Variable refrigerant systems
Variable refrigerant systems are not well-known in the United States yet. But they may be the right HVAC system for your next project.
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Variable refrigerant (VR) systems are a unique solution to a common problem. Every person is comfortable at a different temperature, and VR systems allow each person to control the temperature in his or her environment.
According to John Suzukida's article “ Cooling people, not buildings ” ( Consulting-Specifying Engineer , March 2008), it's more efficient to cool or heat an individual space as needed than to condition all of the space throughout the building all the time. Accordingly, VR systems can save up to 20%; some manufacturers claim that savings can be as much as 30% to 40% over other air conditioning technologies.
Although VR systems were introduced in Japan more than 20 years ago (see “Variable Refrigerant Flow Systems,” by William Goetzler, ASHRAE Journal, April 2007 ), they've been used in the United States for only about five years. So VR systems are not very common—nor very well understood. Though consulting engineers may design VR systems as an HVAC alternative for their clients, building owners just aren't catching on, and many engineers don't have much experience with them.
Here's how they work: Using a control system, refrigerant is pumped through pipes to individually sized/configured evaporators in each space, each of which can have its own thermostat. It's a closed-loop system so refrigerant is continuously circulated, unlike a chilled water system, which requires new chilled water to be pumped into the pipes to make up water lost through cooling towers.
Depending on the manufacturer, VR systems are either a two- or three-pipe system (see Figures 1 and 2, respectively). A two-pipe system is a simultaneous system, which allows for simultaneous heating and cooling, according to Nick Conklin, LEED AP, CITY MULTI mechanical engineer with Mitsubishi Electric HVAC , Suwanee, Ga. The Mitsubishi product offers the same components as other systems: outdoor unit, controller, and indoor unit.
Much like a water source heat pump, portions of the system can be manually turned up or down as needed. Whole-building controls can be set to turn on or off at specific times, such as in an office building with set work hours, or can send additional refrigerant to an area of high need, such as a conference room with a lot of people. In-room thermostats allow occupants to control the temperature of their hotel room, office, or retail store.
VR systems are more common in moderate climates—think of them as following the heat pump market. They make more sense in places where heat can be transferred, such as with a ground or geo-exchange loop. They also tend to work better in retrofit projects, said Dominic Tabrizi, PE, vice president of Environmental System Design Inc ., Chicago. Historic buildings may not have enough ceiling space for distribution ductwork, so VR systems may be the way to go.
Piping refrigerant through spaces does not provide for ventilation requirements. Ventilation can be achieved through a variety of options, all of which depend on the geographic location and the building itself. Some engineers opt for rooftop units, energy recovery ventilators, or outdoor air systems; in some cases, operable windows may suffice.
Is it affordable?
First-cost for VR systems is higher; however, they have a lower maintenance cost than air-cooled systems, and energy savings will reduce operating costs. In some applications, such as historical building retrofits, they may even provide a lower first-cost option. Also, the system may save on plenum height, saving ceiling space and use a smaller footprint than traditional systems.
The lifecycle of these systems can be up to 20 years, stated Lee Smith, director of product, engineering, and applications with Daikin AC Inc ., Carrollton, Texas. A traditional VAV system lasts 15 years. O&M is pretty standard for maintenance personnel: filters must be changed and coils must be cleaned annually.
Some engineers argue that, because the system must move the refrigerant around in pipes, only smaller buildings (10,000 sq ft or smaller) are candidates for VR systems. Manufacturers argue that the system can handle any size; it's a matter of controls and engineering design.
Some applications just aren't cut out for VR systems. Buildings in very cold climates may not be good candidates. Buildings with a high data processing load, like a data center, also may not be appropriate. If special filtration is required, VR systems are not the best option. Large zones, such as theaters or gyms, also may not be good candidates.
Refrigerants and safety
A major concern for consulting engineers is the piping of refrigerant through occupied zones. Sachin Anand, PE, LEED AP, principal, dbHMS , Chicago, recently looked at using VR systems on two projects, but opted not to use the system on either primarily because of safety concerns.
Safety should not be an issue, however, when ASHRAE Standard 15, Safety Standard for Refrigeration Systems , is applied. ASHRAE 15, in conjunction with the tables in ASHRAE Standard 34, Designation and Safety Classification of Refrigerants , establishes acceptable concentration limits based on room volume for different refrigerant types. The current version of ASHRAE 15 gives a value of 25 lb refrigerant/1,000 cu ft for R-410A. All VR equipment manufacturers use refrigerant R410-A in their equipment, which is an A1 safety group refrigerant, the safest category of refrigerant per ASHRAE Standard 34-2007.
VR system manufacturers
There are two big players in VR systems: Daikin AC Inc. ( www.daikinac.com ) and Mitsubishi Electric HVAC ( www.mehvac.com ). Be sure to contact both when working on a building project, as their systems are different.
VR systems: To be, or not to be?
Engineers should ask the following questions when considering a VR system in a building:
• How does the system work?
• What are the building owner's needs?
• Is this a new building or an existing building?
• How many zones are required? Do they all require individual control?
• What will the building restrictions be, such as plenum space, exterior equipment space, mechanical equipment rooms, etc.?
• Are there sound (acoustic) requirements?
• What kind of system maintenance is needed?
• Is a centralized system—tied into the BAS or BMS—required?
• Will electrical sub-metering be required?
•Are there minimum/maximum ambient temperatures? Where (geographically) is this building located?
• What's the priority: energy efficiency, IAQ, or O&M??
• What components—in addition to the controls, pipes, and individual room units—are required?
• How knowledgeable is the system manufacturer? Can it answer all questions, and provide long-term assistance?
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