Condensing Temperature Control Research Articles

Probabilistic analysis of mist cooler effectiveness for air-cooled chiller systems

This study investigates electricity saving potentials of mist coolers for air-cooled chiller systems in commercial buildings in a subtropical climate. Extensive operating data were collected from an experimental chiller with mist cooling of condenser air. The chiller had a nominal capacity of 282kW and was retrofitted with dual condenser fan controls: constant speed with fixed condensing pressure and variable speed with floating condensing temperature. The variation of the cooling effectiveness of the cooler was identified to depend mainly on the wet bulb temperature, followed by the relative humidity and the multiplication of dry bulb temperature and relative humidity under fixed condensing pressure control. While under floating condensing temperature control, the significant independent variables for modelling cooling effectiveness were the wet bulb temperature, followed by the relative humidity and the condensing temperature. Simulation was conducted to find the hourly chiller system load distributions of a reference office building and a hotel in a subtropical climate. The probabilistic distribution of cooling effectiveness was examined with the hourly system load distributions under two system configurations with 6 chillers and 8 chillers. Using mist coolers improved the coefficient of performance of chillers by 0.70–9.35%, resulting in an annual electricity saving of 2.50–2.94% under the diverse patterns of weather and system load distributions. Mist coolers complemented the advanced condenser fan control and improved system configurations with 8 chillers. Higher electricity savings were achieved by prolonging the operation of chillers near full capacity.

Improved energy performance of ammonia recycling system using floating condensing temperature control

Aiming at reducing the energy-consumption of ammonia recycling system, we presented floating condensing temperature control to maximize the coefficient of performance (COP) of the system. Firstly, thermodynamic models for the compressor and evaporative condenser were developed respectively. Then, an evaluation index and a solution scheme were proposed to determine the optimal set point of condensing temperature and the corresponding compressor speed. It is found that the system COP can be maximized by controlling the compressor speed to adjust the set point based on any given operating conditions. When the wet-bulb temperature is 22°C, the system COP could be improved by 19.2–27.6% under floating condensing temperature control.

A Novel Control Strategy for Air-Cooled Twin-Circuit Screw Chillers

Air-cooled chillers are generally the major electricity consumers in air-conditioned buildings in the subtropical climate. To improve the energy efficiency of the air-cooled chillers at part load conditions, two or more refrigeration circuits are designed. This paper considers how the use of optimal circuit loading sequence (CLS) enables these chillers to operate more efficiently. A thermodynamic model for the air-cooled chillers with twin refrigeration circuits was developed using TRNSYS and validated using a wide range of operating data. Based on the sophisticated chiller model, an analysis was carried out on how the chiller COP varied with different CLS and variable condensing temperature control (CTC). A chiller plant designed for a representative office building in Hong Kong was investigated to assess the potential electricity savings by use of CLS. 4.2% of the annual electricity consumption by chillers could be achieved with optimal CLS compared with the base case. Under CTC, the coefficient of performance (COP) was improved further, and CTC coupling with optimal CLS would reduce the annual total electricity consumption for cooling by 9.6%.

Hybrid Artificial Neural Network−Genetic Algorithm Technique for Condensing Temperature Control of Air-Cooled Chillers

Air-cooled chillers are commonly used in commercial buildings in the subtropical climate, which are considered inefficient due to operating under traditional head pressure control. This study presents a hybrid intelligent control technique, including neural networks and genetic algorithms, for the optimal control of the set points of the condensing temperature to improve the coefficient of performance (COP) of air-cooled chillers under various operating conditions. The neural network is used to model the air-cooled chillers, and genetic algorithm is adopted in searching optimal set points of condensing temperature based on the predicted fitness values. Results show that this control technique allows optimal set point of the condensing temperature to be successfully determined, and the chiller performance can be improved considerably.

An analysis on the energy efficiency of air-cooled chillers with water mist system

This paper presents the effect of operating water mist system to enhance the energy efficiency of air-cooled chillers under various operating conditions. A thermodynamic model for an air-cooled chiller with twin refrigeration circuits coupled with a water mist system was developed and validated by measured operating data. The validated model was used to investigate how the thermal properties of the entering condenser air and the performance of the air-cooled chiller varied under different operating schemes with water mist pre-cooling. Under the conventional head pressure control (HPC) with a designed water mist generation rate, the coefficient of performance (COP) increased by up to 21.3%. The chiller performance could be improved further under variable condensing temperature control (CTC). Under CTC, chiller COP was increased up to 51.5% with optimal water mist generation rate. The potential energy savings of these chillers serving a representative office building in subtropical climate was evaluated, and CTC coupling with optimal water mist generation rate would reduce the annual total electricity consumption for cooling by 14.1%. The water consumption of the water mist system was comparatively small, comparing with the total water losses for an open-loop cooling tower system with equivalent heat rejection capacity.

