In an effort to provide greater flexibility, particularly with the goal of reduced energy consumption in data centers, ASHRAE TC 9.9 made an effort in 2008 which resulted in an update to the Equipment Environmental Specifications. The 2004 specifications were bounded between 20 to 25°C (68 to 77°F) and 40 to 55% RH. ASHRAE worked closely with IT manufacturers to expand this region. This included decreasing the dry bulb lower limit to 18°C (64.4°F), and increasing the upper limit to 27°C (80.6°F). The moisture limits were lowered to a 5.5°C (41.9°F) dew point and increased to 60% RH & 15°C (59°F) dew point.
The reasons for the expansion in each direction are different, but the underlying goal was to include a larger operating range with more opportunities for energy savings.
**It should be noted that all conditions are to be measured at the IT equipment inlet.
DRY BULB LOWER LIMIT
There is no concern from an IT standpoint in lowering the operating condition from 20 to 18°C (64.4°F). The goal here was to extend the control range of economized systems by not requiring a mixing of hot return air to maintain the previous limit.
DRY BULB UPPER LIMIT
The justification for increasing the dry bulb limit was to increase hours of economizer use per year. Concerns about increasing inlet temperature causing lower reliability are not well founded. IT Equipment manufacturers test, confirm, and state, through ASHRAE, that operation within the recommended limits will not affect equipment reliability.
MOISTURE LOWER LIMIT
Lowering the moisture limit allows for a greater number of hours per year where humidification is not required. Significant research was done to understand the impact of lower moisture levels as they relate to electrostatic discharge (ESD). It was determined that risks of ESD are impacted by absolute humidity (dew point temperature) and not relative humidity. Additionally, since most heat transfer through computer equipment is sensible (constant dew point), having a limit of relative humidity greatly complicates control and operation of the cooling systems and could require added humidification with a cost of increased energy.
A lower limit of 5.5°C (41.9°F) was chosen which corresponded to the lowest absolute moisture level of the 2004 recommended limits.
MOISTURE UPPER LIMIT
Based on extensive reliability testing of Printed Circuit Board (PCB) laminate materials, it has been shown that Conductive Anodic Filament (CAF) growth is strongly related to relative humidity. It was found that extended periods above 60% can result in failures. As a result, a relative humidity upper limit of 60% was chosen. Additionally, an upper absolute moisture or dew point temperature limit of 15°C (59°F) was chosen to provide an adequate guard band between the recommended and allowable envelopes.
It should be noted that there is an allowable operating range with expanded boundaries compared to the recommended range in which equipment is expected to function for short to moderate periods of time. This is a statement of functionality. This buffer region allows for an expanded envelope where equipment will still function when the recommended limits are exceeded (e.g., equipment failure, extreme outdoor air environmental conditions during economizer use, etc.).
The frequency and duration equipment operates in the allowable environmental conditions must be studied and is an engineering / business decision. The associated risks must be fully understood before decisions are made.
How much of a concern is humidity? Humidity can be a concern if it is outside the recommended range, but equipment will operate reliably within this range. Equipment will still continue to function in the allowable range, but cannot be guaranteed for extended periods of time.
The allowable range provides a buffer zone where equipment will continue to function. Often, these excursions are brief and will not negatively impact equipment operation. Full risk analysis should be conducted to understand what is tolerable in a given installation.
Overheating should NOT be a concern unless equipment is frequently operated outside the recommended region or for long durations.
Does raising the temperature as a way to lower costs increase costs in other areas? This will vary on a case by case basis. Operating within the recommended range does NOT guarantee the data center is operating at optimum energy efficiency. Depending on the cooling system design and the outdoor environmental conditions, there will be varying degrees of efficiency within the recommended zone.
As an example, as the temperature increases, thermal algorithms on IT equipment may increase the speed of air moving devices integral to the equipment. This represents a situation where energy savings by increasing the temperature can increase energy usage. Full analysis will be needed to determine the optimal operating conditions.
Another example would be operating within the upper areas of the envelope which results in latent cooling (condensation) on refrigerated coils. Latent cooling decreases the available sensible cooling capacity for the cooling system and may cause the need to humidify the air to replace moisture removed.
In a case where purchasing new equipment would allow for more efficient operation, a Total Cost of Ownership of various scenarios can be used to calculate payback periods. The determination of an acceptable payback period is then a business decision.
It is generally good practice to monitor temperature and absolute humidity at the server inlet as described in Thermal Guidelines for Data Processing Environments, 2nd Edition. Sensors that ensure the inlet temperature to equipment is within the recommended region should be included in initial cost estimates and should not be considered an added cost. Raising the temperature does not imply the need for new sensors.
Conclusion: It is important to understand that there is no prescriptive operating point for all data centers in all environments. The recommended conditions are intended to provide a reliable operating range for equipment. The point within this range that a given data center operates most efficiently will vary.
The allowable temperature range provides a buffer zone that allows a wider operating range during excursion events such as failures, equipment maintenance, or extreme outdoor environmental conditions. Design should consider business risk and Total Cost of Ownership analysis to understand both where to operate in the recommended range and what excursions into the allowable range are acceptable and for how long.
Proper air management techniques improve the efficiency of air delivery and prevent wasted energy from short-circuiting, recirculation, and bypass air. Any mixture of return and supply air will increase the temperature of the air entering the IT equipment compared to the supply air.
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