Thermal-management lessons for enclosure designers

Brian MordickToni Amenrud

Hoffman Enclosures Inc.
Anoka, Minn.

Electronic equipment that stays cool in its cabinet runs like a champ. But equipment that heats up can slow down, effect productivity, and may fail completely. The scenario can happen any time and is often directly attributed to heat build-up in and around the electronics. Many don't realize how excessive heat shortens the life of electronic equipment. Heat is invisible, but its effects are devastating and costly.

The common solution to hot enclosures is to bring in cool ambient air to lower the internal temperature. But it's not necessarily the best solution. The problem is that condensation can form when warm air contacts cold surfaces. What's more, adding dust to condensation generates a potential for corrosion.

The key to keeping electronics running is to channel or duct cool air to the equipment and provide an exit path for heated air. Testing simulated heat loads in six cabinet configurations taught several design lessons and highlighted which offers the best thermal profile for enclosed electronic equipment.

Test specifications included a cabinet of 2,000 X 600 X 900 mm (78.74 X 23.62 X 35.43 in.) with heating, ventilation, and air conditioning. Ventilation came from two 6-in.-diameter, 115-V fans that generate 480 cfm. Heaters generated hot spots within the cabinet and simulated the heat load from 19-in. vertically mounted equipment. Six thermocouples (three in front and three in back) measured temperatures.

Lesson 1: Fans are a must to move cool air through the cabinet.

Cases 1 and 5 removed the least heat from the enclosures because Case 1 had no cool-air inlets or heated-air outlets, and Case 5 had no exit. Air passing through cabinet 5 is heated and trapped at the top. A boundary along the perforated surface acts as a barrier to airflow in the absence of a clear escape channel for heated air.

Lesson 2: Fully perforated doors work with low heat loads, but simply adding fans isn't the answer for higher loads.

Cases 3 and 4 show that cabinets with fully perforated front and rear doors are viable options with a low heat load. Perforations in front and rear doors let cool air enter and heated air exit, maintaining the temperature of the equipment at an acceptable level (Case 3). Simply adding a vented top improves airflow. However, when the heat load increases, adding fans will not maintain the same acceptable inside air temperature (Case 4). Adding fans changes the airflow patterns but does not ensure an improved airflow.

Lesson 3: Cabinet design can direct cool air to internal "hot spots" and direct warm air out.

Case 2 shows that running fans significantly increases the cooling performance of an enclosure with a solid door. The solid sides of the cabinet, along with the fans, draws in cool air from the floor and provides a path for the heated air to escape. However, only a limited amount of cold air can be drawn into the cabinet.

Lesson 4: Best results come from moving air through a cabinet from bottom to top.

Case 6 works best for the given internal arrangement and heat load. Perforating the entire front door and the lower one-fourth of the back door combined with fans mounted to the top provides the best arrangement for cooling the cabinet. Cool air is drawn through the perforated front door of the cabinet, and heated air is pulled out by the fans mounted on the top. This "chimney effect" is by far the most effective way to keep the inside of a cabinet cool.

Tests also show that the cooling depends on the type of equipment installed in the cabinet. Results in these tests are specific to the configurations described, so each configuration should be reviewed to optimize airflow. Computational fluid dynamics (CFD) software and lab testing can also help determine specific application needs. Another software option is Hoffman's Thermal Management Selection and Sizing Software available on www.hoffmanonline.com/Technical_Info/thermal.