Electronics Enclosures Air Cooling of Electronics Enclosures A Technical Briefing

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

Air Cooling of Enclosures Background 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

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

Air Cooling of Enclosures Background and Problem Description Thermal management requirements for Data/Communications Centers are increasing From Industry Cooling Consortium: IBM, Lucent, HP, Dell, Nortel, Cisco, and Others

Air Cooling of Enclosures Background and Problem Description Thermal management protects against premature failure MIL-HNBK-217 and Bellcore Standard

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

Air Cooling of Enclosures Heat 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

Air Cooling of Enclosures Heat 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)

Air Cooling of Enclosures Fan 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

Air Cooling of Enclosures Fan 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

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

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

Air Cooling of Enclosures How 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

Air Cooling of Enclosures How 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