This book compiles the latest research, development, and application of VRF systems with contributions from various experts who pioneered and contributed to the development of the VRF system. This book presents the fundamental issues related to the real application and behaviour of the VRF system based on the long-term monitoring of the installed system. With our experience of pandemic which COVID-19 is an airborne, the spread of the virus is very fast. With this, the heating, ventilating and air-conditioning (HVAC) system is a major player in the maintenance and control of indoor environment to minimize the spread of the virus. As the variable refrigerant flow (VRF) system is a versatile HVAC system in which it can operate at different conditions, the application of the VRF system is very important to control the indoor environmental conditions. Thus, the publication of this book is important with the present situation and the future possible situation which the control of indoor spaces is very important. With this, this book will serve as a reference for building designer, contractors, building regulators and students.
VRF (Variable refrigerant flow) is an air-condition system configuration where there is one outdoor condensing unit and multiple indoor units. The term variable refrigerant flow (VRF) refers to the ability of the system to control the amount of refrigerant flowing to the multiple evaporators (indoor units), enabling the use of many evaporators of differing capacities and configurations connected to single condensing unit. The arrangement provides an individualized comfort control, and simultaneous heating and cooling in different zones. Currently widely applied in large buildings especially in Japan and Europe, these systems are just starting to be introduced in the U.S. The VRF technology/system was developed and designed by Daikin Industries, Japan who named and protected the term variable refrigerant volume (VRV) system so other manufacturers use the term VRF "variable refrigerant flow". In essence both are same. With a higher efficiency and increased controllability, the VRF system can help achieve a sustainable design. Unfortunately, the design of VRF systems is more complicated and requires additional work compared to designing a conventional direct expansion (DX) system. This 3 -hour quick book provides an overview of VRF system technology. Emphasis is placed on the control principles, terminology, basic components, advantages and design limitations. This course is aimed at the personnel who have some limited background in the air conditioning field and is suitable for mechanical, electrical, controls and HVAC engineers, architects, building designers, contractors, estimators, energy auditors and facility managers.The course includes a multiple-choice quiz consisting of fifteen (15) questions at the end. Learning ObjectiveAt the conclusion of this course, the reader will: * Understand the difference between multi-split air conditioning system and VRF systems;* Understand the operating principle of direct expansion split and VRF system;* Understand the concept of thermal zone;* Understand how VRF with heat recovery are different from ordinary heat pump systems;* Understand the operation of thermostatic expansion valve (TXV) and electronic expansion valve (EEV);* Understand the influence of building characteristics and load profile on selection of VRF system;* Learn the advantages and application of VRF systems;* Understand the design limitations and challenges in design of VRF systems.
A research project "Evaluation of Variable Refrigerant Flow (VRF) Systems Performance and the Enhanced Control Algorithm on Oak Ridge National Laboratory's (ORNL's) Flexible Research Platform" was performed to (1) install and validate the performance of Samsung VRF systems compared with the baseline rooftop unit (RTU) variable-air-volume (VAV) system and (2) evaluate the enhanced control algorithm for the VRF system on the two-story flexible research platform (FRP) in Oak Ridge, Tennessee. Based on the VRF system designed by Samsung and ORNL, the system was installed from February 18 through April 15, 2014. The final commissioning and system optimization were completed on June 2, 2014, and the initial test for system operation was started the following day, June 3, 2014. In addition, the enhanced control algorithm was implemented and updated on June 18. After a series of additional commissioning actions, the energy performance data from the RTU and the VRF system were monitored from July 7, 2014, through February 28, 2015. Data monitoring and analysis were performed for the cooling season and heating season separately, and the calibrated simulation model was developed and used to estimate the energy performance of the RTU and VRF systems. This final report includes discussion of the design and installation of the VRF system, the data monitoring and analysis plan, the cooling season and heating season data analysis, and the building energy modeling study.
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This report addresses the operation, use, and design of air-to-air variable refrigerant flow systems, also known as VRF. Relatively new to the United States, these HVAC systems have potential to reduce energy consumption and utility costs in the correct applications. Although useful in many applications, the best building types for VRF are those requiring a large number of zones and with low ventilation air requirements. The report explains design and system selection considerations and accordingly presents two flowcharts to help designers implement this system. To show how the system compares to traditional technologies in terms of efficiency and cost, the report presents results from several studies comparing VRF to other systems. In addition, an energy modeling study is conducted to clarify the effect of climate on the system; this study established air-to-air VRF as having highest energy consumption in dry, southern climates, based on energy use and operating costs. With this report, HVAC designers can learn when air-to-air VRF is an acceptable method for providing heating and cooling in a building.
