What is a VRV Air Conditioning System

Figure 1: VRV system.

VRV (Variable Refrigerant Volume) air conditioning system was developed and launched by Daikin Industry Co., Ltd. in Japan in 1982. VRV has also become the registered trademark of Daikin variable refrigerant volume multi-split system. Therefore, the word VRF (variable refrigerant flow) is now used in the industry to distinguish similar systems.

1. Components of VRV System

The Daikin VRV system is mainly composed of an indoor unit, outdoor unit, refrigerant pipe and control part. The indoor unit is the terminal device part of the VRV system, and it is a unit with an evaporator and a circulating fan, and the principle is exactly the same as that of a common split air conditioner indoor unit.

Daikin air conditioner three-pipe VRV

1.1 Indoor Unit

(1) Ceiling cassette

(2) Ceiling concealed duct type

(3) Wall-mounted type

1.2 Outdoor Unit

The outdoor unit is a key part of the VRV system. It is mainly composed of an air-cooled condenser, a compressor and other refrigeration accessories. The speed of the compressor is controlled by a frequency conversion controller to automatically change the flow of refrigerant in the system, so as to meet the requirements of indoor cooling and heating load requirements.

Figure 2: Daikin outdoor unit.

1.3 Refrigerant Pipe

The refrigerant pipe, made of copper pipe, connects the outdoor unit to the indoor unit to form a complete system through flexible arrangement.

Figure 3: Y type branch pipe.

1.4 Control System and Equipment

The control is based on computer digital control, and the equipment includes a simple LCD controller, standard LCD controller and centralized controller, etc.

Figure 4: Daikin VRV panel.

2. Installation height requirements of VRV system

Usually, the maximum piping length of the system is 100m between the indoor unit and the outdoor unit, the maximum height difference between the indoor and outdoor units is 50m (40m when the outdoor unit is below), and the maximum height difference between the indoor units of the same system is 15m.

Figure 5: VRV system installation.

3. Switching of Winter and Summer Working Conditions

The VRV system has a reverse cycle heat pump working condition in winter, and the outdoor unit can work at an outdoor temperature of -15°C. As the outdoor temperature decreases, the heating capacity of the VRV heat pump cycle decreases.

Taking a unit of a certain company as an example, when the room temperature is 20°C, the relationship between the heat supply and the outdoor temperature is:

Figure 6: The heat supply and the outdoor temperature are positively correlated.

Usually the heat supply given by the manufacturer is the standard working condition when the outdoor temperature is 7°C (room temperature is 20°C). Therefore, when the outdoor temperature is low, it is necessary to check whether the heat supply can meet the requirements. If not, an auxiliary heat source should be provided or use this system just for cooling.

When the outdoor air temperature is 34~35°C in summer and -3~-5°C in winter, the VRV air conditioning system can basically meet the load requirements both in winter and summer.

4. What are the Pros and Cons of VRV AC system

Since the early 1990s, VRV has developed rapidly. Because of the advantages of flexible use, energy saving and easy installation, the VRV system has been widely used in office buildings, hospitals, villas and other buildings. However, since the VRV system is a unitary air-conditioning system model, there are many differences compared with the traditional centralized air-conditioning system.

4.1 Reduction of Cooling Capacity

Usually, the indoor unit and the outdoor unit of the VRV air conditioning system are connected through refrigerant pipes. The length of the refrigerant pipe and the height difference between the indoor and outdoor units affect the cooling capacity attenuation of the air conditioning system.

Take the VRV air-conditioning system product of Daikin Corporation as an example. The product manual states that the length of the refrigerant pipe between the indoor unit and the outdoor unit can be up to 150m, and the height difference between the indoor unit and the outdoor unit can be up to 50m. The height difference between indoor units can allow up to 15m.

These are the limit values that should be guaranteed in our design. At the same time, it should be noted that with the change of the refrigerant pipe length and the height difference between indoor and outdoor, the cooling capacity attenuation varies greatly.

Figure 7: Cooling capacity reduction diagram.

It can be seen that the capacity correction factor should be considered in the design, and when designing the system, try to keep the distance between the indoor and outdoor units of a system as short as possible and the height difference small. For systems with large height differences and long pipelines, the capacity of the unit should be appropriately increased, so as to ensure the effectiveness of the air conditioner.

4.2 Fresh Air (Ventilation) Problem

The problem of fresh air has always been a difficult point in the design of VRV air-conditioning systems, and it has largely limited the further application of VRV air-conditioning systems. The following briefly introduces several methods of VRV system getting fresh air:

1. Use the ordinary indoor unit of the VRV air conditioning system as a fresh air ventilator to handle the fresh air. This method is widely used in engineering because the system is relatively simple.

2. Use a total heat exchanger as a fresh air ventilator.
Use the total heat exchanger to supply fresh air to the room, and use the cooling capacity of the indoor exhaust air to pre-cool the fresh air. The efficiency of recovering cooling capacity is 60%~70%, which can save energy by 28%. This method is a better solution for the VRV air-conditioning fresh air system.

Figure 8: Total heat exchanger.

3. Use the special fresh air machine provided by the VRV manufacturer.

This type of fresh air fan is usually designed according to the fresh air state, and the number of rows of unit coils is increased, which makes it possible to get the ideal fresh air.
However, the engineering cost of this method is high, which affects the application in engineering. On the other hand, when the outdoor temperature is high, the compressor will run continuously for a long time, which will affect the life of the unit.

4. Set up an air-cooled heat pump unit to provide cold and heat sources for the fresh air system. This method is to separate the fresh air system from the VRV system, and the fresh air system uses a hot and cold water system, thus ensuring the air volume and cooling capacity of the fresh air system.

However, this method uses two types of air-conditioning systems, is cumbersome in operation and management, and does not fully reflect the flexible and energy-saving features of VRV air-conditioning systems.

5. Use directly evaporative air duct air conditioners and rooftop units as fresh air fans.
The principle of this method is the same as the first method, that is, using the indoor unit of the ordinary VRV air-conditioning system as the fresh fan. There are also problems of insufficient cooling and overloaded operation of the main unit, and even burn-in phenomenon occurs in severe cases.

4.3 Control of VRV Air Conditioning System

Figure 9: Daikin VRV system.

The control system of the VRV air conditioning system is more flexible and convenient. The system can be equipped with various remote controls (wired or wireless, simple remote control, centralized remote control, etc.), expansion converters that can centrally control 64 different combinations (up to 1,024 air conditioners), as well as the building management system.

Different control methods can be selected according to the requirements during design. In engineering examples, the control system is often ignored in order to reduce the project cost. In fact, the VRV air-conditioning control system is more convenient and economical than the centralized air-conditioning system to set up an automatic control system, and it can achieve a good control effect without the need to increase too much investment.

In addition, in order to reduce the cost in the project, the same remote control is often used to control several indoor units in one room, which will cause the room temperature to be too cold or overheated, so it is better for each remote control to control one indoor unit.

4.4 Conclusion

The VRV air conditioning system has the advantages of energy saving, flexible use and layout, easy installation, and space saving. But there are problems like refrigerant leakage, fresh air, and it may not meet the use requirements in winter.

However, generally speaking, the production technology of the VRV air-conditioning system is constantly improving, the system capacity is also increasing, and the cost is gradually decreasing. It seems that the VRV air-conditioning system will have a broad prospect.

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