Improved condenser design and condenser-fan operation for air-cooled chillers

Air-cooled chillers traditionally operate under head pressure control via staging constant-speed condenser fans. This causes a significant drop in their coefficient of performance (COP) at part load or low outdoor temperatures. This paper describes how the COP of these chillers can be improved by a new condenser design, using evaporative pre-coolers and variable-speed fans. A thermodynamic model for an air-cooled screw-chiller was developed, within which the condenser component considers empirical equations showing the effectiveness of an evaporative pre-cooler in lowering the outdoor temperature in the heat-rejection process. The condenser component also contains an algorithm to determine the number and speed of the condenser fans staged at any given set point of condensing temperature. It is found that the chiller’s COP can be maximized by adjusting the set point based on any given chiller load and wet-bulb temperature of the outdoor air. A 5.6–113.4% increase in chiller COP can be achieved from the new condenser design and condenser fan operation. This provides important insights into how to develop more energy-efficient air-cooled chillers.     

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.     

Constraints of using thermostatic expansion valves to operate air-cooled chillers at lower condensing temperatures

Thermostatic expansion valves (TXVs) have long been used in air-cooled chillers to implement head pressure control under which the condensing temperature is kept high at around 50 °C by staging condenser fans as few as possible. This paper considers how TXVs prevent the chillers from operating with an increased COP at lower condensing temperatures when the chiller load or outdoor temperature drops. An analysis on an existing air-cooled reciprocating chiller showed that the range of differential pressures across TXVs restricts the maximum heat rejection airflow required to increase the chiller COP, though the set point of condensing temperature is reduced to 22 °C from a high level of 45 °C. It is possible to use electronic expansion valves to meet the differential pressure requirements for maximum chiller COP. There is a maximum of 28.7% increase in the chiller COP when the heat rejection airflow is able to be maximized in various operating conditions. The results of this paper emphasize criteria for lowering the condensing temperature to enhance the performance of air-cooled chillers.     

Modelling of a condenser-fan control for an air-cooled centrifugal chiller

There is a lack of detailed experimental and simulation studies on air-cooled centrifugal chillers. This paper investigates how to optimize the control of condenser fans within the chillers to maximize their coefficients of performance (COPs). A thermodynamic model for the chillers was developed and used to analyse the steady-state COP under various load and ambient conditions. An algorithm is introduced to compute the number of staged condenser fans based on settings of the condensing pressure and outdoor temperature. The model was validated using the experimental data and performance data of an existing chiller running under various operating conditions. It is found that the best strategy for switching condenser fans is to vary their rotating speed by the use of a set point of the condensing temperature, which is adjusted in response to the chiller load and condenser air-inlet temperature. The results of this paper provide an important insight into how to increase the COPs of air-cooled chillers.     

Experimental determination of the energy efficiency of an air-cooled chiller under part load conditions

In cities located in a subtropical climate, air-cooled chillers are commonly used to provide cooling to the indoor environment. This accounts for the increasing electricity demand of buildings over the decades. This paper investigates how the condensing temperature serves to accurately determine the energy efficiency, or coefficient of performance (COP), of air-cooled chillers under part load conditions. An experiment on an air-cooled reciprocating chiller showed that for any given operating condition, the COP of the chiller varies, depending on how the condensing temperature is controlled. A sensitivity analysis is implemented to investigate to what extent COP is responding to changes in operating variables and confirms that the condensing temperature is an adequate variable to gauge COP under various operating conditions. The specifications of the upper limit for the condensing temperature in order to improve the energy efficiency of air-cooled chillers are discussed. The results of this work will give designers and researchers a good idea about how to model chiller energy performance curves in the thermal and energy computation exercises.     

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.     

