Transient simulation of vapour-compression packaged liquid chillers

This paper presents a transient simulation model that is useful for predicting the dynamic performance of vapour-compression liquid chillers over a wide range of operating conditions. The model employs a thermal capacitance approach for specific state variables to account for the dynamics of the chiller and ancillaries. The model accounts for the change in heat transfer coefficients throughout the heat exchangers thereby improving both physical realism and the accuracy of the simulation model. The model requires only a select few initial conditions (eg. the chilled water and condenser water temperatures). A simple compressor model based on empirical regression has been employed in the simulation. The outputs of the model include system performance variables such as the compressor electrical work input and the coefficient of performance (COP) as well as states of the refrigerant throughout the refrigeration cycle with respect to time. The model is validated with data from two in -situ screw chillers. Predictions are found to be within ±10%, although for one of the chillers a degree of empiricism was employed for the evaporator tube wall mass in order to give satisfactory results for the start-up process.     

Steady-state model of centrifugal liquid chillers: Modèle pour des refroidisseurs de liquide centrifuges en régime permanent

A new steady-state model of vapour-compression type centrifugal liquid chillers is presented. The model has a number of advanced features and is capable of simulating both hermetic and open-drive centrifugal compressors. The model accounts for the real process phenomena such as superheating and subcooling in the heat exchangers as well as a capacity control formulation of the inlet guide vanes. The model algorithm is developed with the aim of requiring only those inputs that are readily known to the design engineer, e.g. the general parameters of the chiller, the chilled water flow temperature out of the evaporator and the return water temperature to the condenser inlet. The outputs include the condenser capacity, the refrigeration capacity (at the evaporator), the coefficient of performance, and also the mass flow rates and thermodynamic states of the refrigerant throughout the cycle. The model is validated with the experimental data on part load to full load performance of three different chillers operating at the University of Auckland and the agreement is found to be within ±10%. The model also demonstrates that the COP of the chillers increases with increasing cooling capacity.     

Experimental investigation on the drop-in performance of R407C as a substitute for R22 in a screw chiller with shell-and-tube heat exchangers

A drop-in test of a mixed refrigerant R407C is performed in a commercial screw chiller with shell-and-tube heat exchangers originally designed for R22. The test results show a severe performance reduction when substituting the refrigerant from R22 to R407C. The reason for the performance reduction is analyzed comprehensively, and the influence of thermodynamic properties, compressor efficiency, and heat transfer is evaluated quantitatively. The major factor causing the performance reduction is assessed as the degradation of the heat transfer in using the mixed refrigerant, R407C. The heat transfer degradation in the evaporator is found to be larger and influences more on the chiller performance reduction. The performance reduction caused by the evaporator is approximately two times compared with that of the condenser.     

Optimizing chiller operation based on finite-time thermodynamics: universal modeling and experimental confirmation

The efficiency of chillers (refrigeration and heat pump devices) is limited by the dissipation from their principal components: compressor, throttler, and heat exchangers at the condenser and evaporator. Developing a generalized finite-time thermodynamics model for reciprocating chhillers, we derive analytic formulae for how the fixed finite resources of cycle time and heat exchanger inventory should be allocated so as to optimize chiller performance. Our predictions for optimal operating schemes are compared with detailed experimental data from two different commercial chillers. The agreement between theory and actual performance data attests to the empirical wisdom that has evolved in chiller manufacture. Besides quantitatively documenting the individual sources of irreversibility, we show how the limitations of currently-available chiller components affect optimal chiller design, as well as how potentials steps to improve chiller efficiency can be evaluated within a universal thermodynamic framework.     

Refrigerant performance evaluation including effects of transport properties and optimized heat exchangers

Preliminary refrigerant screenings typically rely on using cycle simulation models involving thermodynamic properties alone. This approach has two shortcomings. First, it neglects transport properties, whose influence on system performance is particularly strong through their impact on the performance of the heat exchangers. Second, the refrigerant temperatures in the evaporator and condenser are specified as input, while real-life equipment operates at imposed heat sink and heat source temperatures; the temperatures in the evaporator and condensers are established based on overall heat transfer resistances of these heat exchangers and the balance of the system.The paper discusses a simulation methodology and model that addresses the above shortcomings. This model simulates the thermodynamic cycle operating at specified heat sink and heat source temperature profiles, and includes the ability to account for the effects of thermophysical properties and refrigerant mass flux on refrigerant heat transfer and pressure drop in the air-to-refrigerant evaporator and condenser. Additionally, the model can optimize the refrigerant mass flux in the heat exchangers to maximize the coefficient of performance. The new model is validated with experimental data and its predictions are contrasted to those of a model based on thermodynamic properties alone.     

