Server Inlet Temperature and Humidity Adjustments
Temperature and Humidity Issues
- In 2008, American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) expanded the recommended temperature range at the inlet of the server from 68°F to 77°F (the 2004 level) to 65°F to 80°F. However, many data centers traditionally have set their temperatures as low as 55°F. As a result, many data centers can save energy simply by raising the thermostat.
- Until very recently, most data center managers believed it was important to tightly control humidity in data centers. Where humidity is too low, it can lead to electrostatic discharge (ESD) failures. Where humidity is too high, water can condense inside servers, causing electrical shorts that trip circuit breakers, damage equipment, or harm electrical circuits within the data center. High humidity and condensation can also cause rust and corrosion in servers, leading to machine failures.
- Because data center CRAC units typically use a great deal of energy to maintain required humidity levels, relaxing those requirements (expanding allowable humidity limits) can save energy. In 2008, ASHRAE expanded the recommended humidity ranges from the 2004 levels. (See table below.)
|ASHRAE Recommended Range
||Low Moisture Limit
||High Moisture Limit
||40% relative humidity
||55% relative humidity
|2008 (new range)
||41.9 degrees F dew point
||59 degrees F dew point and 60% relative humidity
- A typical data center uses a great deal of energy maintaining humidity through multiple CRAC units. Each CRAC unit has the ability to humidify, de-humidify, heat and cool. When humidity gets too low, a typical CRAC unit will raise the humidity by using inefficient infrared or steam canister humidifiers.28 These humidifiers can use an order of magnitude more energy than adiabatic humidifiers (e.g., an ultrasonic humidifier) and can also raise the temperature of the data center because they produce heat to generate steam. When the humidity gets too high, the CRAC unit will subcool the air to remove moisture and then reheat (typically using electric resistance heat) to maintain space temperature. Additionally, each CRAC unit typically bases its temperature and humidity settings on what it measures in the (warm) return air. Given the proximity of the CRAC units' exhausts and intakes, it is common for one CRAC to attempt to cool or humidify the air while another CRAC tries to dehumidify and/or reheat the air.
- There is a growing understanding that even 2008 ASHRAE recommended temperature and humidity ranges are too restrictive:
- A recent ASHRAE journal article found that high humidity is rarely an issue in most data centers and concluded that "it is difficult to make a case for actively controlling humidity in data centers."29 Typically, the temperature of IT equipment is significantly higher than that of the cooling coils' operating dew-point. Furthermore, most IT equipment is rated for operation up to 80% RH. The study states that humidification appears to be unnecessary if you follow best practices for electrostatic discharge: use IT equipment rated and tested for ESD conformance with IEC61000-4-2, and, where personnel handle electronic circuit boards and components, use personnel grounding procedures.
- In August 2008, Intel conducted a 10-month study to assess the effectiveness of using only outside air to cool a data center. The temperature range was 64°F to 92°F. Humidity varied from 4% to over 90% and changed rapidly at times. No increase in server failure was observed30.
- Data center equipment will operate safely at temperatures and humidity levels beyond the recommended ASHRAE ranges (see Figure 12). Consider these operating ranges for the following server models:
- Sun Blade ( 4°F to 90°F)
- Dell Blade (50°F to 95°F)
- IBM Blade (50°F to 95°F)
- NetAppStorage (50°F to 104°F)31
Figure 12: Servers can operate outside the humidity and temperature ranges recommended by ASHRAE. (Courtesy of DOE)
How to Adjust Temperature and Humidity
Savings and Costs
- Data centers can save 4% to 5% in energy costs for every 1°F increase in server inlet temperature.32
- A CIO magazine article claims that data centers can save up to 50% of energy costs by turning off humidification controls on CRAC units and investing in an ultrasonic humidification unit.33
- According to one vendor, ultrasonic humidifiers use only 25 watts of electricity for each pound of mist generated versus 380 watts for electric steam generators/boilers. Annual energy savings for a 100-pound-per-hour system would be $7,100 (based on 2,500 operating hours per year, at 8 cents per kWh).34
- Raising inlet temperature and humidity may lead to uncomfortable working conditions. For example, at the higher end of ASHRAE's operating range in the cold aisle (80.6°F), hot aisle temperatures would be roughly 105°F to 110°F.
- Higher inlet temperatures can cause the internal fans in servers to increase their speed in order to cool the servers—thus raising energy costs. One source states that this happens at temperatures as low as 77°F.
- Data centers with air-side economizers and/or variable speed fan drives in their CRAC units stand to benefit the most from raising set point temperatures. That's because higher set point temperatures increase the amount of time that 1) outdoor air can be used for cooling, and 2) CRAC units can run on reduced fan speeds.
- Ultrasonic humidifiers should only be used with de-ionized water. One manufacturer mentions a de-ionizer system with resin cartridges that need to be replaced every two months.
- Adjusting temperatures may lead to data center hot spots.
Top Twelve Data Center Energy Efficiency Strategies
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