By Michael Young, sales engineer, Trox
Variable Refrigerant Flow (VRF) was introduced in Japan more than 20 years ago.
Since then, VRF systems have become very popular due to high energy savings, flexibility to couple the system with different types of indoor units and the ability to perform heating and cooling simultaneously.
The most common type of arrangement is to have multiple types of indoor units to one central VRF condensing system. The need for higher cooling requirements and higher energy efficiency has resulted in the coupling of a VRF system to an air handling unit (AHU).
VRF systems have gone from cooling commercial office buildings to large data centres and even shopping malls. The past few years has seen a number of AHU manufacturers partnering up with VRF manufacturers to meet the new market trends and demands.
Now before you approach the next AHU manufacturer and request them to couple their AHU to your preferred VRF system, just read this article to the end as there are a few things you need to know.
If you review the VRF design guidelines, you will notice that the maximum amount of cooling capacity that the LEV controller is not identical for each of the various suppliers. You will also notice that the allowable refrigerant pressure loss through the DX coil is also not consistent between each supplier.
So why are these two factors so important when coupling a VRF system to an AHU? To successfully couple a VRF to an AHU, the entire system needs to be balanced.
LEV kits control the amount of refrigerant that enters the evaporator by superheats. Should the cooling load through the DX coil exceed the LEV kit capabilities, the sensors will read high superheats, the expansion valve will open, and the compressor will increase in speed.
The problem is that the expansion valve will open to its maximum capacity but because it is only sized for a certain maximum cooling capacity, insufficient refrigerant will be fed to the evaporator and there will be insufficient cooling.
If the coil is undersized, the sensors will read lower superheats, the compressor will ramp down and insufficient refrigerant will be fed to the evaporator. So not matching the evaporator’s capacity with the LEV kit’s cooling capacity can result in malfunction of the VRF system.
The next thing to note is the refrigerant pressure loss through the DX coil within the AHU. Each VRF manufacturer specifies a maximum pressure loss and erratic operations will occur should this limit not be adhered to.
Exceeding the specified pressure loss through the DX coil causes the refrigerant liquid to boil off at a very early stage within the evaporator. This causes the overall mass flow of refrigerant through the DX coil to decrease which reduces heat transfer.
The air temperature does not reach the desired set point which then increases the compressor speed and causes inefficient operations.
So, coupling a VRF to an AHU is more complex than just adding a condenser to the back of an AHU and laying interconnecting refrigerant piping. To successfully get the system work, the heat transfer aspects and control aspect must be balanced and work in unison.
So next time you require a VRF system to be integrated with an AHU, ask the AHU manufacturer which VRF system they are familiar with and check they have received the correct design training. Should you wish to learn more on VRF systems, email me at michaely@troxsa.co.za
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