Variable Refrigerant Flow (VRF) is an air conditioning technology that achieves extremely high efficiency by varying the flow of refrigerant to indoor units based on the exact demand of each individual area. This ability to control refrigerant flow makes VRF Systems ideal for applications with varying loads. Additionally, VRF systems can also provide space heating, consolidating two building systems into one and saving space.
VRF technology is available in two versions – heat pump systems and heat recovery systems. VRF heat pump systems can cool or heat the entire building, but they cannot perform both functions at the same time. Heat recovery systems do not have this limitation and can serve simultaneous heating and cooling loads, thus leading to 3 different operating modes:
- Cooling mode
- Heating mode
- Simultaneous cooling and heating or heat recovery mode
VRF systems in cooling mode
In cooling mode, VRF operation is not much different from that of an air conditioning system: the indoor units are supplied with liquid refrigerant and an expansion valve inside each unit controls the amount of refrigerant that flows. When the refrigerant enters the cooling coil, it evaporates, removing heat from the indoor air and thus cooling the room. The heat extracted from the interior spaces is then rejected to the outside.
VRF systems are much more efficient than conventional rooftop units (RTU), terminal air conditioners (PTAC) and window units. Ductless mini-split systems and water-cooled chillers offer similar efficiency in cooling mode, but are unable to operate in heating mode.
VRF systems in heating mode
Unlike refrigeration mode, where the indoor units receive liquid refrigerant, here they are supplied with hot gas refrigerant. Gas flow to individual units is controlled with the same electronic expansion valves used in cooling mode, and the heating effect occurs when the hot gas undergoes condensation. The basic principle is still the refrigeration cycle, with the difference that heat is absorbed from the outside and rejected inside; In simple terms, it is how to air-condition the outdoor environment cooler to remove heat from it and use it indoors. This change of operating mode is possible with special 4-way reversible valves, which exchange the evaporator and condenser functions between indoor and outdoor units.
In heating mode, the electrical energy consumed is transformed into useful heat, while in cooling mode it is rejected to the outside. Therefore, a VRF outdoor unit can operate above 100% of its rated capacity when in heating mode. However, it is also important to note that capacity may be reduced, especially when the system is subject to a large variation in outdoor conditions. Longer piping lengths, greater distances between the outdoor unit and the last indoor unit, and greater vertical separation are some other causes of capacity reduction.
VRF systems in heat recovery mode
A heat recovery system is ideal when simultaneous heating and cooling are required. The greatest efficiency is achieved when heating and cooling loads are equal, maximizing the amount of energy transferred from one zone to another using the refrigerant. Heat rejection from cooling spaces can be used for space heating in areas that need it at the same time. Thanks to this configuration, each occupant can choose between cooling or heating mode.
Heat recovery systems are very useful when a building has an east-west or south-north orientation with glass facades . The east-west and south-north orientations cause a large difference in the load requirements for each direction during the morning and evening, providing an opportunity for the VRF system to operate in simultaneous heating and cooling mode. Heat recovery is particularly useful in buildings with electrical rooms or data centers that require permanent cooling, as they also become a constant source of heat.
Heat recovery system piping configurations
Heat recovery systems come in 2-pipe and 3-pipe configurations, where the 2-pipe system is the option that requires the highest refrigerant flow. To operate at reduced flow, the 3-pipe system has a liquid line, a high-pressure gas line, and a low-pressure gas line. The control function is achieved with a Mode Change Unit (MCU) or Mode Select Box, which has three manifolds for high pressure gas, low pressure gas and liquid.
- When there are zones that require a space heater, their internal units function as condensers, providing heat from the condensation of high-pressure refrigerant gas. After heating, the saturated refrigerant is fed into the liquid collector.
- The liquid refrigerant is then supplied to the units in space cooling mode, where it evaporates and absorbs heat, becoming a low pressure gas.
- The low pressure gas returns to its respective collector and then to the compressor, repeating the cycle.
In this case, the outdoor unit should only provide the balance between heating and cooling – whichever is higher will determine the operating mode of the outdoor unit.
- If the refrigeration load is higher, the outdoor unit acts as a condenser, rejecting excess heat to the outdoor air.
- If the heating load is higher, the outdoor units act as an evaporator, extracting the extra heat needed indoors from the outside air.
The best recommendation is to locate the MCU in a publicly accessible area such as a hallway, thus minimizing noise and disturbance to the end user. Maintenance of heat recovery systems is relatively easy, as the external condensing unit is connected only to the MCU, making it easy to separate the system into upstream and downstream portions.
According to ASHRAE 34-2013, the refrigerant concentration limit (RCL) is 26 lb per 1,000 ft3 of room volume for occupied spaces and 13 lb per 1,000 ft3 for institutional buildings. This can be easily achieved by locating the units outside of compact rooms. According to Standard 15, a VRF system is classified as a direct system/high probability system where a refrigerant leak could potentially enter the occupied space.
The use of VRF heat recovery systems proves to be the best option, economically and environmentally. This saves building owners from having to install and maintain two separate systems, whilst achieving very high efficiency. With separate systems for space heating and cooling it is impossible to increase efficiency through heat exchange, even if both systems are very efficient separately.
