VRF offers flexible, energy-efficient heating and cooling
Variable refrigerant flow (VRF) systems can offer flexibility in design-intensive and challenging HVAC projects.
Engineers are tasked with the vocation to bring order from chaos, enhance efficiency from overuse and waste, provide solutions in the midst of difficulty or near impossibility, and contrive innovation out of obsoleteness. Building design offers a variety of challenges for system and equipment determination. Engineers should evaluate the conditions and offer innovative solutions that enhance the opportunity for the building designers and architects to present to the owner and its customers the best state-of-the-art high-performing building. Variable refrigerant flow (VRF) systems with the advancement of technologies continue to deliver those opportunities.
The technology is properly defined as variable refrigerant volume (VRV) or VRF. This particular system circulates in real time the minimum amount of refrigerant compound that is required to satisfy the building load and each individual zone. The technology resides in the direct expansion (DX) family, and is cutting edge with its efficiency and features. The system seeks to eliminate many obstacles including capacity and material overuse, space use, comfort control, and energy savings.
Typically, the size range for each system is 6 to 30 tons for each designated building area. Within these systems, outdoor condensing and indoor fan coil units are networked to heat recovery systems employing branch circuit technology that exchanges energy from cooling zones to provide energy to heating zones and, vice versa, energy from heating zones toward cooling areas. The controls are advanced in their operations to bypass the condenser unit containing the compressor to save electrical consumption.
These energy-efficient operations make the VRF technology very desirable. The systems consist of a network of zones that heats and cools only the zones in use. The amount of refrigerant flow and volume is reduced only to the amount needed per system per building. The equipment swiftly adapts to the changing loads and can be designed for the exact needs. This eliminates the need to oversize each terminal equipment item to cover every zone peak load.
These systems provide a solution to the traditional DX system limitations of the past of on/off control of oversized units, and abundant quantity of equipment and piping indoors and outdoors of constant volume, capacity, and flow. With VRF technology, several indoor units are networked with controls and piping to a heat recovery box and connected to a single condensing unit outdoors. The controls remove energy from one zone that is not using it and applies it to other zones calling for heating or cooling. The compressor speed in the condenser also varies from 15 to 150 Hz for partial and demand controlled operation. The vapor injection technology maximizes the heating performance in heat pump mode through variable stage compression.
In past experience with VRF, the engineering team designed and modeled up to 40% savings over traditional DX systems, and a system measured and verified a system for a building at the University of Arkansas Epley Center for Health Professions for 24% savings over baseline DX system documented by ASHRAE Standard 90.1-2007.
The VRF energy benchmark is best evaluated by the IEER (integrated energy efficiency ratio) rating by ANSI/AHRI 1230. The industry is in the process of recognizing this method over the integrated part load value (IPLV). To obtain ratings using the IEER method, the systems are tested at four capacity levels and outdoor temperature conditions. The formula for IEER is:
IEER = (0.02xA) + (0.617xB) + (0.238xC) + (0.125xD)
A = EER at 100% net capacity at AHRI standard condition (95 F)
B = EER at 75% net capacity at and reduced ambient (81.5 F)
C = EER at 50% net capacity at and reduced ambient (68 F)
D = EER at 25% net capacity at and reduced ambient (65 F).
This method gives a more accurate benchmark for the system because systems rarely operate at full load. When the overall capacity is less, the EER value increases considerably—sometimes higher than 70% of the full load EER value. The values for these systems achieve 20 to 30 IEER depending on the design and how often the zones operate in peak load. VRF design also has opportunity to eliminate ductwork, which greatly reduces the static pressure requirements providing additional energy savings.
VRF systems also are effective because they are flexible with space usage. Traditional split DX systems require one outdoor condensing unit per indoor evaporator fan coil, requiring an abundant amount of outdoor footprint and interstitial space. To reduce the outdoor footprint, most manufacturers can provide a network of up to 64 indoor units connected to one outdoor unit. The piping length for these systems has been extended to at least 130 ft vertical while traditional systems were limited to approximately two floors. Some manufacturers report greater distances of up to 450 ft. This allows all condensers to be tucked away in a small footprint and screened in a yard to serve a building larger than 10 stories.
The engineering team designed a system for the 46,000-sq-ft Epley Center for Health Professions at the University of Arkansas. There were four systems with approximately 80 indoor units consisting of cassettes, fan coils, and wall units. The team supplied outdoor air from two dedicated outdoor air systems (DOAS) located on the roof. The yard housing the unit was located on the side of the building and screened away from view. Four VRF condensing units were installed away from view supporting the building.
One of the most valuable features of VRF systems is the ability to be low-profile and ductless. The cassette-type units can be placed in a lay-in grid that takes up a little more than 2x2 ft of space in occupied areas with high loads such as conference, waiting, training, and meeting rooms. These units can also be located in equipment, control, and data rooms with high cooling loads. The engineering team designed a system for a large hospital in Dallas-Fort Worth area for a series of parking garage vestibules that only had 8 in. of plenum space.
Most manufacturers can compact 4 tons of cooling in a single compact cassette. The cooling capacity of a data or equipment room can be obtained by requiring space equal to only a few ceiling tiles. This design was used for a cancer center renovation for Texas Health Resources in Arlington in a MRI equipment room with a 10-ton requirement. The same ceiling cassette application was implemented for a hybrid operating room, catheterization lab, and an electrophysiology lab with a 6- to 10-ton requirement in the Baylor Heart Hospital in Denton, Texas.