Handling the Heat– Cisco Basic Network Hardware

All equipment used in information technology—hosts, routers, switches, and even copper cables—generates heat during operation. However, operating equipment at higher temperatures can reduce its life span or—if the temperature is high enough—cause immediate failures.

Heat is managed in information technology operations by

• Vendors working to reduce the heat their devices produce.

• Vendors designing devices for efficient airflow.

• Liquid cooling.

• Hot air containment.

The first three of these are outside the scope of this book. Hot air containment is common and easily impacted by everyday maintenance tasks. Figure 10-11 illustrates a simple cooling system without hot air containment.

Figure 10-11 Simple Cooling System

Figure 10-11 shows a pair of rows of racks from the end. In the figure, a computer room air conditioner (CRAC ) cools the air and pushes it into the space between the building and a raised floor.

Equipment racks sit on this raised floor. This cool air is vented in front of each rack through a floor vent and pulled through the rack, front to back, to cool the equipment.

Hot air expelled from the back of each rack rises toward the ceiling. Fans draw hot air back into the CRAC, cooling it, and the cycle starts again.

When pairs of racks are placed back-to-back, a  hot aisle is created. The more fully sealed off this hot aisle is, the more efficient the cooling process is. In a fully sealed system, there is no mixed air; hot air is directed through ductwork back into the CRAC.

Large-scale data centers may pull air from the outside and cool it through an evaporative process, as shown in Figure 10-12.

Figure 10-12 Evaporative Cooling

Figure 10-12 again shows a pair of data center racks from the end. In this figure, air is drawn directly from the outside by fans, through a filter, and then through a waterfall-like evaporative cooling system. As hotter air flows through the water, the water evaporates, consuming heat from the air and leaving the cooler. This air is then pushed into the data center by fans.

Equipment fans draw this cool air through the rack, cooling the equipment by heating the air. The air is expelled into the hot aisle, wholly isolated from the  cold aisle, and then pulled by fans back into the outside environment.