Improved energy performance of air-cooled chiller system with mist pre-cooling

This study investigates how to lower the dry bulb temperature of outdoor air via mist pre-cooling to enhance the coefficient of performance (COP) of an air-cooled chiller system. Based on standard chiller models in commercial simulation software DOE-2E, a modified model was made to analyse how the chiller COP varies under two different condensing temperature controls with mist pre-cooling of air entering the air-cooled condenser. Drawing on the model calibrated by manufacturer and operating data, it was estimated that around 18% decrease in the electricity consumption could be achieved by coupling floating condensing temperature control with an optimal mist generation rate, with regard to a chiller system serving a hotel in a sub-tropical climate. Remarks have been made on how optimal mist control can be implemented to maximize electricity savings while reducing surplus mist carrying over the condenser coil.

Simulation and electricity savings estimation of air-cooled centrifugal chiller system with mist pre-cooling

This paper analyses how to apply mist pre-cooling coupled with condensing temperature control to enhance the coefficient of performance (COP) of an air-cooled chiller system and hence achieve electricity savings. A modified DOE-2.1E chiller model was developed to predict the change of chiller COP due to various set points of condensing temperature and pre-cooling of air stream entering the condenser. The model was calibrated by using manufacturer’s data and used to estimate the annual electricity consumption of a chiller system serving an office building under four operating schemes: traditional head pressure control (HPC); HPC with a fixed mist generation rate; condensing temperature control (CTC) with a fixed mist generation rate; CTC with an optimal mist generation rate. It was estimated that using optimal mist control with CTC could achieve a 19.84% reduction in the annual electricity consumption of the system. Considerations when using mist pre-cooling to maximize electricity savings have been discussed.

Modelling of improved energy performance of air-cooled chillers with mist pre-cooling

Air-cooled chillers are widely used to provide cooling energy and yet pragmatic and simple energy efficient measures for them are still lacking. This paper considers how their coefficient of performance (COP) can be improved by using mist to pre-cool ambient air entering the condensers. The benefit of this application rests on the decrease of compressor power resulting from the reduced condenser air temperature with insignificant consumption of water and pump power associated with the mist generation. Based on a simulation analysis of an air-cooled screw chiller operating under head pressure control, applying such mist pre-cooling enables the COP to increase by up to 7.7%. Precise control of mist generation rate and integration with floating condensing temperature control are the major challenges of using a mist system to maximize electricity savings. The results of this study will prompt low-energy operation of existing air-cooled chillers working for various climatic conditions.

Improved energy performance of air cooled centrifugal chillers with variable chilled water flow

This paper considers how to apply optimum condensing temperature control and variable chilled water flow to increase the coefficient of performance (COP) of air cooled centrifugal chillers. A thermodynamic model for the chillers was developed and validated using a wide range of operating data and specifications. The model considers real process phenomena, including capacity control by the inlet guide vanes of the compressor and an algorithm to determine the number and speed of condenser fans staged based on a set point of condensing temperature. Based on the validated model, it was found that optimizing the control of condensing temperature and varying the evaporator’s chilled water flow rate enable the COP to increase by 0.8–191.7%, depending on the load and ambient conditions. A cooling load profile of an office building in a subtropical climate was considered to assess the potential electricity savings resulting from the increased chiller COP and optimum staging of chillers and pumps. There is 16.3–21.0% reduction in the annual electricity consumption of the building’s chiller plant. The results of this paper provide useful information on how to implement a low energy chiller plant.

A novel energy-saving method for air-cooled chiller plant by parallel connection

A novel method was put forward for improving the energy efficiency of air-cooled water chiller plant operating on part load conditions. The conventional multiple-chiller plant was proposed to be integrated into one refrigeration cycle, by connecting those separate compressors, condensers and evaporators in parallel, respectively. The integrated multiple-chiller plant uses the electronic expansion valve to control refrigerant flow, achieving variable condensing temperature control. A prototype composed of four reciprocating compressors (including one variable-speed compressor), with total nominal cooling capacity of 120 kW was simulated and experimented. Both the simulative and experimental results indicated that applying this novel energy-saving method, the air-cooled chiller plant could get a significant performance improvement on various part load ratio (PLR) conditions, due to the apparent decrease of condensing temperature and some increase of evaporating temperature. Under the same outdoor temperature of 35 °C, when the PLR decreased from 100% to 50%, the COP increased by about 16.2% in simulation and 9.5% in experiment. Also, the practical refrigeration output ratio of the system was 55% on the condition of 50% PLR.

Economic benefits of improved condenser features for air-cooled chillers serving an air-conditioned hotel

Air-cooled chillers can take up around one-fourth of the total electricity consumption of an air-conditioned building. This paper evaluates the potential electricity cost savings of these chillers with improved condenser features of condensing temperature control (CTC), evaporative pre-coolers (EC) and variable speed condenser fans (VSF). A validated model for an air-cooled screw chiller was used to ascertain how the individual and mixed features influence the annual electricity consumption of chillers operating for the cooling load profile of a hotel. With regard to a chiller plant with a 15-year lifespan, the life cycle electricity cost savings are estimated at between HK$ 2,291,247 and 8,320,732, depending on the features used. The simple payback of the features could range from 1.07 to 3.32 years. These findings show how chiller manufacturers and building owners can profit from implementing energy efficient technologies for air-cooled chillers.