Applying condensing-temperature control in air-cooled reciprocating water chillers for energy efficiency

This paper reports on the modelling and findings of the energy performance of an air-cooled reciprocating multiple-chiller plant under the conventional head pressure control and the new condensing-temperature control in a subtropical climate. The simulation model was validated using the operating data of an existing chiller plant. As noted from this existing air-cooled reciprocating chiller plant, there was a substantial efficiency drop at part-load resulting from the head pressure control. If operating at variable lower condensing-temperatures based on the established operating mode of the condenser fans and compressors, it is shown that the chiller consumption can be maintained below 2 kW/refrigeration ton throughout the entire range of outdoor temperature and part-load conditions, giving an average efficiency of 1.08 kW/refrigeration ton. The energy imposition due to cycling on more condenser fans can be compensated by the reduced compressor consumption. Potential energy savings of 18.2 and 29% in the annual chiller consumption are achievable by applying the condensing-temperature control to two existing chiller plants studied. This supports the need to develop the condensing-temperature control as an improvement to the conventional head pressure control.     

Thermodynamic and economic performance of the LiBr–H2O single stage absorption water chiller

The LiBr–H₂O single stage absorption water chiller, installed at the Municipal CHP plant of the city of Poznan, Poland was examined in order to find its energy performance. The 495 kW water chiller is used for CHP plant technological rooms air conditioning (e.g. control rooms, power supply rooms). The superheated steam (p = 1.0 MPa, t = 250 °C) is the heat source for the generator and cooling water for the absorber and condenser is supplied from the existing cooling water installation. The chilled water temperature is set at 6 °C. On the basis of online temperature and media flow measurements, the system’s energy balance was created. Employing the first law of thermodynamics the energy balance equation was solved and used for the derivation of the chiller’s COP factor. The work’s main goal was to establish the influence of the chiller’s actual load on the energy efficiency of the system.The economic evaluation of cooling energy unit price was carried out on the basis of the measured chiller’s COP factor.     

Enhancement of Performance and Energy Efficiency of Air Conditioning System Using Evaporatively Cooled Condensers

This paper presents an experimental investigation of the reduction of energy consumption in a split air-conditioning system employing evaporative cooling of ambient air flowing over the condenser coil. Direct evaporative cooling is employed at the air-cooled condenser of a split air-conditioning system to cool the air flowing over the condenser coils. Different ambient conditions of air were simulated using a heater to mimic typical high temperature environments. The effect of the cooling pad thickness on the performance of the system was investigated by varying the pad thickness from 5 cm to 15 cm in step size of 5 cm. Result shows that the temperature drop experienced by the air is dependent on the thickness of the pad, as well as the condition of the inlet air to the pad. Conditions of the exit air from the pad shows that evaporative cooling can be employed as a stand-alone method for cooling of data centers, with adequate humidity control systems in place, or its output can be used to augment the performance of existing mechanical cooling systems. A decrease in power consumption of the unit is observed, with concomitant increase in coefficient of performance (COP). In addition, results obtained show that up to 44% increase in COP, and a 20% decrease in power consumption can be achieved by employing evaporative cooling. Additionally, the COP was found to increase by about 4% for every 1°C drop in refrigerant condensing temperature. Moreover, a 1°C drop in ambient air temperature causes a drop of 0.6°C in condensing temperature of the refrigerant.     

Steady-state simulation of screw liquid chillers

In order to predict the performance of screw liquid chillers in a wide range of configuration parameters and operating conditions, a steady-state simulation model is presented, which is suitable for non-economized and economized chillers. The model includes sub-models for key components, such as non-economized compressor, economized compressor, shell-and-tube condenser, expansion valve, and flooded evaporator. Sequential modular method and successive substitution methods are combined together to carry out the simulation of the chillers. The convergence properties of this simulation technique are also analyzed. The present model can predict the performance within ±10% for large scale of cooling capacity including four specifications of non-economized chillers and three specifications of economized chillers. With the validated model, a sensitivity analysis on the economizer system is made. It is found that the cooling capacity of screw liquid chiller can be increased apparently by adding the economizer, but only when the volumetric ratio of the first stage internal compression is greater than a certain value, the economized screw liquid chiller can gain a higher COP than that of the non-economized one at the same time.     

