By: Mike Starego, PE
For typical MEP design firms, buzzwords like collaboration and innovation are often used to describe activities that add value to owners and differentiate those firms from the rest of the industry. In reality, collaborating and being innovative are actions that all MEP designers strive to accomplish in their daily routines. Mechanical and electrical engineers routinely collaborate to bring complex designs together and coordinate construction documents, while also creating innovative designs within the context of their own trades. What truly becomes a differentiator is when mechanical and electrical engineers collaborate to innovate designs as partners, operating outside of their individual design silos, and challenging each other’s requirements and needs in an attempt to optimize the whole. Southland Engineering utilizes its in-house mechanical and electrical design engineering departments in this very manner, with the ability to use in-house estimating to provide real-time pricing feedback to evaluate a number of design iterations. One recent example is a confidential data center that required an IT cooling load of roughly 6800 tons of mechanical cooling. The desired system type was air cooled chillers with an integral economizer. For this system type, air cooled chillers have a maximum size of approximately 500-tons, depending on ambient and design conditions. From a mechanical perspective, it was determined that twenty 385-ton air cooled chillers were the optimal quantity and size to meet the specific project conditions based on phasing, redundancy, shipping splits, size, and overall price. However, that was independently optimized without factoring in the electrical design. The electrical system concept design for this facility was comprised of twelve normally powered substations and, for redundancy, three reserve substations. The critical load and associated HVAC loads were distributed amongst the normal substations. With a total of twenty chillers, eight substations would serve two chillers each and four substations would serve one chiller each. This scenario would not present any electrical issues. However, it would leave an imbalance of load across the substations as a whole. This presented three major issues for the owner:
- The “base design” would take away the ability for the owner to measure Power Usage Effectiveness (PUE) on a per-substation basis. PUE is the ratio of total consumed power over critical IT power consumed. The PUE will be high for the two-chiller substations and low for the one-chiller substations, thus making the per-substation PUE figure useless.
- If the owner wishes to bill tenants on energy usage on a per-substation basis, actual metered data could not be directly used. The mechanical power usage would need to be metered on a more granular level, split up, and calculated across all tenants. This would add complexity, cost, and potential errors.
- Data center owners typically desire all substations and associated electrical distribution equipment to be consistently sized across the facility to simplify maintenance and spare parts stock. If this universal sizing scheme was implemented for the base design, four one-chiller substations would be oversized and produce high initial costs and low operating efficiency of the substation transformers.
Through pre-design collaboration, the Southland Engineering team realized that the concept of 20 chillers would create the three major issues mentioned above. Understanding this concept, the team quickly decided that twenty four chillers were the optimal quantity for the system when looking holistically across mechanical and electrical systems. Twenty-four chillers can be distributed evenly across all twenty-four substations by serving two chillers each. This perfect balance of load, combined with the greater efficiency of the system itself, allowed the electrical team to lower substation bus ampacities and backup generator sizes by one nominal. The size reduction resulted in large first cost savings that significantly overshadowed the cost increase of four additional chillers on the mechanical side. Had Southland’s mechanical and electrical engineers only designed and innovated within their own trades, a working building would have been the only result. However, collaborating across both mechanical and electrical trades produced an ideal, holistic mechanical and electrical system that optimized functionality and cost savings.
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