Thermodynamic performance limit considerations for dual-evaporator, non-azeotropic refrigerant mixture-based domestic refrigerator-freezer systems

Non-azeotropic refrigerant mixtures (NARMs) are investigated for a two-temperature level heat exchange process found in a domestic refrigerator-freezer. Ideal (constant air temperature) heat exchange processes are assumed. The results allow the effects of intercooling between the evaporator refrigerant stream and the condenser outlet stream to be examined in a systematic manner. For the conditions studied, an idealized NARM system will have a limiting coefficient of performance (COP) that is less than that of the best performing pure refrigerant component. However, for non-ideal heat exchange processes (gliding air temperature), the NARM-based system can have a higher limiting COP than a system running on either pure NARM component. Intercooling significantly affects the COP of NARM-based systems; however, depending on the location of ‘pinch points’ in the heat exchangers, only one intercooling heat exchanger may be needed to obtain a NARM's maximum refrigerator COP. The results are presented for mixtures of R22–R142b, R22–R123 and R32–R142b.     

Boiling hysteresis at low temperature on enhanced tubes

The boiling hysteresis phenomenon is studied for a real scale enhanced evaporator tube (2 m long Turbo-B type) with R134a refrigerant used in the flooded evaporator of a centrifugal brine chiller for the ice-making facility. Unlike previous studies of the boiling heat transfer with uniform heat flux and uniform wall temperature, the wall temperature varies along the tube in the present experiment. To see if the similar hysteresis occurs as in the case of uniform wall temperature, a careful control of refrigerant temperature and heat flux is made. We have found hysteresis of the temperature overshoot (TOS) at the onset of nucleate boiling initially at the inlet section of the tube, before it gradually moved downstream section of the tube until the nucleate boiling occupied the whole section of the tube as the inlet temperature increased. The hysteresis became stronger at low refrigerant temperatures. The decreasing trend of heat flux after the contents of the whole tube boiled was different from the increasing trend. This paper provides a guideline how to design the evaporator in order to avoid the abnormal operation of the chillers.     

Study of a novel silica gel–water adsorption chiller. Part I. Design and performance prediction

A novel silica gel–water adsorption chiller is designed and its performance is predicted in this work. This adsorption chiller includes three vacuum chambers: two adsorption/desorption (or evaporation/condensation) vacuum chambers and one heat pipe working vacuum chamber as the evaporator. One adsorber, one condenser and one evaporator are housed in the same chamber to constitute an adsorption/desorption unit. The evaporators of two adsorption/desorption units are combined together by a heat-pipe heat exchanger to make continuous refrigerating capacity. In this chiller, a vacuum valve is installed between the two adsorption/desorption vacuum chambers to increase its performance especially when the chiller is driven by a low temperature heat source. The operating reliability of the chiller rises greatly because of using fewer valves. Furthermore, the performance of the chiller is predicted. The simulated results show that the refrigerating capacity is more than 10 kW under a typical working condition with hot water temperature of 85 °C, the cooling water temperature of 31 °C and the chilled water inlet temperature of 15 °C. The COP exceeds 0.5 even under a heat source temperature of 65 °C.     

An NTU-ε model for alternative refrigerants: Un modèle NTU-ε pour les frigorigènes de remplacement

This paper presents a steady state simulation model to predict the performance of alternative refrigerants in vapour compression refrigeration/heat pump systems. The model is based on the NTU-ε method in analysing the heat exchangers following an elemental approach. The model extends its applicability to new refrigerants including hydrocarbons and uses a large database of REFPROP package for refrigerant properties. The main inputs to the model include the physical details of the heat exchangers, compressor efficiency, mass flow rates of heat transfer fluids and their inlet temperatures to the evaporator and the condenser, the pressure drops across the heat exchangers and the capacity of either the evaporator or condenser (in kW). The model results are validated with a wide range of experimental data of HCFC22 and propane (HC290) on a heat pump test facility for a number of parameters, e.g. coefficient of performance, condenser capacity, mass flow rate of the refrigerant and compressor discharge temperature. Although the model is currently tested for pure refrigerants using compact brazed plate (counter flow type) heat exchangers, it can also be applied to mixture of refrigerants as well as to other types of heat exchangers.     

