Choice Between VRF System and Chillers in HVAC Design
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When designing an HVAC project, we need to understand the project requirements, the type of application, the duty conditions, and the compliance to standards. To decide which air conditioning system is to be used for a particular project, we need to evaluate different HVAC technologies & systems that will be best suited for that project. Nowadays, the designer has many choices for HVAC system design, and it could be unclear to identify the correct system for any given application. To understand which system is best suited for an application, the designer must consider the various aspects such as the life cycle cost analysis, the system architecture, the space availability, etc.
A dilemma often faced is whether to design a system for a central plant with a chilled water system or go for the VRF System. The following are generally considered during HVAC designing.
Technology – Merits & Demerits
System Architecture
Reliability
Life Cycle cost
Technology – Merits & Demerits
Variable Refrigerant Flow (VRF)
VRF system comprises one or more ODUs (outdoor units) and many IDUs (indoor units) connected through a refrigerant piping network. The ODUs are installed conveniently on the rooftop or in the shaft space, etc. The refrigerant is circulated through the pipes to the evaporator units. The inside areas are cooled using a refrigerant circulated through the evaporator units. Generally, we use an Electronic Expansion Valve is used as an expansion device.
Merits –
These systems are around 30% more efficient as compared to conventional HVAC systems. In addition, VRF provides energy savings by the varied compressor speed and only delivering the required temperature.
We can use different IDU options like the Hi-wall unit, cassette unit, concealed unit, ducted type units, etc., which can help blend the air conditioning system with the appropriate interior design of the space.
They come in a multitude of types and sizes, which means they can fit any application. In addition, you can find sleek and compact equipment, so this could be an excellent choice if you have limited space.
Its operation is effortless. It is, in fact, as simple as operating a split AC system, and hence no separate staffing is required for using the system.
A good option for providing air conditioning to apartments/villas is small installations wherein the system complexity (a significant demerit) can be kept to a minimum.
Demerits –
It is a relatively new technology with a history of only two decades and is therefore not time tested.
It has a complex system architecture, a large amount of refrigerant piping running around in conditioned space.
When we use it for larger capacities, it requires more space to install any number of ODUs. Access to ODUs is a necessity as all the maintenance work required will be mainly on the ODUs.
It won't benefit the cooling load with limited diversity, such as in IT / ITES offices, malls, theaters, etc. The diversity is limited to some conference rooms and cafeterias, etc.
VRF can be hazardous for the occupants. Since the refrigerant is circulated through pipes, it is susceptible to leakages, with the refrigerant is possibly entering the conditioned space.
It requires highly skilled labor for installation, commissioning, and maintenance, especially identifying and correcting leakages.
Different evaluation standards are referred to for performance evaluation.
There is a limitation on CFM delivered inside the space, up to 350CFM / TR. It can be a severe limitation from indoor air quality as air circulated can be insufficient.
Water Cooled Screw Chillers
Water-cooled chiller systems have a cooling tower. Thus they feature higher efficiency than air-cooled chillers. Water-cooled chillers are more efficient because they condense depending on the ambient temperature bulb temperature, which is lower than the ambient dry bulb temperature. The lower a chiller condenses, the more efficient it is. These chillers also feature higher efficiency and last longer than the mentioned alternative. Those who would like the equipment to be placed indoors may find the water-cooled machine desirable.
Merits –
It is a mature technology with a history of over six decades.
The international standard referred for performance evaluation is AHRI 550/590
Highly energy efficient enables comfortably complying to Green Building norms such as LEED – Platinum.
With the right system design and the installation of multiple chiller units, even in case of high diversity, chillers have become a good option due to their excellent efficiency even at part load conditions.
Project size is not a constraint at all.
They are non-hazardous for the occupants, as water is circulated throughout the refrigeration circuit, with no chance of hazardous material entering the conditioned space.
Their advantage on efficiency can be multiplied by using thermal storage and the usage of differential tariffs.
Demerits –
Availability of water is a must
It requires additional space for plant room for chillers, AHU rooms, and space for mounting the cooling towers.
It requires a trained workforce for operating the central chiller plant.
System Architecture
The VRF system essentially requires a high electrical & electronic control to achieve capacity modulation, most of which are fitted in the ODUs panel board. In addition, each outdoor unit consists of refrigeration components like the compressor, condenser, fan, refrigeration components & controls, and quite a lot of copper piping.
The system also needs a communication cable to run through from outdoor boxes to each & every indoor unit to exchange electronic signals between the ODUs and every single IDU. Having copper piping, electrical cabling, communication cables, etc., makes the refrigeration network a bit complex.
On the other hand, the water-cooled screw chiller system architecture & its functioning is quite simple. It does not require complex electrical–electronic controls, refrigeration network for the system to operate, and capacity modulation. Further, the water circulation to the AHUs is also a straightforward system. All the designer needs to follow is the simple rules of piping.
We all know that the simpler the system architecture, the more reliable it will be & also be better from the serviceability standpoint.
Reliability
Oil Management
VRF – In VRF, the refrigerant is circulated throughout the refrigeration circuit. The oil and the refrigerant leave the compressor through every compressor discharge, distributed throughout the refrigeration circuits. It is essential that whatever oil has been left out of the compressor should be returned to a compressor to facilitate lubrication.
At low part-load conditions, the velocity of the refrigerant & the oil is not adequate to bring both the oil & the refrigerant back to the compressor. Therefore, it becomes almost essential to run the entire system at a high load, for few minutes, after every particular hour.
Sometimes due to the application requirements, the liquid line distances are quite long, say 400 ~ 450+ feet, which further creates more challenges for oil recovery. Also, the longer the refrigerant lines, the lesser efficient the system is.
Chillers have an in-built oil management system, which eliminates any issues of oil recovery.
Life Cycle Cost Analysis
Life Cycle Cost = Initial CapEx + Operational Cost + Maintenance Cost
Calculation References:
Total CapEx: Average cost/TR
Total operating cost: Lowest figure for kW/TR; Running hours 15 Hrs/day x 300 Days
Total maintenance cost: Lowest cost/TR/Yr
Conclusion
For smaller systems, up to 100 ~ 200 TR, with a limited number of ODUs and IDUs, the VRF system can be the right solution. However, for smaller central plant systems, the space available for chillers and AHUs may not be available. Additionally, for such small systems, the VRF system can remain simple with little refrigerant piping without many complexities.
However, as the installation size starts increasing, VRF systems start becoming complex. In such cases, planning to install a central chiller plant is beneficial. For such a system, the life cycle cost is lower than the VRF system. It also avoids safety-related issues such as leakage of refrigerant in conditioned space, better air circulation, better indoor air quality, etc.
Written by SM Rajendran