One‐zone simulation model of an oil‐injected screw chiller

This paper presents a one‐zone steady‐state system model of an oil‐injected screw chiller. The model can be used as a design and optimization tool for system performance of multiple‐chiller plant in process industries. All major components of the system are modelled in a modular format including the oil‐injected screw compressor, shell and tube condenser, flooded evaporator and a high side‐float value. The model results are validated with the experimental data from a multiple‐chiller plant at a process industry. The validated results show that the part‐load ratio and the glycol–water temperature at the evaporator inlet significantly affect the system performance as compared to the temperature of cooling water entering the condenser. Copyright © 2004 John Wiley & Sons, Ltd.     

Performance modelling of air-cooled twin-circuit screw chiller

A large variety of chiller models are available in the public domain but none can model chillers that comprise multiple and separate refrigerant circuits, despite that chillers of this type are already widely used for their good part-load performance. Presented in the paper is a mathematical model for an air-cooled twin-circuit chiller, with two screw compressors per circuit. The chiller model comprises a series of linked mathematical modules, each made up of a set of thermodynamic and empirical equations for modelling the major chiller components. The coefficients in the component models were evaluated using rated operating conditions obtained from the manufacturer and measured performance data of an existing chiller. The chiller model had been applied to simulate the performance of another set of chiller of the same make and model. Comparison of the predicted and measured performance of the chiller showed that the model could yield accurate energy use predictions over a wide range of operating conditions. The model could also provide good predictions of the variation in chiller performance due to staged operation of the separate refrigerant circuits in the chiller and of compressors in each circuit, which matched with observations made with measured chiller operation data.     

Numerical investigation of a two-stage air-cooled absorption refrigeration system for solar cooling: Cycle analysis and absorption cooling performances

Two-stage air-cooled ammonia–water absorption refrigeration system could make good use of low-grade solar thermal energy to produce cooling effect. The system simulation results show that thermal COP is 0.34 and electrical COP is 26 under a typical summer condition with 85 °C hot water supplied from solar collector. System performances under variable working conditions are also analyzed. Circular finned tube bundles are selected to build the air-cooled equipment. The condenser should be arranged in the front to get an optimum system performance. The mathematical model of the two-stage air-cooled absorber considering simultaneous heat and mass transfer processes is developed. Low pressure absorber should be arranged in front of middle pressure absorber to minimize the absorption length. Configuration of the air-cooled equipment is suggested for a 5 kW cooling capacity system. Temperature and concentration profiles along the finned tube length show that mass transfer resistance mainly exists in liquid phase while heat transfer resistance mainly exists in cooling air side. The impacts on system refrigeration capacities related to absorption behaviors under variable working conditions are also investigated. Both cycle analysis and absorption performances show that two-stage air-cooled ammonia–water absorption chiller is technically feasible in practical solar cooling applications.     

Experimental and theoretical investigation of an innovative evaporative condenser for residential refrigerator

In this study, an innovative, evaporative condenser for residential refrigerator was introduced. A vapor compression cycle incorporating the proposed evaporative condenser was tested to evaluate the cycle performance. To allow for evaporative cooling, sheets of cloth were wrapped around condenser to suck the water from a water basin by capillary effect. The thermal properties at the different points of the refrigeration cycle were measured for typical operating conditions. The experimental results showed that the condenser temperature increases 0.45 °C for each degree increase in evaporator temperature when the air velocity is 2.5 m/s, and the ambient condition is 29 °C and the relative humidity is 37.5%. Meanwhile, the condenser temperature increase is 0.88 °C in the case of air velocity 1.1 m/s and ambient conditions of 31 °C and relative humidity of 47.1%. A theoretical model for the evaporative condenser was developed, and validated by experimental results. The theoretical model showed that the evaporative condenser can operate at a condensing temperature of 20 C lower than that of the air-cooled condenser for heat flux of 150 W/m², and at air velocity 3 m/s. The effect of the different parameters on the condenser temperature was studied too.     

