Though VRF systems often prove to be better than air-cooled chillers and traditional ducted splits, they have certain inherent limitations. Dharmesh Sawant demonstrates that connecting fresh air systems and VRF systems will yield greater benefits.

With the recent EHS and ESTIDAMA regulations, stakeholders of most projects are racing against time to earn maximum credits to get the approval from the authorities. This trend has encouraged the real estate and construction sectors to adopt green building practices that are ultimately beneficial to the end-users, who had earlier been sidelined and left out of the value chain.

Given the scenario, quite often, the Variable Refrigerant Flow (VRF) system has been found to be useful, thanks to the high EER (above 12, as per ASHRAE 90.1) and the use of R-410A, which is an eco-friendly refrigerant with zero ODP. Apart from that, the part-load efficiency of VRF systems is also better than that of air-cooled chillers and traditional ducted splits, due to their innovative DC inverter compressor technology and precise control of refrigerant flow to the evaporator coil, with the help of multiple sensors in the refrigerant line.

Earlier, however good the VRF system was, it had an inherent limitation when it had to be used for fresh air systems. Since the VRF system works on the control logic integrated with the evaporator coil, which controls the frequency of the DC inverter compressor, it was, therefore, used only with the factory-tested and designed indoor units.

These indoor units have inbuilt electronic expansion valves, which throttle as per the cooling demand. Also, it has its PCB in the indoor unit, which communicates constantly with the outdoor units at an interval of two seconds. Owing to this limitation of the VRF system in the fresh air system, it is either connected to chilled water, making it difficult to maintain two different systems – VRF for the re-circulating FCU and chilled water for the fresh air system. Or else, the entire system is connected to the chilled water system, increasing capital as well as operating costs.

Looking at the positive side, if it is possible to connect the fresh air system to the VRF system, the benefits compared to chilled water system are as follows:

Reduced capital cost up to 25%

A VRF system needs fewer components – VRF condensing unit, indoor units and connecting copper pipes. The valve packages are all in-built, unlike chillers, which need pumps, chemical dosing plants, valve packages, three-way valves, chillers, chiller manager, makeup water arrangements, etc. This increases the cost of the chiller system. Owing to this characteristic, a VRF system proves less cumbersome for contractors, as they only have to coordinate with fewer vendors.

Chillers are normally kept in remote chiller yards to keep the noise levels away from the main building. This increases the pipe network cost. In the VRF system, however, the condensing units are kepts close to the AHU, thus reducing the pipe cost.

Reduced operating cost by up to 30%

The daily range of temperature in the Middle East is quite high. This translates into a highly variable fresh air load. The DC inverter varies its frequency as per the ambient temperature (fresh air load), thus optimising power consumption. The NPLV figures of the VRF system go as low as 0.58 KW/TR for the horizontal development, with reduced separation between indoor and outdoor units.

Reduced connected electrical load

The VRF system has lower connected electrical load of 1.2 KW/TR, compared to 1.7KW/TR, including pumps at 460C for horizontal development with minimal separation between the indoor and outdoor units. This reduces the size of the transformer, lowering the infrastructure cost.

Ease in commissioning

One of the nightmares of commissioning agents in the chiller system is its hydronic balancing. In the case of the AHU connected to the VRF system, it can be commissioned with the help of a software. As a result, all the important parameters can be viewed on the laptop.

Better control of finances in a phased development

In the case of a VRF system connected to the AHU, the project finance is directly proportional to the development phase. But in case of a chiller system, you need to pay a higher cost than the development phase. For example, in the final stages, if a 50 classroom school is made in two phases of 35 and 15 respectively, within a year’s gap, the capital cost in the case of the VRF will be 70%. But in a chiller system, you need to lay the pipe an install the chiller for the total capacity of 50 classrooms.

All the above benefits are made possible by integrating the AHU and the VRF condensing units with the help of a control kit and an expansion kit. The architecture of the system is as follows below.

A total of eight outdoor units amounting to 700 KW can be connected to a single AHU, giving more flexibility to contractors/consultants. The following options are available with the AHU:

• a) Heat recovery wheel or heat pipe
• b) Temperature/humidity sensors in return and supply air
• c) Damper actuator for fresh air and return air

A total of eight outdoor units amounting to 700 KW can be connected to one AHU, giving more flexibility to the contractors/consultants. The following options are available with the AHU:

• a) Heat recovery wheel or heat pipe
• b) Temperature /humidity sensors in return and supply air
• c) Damper actuator for fresh air and return air
• d) Differential pressure switches across the filters
• e) Valves for the humidifiers
• f) Operation of the motorised damper, as per the CO2 sensors
• g) Operation of the motorised damper, as per the smoke detector

The entire system is BMS-compatible through LONWORK or BACNET gateway. Thus, it offers the benefit of high EER of up to 12~12.5 due to DC inverter scroll compressor, high efficiency DC inverter condenser fan motor, high efficiency sub-cooling heat exchanger and wide louvre condenser fins. Above all, it offers reversible heating cycle.

The condensing unit comes in modular design, with each module having two compressors – one inverter and the other, constant-speed scroll. Therefore, even if one compressor fails, the other compressor comes on automatically, with auto back-up function.

The condensing system is easy to commission with the help of a Monitoring Viewer by hooking the system to the laptop. This provides a real-time view of various important parameters of the air conditioning system, like suction, discharge pressure, sub-cooling temperature, superheat temperature, ambient temperature, and refrigerant inlet and outlet temperature (see Figure1 and Figure2 below).

In addition to this, with the help of the control kit that integrates AHU and VRF system, it is possible to control as well as monitor the AHU parameters using an AHU Viewer.

As shown in the screen shot (Figure 3 below), we can easily check the following:

• Fan and damper condition
  • — Fan ON/OFF, damper opening angle
• AHU condition by sensor installed in each part
  • — Mandatory sensor (RA/SA temperature)
  • — Other sensors (SA/RA humidity/OA/Mixing air, CO2, etc.)
• Control condition
  • — Operation mode (cooling, heating, fan, power saving)
  • — Setting temperature/humidity
  • — Supply and return fan operation

Green fresh air solution with VRF technology has found increasing acceptability in the region, thanks to the enforcement of green building practices by ESTIDAMA, as many stakeholders look for a solution in the design stage to achieve the required credits.

A few of the applications where fresh air units are feasible are schools, mosques, villas, horizontal office buildings (up to 10 storeys), cultural centres and museums.

The writer is Senior Manager, LG Commercial AC. He can be contacted at: dharmesh.sawant@lge.com.