Mechanical engineering is a science relatively unchanged over the past 50 years. Conversely, equipment selection for a mechanical engineer is as much an art of application as a science of technology. Today, refinements to manufacturing, increasingly advanced controls, and changing end-user needs determine both the science of technology and the roster of equipment for selection. Compounding this, over the past 15 years, a strong increase in customer needs related to best-value considerations, such as risk, aesthetics, longevity, maintenance, and efficiency, have added complexity to the determinants that need to be evaluated in equipment selection.
To provide maximum value to customers, mechanical engineers must have a strong understanding of owners' needs and the ability to evaluate key aspects of mechanical system selection to meet those needs. In other words, there is an art and science to defining and evaluating key aspects in order to choose the proper equipment.
Demystifying the "wants" and "needs"
An interesting paradigm exists when defining the aspects that should be evaluated in equipment selection. The typical pattern involves building owners and/or end users simply expressing their "wants," from robust to redundant to inexpensive. Difficulty can arise when these wants are discussed and prioritized against the needs identified to drive evaluation aspects. Core aspects exist that often are purely technical variables that require evaluation and satisfaction. Additionally, engineers' needs sometimes vary from the owners', which can create another complicating factor. Regardless, the subjective or intangible wants should not be ignored because of difficulties in quantifying the value. Instead, they should be distilled into needs and evaluated as key aspects in equipment selection.
This can be a hard reality for building owners (customers). For example, owners typically desire brand names and advanced equipment that will integrate into their building system, but they want it to be inexpensive to buy and operate and easy to replace. Brand equity is not so much a need as it is a method of ensuring a reputable warranty, parts availability, proven application, and a wide field of technicians able to service the equipment. The most advanced equipment is not a need, but quality is—and, unfortunately, sometimes top name brands include cheaper heat-transfer materials, unreliable bearings, or statistical quality control rather than start-up and test-inspection techniques.
While inexpensive is a want, the first cost or perhaps total cost of ownership is the need. For example, heat pumps are not very expensive to buy and install, but they do require invasive and time-consuming maintenance (versus a fan coil or variable air volume box); and they become loud and clunky over time. So would noise criterion levels or minimal interruption of the benefitted space trump costs? Not absolutely, but relatively to a point. Those needs must be emphasized and prioritized as necessary for evaluation in equipment selection.
Creating the roadmap for equipment evaluation
The best roadmap for what to consider, and how, results from the lifecycle cost analysis (LCCA) approach and its sum total of satisfying aspects. A total cost-of-ownership approach that identifies needs and assigns values to be evaluated can balance the limitations of first-cost considerations on total comfort, satisfaction, and long-term costs. To perform this, engineers must be able to specify the best equipment for a design as well as be subject matter experts on constructibility, operations, maintenance, human behavior, economics, and manufacturing. One challenge is identifying aspects for consideration. This diverse knowledge is necessary to create the roadmap for equipment evaluation.
The single greatest pressure on any evaluation is typically cost, and more commonly first cost.The first cost is comprised of the capital costs to design, furnish, and install a specific piece of equipment, and it is affected by project speed. Engineers are the subject matter experts that select based on the criterion to be evaluated, not only first cost.
In many instances, an owner structures and selects engineering firms, architects, and contractors to satisfy first cost. Therefore, there is no better arbitrator than the engineer to educate, evaluate, and recommend the selection of equipment that considers all aspects rather than only first cost. The engineer must have a good grasp of these aspects for equipment selection in the factors of their evaluation.
Factors of evaluation
A multitude of varying factors exist for every project and owner, including but not limited to:
First cost: Budgets are a strong consideration, and engineers must limit the equipment options to meet first-cost requirements. The total cost of installation including time, material, infrastructure, and opportunity costs must be evaluated.
Suitability: Equipment selection must be suitable to the application and building. For example, variable refrigerant flow or chilled beams are technologies that either do or do not work well. An example of unsuitability is chilled water in a data center. It is efficient at moving heat, but the presence of water (even with containment) is a risk that must be evaluated.
Constructability due to schedule, lead time, start-up/commission-ability: Aspects such as equipment procurement or tradesman installation time must be evaluated. For example, a piece of equipment that requires a highway shutdown so it can be transferred to the site will have an impact, as will the job site if the equipment must be moved via crane into place. Also, consider whether a piece of equipment can reside in the factory for an extra week if the construction schedule is unexpectedly impacted. Additionally, once the project is launched and commissioned, can the equipment sit unoccupied and not used for 3 months before occupancy?
