ASHRAE, the American Society of Heating, Refrigeration and Ventilation Engineers, is the world’s most respected authority on what constitutes the best environmental parameters within a computer room environment.
Their key document, Thermal Guidelines for Data Processing Environments, originally published in 2004, has just been re-issued as the Fifth Edition, March 2021.
In this paper we will review some of the key highlights and changes in this new document.
Humidity control and atmospheric pollution
ASHRAE has considered the effects of atmospheric pollutants on electronic components and the relationship with humidity. The pollutants considered are three chemicals that are pervasive throughout the planet (SO2, NO2, and O3) and two catalyst pollutants (H2S and Cl2).
It is recommended that data centre operators should use silver and copper coupons inside their data centers at least twice a year (once in the winter and once in the summer) to detect the level of corrosion in the environment. The coupon is literally a strip of silver and a strip of copper that you leave out in the computer room for thirty days. At the end of this period you send it back to as specialist laboratory that will measure the growth of sulphides and oxides on the surface.
These sulphide and oxide contaminants ultimately cause printed circuit board failures by a combination of failure modes but typically copper creep corrosion. ASHRAE’s research found that copper corrosion in particular is strongly dependent on relative humidity.
The units of measurement for this kind of pollution effect on PCBs is Angstroms per month (Å/month) where an Angstrom is 10-10 metres. Although a metric unit this is not a recognised SI unit. ANSI/ISA-71.04:2013 defines a build-up of corrosion of less than 300 Å/month as ‘mild’ whereas >2000 Å/month would be classified as ‘severe’.
Correspondingly the ASHRAE standard states that for data center environments tested with silver and copper coupons that are shown to have corrosion levels less than 300 Å/month for copper and 200 Å/month for silver, the recommended moisture limit has been raised from 60% RH to 70% RH. For data center environments tested with silver and copper coupons that are shown to have levels of corrosion greater than 300 Å/month for copper and 200 Å/month for silver, then the recommended moisture levels should be kept below 50% RH.
Although dust is problematic it can be easily removed with air filtration whereas chemical pollutants are not so simply dealt with. Not many data centre operators actually go to the lengths of measuring silver and copper corrosion for two months every year and ASHRAE suggests that, If coupon measurements are not performed to aid in understanding the possible corrosion impact on ITE, the data center operator should consider maintaining a lower humidity level to protect the ITE, either below 60% as specified in the fourth edition of this book or below 50%.
In the last edition of Thermal Guidelines ASHRAE lowered the bottom end of allowable humidity and opted for the units for Dew Point as the principal measurement. The recommended range is -90 C to 150 C dew point and 18-270 C in the cold aisle. This is a very low level of humidity and equates to between 8 and 15% relative humidity. Not everybody in the industry, including Capitoline, believes that such a low level of humidity is conducive to the long life of the electronic equipment.
It is worth noting that in the notes that accompany the tables in Thermal Guidelines ASHRAE states that at such low levels of humidity a conductive and grounded floor must be in place and, during maintenance on any hardware, a properly functioning and grounded wrist strap must be used by any personnel who contacts ITE. Data centre managers and users may wish to consider the practicality of the latter requirement!
New Environmental Class Definition for High-Density Air-Cooled Equipment
The fifth edition introduces a new class of operation for data centres, H1, aimed at high density data centres. Unfortunately they don’t define what high density means and leaves that to the user to decide.
H1 environment is 18 to 220 C with the same humidity ranges as already discussed.
New facility water supply temperature classes
In the fifth edition ASHRAE introduces a ‘W’ rating to define input water temperature range when using water cooled systems. For example Class W17/W27: Typically data centers that are traditionally cooled using chillers and a cooling tower, but with an optional water-side economizer to improve energy efficiency, depending on the location of the data center.
All of the liquid-cooling classes allow the water supplied to the ITE to be as low as 2°C, which is below the ASHRAE allowable room dew-point guideline of 17°C for Class A1 enterprise datacentres. If low fluid operating temperatures are expected, careful consideration of condensation should be exercised. It is suggested that a Cooling Distribution Unit with a heat exchanger be used to raise the coolant temperature to at least 18°C to eliminate condensation issues.
Data Centres finally defined?
It is interesting to see that ASHRAE defines a data centre as a room or building, or portions thereof, including computer rooms served by data center systems, serving a total ITE load greater than 10 kW or 215 W/m2. Whereas a computer room is a room or portions of a building serving an ITE load less than or equal to 10 kW or 215 W/m2 or less of conditioned floor area.
Is PUE a trademark?
On a final point I note that ASHRAE now writes Power Usage Effectiveness as PUETM.
Trademarked by who?
I thought that would be an interesting position as the term PUE appears in ISO and EN standards. If it is going to be trademarked anywhere it must be in the USA so I checked with the United States Patent and Trademark Office and found that the terms PUE and pPUE are not trademarked by anyone.