05/02733 Analysis of the component characteristics of air-cooled chillers for modelling floating condensing temperature control

05/02733 Analysis of the component characteristics of air-cooled chillers for modelling floating condensing temperature control

Application of Direct Evaporative Coolers for Improving the Energy Efficiency of Air-Cooled Chillers

This paper describes how direct evaporative coolers can be used to improve the energy efficiency of air-cooled chillers in various operating conditions and with different strategies for staging condenser fans. These coolers are installed in front of air-cooled condensers to precool outdoor air before entering the condensers. A simulation analysis on an air-cooled chiller equipped with a direct evaporative cooler showed that when head pressure control is used, the cooler enables the condensing temperature to drop by 2.1–6.2°C, resulting in a 1.4-14.4% decrease in chiller power and a 1.3–4.6% increase in the refrigeration effect. When the chiller with the cooler operates under condensing temperature control, where condenser effectiveness is enhanced by staging all condenser fans, there is a savings in chiller power of 1.3-4.3% in some operating conditions in which the drop in compressor power exceeds the additional condenser fan power due to the pressure drop across the cooler.

Life cycle analysis of enhanced condenser features for air-cooled chillers serving air-conditioned buildings

Air-cooled chillers are commonly used to provide cooling energy for air-conditioned buildings at the expense of considerable electricity. This paper examines the life cycle electricity cost of these chillers with the improved condenser features of condensing temperature control (CTC), evaporative pre-coolers (EC) and variable speed condenser fans (VSF). A validated model for an air-cooled screw chiller was used to ascertain how the individual and mixed features influence the annual electricity consumption of chillers in various operating conditions. It is estimated that the life cycle electricity cost savings range from HK$ 2,099,742 with EC to HK$ 6,399,564 with all the three features, with regard to a chiller plant serving an office building for 15 yr. The life cycle analysis reported here provides important insights into how to reap the benefits of energy efficient technologies for air-cooled chillers.

Modelling of the coefficient of performance of an air-cooled screw chiller with variable speed condenser fans

Air-cooled chillers are generally recognized as energy intensive equipment in air-conditioned buildings in the subtropical climate. This paper considers how the use of variable speed condenser fans enables these chillers to operate more efficiently. The thermodynamic model of an air-cooled screw chiller was developed using the simulation program TRNSYS and validated using the field data and specifications of the chiller. The staging of condenser fans and the control of their speed in various operating conditions were described. A comparison was made on the coefficient of performance of the chiller in the steady state with various control strategies: head pressure control with constant or variable speed condenser fans; condensing temperature control (CTC) with constant or variable speed condenser fans. Potential improvements in the chiller COP due to the use of CTC with variable speed condenser fans were discussed. The findings of this paper are useful in developing more energy efficient air-cooled chillers.

Condensing temperature control to enhance the efficiency of air-cooled chillers

This paper describes an operating strategy of condensing temperature control (CTC), which means regulating the set point of condensing temperature based on the outdoor temperature, to enhance the efficiency of air-cooled chillers used in air-conditioned buildings. A mathematical model of an air-cooled chiller is developed for simulation and is validated by using the operating data of an existing chiller and of an experimental chiller. Compared to head pressure control (HPC) with a fixed set point of condensing temperature, CTC brings a moderate rise in the condensing temperature above the outdoor temperature, and hence enables the compressors to operate at a lower condensing pressure through staging more condenser fans. While more condenser fan power is consumed, compressor power can drop considerablely, reducing the annual chiller power consumption by 18.4%. It is suggested that the condensing temperature can be coupled with the outdoor temperature and the part load ratio of chillers to gauge the achievable part load efficiency.

Electricity end-use characteristics of air-cooled chillers in hotels in Hong Kong

This paper presents the operating efficiency of air-cooled chillers in three existing hotels and investigates the extent to which the annual electricity consumption can decrease by improving their efficiency. Chillers in these hotels tend to be improperly staged, causing their seasonal efficiency to rise by 0.05–0.12 kW/kW from a full load efficiency of 0.32 kW/kW. When chiller sequencing is restored, their seasonal efficiency could be enhanced to 0.34 kW/kW, which corresponds to an 8.8–22.7% drop in their annual electricity consumption. It is possible to further decrease the annual electricity consumption by 27.0–38.6% when the chillers operate under floating condensing temperature control instead of head pressure control. The implications of improved chiller efficiency for reducing the electricity demand of hotels are discussed.

Analysis of the component characteristics of air-cooled chillers for modelling floating condensing temperature control

This paper considers how the components of air-cooled chillers interact with each other and how this understanding helps model floating condensing temperature control for these chillers. Existing strategies for modelling chillers were reviewed. To assess chiller efficiency, it is important to examine how the heat transfer coefficient of evaporators varies at different part load ratios of the chillers. A mechanistic model has to be used to simulate accurately the heat transfer characteristics of air-cooled condensers, taking into account the local heat transfer coefficients of the air side and the refrigerant side. Floating condensing temperature control is proposed as an alternative to head pressure control to optimize the heat rejection effect of the condensers and, hence, to decrease the condensing temperature. The implications of reducing the condensing temperature for the improvement of chiller efficiency are explained. Strategies for implementing this control by means of simulation and experiment are discussed.