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 2×2 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.
For these particular facilities, any other system would have been impossible. Normally, an abundant amount of open ceiling space will be required outside the room and large ductwork connected to supply and return outlets of the unit and room to plentiful wall space, or floor space will need to be provided for a floor-mounted vertical data computer room air conditioning (CRAC) unit. Finding floor space in the equipment room where radiology manufacturers are filling every square inch with equipment is a challenge. In the Heart Hospital, the radiology rooms were compact in the surgery department sterile area, so little to no ceiling space would be available for a unit or ductwork. The sterile corridor above the ceiling space was occupied by the operating room air systems that served the procedure and operating rooms. The cassettes were to be placed in the ceiling grid where no equipment is located below where condensate can spill in an unusual circumstance and damage the equipment.
At the cancer center, the team designed a combined cassette and ducted system. The ducted indoor units were used to serve a small zone of offices and the MRI room that required a unit outside the space with nonferrous duct and diffuser system connecting for safety measures from the MRI’s magnetic forces. The equipment and control rooms consisted of cassettes, and the electrical rooms had a wall-mounted unit above the doors cooling large transformer loads. The wall unit mounted above the door is in an ideal location because there is no electrical equipment below for condensate to damage.
The main parameter that forced the decision to use VRF was the floor-to-floor ceiling height. This building was 11 ft 6 in. floor-to-floor with large steel beams extending the length of the building. Providing a duct system in the plenum for the increased load requirement for a radiology department would have been impossible. There was also no floor space outdoors for rows and columns of condensing units. The team fit two stacked condensing units outdoors under the soffit in a smaller footprint than the condensing units to be removed during demolition. The outdoor air requirements were low enough that the engineers supplied unconditioned air to the units that handled the outdoor and indoor sensible and latent loads. The outdoor ductwork sizes were small enough to be routed to the units in the limited space available in the plenum. For facilities with high outdoor air requirements, designers shall evaluate the need for conditioned air with a DOAS.
Pressure, control requirements
When evaluating the amount of rooms and floor space to be placed on a zone, the designer must consider the static pressure limitations of the indoor units. The indoor units’ external static pressure capabilities are approximately 0.39 to 0.55 in. w.g. The lower the airflow requirements, the smaller the amount of static pressure that is available from the unit. Large zones that require supply and return ductwork should be evaluated closely. Also, filter requirements per space shall be examined andASHRAE Standard 52.2 consulted. The VRF units typically provide only a washable filter that satisfies the lowest level code requirement of 25% to 30% dust spot efficiency. Designers will need to seek custom applications for spaces (such as hospitals) that require more stringent filtration, such as by consulting TDSHS Title 25 Chapter 133, and FGI Guidelines for Design and Construction of Health Care Facilities.
Capacity limitations shall also be noted when laying out the zones for the building. The cut-off limit for the indoor fan coil units is typically 4 to 5 tons. Heating capacity is also limited to approximately 52 MBtu per unit. Zones may require supplemental electric heat, which could complicate the controls and design. However, some manufacturers now provide an option to include additional electrical heat in the unit, which could be very desirable for frigid climates and exterior zones with high heating loads. Some manufacturers have more capacity than others and are able to include more units per system.
Control systems are proprietary and complex. They control the complexity of the refrigeration exchange between the evaporators and condensing units with the heat recovery units. They issue alarms including filters, components, and sensors that will inform the facility if filters need to be changed, coils need to be cleaned, refrigerant levels are low, or components are malfunctioning. The control systems are good for both facilities needing stand-alone controls and a central building operation system that can either monitor or take over the control of the VRF system.
Another smart feature of these VRF systems is a sensor that compares air and floor temperatures and adjusts the diffuser blade direction to provide distribution throw for uniform surface temperatures. Condensing units have a defrost mode to remove the frost on the condenser coil, reducing heating delays until the unit is started and completely thawed out. The VRF units are quieter than other DX technologies. Indoor units can operate at sound levels as low as 23 dB and outdoor ones at as low as 50 dB. That is equivalent to a soft whisper indoors and a normal conversation at 3.2 ft outdoors.
Indoor units are equipped with condensate pumps so that there is no need for gravity condensate design. Also, the pump can consist of a high head sensor programed to the building management system for secondary condensate system requirements. Some manufacturers include multi-zone dampers on the front of the unit to avoid costs of providing a unit per zone. The thermostats for each zone control the opening of the dampers. Some manufacturers allow for a “raw” coil to be placed in other equipment to incorporate into the VRF design. Instead of replacing a unit, a coil can be retrofitted and added to a VRF system and single condenser with variable flow technology used with corresponding energy savings. The systems are also available in water source heat pumps. This provides a great application for data rooms in office buildings with cooling towers that serve water cooled air handling units. The condensing units that are water sources are extremely compact and can be placed in a local equipment room requiring very little floor space.
Each manufacturer uses its own proprietary selection software. The designer should run its loads per zone separately with airflow, entering and leaving temperatures, sensible and latent cooling loads, and heating loads. The manufacturer will enter the designer’s data into the selection software that will match indoor and outdoor unit size, calculate piping sizes and piping line lengths, determine refrigerant charge volume, match heat recovery boxes, and provide electrical wiring diagrams.
A challenge with laying out the system around the design, if another manufacturer wins the project, is that there may be quite a bit of redesign. The redesign mainly comes from the fact that various manufacturers have different capacity cut-off points for indoor and outdoor units and piping systems. Some manufacturers can daisy-chain their piping from one unit to the next and home run to the heat recovery units. Some have to run all their piping back to one box, and others run their piping to several boxes. There are also two- and three-pipe systems. With these different designs, refrigerant line sizes, line lengths, refrigerant volume, space to locate all of the heat recovery units, and the electrical design will change from one manufacturer’s layout to another.
Discussions with the owner and design team concerning the cost of the system should be scheduled for the beginning of the project. These complex systems may not have a desirable payback for certain facilities and applications. VRF systems are more expensive than traditional DX systems that split or rooftop units. The VRF systems usually achieve their payback by reduced electricity costs over a period of time and decreased maintenance and installation costs. Since these systems contain fewer components and less ductwork, they are easier to install and maintain. Longer lengths of refrigerant piping are easier to install and maintain than several small branch piping segments required for constant flow split systems. In addition, most of the components and parts that need service and replacement are located at the units installed in accessible locations.
The installation and operation manual should be read carefully for installation requirements. Most manufacturers provide training for procedures to install the units and piping. It is very important to attend the training. If this step is skipped and the refrigerant piping is not spaced and anchored correctly, the piping can expand and stress against the other materials. In this situation, leaks have occurred that compromised system performance, and the piping needed to be removed and reinstalled in several sections.
Around the world, engineers and facility owners are enjoying the advancing technology of DX systems. Engineers are using creativity to provide applications for building conditioning increasing capacity capability, energy efficiency, and space use flexibility while decreasing the complexity of installation and maintenance. The benefits are not only reaped by the engineers and owners, but also by the architects and interior designers who optimize the design and use of the facilities.
Consulting Specifying Engineer Magazine
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