Building heating ventilation and air conditioning (HVAC) systems have significant impact on the energy consumption of residential and commercial buildings. The Variable Refrigerant Flow (VRF) systems, by distributing refrigerant instead of air flow, have emerged as an appealing class of HVAC system that features quieter operation, higher flexibility, and lower cost of installation and maintenance. However, such systems also present higher challenges for controls that can realize its achievable performance. This dissertation research proposes to investigate the dynamic simulation modeling and modelfree control strategies for energy efficient operation of VRF systems with single or multiple outdoor units (ODUs) under fluctuating and uncertain load and ambient conditions. Modelica based dynamic simulation models are developed for VRF systems of different configurations, which involve modeling of indoor units (IDUs), ODUs, and various control valves for thermal regulation. Motivated by a preliminary study on applying multi-variable extremum seeking control (ESC) for an air-source heat pump (ASHP) system, a model-free self-optimizing control strategy is investigated for efficient operation of a multi-functional VRF system under both heat pump modes and heat recovery modes. With the feedback of the total power only, the multi-variable ESC takes different combinations of manipulated inputs, including IDU superheat setpoints, compressor pressure setpoints, ODU fan speed, and ODU superheat setpoint. Input selection is carried out based on their respective impact on the total power. To realize automatic and smooth switching between different operation modes of multi-functional VRF system, the mode switching strategy is proposed. Whether to turn on or off an IDU is determined by the zone temperature and a preset hysteresis band about the temperature setpoint. Based on the thermodynamic analysis, a decision variable for determining the mode of ODU heat exchanger (HX) is proposed as the ODU-HX air-side temperature differential normalized by the dimensionless outdoor unit fan speed. For the smooth switching between two compressor pressure controllers, two bumpless transfer methods are applied. The simulation results validate the effectiveness of the proposed strategy and performance of bumpless transfer strategies. For the multi-ODU VRF systems, an integrated efficiency operation strategy is proposed to optimize the energy efficiency, which consists of three respects: i) for a given operating condition, a multi-variable ESC strategy is used to optimize the settings of manipulated inputs of operating ODUs, by use of a load-sharing valve array and feedback of the normalized total power; ii) a model-free ODU compressor staging strategy with ESC integrated control logic; and iii) a modelfree ODU heat exchanger mode switching strategy with ESC integrated control logic. For staging on additional ODU compressor, the saturated operation of compressor speed (i.e. saturated to the higher limit) is used as the indicating variable. With ESC-alike real-time optimization strategy in operation, the least efficient compressor would be driven, which will then be staged off. As for automatic mode switch of ODU heat exchangers during heat recovery operation, the saturation of IDU EEV opening is utilized as the indicating variable, and the ODU heat exchanger (or fan-coil unit) with least efficiency under ESC operation will switch its operational mode. Similar to many other HVAC systems, the ESC operation of VRF systems is subject to state and/or input constraints. In this dissertation research, the general problem of constrained ESC is studied. The penalty-functions based framework of constrained ESC is studied. The dither-demodulation design is modified for penalty-function based ESC with assumption of Wiener-Hammerstein system composition. An online penalty-weight adaptation scheme is proposed with online Hessian estimation, and its convergence analysis is conducted in the context of numerical optimization ESC (NOESC).
In a world increasingly aware of energy efficiency, comfort, and the need for sustainable solutions, Variable Refrigerant Flow (VRF) systems are emerging as a transformative force in climate control. This book delves into the heart of VRF technology, offering a comprehensive guide for all who seek to understand, design, or implement these innovative systems. Whether you're a seasoned building professional or a homeowner curious about cutting-edge HVAC options, this book welcomes you. We'll embark on a journey that demystifies the complexity of VRF systems, exploring their intricate components, operational principles, and diverse applications. Why VRF? The need for a paradigm shift in climate control solutions is undeniable. Traditional systems often struggle with inefficiencies, inflexible zoning, and bulky installations. VRF technology addresses these challenges head-on, offering: Unmatched energy efficiency: VRF systems boast inverter-driven compressors and sophisticated control systems, leading to significant energy savings and reduced environmental impact. Zonal comfort control: Each space within a building can be individually conditioned, ensuring occupant comfort while minimizing wasted energy. Flexible design and installation: VRF systems adapt to diverse architectural constraints with their branch-based piping network, requiring minimal ductwork and offering space-saving advantages. Cost-effectiveness: While requiring some initial investment, VRF systems often deliver long-term cost savings through energy efficiency, reduced maintenance, and extended longevity. Beyond the Hype: This book will not simply tout the benefits of VRF systems. We'll delve into their limitations, design considerations, and installation complexities, ensuring you make informed decisions. Understanding the challenges alongside the advantages empowers you to navigate the VRF landscape with confidence.
