What is VRF Air Conditioning System?

VRF air-conditioning systems owe their growing popularity to their ability to meet a wide range of requirements, as Tony Nielsen explains.

The primary function of all air-conditioning systems is to provide thermal comfort for building occupants. There is a wide range of system types available, staring with the basic window-fitted unit through to the very latest VRF (variable refrigerant flow) equipment. Deciding which system best suits the application will depend on several variables. For example, in a modern, design-conscious office the aesthetic requirements of the client may prove of greater importance than the number of control options they provide. While a client looking for the best life-cycle cost will need to balance capital cost with long-term operating costs, efficiency and predicted
VRF systems provide cooling and heating using refrigerant (R407C or R410A) as the working fluid. There are two basic types of VRF system — cooling/heating-only and energy-recovery.

Cooling/heating-only can provide just cooling or, with the incorporation of a heat pump, cooling or heating to the space — but not both at the same time.

The energy-recovery system, can provide cooling and heating simultaneously, to different parts of the building. This feature has the potential to deliver maximum energy savings. Further, a heat-recovery system can transfer the heat extracted from those spaces requiring cooling to areas demanding heating — resulting in extremely high primary-energy ratios and low operating costs.

Outdoor units are available with different cooling and heating capacities to suit the application. A host of controller options is available, from simple wireless controllers to powerful PC software packages. Finally, all the units are linked using transmission cable, providing a fully networked air-conditioning system.

Basic principle of operation:

When in the cooling mode, indoor units are supplied with liquid refrigerant. The amount of refrigerant flowing through the unit is controlled via an electronic expansion valve located inside the unit. When the refrigerant enters the coil it undergoes a phase change (evaporation) that extracts heat from the space, thereby cooling the room. The heat extracted from the space is exhausted to ambient air.

When in the heating mode, indoor units are supplied with hot gas refrigerant. Again, the amount of hot gas flowing through the unit is controlled via the same electronic expansion valve. As with the liquid refrigerant, the hot gas undergoes a phase change (condensation), which releases heat energy into the space

Inverter (air conditioning)

The inverter tag found on some air conditioners signifies the ability of the unit to continuously regulate its thermal power flow by altering the speed of the compressor in response to cooling demand.

Traditional reverse-cycle air-conditioners use a heat pump that is either working at maximum capability or switched off, as the compressor's speed cannot be varied. In order to regulate temperature a thermistor is used to measure the ambient air temperature and switch the compressor on when the ambient air temperature is too far from the desired temperature

Air-conditioners bearing the inverter tag use a variable-frequency drive to control the speed of the motor and thus the compressor. The variable-frequency drive uses a rectifier to convert the incoming AC current to DC and then uses pulse-width modulation of the DC current within an inverter to produce AC current of a desired frequency. The AC current is used to drive a brushless motor or an induction motor. As the speed of a brushless motor is synchronised to the frequency of the AC current, it is thus possible to build a compressor that can be run at different speeds. Similarly, the voltage and frequency can be varied as needed to efficiently run an induction motor at different speeds. A microcontroller can then sample the current ambient air temperature and adjust the speed of the compressor appropriately.

Eliminating stop-start cycles increases efficiency, extends the life of components, and helps eliminate sharp fluctuations in the load the air-conditioner places on the power supply.

While at the beginning of the 1990s inverter air conditioners had some drawbacks,[1] this is no longer the case. Running at full load conventional air conditioners tend to be more efficient and outperform inverters. Inverter-based air conditioners have their strengths in environments where part load is common as they are significantly more efficient than conventional air conditioners in these situations. For most households inverters are the preferred option as part load is the common mode there. The higher initial expense is balanced by lower energy bills. In a typical setting the pay-back time is about two years.

Therefore, it depends on usage what type will be better. In any case though, it pays to buy an energy efficient model, i.e. one with a high SEER, EER or HSPF factor and at least an EnergyStar certified product, as this will significantly reduce the electricity bill.

The inverter principle has been applied to a wide range of household appliances including microwave ovens, washing machines, and so on

High-Performance HVAC-part1

Heating, ventilating, and air-conditioning (HVAC systems) account for 39% of the energy used in commercial buildings in the United States. Consequently, almost any business or government agency has the potential to realize significant savings by improving its control of HVAC operations and improving the efficiency of the system it uses.

The use of high performance HVAC equipment can result in considerable energy, emissions, and cost savings (10%-40%). Whole building design coupled with an "extended comfort zone" can produce much greater savings (40%-70%). Extended comfort includes employing concepts such as providing warmer, but drier air using desiccant dehumidification in summer, or cooler air with warmer windows and warmer walls in winter. In addition, high-performance HVAC can provide increased user thermal comfort, and contribute to improved indoor environmental quality (IEQ).

Given the range and complexity of the subject, this information should be viewed as only a starting point to access information from the many trade associations, agencies, and manufacturers linked throughout the text.

