A friend and client of mine recently sent me a copy of a proposal he had solicited from an equipment manufacturer for a turn-key installation of three air handling units and asked if I would review from a commissioning and engineering perspective. The units were intended to replace three aging direct expansion package systems serving a hotel ballroom. The new equipment would be double wall, modular rooftop air handling units served by a recently renovated and expanded chilled water plant. The project and proposal included the expansion of the facility’s DDC system to serve the air handlers and related utility systems.
All-in-all, the new equipment has the potential to take a major step forward in terms of providing improved performance and efficiency, better turn-down capability and higher quality equipment to serve a critical hospitality industry function. But, the upgrade represented a repair by replacement of existing machinery and the design criteria was simply to match the existing equipment in terms of performance. This is not an uncommon approach for facilities engineering groups to take when face with the need to replace aging machinery in a timely fashion and on a limited budget with very little time for direct involvement in the process. Thus a trusted supplier is a valuable asset because they know they can turn a replacement project over to them and get a proposal that will have all of the bases covered and deliver a system that will work.
But a working system is not necessarily an efficient system, and may not represent the best value when viewed in a life cycle perspective. Unfortunately, the pressures of a short time line and tight budget often lead to lost opportunities in terms of securing the best value for the dollar, especially when the only real design criteria is to match what’s there as cost effectively as possible. Frequently, the term “cost effective” is interpreted to mean that the lowest first cost will be the right answer. And, on many occasions, that is exactly what is meant. But, as Tom Stewart, another friend of mine would often say, there are a lot of right answers out there; we’re looking for the best one.
In my experience, taking a life cycle perspective on the term “cost effective” is one way identify the best right answer. Tracy, having been around the block a couple of times himself had a similar opinion; thus his request that I take a look at the proposal. While he was happy with the price and the improvement in quality and performance it represented, he was wondering if there might be a few “gems” in there, with a “gem” being something were spending a few extra dollars up front would represent a long term value that could be readily accommodated when the proposed equipment was a concept on paper, but would become inviable once the concept became a physical reality.
It turned out that there were a few gems and that it was not really that hard to identify them, given a bit of time and a bit of familiarity with the fundamental physics behind HVAC. So, I thought it would be interesting to share a few of the them with you via this blog in the hope that the insights I had while reviewing Tracy’s proposal might also be valuable to others. So this post and subsequent posts will look at some of the opportunities that were identified, one at a time.
Same Frame Size, Different Fan Sizes – Part 1
Given that the new systems were to match the existing systems, one of the first things I looked at was the flow rate proposed for the new equipment vs. the flow rate provided by the existing equipment. I discovered that while the proposal matched the original equipment schedule in terms of cubic feet per minute of air delivered to the load, there were several opportunities to optimize the process. One of them was related to the prime mover behind delivering the air; i.e. the fan.
Most modular equipment lines include options for a number of different fan sizes with-in a given unit size, and such was the case for the units proposed for Tracy’s project. Specifically, there were six different fan options; two forward curved double width, double inlet fans (DWDI) two airfoil DWDI fans and two axial fans. Given the existing distribution system configuration, the axial fans were not a viable option, so I focused my attention on the forward curved and airfoil blade options.
When I did my review, I did not have the manufacturer’s fan performance data available to me. But, I did have Greenheck’s CAPS selection software. So, I assumed geometric similarity and used the software I had to project what the performance of the different fan options might be for the fan sizes available in the product line associated with the proposal.
Incidentally, Greenheck isn’t the only manufacturer to offer software selection packages at no cost. For instance (in alphabetical order):
- If you log into the Buffalo Forge website and look in their on-line library, you will find a page called How Things Work that has some nice graphics and explanations about different fan technologies. You can also order the Buffalo Fan Engineering Manual in paperback or CD form, which is somewhat of a classic in the industry.\
- If you log into the Chicago Blower web site, you will find a place to ask for a copy of their Fan.Net software as well as a place to request a copy of the fan engineering guide.
- Lawrence Berkeley National Labs put together a Fan Source Book that provides basic information on fans and fan systems theory, guidelines for improving fan system performance, and links to additional resources which is available for download at no cost.
- If you log on to the New York Blower website, you can request an electronic copy of their catalog on CD, complete with their selection application and download their Engineering Newsletter series.
- If you log onto the Trane Company’s website, you can order the Trane Fan Manual for a very modest cost and learn a lot by reading it.
The point is that there are a lot of very useful resources out there that you can easily obtain which will provide not only assistance when you are reviewing fan selections and designs, but also will provide a way for you to learn more about the hardware you are working with everyday.
Anyway, getting back to my little review project, since the units were to be sized to match the original equipment, I simply used the original equipment’s scheduled operating point as the input for my selection. The results of my analysis are presented in the table below, along with a screen shot from the selection software.
The Greenheck software did not have any DWDI forward curve fan selections available, so I just compared the backward inclined (BI) and airfoil (AF) selections. In general terms, for a given fan wheel size and a given operating point, a forward curved fan will be the least efficient but lowest cost option while an airfoil wheel will provide the best efficiency but at the highest cost. A backward inclined wheel will lie somewhere in-between.
The ASHRAE Equipment Handbook provides a really nice table in the chapter on fans that contrasts the various fan types as illustrated below.
The table is also included in the recent versions of the ASHRAE Pocket Guide.
As you can see from my analysis, there is a significant difference in fan efficiency associated with the proposed AHU depending on which of the five fan wheels are selected. And, while I did not have pricing data available to me for the different fan options, its likely that the least expensive fan is the smallest forward curved wheel available, which is likely the reason that the lowest first cost based proposal indicated that AHU would be provided with the smallest forward curved wheel available.
In the next post, I will look at what the different fan selections mean in terms of operating cost.
Senior Engineer – Facility Dynamics Engineering