Centrifugal Water Chillers Research Articles

Buffer Tank Discharge Strategies in the Case of a Centrifugal Water Chiller

In this article, energy optimization of the cooling system of IKEA Budaörs is carried out. The cooling system is served by a centrifugal water chiller and includes a large-volume cooling buffer tank. The facility operates the hydraulic system of the buffer storage tank only during the transitional period. The main goal is to reduce energy consumption by changing the operating strategy of the existing system. To test the operating strategies, the operation and the thermal load of the shopping center during the summer season had to be simulated to find the best operation strategy. A hybrid method (real data and calculated values) was used in the simulation. The three operating scenarios examined show that the annual energy consumption and the number of operating hours of the chiller can be reduced by using the buffer tank with the right strategy. In the examined scenarios, a 30% energy improvement was achieved. The possibility of using a buffer tank is significantly limited by the fact that the heat exchangers were sized for low forward water temperatures. By re-sizing the heat exchangers, the utilization of the buffer tank could be considerably improved in conditions close to peak heat load.

Evaluating the Performance of Water Chillers Equipped with Constant- or Variable-Frequency Centrifugal Compressors

The cooling coefficient of performance (COPR) and energy efficiency ratio (EER) of refrigerant R-134a compressors (single- and double-compressors) with different refrigerant tonnage (200, 250, 300, 380, 500, and 700 RT) for centrifugal and Maglev centrifugal compressors change with different operating performance load percentages (10–100%), and constant-frequency and variable-frequency operation, resulting in performance differences. In particular, a water chiller can have a fixed cooling water inlet temperature of 32 °C and a variable cooling water inlet temperature between 18.33 °C and 32 °C. According to the actual test results, the commercial performance code program and parameter table of the water chiller were established. Based on the performance matching of different load chillers, the on-site load capacity was analyzed and the effective water chiller performance and model matching were determined as the best choice for the tonR number of the deicing machine and unit matching, providing a reference for a future large water chiller that cannot be used on site for a single unit tonR. To achieve energy-saving benefits, different types of compressors, different refrigeration tonR operation, constant-frequency unit and variable-frequency unit alternate operation, and different operating performance load percentage operation can be allocated. Finally, the results show that, when the cooling water inlet temperature is fixed, the Maglev variable-frequency centrifugal compressor water chiller is better than the constant-frequency centrifugal water chiller, and also better than the variable-frequency centrifugal water chiller. The larger the freezing tonR of the variable-frequency centrifugal water chiller, the smaller the difference between COPR and EER. When the cooling water inlet temperature changes, the Maglev variable-frequency centrifugal water chiller is better than the constant-frequency centrifugal water chiller, and it is also better than the variable-frequency centrifugal water chiller. The larger the freezing tonR of the variable-frequency centrifugal water chiller, the smaller the difference between COPR and EER. Moreover, the operating performance of the constant-frequency centrifugal water chiller is between 60% and 90%, which can maintain relatively high COPR and EER values. The operating performance of the variable-frequency centrifugal water chiller is between 40% and 70%, which can maintain relatively high COPR and EER values. Compared with the constant-frequency and variable-frequency, the Maglev variable-frequency centrifugal water chiller can maintain higher COPR and EER values when the operating performance is between 10% and 100%. When the operating performance is between 10% and 70%, it can maintain very high COPR and EER values. When the water chiller is selected in the field, the energy-saving of COPR and EER will be given priority. Therefore, the load capacity can be used to effectively manage the water chiller performance and model selection, so that the operation performance can reach the best percentage and energy saving can be achieved.

A Comparative Study of the Oil-Free Centrifugal Water Chillers with the Flooded or Falling Film Evaporator—A Case Study

A comparative study regarding the performance of real-scale oil-free centrifugal chillers having the flooded evaporator or falling film evaporator was conducted in this study. The nominal capacity for the test chillers was around 200~230 USRT (US refrigeration ton) (703~809 kW). The compressors of the two chillers were identical and R-134a was used as the working fluid. Both evaporators employed the same enhanced tubes (GEWA-B) to fulfill phase change. Tests were conducted in full, 75%, 50%, and 25% loading. Test results indicate that both chillers contained a comparable system performance with an integrated part-load value of around 8.62~8.63. The overall heat transfer coefficient for the flooded evaporator was appreciably higher (20~40%) than the falling film evaporator. This is because the falling film flowrate was below the threshold value and the heat transfer was dominated by evaporation mode. Yet, the heat transfer performance for the falling film evaporator was further jeopardized due to starvation of the film flowrate (partial dry-out), especially in the middle or bottom of the tube bundle. This phenomenon became even more pronounced at partial loading (25%), whereas the flooded evaporator did not reveal such a performance dip at partial loading.

