Design Review – Taking a Look at Coil Options

In previous posts, I started writing about some of the opportunities for improved life
cycle cost that were identified in a recent review of a turn key proposal for air handling equipment that was solicited by one of my clients. Specifically, I looked at how the different fan options available in the proposed equipment could have a significant impact on its life cycle cost due to the different fan efficiencies associated with each option. This post and others to come will discuss some of the other opportunities that were identified by the effort.

The equipment proposed on the project not only can accommodate a number of different fan options, which were the topic of discussion in the earlier posts.  It can also accommodate a number of different coils. So, I decided to take a look at the implications of some different coil selections in terms of over-all performance as well as in terms of coil face area. In this post, I’ll take a look at the face area issue and will explore the over-all performance issues in a different post.

The coil face area has an operating cost implication in terms of pressure drop. At a given airflow rate, the coil face velocity and pressure drop will increase as the coil face area is reduced, a familiar concept to most of us involved in the HVAC industry. What may not be as familiar is that many manufacturers offer a number of different coil face area options with-in a given package or modular equipment frame size. The table below illustrates this for the equipment I was looking at.


Given that the proposal pricing was generally first cost driven, lacking any specific performance parameters other than to match the existing system in terms of capacity, the proposed coil ended up being the smallest coil available in the unit.

Small coils typically translated to the lowest first cost. But, they can also translate to higher pressure drops on both the air and water side and marginal performance when contrasted with some of the other options that might be available.

As you can see from the table, in this particular product line and unit size, it is possible to increase the available coil face area by 60% by using the largest available coil. Of course, this will have some significant implications in terms of performance, fan energy, and first cost.

As a starting point, I decided to look at the fan energy savings that would be associated with the larger coil. My results are summarized in the tables below.



As you can see, the savings in fan energy are about $1,400 per year, and that is just for one of the three units that were to be purchased.

The smaller coil has another implication that is important but difficult to pin a cost on. Specifically, the smaller face area results in a higher face velocity across the coil, which is in
fact, the driver behind the fan energy savings. But the higher face velocity also means it is more likely that water will be carried off of the coil when it is operating in a condensing mode. For this reason, most designers try to limit face velocities on dehumidifying coils to the 500 – 600 fpm range even though the governing ARI standard would rate the coil at
up to 800 fpm.

Preventing water from being blown off of the coil is important because if water is blown off of the coil, it can cause problems that include:

  • Corrosion of downstream components,
  • Belt slippage,
  • Water damage at the area served when water starts running out the diffusers, and
  • Saturation of filters and duct liner.

The latter two points are particularly important because they have indoor environmental quality implications, which can be costly to correct once the damage occurs and costly to litigate if someone gets sick and takes legal action.

It should be noted that paying attention to coil face velocity is only part of the trick for controlling carry over. Coil height can also come into play; the taller the coil, the more water accumulates as it flows down the fins towards the drain pan. The larger the accumulated drop of water is, the more likely it is to be blown off of the fin, especially if it gets large enough to bridge the space between the fins. So, providing intermediate drain pans on large coils is an important part of controlling carry-over. If you want to know more about this or coils in general, the ASHRAE Handbook of Equipment and Systems has an entire chapter dedicated to cooling and dehumidifying cools.

Anyway, returning to my discussion about the cost/benefit of the larger coil, I didn’t have access to pricing for the different coil options from the equipment manufacturer. But, I did have USA Coil’s free coil modeling software which includes pricing in addition to coil performance.

So, I assumed geometric similarity and modeled both coils using the software, changing only the size of the coil to match the manufacturer’s specifications while holding everything else constant. The illustrations below show the input and output screens for the modeling program for the larger coil model as well as a summary comparing the two coils.




The bottom line is that the larger coil has an attractive simple payback simply in terms of the fan energy it saves.

The extra money also buys you several other things. For one thing, it buys more heat transfer performance. Granted, that can cost you money at the central plant, but only if you use it. By properly controlling the coil, it will only use the amount of cooling necessary for the current load condition. But, the larger coil could deliver more performance if you needed it. This is likely attractive to Tracy, my client, since the units serve ball
rooms in a hotel and the loads can be highly variable and unpredictable. A little spare capacity can be worth its weight in gold on a hot day with an event in the ballroom that is using a lot of special lighting, serving a large crowd, or both.

The larger coil also has the potential to save pump energy relative to the smaller coils as indicated by the lower water side pressure drop. There is some flexibility here to reduce
the pumping head for the smaller coil by changing the circuiting. For instance, if I change the circuiting from single serpentine to double serpentine, the pressure drops for the smaller coil drops to 3.6 ft.w.c.

\There is also a modest improvement in leaving air conditions to 57/56.4°F twb/tdb. But
changing the circuiting increases the cost of the coil bank to about $7,874 or with-in 8% of the cost of the larger coil with none of the other benefits.

Related to the coil face velocity is the selection point for the unit proposed in general. If you look at the table below, which is the quick selection table for the proposed equipment, you will notice that the proposed equipment is actually being applied at about 6% above its nominal rating (18,000 cfm application vs. a nominal rating of 17,050 cfm).


Increasing the unit size certainly has cost implications both for the unit and for the related utilities and structure to support it. Thus it may not be a practical consideration for Tracy’s replacement project.  But, for new construction, given that the unit may be in place for 20-25 years or more and the savings would compound over that time, it may be worth considering.

The rated capacity of a size 40 system would provide approximately 9% reserve relative to the original equipment while a size 50 system would provide 37% reserve. At the current operating conditions, the larger units would reduce operating cost approximately $500 to $900 per year due to lower pressure drops in addition to the providing reserve capacity and flexibility.

In general terms, the size 40 unit appears to be about the same height as a size 35 unit and a bit over a foot wider and increases the weight of the unit by about 10% based on the fan and coil section weights.  Thus, the structural implications of upgrading one size may not be that significant.  The size 50 unit is about a foot taller than the size 35 unit and about 2 feet wider and increases the weight of the unit by about 40%.

The bottom line is that many of the current modular air handling product lines provide a lot of flexibility and options. Spending a little time or engineering budget to explore
the possibilities from a life cycle cost perspective instead of purchasing based on first cost can open the door to significant benefits in terms of operating cost, performance improvements and reserves, and flexibility.


David Sellers
Senior Engineer – Facility Dynamics Engineering

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One Response to Design Review – Taking a Look at Coil Options

  1. Brian Derby says:

    David, Thanks for the article. This is great analysis for common design review situations. I just want to point out that the water pressure drop listed in the comparison table appears to show a higher pressure drop for the larger coil. At first it looked like the fan savings would be offset by a pumping penalty, but I can see from the two sizing screenshots that there is additional pump savings by switching to the larger coil. Thanks!

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