How Coil Design Can Make or Break Your Commercial Unit's Efficiency

In the world of commercial HVAC-R systems, energy efficiency is paramount – whether your goal is to satisfy industry-mandated efficiency ratings or win customers by saving them money on their electric bill. Overall unit/system efficiency is what end users are after, but if you’re an OEM of commercial RTUs, AHUs, etc., and don’t have a deep understanding of how subcomponents - especially coils - impact unit efficiency, you could be costing your customers money – a lot of money.

Based on the experiment outlined in this article, poor coil design can lead to more than a 100% increase in annual fan electricity usage per coil per year.

However, while suboptimal coil design can lessen unit efficiency, a well-designed coil can be a boon to your units’ economy.

For our experiment, we’ll use the same dimensions for each coil – a 20x32 6-row water coil with 0.5” OD copper tubing. Each coil will also be designed to meet the same performance requirement (73,500 Btu/hr.). In terms of appearance and capacity, these four coils are nearly identical – and that’s by design. The goal of this article is to illustrate the importance of a granular approach to coil design and how even minor design adjustments can have a sizable impact on overall unit efficiency.

Jump to:

• Coil requirements and conditions
• Hypothetical coil selections
• Required fan power based on coil designs
• Cost implications and conclusion

For each coil, we’ll make a change to either the coil’s fin density, fin geometry, or number of circuits - changes which will, in turn, affect the coil’s static pressure and fluid pressure drop. Then, we’ll examine how changes in those values impact fan selection, which will effectively allow us to assign a rough dollar value to those design choices with respect to electricity usage.

Below are the airside and tube-side requirements for the hypothetical application.

Looking for help optimizing coil designs for your OEM equipment? Drop us a line. We'd love to hear about your application.

Coil Selections

Coil #1: Corrugated Fin, 16-circuit, 13 fins per inch

Coil #2: Corrugated Fin, 8-circuit, 8 fins per inch

Coil #3: Sine wave fin, 16-circuit, 12 fins per inch

Coil #4: Sine wave fin, 8-circuit, 7 fins per inch

Required Fan Power

Now that we’ve got an idea of how these coils will perform in our hypothetical application, we can calculate the amount of electricity the coil's fan will require to overcome the coil's pressure drop and satisfy airflow requirements. Those values can be found in the table below.

From here, if we assume that 1 HP = 0.7457 kW and we factor 4,000 hours of operation per year, we can get an idea of the annual price of the electricity needed to power each fan.

Conclusion

Coil design is a critical phase when designing a commercial HVAC unit. Remember, each of the coils above meet the hypothetical requirement, but depending on coil design, users could be paying more than twice as much as they need to per coil. For organizations with thousands of coils, that can add up to some serious savings.

For example, let’s say a hospital campus has 500 coils installed in units around their grounds. If those units feature coil & fan selection #3, their electricity usage from fans alone would total $69,170. If their HVAC vendor had used a coil/fan design closer to selection #2 above, their electricity cost for the same power would be $30,455.

These annual totals are based on a price of $0.15/kWh, which is roughly the average price across the US. However, in a state like California, where electricity costs come out closer to $0.20/kWh[1], the cost of poor coil design is magnified, as shown in the table below.

Using the same hospital campus example, fan power consumption for coil #3 would be $92,225 per year, whereas selection #2 would result in an annual cost of $40,605, a difference of $51,0620 per year.

At SRC, we specialize exclusively in coils, which allows us to treat each project with the engineering rigor it deserves. We understand that sometimes “good enough” is an acceptable standard, but for instances like the application above, there’s real opportunity to gain efficiency. Furthermore, if there’s an opportunity to use less power to do the same job, we feel it’s our responsibility as engineers to help play our part in making that happen. And we think that examples like the ones above show the value of having a coil supplier who’s also an engineering partner. If you’re developing a unit and could use some help with coil design, give us a call. We’ll work with you to make sure your heat exchangers do their job and do it efficiently.

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[1] https://www.electricchoice.com/electricity-prices-by-state/