Analysis and prediction of chilled water plant performance based on multivariate statistical methods and large historical data

The large datasets resulting from operating HVAC&R systems are currently scrutinized to find ways to exploit the useful information that they might contain. In this work, historical data of a centrifugal water chiller over the course of more than 1.5 years of operation is used to learn about the system and to suggest modifications to its operation scheme. The results show that principal component analysis (PCA) captures well the variance in the historical data. The first two principal components explained between 62 and 80% of the variance, depending on the cases considered. The main factors responsible for the variation of the chiller operation are found to be the weather and the cold water temperature setpoint. The effect of the sampling time step on the results is also studied. Moreover, this work demonstrates that partial least squares (PLS) regression can adequately predict an important indicator of the chiller performance, namely the coefficient of performance (COP), one time step ahead with an R² of 77.49% and root-mean square error of estimation (RMSEE) of 0.463 using a separate validation set of data. The PLS model was also able to predict future COP values up to 2 time steps (∼3 h) in advance.

Energy Performance and CO2 Emissions of HVAC Systems in Commercial Buildings

Energy performance of buildings has attracted much attention among building physicists and engineers worldwide. The effects of building heating; ventilation; and air conditioning (HVAC) systems’ design upgrade on the building energy performance are the focus of the current study. The adopted HVAC system consisted of chilled ceiling and chilled beam systems served by a centrifugal water chiller. An energy simulation study was undertaken in accordance with the national Australian built environment rating system-rules for collecting and using data. A three-dimensional simulation study was carried out utilizing the virtual environment-integrated environmental solutions software. Results from the current study have shown the importance of utilizing energy-efficient HVAC systems and HVAC strategies for achieving a high building energy star rating. Recommended strategies in order to achieve the nominated star rating; as predicted by the simulation analysis; were presented. Moreover; the effects of solar radiation inside the building atrium were significant; which cannot be overcome by simply installing a low shading coefficient glazing type at the atrium skylight. In addition to providing chilled ceiling technology; a high efficiency chiller and low energy lighting; it is recommended that the building be well tuned during the commissioning period. The current approach could be extended to accommodate higher energy ratings of commercial buildings at different locations worldwide.

A steady-state empirical model for evaluating energy efficient performance of centrifugal water chillers

A multivariate polynomial empirical model for evaluating the coefficient of performance (COP) of centrifugal chillers was proposed in this work. The model variables are common parameters (i.e. pressures and temperatures) on chilled water side in chiller systems. It is independent of the cooling load or partial load ratio (PLR) required by most of empirical chiller models so that it can provide a new method to evaluate the COP of a chiller without measuring the chilled water flow rate which is anyway difficult to measure directly and accurately in many on-field operating chiller plants. This makes it easy and convenient to use for on-field engineers. On-field measurement and analysis indicate that the accuracy of the proposed model is near to the compared three empirical models quoted from literatures—Gordon-Ng universal (GNU) model, Bi-quadratic regression (BQ) model, and Multivariate polynomial regression (MP) model when internal data is considered. When external data is adopted, the proposed model is obviously superior to the other three models. When internal data is used, values of the coefficient of variation of root-mean-square error (CV) of the novel model for chillers 1–3# are respectively 2.985%, 4.295%, and 2.956% with the corresponding mean relative error (MRE) values varying from 2.14% to 3.41%. While external data is adopted, the CV values of the novel model for chillers 1–3# are respectively 5.13%, 8.84%, and 4.33% with the corresponding MRE values varying from 3.19% to 6.84%. The result is acceptable for engineering applications such as building energy audit, energy conservation management, and operation management of multi parallel centrifugal chillers.

터보냉동기의 고효율 운전을 위한 협조 방식 기반의 압축기 대수제어

This paper discusses compressors operating number control strategy using cooperative logic to cope with variable partial load for high efficiency of a centrifugal water chiller. The cooperative logic is composed of a speed-up and speed-down controller, enabling smooth operation of compressors and equivalent distribution of thermal load in each compressor. This centrifugal water chiller design can be operated with high efficiency without incurring excessive energy waste and large transient phenomena at partial load states. Simulations in MATLAB and experiments in a real chiller system were conducted and verified the high efficiency control of a centrifugal water chiller achieved by the suggested strategy.

터보냉동기를 위한 실용적 모델링과 PI 제어기 설계

This paper describes the PI controller design based on a practical transfer function model for centrifugal water chillers. The rotational speed of a compressor and the opening angle of an electronic expansion valve were simultaneously regulated as manipulated variables to maintain temperature reference and to ensure high efficiency of the chiller. The COP according to the change in each variable was investigated by performing some static experiments, and it was reflected in the PI controller design to accomplish the high efficiency control. Especially, the practical transfer function model of the chiller was built based on the dynamic experimental data considering the strong inherent non-linearity and complexity of the chiller system. The validity of the designed PI controller was proven by some experimental results using the test facility and the results were also compared to the conventional evaporating pressure control results.

Energy Saving Potential on Centrifugal Water Chillers with Frequency Converter System: A Case Study in China

In order to evaluate the energy saving potential during centrifugal water chillers’ operation, centrifugal water chillers from an industrial enterprise in China are investigated. Based on the actual operation data gained from 3 different water chillers, their working performances and the energy consumption situations are reported and researched in this paper. Meanwhile, the energy saving potential and economic performance of the chillers are also reported and analyzed, when the appropriate variable frequency systems are introduced.