Heating, Ventilating, and Air-Conditioning (HVAC)
The term HVAC refers to the three disciplines of Heating, Ventilating, and Air-Conditioning. A fourth discipline, Controls, pervades the entire HVAC field. Controls determine how HVAC systems operate to meet the design goals of comfort, safety, and cost-effective operation.

Heating can be accomplished by heating the air within a space (e.g. supply air systems, perimeter fin-tube "radiators"), or by heating the occupants directly by radiation (e.g. floor/ceiling/wall radiation or radiant panels).
Ventilating maintains an adequate mixture of gases in the air we breath (e.g. not too much CO2), controls odors, and removes contaminants from occupied spaces. "Clean" air helps keep occupants healthy and productive. Ventilation can be accomplished passively through natural ventilation, or actively through mechanical distribution systems powered by fans.
Air-conditioning refers to the sensible and latent cooling of air. Sensible cooling involves the control of air temperature while latent cooling involves the control of air humidity. Room air is cooled by transferring heat between spaces, such as with a water loop heat pump system, or by rejecting it to the outside air via air-cooled or water-cooled equipment. Heat can also be rejected to the ground using geothermal exchange. Cool air is not comfortable if it is too humid. Air is dehumidified by condensing its moisture on a cold surface, such as part of mechanical cooling), by removing the moisture through absorption (desiccant dehumidification). In dry climates, humidification may be required for comfort instead of dehumidification. Evaporative humidification also cools the air. Further, in such climates it is possible to use radiant cooling systems, similar to the radiant heating systems mentioned above.
Controls ensure occupant comfort, provide safe operation of the equipment, and in a modern HVAC control system enable judicious use of energy resources. HVAC systems are sized to meet heating and cooling loads that historically occur only 1% to 2.5% of the time. It is the function of the controls to ensure that the HVAC systems perform properly, reliably, and efficiently during those conditions that occur 97.5% to 99% of the time.
Each HVAC discipline has specific design requirements and each has opportunities for energy savings. It must be understood, however, that energy savings in one area may augment or diminish savings in another. This applies to interactions between components of an HVAC system, as well as between the HVAC system and the lighting and envelope systems. Therefore, understanding how one system or subsystem affects another is essential to making the most of the available opportunities for energy savings. This design approach is known as whole building design.

Impact on Building Energy Performance Goals
Employing high-performance HVAC equipment in conjunction with whole building design can result in significant energy savings. Typically, a 30% reduction in annual energy costs can be achieved with a simple payback period of about three to five years. And, if the payback threshold is extended to seven years, the savings can be about 40%. These figures apply to buildings that offer conventional comfort (e.g., 70°F in winter, 76°F in summer). (For more information on cost-effectiveness, see WBDG Cost-Effective Branch).

If the comfort zone is extended through natural ventilation and air movement in summer, and through lower air temperatures in winter (made possible by highly-insulated and, therefore, warmer wall and window surfaces), even higher savings can be achieved. For example, a typical office building minimally complying with the ASHRAE Standard 90.1-1989 might use 75,000 Btu/sq.ft./yr. The goal for many federal buildings is 50,000 Btu/sq.ft./yr. A highly energy-efficient building using conventional comfort could have an energy use of 40,000 Btu/sq.ft./yr. or even less. A building designed and operated with extended comfort strategies might only use 20,000 to 30,000 Btu/sq.ft./yr.

However, note that highly energy-efficient design utilizing high-performance HVAC equipment often requires more effort and more collaboration from the design team than a conventional, sequential approach.

