of components or the swapping of cards and other parts can generate a charge of hundreds to thousands of volts. A discharge through sensitive electronics (ESD — Electrostatic Discharge) causes immediate or latent damage. Latent damage is the more insidious kind, because the component still works but has a shortened lifetime.

The lower ASHRAE TC 9.9 limit for classes A1–A2: a dew point of −12 °C or φ = 8% (the stricter one applies). In practice, facility managers keep RH between 40% and 55% for a safe margin.

High humidity shows up as corrosion and condensation

Above 60% RH, electrochemical corrosion of the metal surfaces of printed circuit boards begins. Humidity eases the migration of ions and the formation of conductive paths (CAF — Conductive Anodic Filament). More critical is condensation: if warm humid air comes into contact with a surface colder than its dew point, water condenses directly on the hardware. The upper ASHRAE TC 9.9 limit: a dew point of 17 °C and 80% RH (the stricter one applies); in practice, RH is kept below 55–60%.

ASHRAE TC 9.9 and the envelope for classes A1–A4

The classes define the conditions at the intake to the IT equipment (not in the whole space). The higher the number, the more relaxed the conditions:

Class	Application	Temperature	Dew point	RH
A1	Critical infrastructure	15–32 °C	−12 to 17 °C	8–80%
A2	Typical server room	10–35 °C	−12 to 21 °C	8–80%
A3	Robust hardware	5–40 °C	−12 to 24 °C	8–85%
A4	Outdoor / edge	5–45 °C	−12 to 24 °C	8–90%

The stricter of the two limits always applies — the dew point, or the RH.

These envelopes can be plotted directly on the h-x diagram as regions of allowable states. The air conditioning must ensure that the air state at the intake to the racks stays inside the envelope under all outdoor conditions — from a summer day of 35 °C / 50% RH to a winter night of −15 °C / 80% RH.

Free cooling and its psychrometric analysis

Free cooling uses outdoor air without an active compressor, when the outdoor conditions are favorable enough:

• Air-side free cooling — outdoor air directly (after filtration) into the hall; valid when the temperature and humidity are within the allowable envelope.
• Water-side free cooling — a cooling tower cools the circulating water without a compressor.

For Prague climate data, the outdoor air lies outside the class A1 thermal envelope for roughly 2,000–3,000 hours a year; in the remaining 5,700–6,700 hours, direct air-side free cooling is potentially possible. The humidity condition (φ < 80%) is less limiting than the temperature one in a Central European climate.

Adiabatic (evaporative) cooling

Large operators have extended free cooling with adiabatic pre-cooling: the outdoor air is cooled by water evaporation before it enters. On the h-x diagram the air moves along an isenthalp to the right and down — temperature falls, humidity rises. This can add 500–1,500 free-cooling hours a year. The risks are a rise in the RH of the supply air and possible contamination with minerals from the water, so water treatment and regular maintenance of the porous panels are necessary. The physical principles are covered in the article Evaporative cooling.

PUE and the impact of the humidity strategy

PUE (Power Usage Effectiveness) = the total data center energy use ÷ the IT equipment energy use.

The global average PUE is around 1.55; hyperscale centers reach 1.1–1.2. Air conditioning accounts for 30–40% of the consumption, and the choice of humidity strategy directly influences the share of hours with free cooling. Widening the allowable temperature window (moving from A1 to A2 or A3) extends the free-cooling season — every degree of increase in the intake temperature lowers the PUE by roughly 0.02–0.04.

Frequently asked questions

Why is both the upper and the lower humidity limit watched in a data center? The lower limit against electrostatic discharge (below 30% RH), the upper against corrosion and condensation (above 60% RH). The allowable corridor is therefore narrow and is defined by ASHRAE TC 9.9.

What do classes A1–A4 mean? They define the allowable temperatures and humidities at the intake to the IT equipment. A higher number means more relaxed conditions suited to more robust hardware, which allows more free-cooling hours and a lower PUE.

How does the humidity strategy affect operating costs? The wider the allowable window, the more hours can be cooled with outdoor air instead of a compressor. This lowers both the PUE and the consumption — which is why ASHRAE recommends operating in the upper band of allowable temperatures.