Variable Refrigerant Flow systems

Variable refrigerant flow (VRF) systems vary the flow of refrigerant to indoor units based on demand. This ability to control the amount of refrigerant that is provided to fan coil units located throughout a building makes the VRF technology ideal for applications with varying loads or where zoning is required. VRF systems are available either as heat pump systems or as heat recovery systems for those applications where simultaneous heating and cooling is required.

In a VRF system, multiple indoor fan coil units may be connected to one outdoor unit. The outdoor unit has one or more compressors that are inverter driven, so their speed can be varied by changing the frequency of the power supply to the compressor. As the compressor speed changes, so does the amount of refrigerant delivered by the compressor. Each indoor fan coil unit has its own metering device that is controlled by the indoor unit itself, or by the outdoor unit. As each indoor unit sends a demand to the outdoor unit, the outdoor unit delivers the amount of refrigerant needed to meet the individual requirements of each indoor unit

These features make the VRF system ideally suited for all applications that have part load requirements based on usage or building orientation, as well as applications that require zoning.

Variable Air Volume (VAV)

Variable Air Volume (VAV) is a type of heating, ventilating, and/or air-conditioning (HVAC) system. The simplest VAV system incorporates one supply duct.

The air handling system delivers heated or cooled supply air to the multiple VAV systems attached to the building’s air ducts. The VAV system control is dependent on the operation of the air handling system.

The air handling unit consists of two main components: • The air handling equipment, which consists of outside air dampers, mixed air dampers, heating coils, cooling coils, and supply air fans. • The air handling controls, which consists of the electronic control hardware that sequences the operation of the air handling system.

The air handling control operation, as shown in the figure above, is described by the following four steps: 1. Mix outdoor air with return air 2. Control the supply air pressure 3. Heat/cool the mixed air 4. Control the exhaust air pressure

The VAV control concept was derived from the realization that most of the air conditioning operation in a building is cooling only. Independently of the outdoor air climate conditions, most occupied rooms need cooling only to eliminate the energy loads caused by solar radiation, human occupancy, equipment and utility operation.

The operation of the VAV cooling-only control system, assuming that there is enough air pressure in the duct system and that the temperature of the supply air is cold enough, is described by the following steps:

1. The VAV controller monitors the temperature in the room.
2. If the room is warm, the VAV controller opens the supply air damper to allow more cold air into the room.
3. If the room is cold, the VAV controller closes the supply air damper, thus allowing the interior loads to heat up the room space.
4. If the room is occupied, the VAV supply air damper cannot be fully shut. The VAV controller has to maintain a minimum amount of fresh air, specified by the building designers, so that people occupying the room do not suffocate.

Inverter Units

The Inverter technology (DC) is the latest evolution of technology concerning the electro motors of the compressors. An Inverter is used to control the speed of the compressor motor, so as to continuously regulate the temperature. The DC Inverter units have a variable-frequency drive that comprises an adjustable electrical inverter to control the speed of the electromotor, which means the compressor and the cooling / heating output. The drive converts the incoming AC current to DC and then through a modulation in an electrical inverter produces current of desired frequency. A microcontroller can sample each ambient air temperature and adjust accordingly the speed of the compressor. The inverter air conditioning units have increased efficiency in contraction to traditional air conditioners, extended life of their parts and the sharp fluctuations in the load are eliminated. This makes the inverter AC units quieter, with lower operating cost and with less broke downs.

The inverter technology works like an accelerator in a car. When compressor needs more power, it gives it more power. When it needs less power, it gives less power. With this technology, the compressor is always on, but draws less power or more power depending on the temperature of the incoming air and the level set in the thermostat. The speed and power of the compressor is adjusted appropriately. This technology was developed in Japan and is being used there successfully for air conditioners and refrigerators. This technology is currently available only in split air conditioners.