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Humidity in Data Centres – Part 2

Humidification – the cost effective way

In Part 1 of this blog we looked at why, if we are going to stay within ASHRAE guidelines, we do need some means of adding moisture to the air in a data centre i.e. a humidifier.  This is because in winter the outside air has a very low moisture content and when this warms up to data centre temperatures it will translate into a very low relative humidity (RH), as shown in the table below:

Air temperature °C

Dry Bulb / Wet Bulb

(typical for central England)

RH at 21°C

(if no moisture is added)

-10 / -10


-5 / -5


0 / -2


5 / 0


10 / 6


In most data centres constructed over the last 30 years the means of adding moisture was with an electrode boiler type of humidifier (often referred to as “kettle” humidifiers).  These are very effective but also consume a lot of power, so why did they become so ubiquitous?

It was partly because electricity was (relatively) cheap, partly because unless you have direct fresh air free cooling the numbers involved are very small and partly because if you wanted to run your data centre at 21°C and 50% RH, which were typical design conditions for the time, you really didn’t have much choice.  Before the advent of segregated hot and cold aisles the return temperature to the CRAC units was around about the same as that for the data centre space in general e.g. 21°C.  The supply air temperature would have been around 10°C and to achieve this and the cooling coil temperature would have to be about 5°C.  As we covered in Part 1, air at 21°C and 50% RH has Dew Point (DP) of 10°C so if you try to put the moisture in on the inlet side of the coil most of it will condense out again when it hits the cold surface.  So you have to put the humidifier after the coil but here the air is at 10°C and virtually saturated.  The only way to add more moisture is as vapour which means supplying energy to the water, i.e. boiling it, before it is introduced to the air (hence the “kettle”).

Today, however, the situation is very different.  With segregated hot and cold aisles we are likely to have a return temperature to the CRAC units of 30°C or more and a supply of 20°C or more.  We are also now only trying to achieve 36% RH if we opt for the ASHRAE “recommended” envelope or 20% RH if we choose the lower “allowable” humidity limit.  There is now sufficient energy in the air to do the evaporation for us and we no longer need to supply energy to the water in the form of electricity.  Indeed using the air to do the evaporation has an added benefit in that this cools the air giving us a little bit of free cooling in the process (this is exactly the basis on which evaporative or adiabatic cooling works).

So how big a humidifier do you need and how much will you save?

This depends upon two factors, firstly, what RH do you want inside your data centre and secondly, how much fresh air are you bringing in?

Although ASHRAE now allows for the RH to go down to 20% and although most hardware manufacturers are happy with this, it is, in my personal opinion, a bit low as a set point.  I would suggest aiming for 30% RH with an alarm at 25% so as to remain comfortably within the allowable range.  The amount of moisture to be added to bring air which was at -10°C and saturated (our worst case in the UK) up to 21°C and 30% RH is 0.003 kg/kg.  This brings us to our second factor: how much fresh air are you bringing in?

Unless you are using a direct fresh air free cooling system, which we will come to later, you will probably be bringing in a relatively small amount of fresh air.  The amount of fresh air required under building regulations and CIBSE guidelines is based on the number of occupants but as most data centres are “lights out” they do not have an “occupancy” on which to base this.  Typically the designer will have calculated perhaps 1 or 2 air changes an hour for fresh air.  What this equates to in kg of air obviously depends on the volume of your particular data centre but very roughly if you multiply the floor area in m2 by 3 that will be equal to the kg per hour for 1 air change an hour.  So if we have a 500m2 data centre with 2 air changes an hour that will be 1,000 kg per hour of fresh air.  Therefore our 0.003kg/kg of moisture to be added requires a humidifier capable of delivering 1,000 x 0.003 =30kg per hour.  The energy required to produce 30kg per hour of water vapour is 22kW and in a typical winter you can expect this to be running for about 500 hours so that’s 6,600kW hours per year.  An evaporative humidifier, in contrast, uses heat from the air to do the evaporation and the power used is minimal – just a small pump consuming a few hundred watts.  However as mentioned previously these numbers are small – if you are paying a typical 12pence per kW hour (including carbon tax) then that 6,600 kW hours is only costing you £792 p.a. so not worth worrying about.

What happens if you have direct fresh air free cooling?

Free cooling systems of all types have had a great resurgence in recent years and understandably so.

If you have an indirect system, either air or water based, then there is no change to the humidification load.  If however you are using direct fresh air then the situation is very different.  Let’s say for example that our 500m2 data centre has an IT load of 500kW.  To cool this using direct fresh air when the outside is -10°C (our best case for free cooling efficiency but our worst case for humidification) will require 12.5kg/sec (based on a hot aisle temperature of 30°C).  The moisture required to bring this to our desired RH of 30% in the cold aisle is 135kg per hour.  Using electrode boiler humidifiers this will use almost 100kW of power which is about 6 times what the fresh air supply fan is using to do the cooling!  Your fantastically low PUE of 1.03* has just become 1.23.

*this is an instantaneous PUE and is relevant ONLY for the comparison given

The good news is that an evaporative humidifier can add the moisture you need using minimal power and at the same time cool the air thus reducing the fresh air volume required and further improving your PUE.  The even better news is that if you are using evaporative (adiabatic) cooling you already have the equipment you need because an evaporative cooler is the same thing as an evaporative humidifier.  The bad news is (you know there had to be some) is that in many evaporative cooling systems the evaporator is in the wrong place!  To be able to humidify the evaporative element has to be in the tempered air i.e. after the recirculation duct, where the air is mixed before entering the data centre (see diagram).  If you already have an adiabatic fresh air system, or are considering one, check to see where the evaporator is installed.

As long as the evaporator is in the tempered air you can have the best of both world’s – evaporative cooling in the summer and evaporative humidification in winter.

Take care though – direct evaporative cooling when applied to data centres can give rise to problems at the upper end of the humidify range.  How to avoid these will be covered in Part 3 of this series of articles.

Writer bio: My name is Frank Wilman and I was one of the founders of Future-tech back in 1982. I worked on my first “data centre” in 1973 and have design and installed almost every cooling solution available over the years. My passion is mechanical cooling systems as they give us an opportunity to be truly innovative and continually push the boundaries of resilience and efficiency.