Air Cooling of Electronics Enclosures A Technical Briefing

1. Air Cooling of Electronics Enclosures A Technical Briefing

2. Topics • Background and problem description • Heat transfer • Fan characteristics • How servers are cooled • Wrap-up

3. Air Cooling of EnclosuresBackground and Problem Description • Advances in electronics are pushing cooling issues to the forefront • More applications • Smaller components More heat in less volume • Faster speed • Previous methods of thermal management are becoming insufficient

4. Air Cooling of EnclosuresBackground and Problem Description Major causes of electronic failures Source: BCC, Inc. Report GB-185R

5. Air Cooling of EnclosuresBackground and Problem Description Thermal management requirements forData/Communications Centers are increasing From Industry Cooling Consortium: IBM, Lucent, HP, Dell, Nortel, Cisco, and Others

6. Air Cooling of EnclosuresBackground and Problem Description Thermal management protects against premature failure MIL-HNBK-217 and Bellcore Standard

7. Air Cooling of EnclosuresBackground and Problem Description 2% Clockspeed Increase for every 100C CPU Reduction Data for a 1,500 MgHz CPU Kyrotech and IBM Report

8. Air Cooling of EnclosuresHeat Transfer • Three types of heat transfer – conduction, convection and radiation • Conduction is heat transfer by molecular interaction (think of direct contact) • Convection is heat transfer by a combination of fluid mixing and conduction (think of heat transfer from a surface to a fluid like air) • Free convection – fluid movement caused by density changes • Forced convection – fluid movement caused by a pump or other device • Radiation is emission of electromagnetic waves (for example: the sun) • Conduction and convection are the primary heat transfer mechanisms involved in enclosure cooling – radiation is a small player • Our focus today is on forced convection (air cooling) • Convection is most easily controlled by users • Conduction is a function of the enclosure construction and location in the facility

9. Air Cooling of EnclosuresHeat Transfer • Convection heat transfer from a surface to an airflow is governed by the equation q = hA(Tw – Tf) Where q = the heat transfer rate h = the convection heat transfer coefficient A = the surface area Tw = the temperature of the surface Tf = the temperature of the fluid • In enclosure cooling, the factors h, A and Tw are determined by the server • We can affect q (the rate at which heat is transferred to the air) by helping determine Tf (the temperature of the air entering the server)

10. Air Cooling of EnclosuresFan Characteristics • Fan theory • Fans work against a resistance called static pressure • Static pressure can be likened to blowing through a drinking straw – how hard the fan has to push to get the air to move through the path • Each air path has its own unique static pressure (think of different sized drinking straws) • Fan performance is dependent on the static pressure of the air path

11. Air Cooling of EnclosuresFan Characteristics • Axial fans • Most commonly used fans in electronics cooling • Wide variety of sizes and capacities are used • Good application for air movement in low static pressure environments • Centrifugal fans • Work well against higher static pressure • More readily adaptable to ducting to provide for better delivery of air • More readily adaptable to filtration • Typically more efficient than axial fans – as power requirements rise in server facilities, these fans will become more popular • Typically allow operation at lower speeds which helps manage noise

12. Enclosure with few servers Additional cooling air that a centrifugal blower provides Air Cooling of EnclosuresFan Curve Centrifugal Blower Static Pressure Axial Fan Airflow (CFM)

13. Enclosure with more servers The more resistance, the better a blower works compared to a fan Air Cooling of EnclosuresFan Curve Centrifugal Blower Static Pressure Axial Fan Enclosure with few servers Airflow (CFM)

14. Air Cooling of EnclosuresHow Servers Are Cooled • Servers (and other electronic equipment designed for rack-mounted use) are typically fan cooled • Fans are integral to the servers • Air inlet is at the front of the server • Air inlet temperature is affected by room temperature and enclosure front door configuration • Air inlet temperature typically varies from the top of the enclosure to the bottom • Air discharge is at the rear of the server • Discharge path often congested by cable management • Warm discharge air not forcibly exhausted very well from the enclosure

15. Air Cooling of EnclosuresHow Servers Are Cooled • Airflow is low within the server • Small fans • Congested air path • Server room air conditioning provides the cool air the servers use • Cooling the entire room requires tons of refrigeration • As servers require more cooling, adding room air conditioning capacity without improving airflow to the server is: • Costly • Inefficient • Can make the room uncomfortable for workforce • Adds to room noise • Improving server cooling needs a more direct solution