Variable-refrigerant Flow (VRF) Systems: Air-cooled And Water-Cooled
March 19, 2014
Variable-refrigerant flow (VRF) systems have been gaining new attention among facility executives in the United States. The systems are energy efficient and can simultaneously heat and cool separate spaces in a building. But facility managers have to evaluate each project separately to decide whether a VRF system is appropriate. There are two basic systems, air-cooled and water-cooled, and a simple VRF system consists of an outdoor condensing unit and multiple indoor evaporators. The condenser and evaporators are connected by a complex set of oil and refrigerant pipes, all governed by individual thermostat controls.
Ramez Affify, principal at E4P consulting engineering, has worked extensively with VRF systems, including ASHRAE subcommittees addressing variable refrigerant flow, and notes that basic questions need to be asked before installing a VRF system.
"There are major decisions in the beginning of each project to choose the most suitable HVAC system for a building," Affify says. "When VRF are considered, the very first question is: Will the VRF units be air cooled or water cooled?"
If they are air cooled, Affify says, exterior space is required for installation of the condenser unit. Furthermore, the space/site selected for installation has to be away from windows, accessible for maintenance, and able support the weight of the units.
In one case, "the height of an exterior [air-cooled] VRF units caused the neighbor, which was an adjacent restaurant, to complain because they said the unit blocked their view," Affify says. "Lesson learned is to think ahead before installing a 6-foot outdoor VRF section, especially in low rise communities."
He further notes that architectural enclosures can be considered; while the enclosures might not mitigate concerns neighbors have with blocked views, they can hide the condenser units.
If a water-cooled VRF system is used, Affify says, water source units that help comprise the system can be placed in small closets.
With both air- and water-cooled units, a feasible path to route the network of refrigerant pipes needs to be identified.
One challenge when specifying VRF systems is providing a separate outside air supply to each indoor unit to comply with ASHRAE Standard 62.1 and building codes. For larger buildings, that means that a separate outside air fan and control system is usually required, and in humid climates, providing preconditioned outside air to each indoor unit helps ensure good indoor air quality.
Today's quick read came from , contributing editor for
Building Operating Management.
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