Functioning Modes of VRF Systems – VRF HVAC Technology

Today we will discuss a type of air conditioning technology that delivers very high performance. And by changing the flow of refrigerant in indoor units, based on the need of each specific area. The technology is called Variable Refrigerant Flow. (VRF HVAC Technology) The fact that VRF can regulate the discharge of refrigerant, makes VRF systems ideal for utilization with varying loads. Additionally, VRF systems can also produce space heating. And that by incorporating two cooling methods in one, and conserving space at the same time.

VRF technology is available in two variants – heat recovery systems and heat pump systems. Heat recovery systems do not have this control and can provide simultaneous heating and cooling duties. Thereby presenting 3 distinct modes of operation:

Heat pump VRF systems can handle cooling or heating an entire building. But they cannot achieve both purposes at the same time.
Heating mode
Cooling mode
Simultaneous cooling & heating, *also known as (heat recovery mode)

VRF HVAC Technology – Heating Mode

VRF in heating mode is supplied by the hot gas refrigerant. Gas movement to specific units is regulated with the identical electronic expansion valves. Those are used for VRF Systems in cooling mode. The heating effect happens when hot gas encounters condensation.

The fundamental principle, nevertheless is the refrigeration cycle, with the exception that heat is absorbed from outside and released inside. The way that this happens is comparable to an air-conditioning unit. The unit cools the outdoor environment to transfer heat from it and use it inside. The heating mode switch is possible by means of unique 4-way reversing valves. That inverts the functions of condenser and evaporator among both outdoor and indoor units.

In addition, with VRF Systems in heating mode, the electrical energy consumed converts into beneficial heat. with VRF Systems in cooling mode, the electrical energy is released outdoors. Therefore, a VRF outdoor unit is able to perform past 100% of its rated capacity while it’s in heating mode.

It is important to take note, however, that the overall capacity may be underrated. This seems to be the case especially when the system is subjected to considerable variations in outdoor conditions. Longer lengths between the outdoor and last indoor unit, longer piping lengths, and higher vertical distance are some of the additional causes of underrating.

VRF HVAC Technology – Cooling Mode

These Systems in cooling mode has a VRF operation that is not much dissimilar from that of a regular air conditioning system as well. In other words, indoor air conditioning units are outfitted with liquid refrigerant, and then an expansion valve inside the unit regulates the quantity of refrigerant that flows through the unit. When the refrigerant enters into the coil of the cooling unit, evaporation takes place, which then removes any heat from indoor air and thereby cools the indoor space. The heat that is extracted from indoor spaces at that point is released outdoors.

Systems tend to be much more efficient than packaged terminal air conditioners, (PTAC) traditional packaged rooftop units, (RTU) AND normal window units. Water-cooled chillers & ductless mini-split systems both offer comparable performance while in cooling mode. But they are both incapable of operating in heating mode.

VRF Systems in Heat Recovery Mode

Heat recovery systems are useful when synchronous heating and cooling are needed. The most prominent performance is obtained when the heating and cooling amounts are identical, by maximizing the quantity of power carried from one zone to another using refrigerant. The heat released from cooling areas can be used for space heating in areas that may need to be heated. Because of heat recovery mode, every tenant is free to decide whether or not they want cooling or heating mode, offering a more personalized level of control in different areas of a building.

Heat recovery systems become extremely useful in situations where a building has a south-north or an east-west positioning with glass windows. South-north or east-west positionings create a large variation in load demands for all directions in both the morning and evening, presenting a chance for the system to function in heating and cooling mode at the same time. Heat recovery is especially beneficial in structures because they put out a steady source of heat.

Heat Recovery System Configurations for Piping

These systems are available in 2-pipe and 3-pipe configurations. The 2-pipe system is the one that uses the highest amount of refrigerant flow. To operate with reduced flow, the 3-pipe system contains a high-pressure gas line, a low-pressure gas line, and a liquid line, The key function is executed via a Mode Selection Box, also known as a Mode Change Unit, (MCU) that contains three separate headers: one for liquid, and one for high-pressure gas, and one for low-pressure gas.

● If there are areas in need of space heating, indoor units will work in a similar manner as condensers do, providing heat from high-pressure refrigerant gas condensation. After heating, any refrigerant left over is fed to the liquid header.
● Next, liquid refrigerant is provided to the units in space cooling mode, at which point the refrigerant evaporates. Absorbing the heat and converting into a low-pressure gas.

● Finally, the low-pressure gas is then passed back to its corresponding header. Then on to the compressor to repeat the cycle.
In cases like this, outdoor units need only maintain the balance between the heating and the cooling. Whichever one is higher will then define which operating mode that the outdoor unit uses.
● If the cooling load is greater, the outdoor unit will take on the function of a condenser, releasing the leftover heat outdoors.
● If the heating load is greater, on the other hand, the outdoor units will take on the function of an evaporator, pulling air from outdoors to supply the extra heat that is needed inside.

conclusion:

It is recommended to find the MCU in an unrestricted access area like a corridor. Thereby reducing noise and disruption for the occupants. The maintenance of heat recovery systems is moderately simple. Since the outside condenser unit is only connected to the MCU, facilitating system separation into upstream and downstream parts.
According to the ASHRAE Standard 34-2013, the refrigerant concentration limit (RCL) is 26 lbs per 1000 ft3 of room volume that exists for spaces with occupants in them and 13 lbs per 1000 ft3 for institutional buildings.

This metric can be achieved by finding the units that are present outside of compact rooms. According to Standard 15, a VRF system is listed as a high-probability system or a direct system. In cases where a refrigerant leak has a possibility of entering an occupied space.
The application of heat recovery systems in VRF has proven to be a better option, both environmentally and economically. This saves the building owners from needing to install two different systems while delivering very high performance with one.

In addition, by utilizing different systems for space cooling and heating, it is difficult to boost performance by heat exchange. Even in cases where both systems are highly efficient individually.