Ease and cost of operations and maintenance: Do the evaluated equipment-selection aspects account for how preventive maintenance technicians will access the equipment? How accessible are the filters? Does special attention need to be placed on the design of the strainer locations? If the reversing valve fails in year eight, how dire will the beneficial space be to replace it? Are the economizer/outside-air dampers easy to access for maintenance?
Total cost of ownership: This entails first cost and all other major fixed and variable costs associated with the lifetime of the equipment evaluated at net-present value (NPV) against alternatives for selection. This aspect allows engineers to look at incremental factors, such as the benefit of variable frequency drives (VFDs) on the condenser water pumps or whether 1/10 less kW/ton material affects the NPV versus alternatives.
Experience and reputation of the equipment manufacturer: This aspect examines the potential of sourcing partners for equipment. Engineers, owners, and contractors have preferred partners. These manufacturers have gained favor through positive experiences. An engineer must understand the needs and be wary of marketing or prejudiced specifications.
Impact on other building design elements (size, location, interference): Engineers refer to this as coordination, or developing a method of evaluating the coordination with mechanical, electrical, plumbing (MEP), and other system design and installation. Engineers evaluate the risk of change orders, time delays, and other impacts in equipment selection that must be foreseen. For example, the contractor may have to reroute or core a hole in the floor because elevator hydraulic lines are already in the proposed path for the chilled-water supply and return.
Noise criteria (NC): This is a key aspect to be evaluated. Different scales for different frequencies of noise should be understood and evaluated, especially if equipment starts and stops routinely. Engineers must understand ambient noise, and come in under recommended or specified NC targets.
Lifespan: The average age of commercial or school buildings is slightly more than 40 years. Mechanical equipment with proper maintenance can last more than 20 years, and others even longer Evaluating the requisite lifespan is an important aspect of equipment selection. A chiller can easily provide service for 15 years, while cooling tower life varies. A new programmable thermostat may need to be replaced in 8 years due to persistent button pushing. Realistic evaluation is important to achieve the project needs and secure return on investment; it affects total cost of ownership assumptions greatly.
Energy benefits (code requirements, energy efficiency, or value of the property): These types of evaluation variables are abstract and can be difficult to quantify, albeit not to be over-looked. A curious example exists in the Bank of America Tower in New York City, which is a notoriously energy-consumptive building despite having achieved the highest U.S. Green Building Council LEED certification available. Still, the building attracts major environmental-advocating tenets, demonstrating the value of its purported energy benefits.
Scalability, staging, and modularity of equipment: This involves aspects of future planning and optimum use. A cooling unit that runs near full load reaches peak efficiencies and likely achieves good investment economy of scale. However, the same unit that runs at part load does less so. And a unit that short-cycles may not be ideally efficient or cost-effective, but necessary. For projects with phased development and occupancy, perhaps evaluate for what is needed soon and consider scaling. For owner projects with wildly varying load requirements, consider evaluating the equipment needs to satisfy only 85% of those needs. For projects such as data centers with abrupt and rapid expansion needs, consider evaluating what equipment will work over time with the equipment selected now, and vice versa.
Redundancy and failure-node risk: Evaluate areas where weakest-link scenarios arise. There may be value in robust equipment in areas where a failure could lead to difficulties in the facilities. For example, valves, chillers, and pumps associated with a large thermal-energy storage system may require special consideration because the failure of any point therein could result in a facility unable to meet cooling requirements early the next afternoon.
Environmental health attributes (i.e., R-123, ammonia): These evaluation criteria should be evaluated with owners, factory reps, and other authorities having jurisdiction (AHJ) requirements. For example, R-123 refrigerant has been a phenomenal performer through a wide range of compressor load levels, but it is unfavorable by some who cite its potential damage to the environment if leaked. Contrary, ammonia refrigerant is specialized and deadly, but favored by a few for its unique properties and relative friendliness to the environment.
Safety: This is an area every engineer must consider in equipment selection. What is safe to construct, operate, and maintain must be evaluated. For example, discussions with owners and contractors over what and where with regard to safety concerns can integrate project delivery and increase health and safety.
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