It is June and we are half-way through our Engineers Newsletter 50th anniversary celebration! As the temperature and humidity begin to rise, our appreciation for air-cooling equipment increases. One of the most prevalent cooling systems applied in our homes and small buildings are direct expansion (DX) split systems. Over the years, seven percent of the Engineers Newsletter have covered topics involving split systems. According to one EN, a split-system is nothing more than a packaged unit split in half, one half staying on the roof and the other half moved indoors into a central station air handler. While this is not always the case, it is a good place to start.
From 1975 through 1976, a comprehensive series of ENs focused on DX / VAV systems covering general building criteria, system zoning, air terminal selection, air design criteria, control considerations, and energy conserving modifications. The latter was published just after the first publication of ASHRAE® Standard 90 “Energy Conservation in New Building Design”, and covered strategies for night setback, morning warm-up, air-economizing, and reheat lock out. Since then, ENs have included guidance on controls, coil selection, and systems.
Controls
In 1980, two Engineers Newsletters titled “How do you Control DX/VAV? Part I and II“ (volumes 9-3 and 9-4) were based on the application manual on a similar topic and included guidance for interlocks, crankcase heaters, compressor sequencing and expansion valve selections. Note that while not covered in a recent EN, modern DX controllers and control strategies are the basis for the 2019 Trane Engineers Newsletter Live program “Controls for Small Rooftop Systems.”
Coils
In 1980, before intertwined coils became popular, the EN titled “DX Cooling Coils: Horizontal vs Vertical Split” (volume 9-6) compared the pros and cons of each coil configuration and which arrangements were best for which application. It also covered techniques for ensuring reliable multi-circuit designs. Intertwined coils came about in the mid-80s and significantly improved selections for comfort. In 1988, the EN “The DX Refrigerant Cooling Conundrum: Q: What is the best kind of coil for DX/VAV? A: It depends” (volume 17-1), encouraged split system designers to select intertwined coils just as their “parent” packaged units had been doing for years. This would lead to achieving better leaving air temperature control, easier humidity control and improved system reliability. In “Understanding the Selection of Direct Expansion (DX) Evaporator Coils” (volume 48-4, 2019), Paul Solberg laid out the framework for selecting coils and condensing units to ensure reliable operation. Paul reintroduced us to cross plots and split-system balance points and stressed their importance on the coil and condensing unit selection process. Note that much of his guidance along with the associated piping and refrigerant specialty application guides have now been built into the split selection routines within Trane Select Assist®.
Systems
The 1982 EN “The System Choice: Chilled Water/VAV vs DX/VAV” (volume 11-5) compared the two systems for a multi-level commercial building. It favored the DX/VAV in most categories but did concede chilled water/VAV was more efficient.
In 1990, “A Decade of Change…Enhancements in Split System Design” (volume 19-4) the advent of scroll compressors enabled several changes in system design practice including unloading, oil management, and coil selections which gave opportunity to “design and install less costly and more reliable split systems.”
Also authored by Paul Solberg, the EN “Split DX: Make Sure It’s the Right Choice” (volume 33-3, 2004) begins with a list of seemingly unrelated items.
“TVs, VCRs, automobiles and paint…”
to make the point that:
“…there are a plethora of products and services competing for our attention and our wallets. Choosing the “right” product or service is a matter of knowing the requirements and garnering enough information about the available options to compare their benefits and shortcomings.”
Great advice for nearly any comparison effort. The same newsletter laments that split systems are historically bid in two parts with the “middle parts” like piping, refrigerant specialties, and system balance left for someone else to figure out. Yet the middle parts have significant impacts on the system operation, cost, and ultimately customer satisfaction. If they are unable to be engineered, it may be better to specify a packaged solution like chilled water.
Over the years the DX split system guidance has evolved with the ever-changing equipment landscape. We hope that the Engineers Newsletters continue to meet your needs and if you have ideas for future topics, do not hesitate to make suggestions.