Fundamentals of Energy- and Resource-Efficient HVAC Design
Consider all aspects of the building simultaneously Energy-efficient, climate responsive construction requires a whole building perspective that integrates architectural and engineering concerns early in the design process. For example, the evaluation of a building envelope design must consider its effect on cooling loads and daylighting. An energy-efficient building envelope, coupled with a state-of-the-art lighting system and efficient, properly-sized HVAC equipment will cost less to purchase and operate than a building whose systems are selected in isolation from each other.
Decide on design goals as early as possible A building that only meets energy code requirements will often have a different HVAC system than one that uses 40% less energy than the code. And the difference is likely to be not only component size, but also basic system type. See WBDG Functional—Meet Performance Objectives.
"Right Size" HVAC systems to ensure efficient operation Safety factors for HVAC systems allow for uncertainties in the final design, construction and use of the building, but should be used reasonably. Greatly oversized equipment operates less efficiently and costs more than properly sized equipment. For example, oversized cooling systems may not dehumidify the air properly, resulting in cool but "clammy" spaces. It is unreasonable and expensive to assume a simultaneous worst-case scenario for all load components (occupancy, lighting, shading devices, weather) and then to apply the highest safety factors for sizing.
Consider part-load performance when selecting equipment Part-load performance of equipment is a critical consideration for HVAC sizing. Most heating and cooling equipment only operate at their rated, peak efficiency when fully loaded (that is, working near their maximum output). However, HVAC systems are sized to meet design heating and cooling conditions that historically occur only 1% to 2.5% of the time. Thus, HVAC systems are intentionally oversized at least 97.5% to 99% of the time. In addition, most equipment is further oversized to handle pick-up loads and to provide a factor of safety. Therefore, systems almost never operate at full load. In fact, most systems operate at 50% or less of their capacity.
Shift or shave electric loads during peak demand periods Many electric utilities offer lower rates during off-peak periods that typically occur at night. Whenever possible, design systems to take advantage of this situation. For example, energy management systems can shed non-critical loads at peak periods to prevent short duration electrical demands from affecting energy bills for the entire year. Or, off-peak thermal ice storage systems can be designed to run chillers at night to make ice that can be used for cooling the building during the next afternoon when rates are higher.
Plan for expansion, but don't size for it A change in building use or the incorporation of new technologies can lead to an increased demand for cooling. But, it is wasteful to provide excess capacity now—the capacity may never be used or the equipment could be obsolete by the time it is needed. It is better to plan equipment and space so that future expansion is possible. For example, adequately size mechanical rooms and consider the use of modular equipment.
Commission the HVAC systems Commercial HVAC systems do not always work as expected. Problems can be caused by the design of the HVAC system or because equipment and controls are improperly connected or installed. A part of commissioning involves testing the HVAC systems under all aspects of operation, revealing and correcting problems, and ensuring that everything works as intended. A comprehensive commissioning program will also ensure that O&M personnel are properly trained in the functioning of all systems.
Establish an Operations and Maintenance (O&M) Program Proper performance and energy-efficient operation of HVAC systems can only be ensured through a successful O&M program. The building design team should provide systems that will perform effectively at the level of maintenance that the owner is able to provide. In turn, the owner must understand that different components of the HVAC system will require different degrees of maintenance to perform properly.

Design Recommendations
Consider all aspects of the building simultaneously. The building should incorporate as many features as possible that reduce heating and cooling loads, for example:

In skin-load dominated structures, employ passive heating or cooling strategies (e.g., sun control and shading devices, thermal mass).
In internal-load dominated structures, include glazing that has a high cooling index.
Specify exterior wall constructions that avoid thermal bridging.
Detail the exterior wall constructions with air retarder systems.
Incorporate the highest R-value wall and roof construction that is cost-effective.
Design efficient lighting systems.
Use daylight dimming controls whenever possible.
Specify efficient office equipment (e.g., EPA Energy Star® Office Equipment).
Accept life-cycle horizons of 20 to 25 years for equipment and 50 to 75 years for walls and glazings.
Decide on design goals as early as possible. It is important that the design team knows where it is headed long before the construction documents phase.

Emphasize communication between all members of the design team throughout the design process (see WBDG Project Management).
Develop a written "Basis of Design" that conveys to all members of the project goals for energy efficiency. For example, such a BOD might highlight the intent to incorporate daylighting and the attendant use of high-performance glazing, suitable lighting controls and interior layout.
Establish a quantitative goal for annual energy consumption and annual energy costs.
Clarify goals to meet or exceed the minimum requirements of codes or regulations during schematic design.
Right Size" HVAC systems to ensure efficient operation.
Accept the HVAC safety factors and pick-up load allowance stated in ASHRAE/IES 90.1-1989 as an upper limit.
Apply safety factors to a reasonable baseline. It is unreasonable to assume that on the hottest clear day if no shades are drawn and all lights are on that each room is occupied by the maximum number of people allowed by fire codes (thus, far in excess of the maximum number of people that can be expected in the building), and then apply safety factors. Safety factors should be applied to a baseline that was created using reasonable assumptions.
Take advantage of the new generation of dependable computerized analysis tools, such as DOE 2.1E, to reduce uncertainty and eliminate excess oversizing. Hour-by-hour computer simulations can anticipate how building design and operation affect peak loads. Issues such as diversity, pick-up requirements, and self-shading due to building geometry can be quantified. As uncertainties are reduced, oversizing factors can also be reduced or at least can be applied to a more realistic baseline

Consider part-load performance when selecting equipment.
Select systems that can operate efficiently at part-load. For example:
Variable volume fan systems and variable speed drive controls for fan motors;
Variable capacity boiler plants (e.g., step-fired (hi/lo) boilers, modular boiler plants, modulating flame boilers);
Condensing boilers operate more efficiently (95%-96%) as the part-load decreases to the minimum turn-down ratio;
Variable capacity cooling plants (e.g., modular chiller plants, multiple compressor equipment, and variable speed chillers);
Variable capacity cooling towers (e.g., multiple cell towers with variable speed or two speed fans, reset controls);
Variable capacity pump systems (e.g., primary/secondary pump loops, variable speed pump motors); and,
Temperature reset controls for hot water, chilled water, and supply air