Design and Construction Using Pre-Fabricated Systems- Part 1

Introduction
A growing source of productivity in the Design and Construction Industry is the use of prefabricated systems or materials. These can range from a simple wall section that is built off-site and then set into place– to the complete structural system of the building, which can be designed and cut (assuming a steel structure) off-site then assembled on site. Prefabricated systems and materials offer a host of benefits to each project including;

– The material can be assembled in a controlled environment which provides a much higher level of quality control.
– The systems can have all plumbing, wiring, and finishes installed in a clean environment, then all of the systems can be tested and inspected—before they arrive on site. This is especially useful when the system might be used in a specialized environment like a hospital, or with repetitive assemblies that require a more rigorous inspection.
-The manufacturer might be the designer and builder of the system for a specific building, then it is simply installed and final tested on site—which can simplify the installation, testing and warranty period. This leaves the manufacturer the option of offering and providing long term maintenance, and even of the option of “leasing” that component.

McGraw Hill has a free document discussing the impact of prefabrication in much greater detail at http://construction.com/market_research/FreeReport/PrefabSMR/.
In a time when craft labor is becoming more scarce- like today—the ability to manufacture systems and materials in a clean environment typically leads to a better, more productive use of the available, labor pool, and provides an environment where the craft labor component becomes less thru the use of automated processes and robotics.

This post is the first of a series of three posts on the same topic, the use of prefabrication. The posts will describe three specific types of pre-fabricated systems and how they were developed and installed on a project. They include a discussion on the installation of a major Variable Refrigerant Flow (VRF) system for heating and cooling; a discussion related to a hospital headwall system and how the design evolved, how they were built and how they were installed- saving significant time, and producing a better product; and a discussion on a prefabricated structural system for a complete building.

Variable Refrigerant Flow (VRF) for Heating and Cooling a Major New Building

At Denver Union Station there are two buildings that stand to the North and the South of the Historic Union Station Building. See http://denverunionstation.org to see that development and how amazingly it has changed the center of downtown Denver. The buildings are called the North Wing Building and South Wing Building and they both fall within Denver’s Landmark Zoning. That Zoning requires a “hard cap” on the building to the height of 65 feet, which is typically sufficient for a four story building. The developers of both buildings worked together in determining if there would be a way to build five story buildings within that envelope and still maintain a Class A business environment in each tenant space. Class A translates to 9 foot ceilings (minimum), significant glass for sunlight, high end finishes throughout, and an exterior design that is compatible to and compliments the surroundings
It turns out there are two mechanical system approaches that can be used to accomplish the height and still maintain the 9 foot ceilings. The building could use an underfloor distribution system or a variable refrigerant flow system. Both would provide heating and cooling and allow the floor to floor height to be decreased. After significant research, the VRF system was chosen for both Wing buildings.

The VRF system has a number of great attributes; it is essentially a completely prefabricated system- excepting for a fairly straightforward condensing water loop; the detailed design is done off-site, by the manufacturer including controls; the system saves a significant amount of energy in the day to day use; the system allows significant flexibility in temperature control for different areas; and it allows great flexibility in after-hours use. In short, the VRF system had all of the attributes of a prefabricated system listed above and was a perfect fit to the building’s geometry.

The Progression from Design to Construction

The VRF system was proposed very early in the design process by Greg Koenig of Haynes Mechanical Systems and Paul Dense of MTech Mechanical and Plumbing and both championed the system’s use throughout the design and construction period. (Paul has subsequently joined Trautman and Shreve). Haynes Mechanical is a specialized mechanical service company and they developed a service program for the VRF systems as provided by Mitsubishi. A second service firm in Denver represented the major competitor for the VRF systems- Daiken.

The use of the VRF system in two new Class A buildings in Denver required a great deal of research and collaboration between the project Engineers, ME Engineers; the Architects , AMD; the Mechanical Subcontractor, M-Tech; the General Contractor , Haselden Construction; the building ownership; and the City and County of Denver. The VRF system is a relatively new technology in the United States, though it is a tried and tested technology in Asia and Europe. You can find a complete introduction to the use of VRF technology in Class A Buildings at http://www.mitsubishipro.com/en/professional/overview-and-benefits/overview. You can find examples of the installation of the VRF system in a number of different building types and sizes at the site.

VRF in a Nutshell

The VRF system is a sophisticated, water cooled, heat pump system, with 50 tons of compressors on each floor. The transfer medium for heating and cooling is a gas, instead of forced air as found in the more traditional heating and cooling systems. The local heat pump reduces the overall use of medium pressure ductwork by 75%. Additionally, It is possible to heat one area and cool another so the system delivers good comfort throughout the building, and the temperature can be controlled at the single office level . The compressors are extremely quiet and vibration free. In fact, they are located on each of the office floors and cannot be heard or felt by the tenants on that floor.

All of the gas piping is pre-cut for each office and the ends are complete with couplings. Installation is relatively easy compared to a ductwork system and the height required above the ceiling is approximately 12 inches versus a 24 to 30 inch height requirement in a typical ducted air system. The City Building Department had concerns for the amount of refrigerant in the piping and what the impact would be if there were a failure—and again—the City Engineers worked closely with the team defining exhaust pathways and air volumes as well as understanding the chemical makeup of the refrigerant. The total team worked to make certain the system would be safe for the tenants and users.

The Impact/ Benefits of the VRF Technology and Innovation

The entire design and specification process for the building took over a year and a half, and involved a very diverse set of professionals working together in the process—and it was well worth that effort. The benefits from using the VRF system for the ownership and tenants included the following;

1- The building design could easily accommodate 5 floors within the 65 foot hard-cap.
2- The system is modular and installation was simplified because so much was built off-site in a manufacturing environment. In fact, the detailed system design was by the manufacturer and the building engineers and architects worked within that spec.
3- We eliminated 75% of the medium pressure ductwork found in a traditional office building and a majority of the vertical shafts- essentially adding several thousand rentable sf to the building at no marginal cost.
4- The control system was modular and programmed for this application. The building only needed to provide an appropriate control interface to the “world”.
5- The mean-time-to-failure on the compressors is listed as 18 years and they can be taken out with a handcart in the elevator—all while the balance of the building VRF remains operational. Quite different than a traditional roof top system, where the entire building is shut down to replace a compressor. A crane is typically required to even move the compressors.
6- The Building attained a LEED Gold Certification in part because of the efficiency of the VRF system.
7- Time was saved in the installation, and testing. The system does not require air-balance testing and minimal adjustments. The VRF can be considered plug-and-play at scale.
8- The over-all system operates quietly, efficiently and vibration free. It is a perfect application in an office environment.
9- There is a cost difference but that easily washed out with the gain in rentable square feet from eliminating the vertical shafts and the ability to add the fifth floor.
10- The reduced energy cost helped the building gain a financial incentive from XCEL energy for the reduced power system demand and power consumption. In addition, the electrical wiring , breakers, panels, etc. were smaller- yet another savings.

In Summary

Given the modularity of the VRF system; the fact that the manufacturer designs the piping, then cuts and terminates it; the fact that it requires very little testing before turn-on; and the limited number of other trades required for the installation makes the VRF system a “poster child” for the benefits of using prefabricated systems on a construction project. The innovative technology and productivity gains during installation go to the heart of the benefits of a VRF system. The system has only been accepted in the US at any scale and the North and South Wing buildings have established a new standard for delivering Class A space to their tenants.

The next posting will be Design and Construction Using Pre-Fabricated Systems – Part 2—and will feature a unique Prefabrication process for delivering headwalls for new Hospital rooms on an accelerated schedule.