A Review of Electronic Expansion Valve Correlations for Air-conditioning and Heat Pump Systems

Split-type air-conditioning systems with multiple indoor units and variable capacity feature, also known as variable refrigerant flow systems (VRF), are growing in both residential and commercial buildings for its flexibility in configurations and high energy efficiency through zoning control and variable capacity control. The VRF system controls its capacity by the combination of drive frequency of variable speed compressor and opening of electronic expansion valves (EEV). According to fluid dynamics analysis, the mass flow rate is a function of the EEV opening. Therefore, the accurate correlation of mass flow rate through an EEV is important in accurate prediction of the mass flow rate in the VRF systems and design the control system. The simulation and experiment studies are reviewed in this paper. Literature study showed that the most commonly used approach is power-law correlation which is recommended by ASHRAE. The relative deviation of the predicted mass flow of this method is around 5% to 40% depending on the refrigerants used. Different refrigerants include R-22, R-245a, R-134a, R-410A, R-407C and R-744 have been studied since 2005. Second type is polynomial fit correlation is a simplified model, which has a relative deviation of 5% for 98% of the data and developed in 2013. Third type is using Artificial Neural Network (ANN) algorithm, which has a relative deviation from 4% to 7% and has been used since 2015. In addition, a dynamic model of EEV has been developed since 2016. In conclusion, the most accurate method is ANN curve fitting, which is a novel approach to be utilized in EEV correlation. However, only a few researches have been conducted on this method. Therefore, more research is needed

Review of Temperature and Humidity Control Technology for Heat Pump and Air Conditioning Systems

Nowadays, indoor temperature and humidity controls have been more important to provide thermal comfort to occupants. In residential and commercial buildings, split-type air conditioning systems have been widely used for its compactness and installation flexibility. However, the current split-type systems could not effectively control the thermal comfort of the occupants due to its insufficiency in humidity control. In this paper, the existing independent temperature and humidity controllers of the single unit split-type system are reviewed. Then, the study has extended to the variable refrigerant flow (VRF) system, which is a typical multiple-unit split-type system. Several VRF systems that has the humidity control option have been reviewed and their performance is discussed. Based on the literature review, the feasibility of VRF systems with independent temperature and humidity controllers is discussed

Development of Dynamic Modeling Framework Using Convolution Neuron Network for Variable Refrigerant Flow Systems

Modeling the air conditioning system provides an excellent tool for system design, control, operation, and fault diagnosis. Such models were developed as either steady-state and transient models or knowledge-based and physics-based models. Most of the current studies mainly concentrated on physics-based models or steady-state models. Knowledge-based dynamic models were rarely discussed. In this paper, a knowledge-based dynamic model using a Convolutional Neural Network was developed for the air conditioning system. Instead of using operating parameters at a time point, we used the numbers in a time window as input data. We conducted a case study of the variable refrigerant flow system with field tests in an office building to validate this approach. It was found that the new method has a better accuracy within 2% deviation and a faster simulation speed in less than 1 second than the traditional physics-based model. The proposed method, which does not have a convergence problem, is user-friendly for non-experts. This approach also provides a way for existing systems to adjust operation parameters and detect faults. Future work can be making the current model more robust and reliable. In addition, how to combine the strengths of the knowledge-based method and physics-based methods needs to be further studied

Experimental Investigation of Multi-Functional Variable Refrigerant Flow System

The VRF systems have a better flexibility in the controlling strategy and a wider range of capacity modulation. In this study, a multi-functional VRF (MFVRF) system which is capable of heat recovery operation and water heating was experimentally investigated. The MFVRF system could supply space cooling and heating simultaneously in multiple zones, as well as providing hot water. The system performance was experimentally measured and discussed. It was found that the partial load performance of the system was improved with hot water demand, which increase the daily performance factor (DPF) by 17%. Moreover, it was also found that the system performance was enhanced by the heat recovery operation which could increase the hourly performance factor (HPF) by reducing the pressure difference across the compressor

The evaluation of energy saving performance for the modular design centrifugal chiller

The modular concept design and operation of a centrifugal chiller offers the potential of increased cycle efficiency both at full load and off load conditions compared to the single compressor chiller. Modular chiller design is that a smaller, individual chiller can be connected with another, to make a larger capacity system. In the case of a tandem chiller, there would be faced to the higher initial cost or bigger foot print than a single compressor chiller at the same capacity because manufacturing cost, weight and size will be decreased according to increase of the refrigeration capacity. Therefore, it is obvious that a modular chiller have to be improved its efficiency first by both the operation algorithm and major cycle components. Eventually, the efficiency of the modular chiller achieves up to 6% at the full load condition after added series counter flow effect as much as 3% and more 3% added by improved aerodynamic design for impeller. Moreover, maximize off design potential through the capacity combination algorithm as the way of uneven loading makes 24% improved based on AHRI 550/590