A steady‐state model for the design and optimization of a centralized cooling system

A simplified steady‐state model has been developed to describe thermodynamically the operation of a centralized cooling system. This model resolves the mass and energy equations simultaneously and uses inputs that are readily available to the design engineer. The model utilizes an empirical relationship for the compressor power as a function of cooling load and key temperatures. The outputs include the chiller coefficient of performance (COP) and the compressor actual power. The model simulation results are validated with a manufacturer performance data and compared with the experimental data collected at Hewlett‐Packard Laboratories site for two chillers: a variable speed and a constant speed chiller. The results of the model are found to closely match the current experimental data with less than 5% average deviation for chiller load over 10% and with a maximum deviation of 18% at 95% chiller load. For the constant speed chiller, the chiller efficiency increases with increasing load and peaks at full load. For the variable speed chiller, the chiller efficiency peaks at part loading between 40 and 80% of the chiller full load depending on the condenser water temperature. This indicates that for variable speed chillers, the chiller design has been optimized for loading less than 100% depending on the ambient conditions, customer specifications and size of the chiller. Copyright © 2010 John Wiley & Sons, Ltd.     

Modelling of vapour-compression liquid chillers with neural networks

A new approach to steady state modelling of vapour-compression liquid chillers is presented in this paper. The model uses a generalised radial basis function (GRBF) neural network to predict chiller performance. The GRBF chiller model is developed with the objective of requiring only those input parameters that are readily known to the operating engineer, i.e. the chilled water outlet temperature from the evaporator, the cooling water inlet temperature to the condenser, and the evaporator capacity. The GRBF chiller model predicts relevant performance parameters of a chiller, especially the coefficient of performance. The neural network is applied to two different chillers operating at the University of Auckland, New Zealand and the agreement is found to be within ±5%. It is inferred that neural networks, in particular the generalised radial basis function, can be a promising tool for predicting the chiller’s performance for fault detection and other diagnosis purposes.     

Study on a silica gel–water adsorption chiller integrated with a closed wet cooling tower

A silica gel–water adsorption chiller integrated with a closed wet cooling tower is proposed. This adsorption chiller consists of two vacuum chambers, each with one adsorber, one condenser and one evaporator. Vacuum valves were not adopted in this chiller in order to enhance the reliability. A novel heat recovery process was carried out after a mass recovery-like process to improve the coefficient of performance (COP). Integration of the closed wet cooling tower into the chiller could ensure the cleanliness of cooling water circulating in the chiller and also promote the convenient setup of the chiller. A transient one-dimensional mathematical model was adopted to study this adsorption chiller. The simulated results showed that the cooling power and COP were 10.76 kW and 0.51 respectively when the hot water inlet temperature, the chilled water inlet temperature, the air inlet wet bulb temperature and dry bulb temperature were 85, 15, 28 and 30 °C respectively.     

Steady‐state model of a variable speed vapor compression system using R134a as working fluid

This paper presents a steady‐state physical model for a variable speed vapor compression system. Its development and validation for a wide range of operating conditions are presented. The model requires as input parameters: compressor speed, static superheating degree and volumetric flow rates and temperatures of secondary fluids at the evaporator and condenser inlet. Using these input parameters, which can be easily obtained in this kind of facility, the model predicts the operating pressures, the temperature of secondary fluids at the evaporator and condenser outlet, the evaporator and condenser thermal capacities, the electric power consumed by the compressor and the coefficient of performance, COP. The experimental validation of the model has been carried out with 177 tests using R134a as working fluid, concluding that the model can predict the energetic performance of a variable speed vapor compression chiller with an error lower than ±10%. Copyright © 2009 John Wiley & Sons, Ltd.     

Optimization of water-cooled chiller system with load-based speed control

This study investigates the energy performance of chiller and cooling tower systems integrated with variable condenser water flow and optimal speed control for tower fans and condenser water pumps. Thermodynamic-behaviour chiller and cooling tower models were developed to assess how different control methods of cooling towers and condenser water pumps influence the trade-off between the chiller power, pump power, fan power and water consumption under various operating conditions. Load-based speed control is introduced for the tower fans and condenser water pumps to achieve optimum system performance. With regard to an example chiller system serving an office building, the optimal control coupled with variable condenser water flow could reduce the annual system electricity use by 5.3% and operating cost by 4.9% relative to the equivalent system using constant speed fans and pumps with a fixed set point for cooling water temperature control.