Evaluation of the suitability of empirically-based models for predicting energy performance of centrifugal water chillers with variable chilled water flow

This study evaluates the performance prediction ability and model suitability of eleven empirically-based performance models for centrifugal water chillers. Specifically, this study uses over 2000 datasets with a constant or variable chilled water flow rate for fixed or variable speed drive centrifugal liquid chillers. The best regression coefficients for each empirical-based model were obtained using the ordinary least squares (OLSs) method. The model prediction accuracy of each empirical-based model is based on the coefficient of variation of root-mean-square error (CV). The evaluation for model suitability is based on the considerations of prediction ability, the complexity in training datasets, the effort needed to calibrate, the generality of the model, and its ability to physically interpret the model regression coefficients in this study. Results show that among the eleven empirical-based models, the BQ (CV=0.54%), MP (CV=0.61%), SMP (CV=0.70%), and MDOE-2 (CV=0.63%) models have overall prediction CV values under 1% for all kinds of datasets and achieve extremely good prediction accuracy. Because the MDOE-2 model has a more complicated datasets training process than the BQ, MP, and SMP models, and it has no ability to physically interpret the model regression coefficients, the BQ, MP, and SMP models have the best suitability. The results of this study provide important reference values for selecting empirically-based performance models for energy analysis, optimal operating control, energy efficiency measurement and verification (M&V), and the development of fault detection and diagnosis (FDD) systems in centrifugal water chillers.

Application of the cumulative exergy approach to different air conditioning systems

The cumulative exergy approach is applied to evaluate two cases with different air conditioning systems. The first case includes three air conditioning systems, one is the gas direct fired air conditioning system, and the other two have air cooled heat-pump chiller and water chiller, respectively; the second case consists of four air conditioning systems, one is the gas direct fired air conditioning system, and the others have centrifugal water chiller, screw water chiller, air cooled water chiller, respectively. The results of the first case show that the air conditioning system with the air cooled heat-pump chiller maybe inferior to that with the water chiller, as the cumulative exergy efficiency of the former is 11.28%, which is less than that of the latter (11.92%). The second case shows that the gas direct fired air conditioning system, whose cumulative exergy efficiency is 14.86%, is better than the system with the air cooled water chiller, whose cumulative exergy efficiency is only 11.56%. The results are different from those of the exergy analysis, indicating that cumulative exergy analysis is an effective method to quantitatively evaluate different air conditioning systems according to resource utilization.

06/01289 Exergy analysis of air cooling systems in buildings in hot humid climates: Alpuche, M. G. et al. Applied Thermal Engineering, 2005, 25, (4), 507–517

The cumulative exergy approach is applied to evaluate two cases with different air conditioning systems. The first case includes three air conditioning systems, one is the gas direct fired air conditioning system, and the other two have air cooled heat-pump chiller and water chiller, respectively; the second case consists of four air conditioning systems, one is the gas direct fired air conditioning system, and the others have centrifugal water chiller, screw water chiller, air cooled water chiller, respectively. The results of the first case show that the air conditioning system with the air cooled heat-pump chiller maybe inferior to that with the water chiller, as the cumulative exergy efficiency of the former is 11.28%, which is less than that of the latter (11.92%). The second case shows that the gas direct fired air conditioning system, whose cumulative exergy efficiency is 14.86%, is better than the system with the air cooled water chiller, whose cumulative exergy efficiency is only 11.56%. The results are different from those of the exergy analysis, indicating that cumulative exergy analysis is an effective method to quantitatively evaluate different air conditioning systems according to resource utilization.

Comparison of Enhanced and Standard Finned Tubes: Field Test of 250-Ton Centrifugal Water Chillers

Today, the refrigeration industry can choose among a wide variety of enhanced tubes for evaporators and condensers of chilled water systems. To evaluate the performance of such tubes under field conditions, the standard finned tubes of one of the two 250-ton (879-kW) water chillers in a building have been replaced by specific enhanced finned tubes for comparison. The 2-pass evaporators of the two R-11 water chillers have 170 tubes, and two 2-pass condensers 178 tubes each, all of which are 12.6 ft (3.86 m) long. The standard copper finned tubes have a plain inside surface and a fin diameter of 0.75 in. (18.7 mm) with 23 fins/in. (905 fins/m). The enhanced finned tubes have a wavy inside surface, the same fin diameter, but only 19 fins/in. (740 fins/m) and a T-shaped fin cross section on the outside surface. In experiments conducted over a period of 3 years and approximately 9000 operating hours, the use of enhanced finned tubes improved the UA value in the condenser by an average of 60% and in the evapora...

Computer modelling of the performance of centrifugal water chillers in mine refrigeration installations

The non-specific behaviour of centrifugal water chillers is investigated. The conclusions reached are mainly based on calculations performed by the computer programs developed for the project. A better understanding of the influence of the chiller operating parameters will enable designers to optimize part-duty operation during the specification phase and enable operators to improve the running of existing installations.