Handling the heat

Using heat load information with thermal characteristics of the electrical room envelope engineers to establish a cooling load allows mechanical engineers to accurately select the size of their cooling system

By Luis Madrigal, PE, M+W Group, Chicago March 8, 2011

Mechanical engineers will use the heat load information—together with the thermal characteristics of the electrical room envelope—to establish a cooling load. This cooling load will be used to size the selected cooling system. The cooling system selection is based on the geographic location of the building and the maximum inside and outside air (OA) temperatures.

If the building is located in a moderately cool and dry climate, OA ventilation may suffice. However, if the location is in a hot and/or humid climate, mechanical cooling will be needed.

When using ventilation for cooling, the cool OA is drawn into the space by exhaust fans without any portion of the air recirculating into the space. The amount of airflow required is inversely proportional to the difference between the design indoor temperature and the design OA temperature. A commonly used value for this difference is 10 F. This translates to a cooling load capacity at sea level of 3.22 W/sq ft at 1 cfm/sq ft.

The term "cfm" is the imperial unit commonly used by mechanical engineers for volumetric air. Note: Volumetric airflow requirements are highly dependent on the elevation above sea level. Therefore, the amount of cfm of cooling air required for an electrical room in Denver will be much higher than for the same electrical room located in Seattle.

When the OA cannot cool the electrical room, mechanical cooling will be required. In that case, the amount of cfm will be based on the temperature difference between the space and the supply air from the mechanical cooling unit. A common value for this difference is 30 F. Hence, when cooling with mechanical equipment, the amount of CFM may be as low as one-third the amount of cfm if using OA alone.

Madrigal is the mechanical engineering platform leader in Chicago for M+W Group. He has more than 20 years of experience in designing mechanical systems. His area of expertise is in the design of HVAC systems for critical environments in R&D laboratories, cleanrooms, high-energy accelerators, and mission critical facilities.