Performance evaluation of a heat pump cycle using NARMs by a simulation with equations of heat transfer and pressure drop

Simulation analyses for a vapour compression heat pump cycle using nonazeotropic refrigerant mixtures (NARMs) of R22 and R114 are conducted under the condition that the heat pump thermal output and the flow rate and inlet temperatures of the heat sink and source water are given. The heat transfer coefficients of the condensation and evaporation are calculated with empirical correlations proposed by the authors. The validity of the evaluation method and the correlations is demonstrated by comparison with experimental data. The relations between the coefficient of performance (COP) and composition are shown under two conditions: (1) the constant heat transfer length of the condenser and evaporator; and (2) the constant temperature of refrigerant at the heat exchanger inlet. The COP of the NARMs is higher than that of pure refrigerant when the heat transfer lengths of the condenser and evaporator are sufficiently long.     

A novel special distributed method for dynamic refrigeration system simulation

A novel dynamic mathematical model based on spatially distributed approach has been developed and validated in this paper. This model gives good agreement in predicting the system COP and other parameters. The validated model has been used to enhance the prediction of the micro variations of superheat and sub-cooling. The novel spatial distributed model for the condenser and evaporator in refrigeration system, calculates the two-phase region in gas and liquid field separately since the gas and liquid in the two-phase region have different velocities. Previous researchers have used a pre-defined function of the void fraction in their spatially distributed model, based on experimental results. This approach results in the separate solution of the mass and energy equations, and less calculation is required. However, it is recognized that the mass and energy equations should be coupled during solving for more accurate solution. Based on the energy and mass balance, the spatial distribution model constructed here solves the velocity, pressure, refrigerant temperature, and wall temperature functions in heat exchangers simultaneously. A novel iteration method is developed and reduces the intensive calculations required. Furthermore, the condenser and evaporator models have shown a parametric distribution along the heat exchanger surface, therefore, the spatial distribution parameters in the two heat exchangers can be visualised numerically with a two-phase moving interface clearly shown.     

Comparative study of irreversibilities in an aqua-ammonia absorption refrigeration system

Irreversibilities in components of an aqua-ammonia absorption refrigeratio system (ARS) have been determined by second law analysis. The components of the ARS are as follows: condenser, evaporator, absorber, generator, pump, expansion valves, mixture heat exchanger and refrigerant heat exchanger. It is assumed that the ammonia concentration at the generator exit is, independent of the other parameters, equal to 0.999 and at the evaporator exit the gas is saturated vapour. Pressrre losses between the generator and condenser, and the evaporator and absorber are taken into consideration. In the results the dimensionless exergy loss of each component, the exergetic coefficient of performance, the coefficient of performance and the circulation ratio are given graphically for each different generator, evaporator, condenser and absorber temperature.     

Multi-bed regenerative adsorption chiller — improving the utilization of waste heat and reducing the chilled water outlet temperature fluctuation

A multi-bed regenerative adsorption chiller design is proposed. The concept aims to extract the most enthalpy from the low-grade waste heat before it is purged into the drain. It is also able to minimise the chilled water temperature fluctuation so that downstream temperature smoothing device may be downsized or even eliminated in applications where tighter temperature control may be required. The design also avoids a master-and-slave configuration so that materials invested are not under-utilised. Because of the nature of low-grade waste heat utilization, the performance of adsorption chillers is measured in terms of the recovery efficiency, η instead of the conventional COP. For the same waste heat source flowrate and inlet temperature, a four-bed chiller generates 70% more cooling capacity than a typical two-bed chiller. A six-bed chiller in turn generates 40% more than that of a four-bed chiller. Since the beds can be triggered into operation sequentially during start-up, the risk of ice formation in the evaporator during start-up is greatly reduced compared with that of a two-bed chiller.     

Refrigerant and lubricant charge in air condition heat exchangers: Experimentally validated model

The paper presents a semi-empirical model to predict refrigerant and lubricant inventory in both microchannel condenser and plate-and-fin evaporator of an air conditioning system. Validated against experiments, where mass of refrigerant and lubricant are isolated under steady state and afterwards measured within ±1% and ±2.5% uncertainty, respectively, the model predicts refrigerant mass in both heat exchangers within 20%. As for lubricant, initial application of the model predicts mass well for evaporator but consistently under-predicts for condenser. Analysis shows that the lubricant might be separated in the condenser inlet header. Accumulated in the bottom, the lubricant-rich liquid may start to fill in the microchannel tubes. The temperature profile in the infrared image supports this hypothesis, as the temperature of the bottom channels is much lower. After modifying the model by counting in these liquid filled channels, both refrigerant and lubricant mass in the condenser can be modeled within 15% error.     

Experimental investigation of a vapor absorption refrigeration system

An experimental investigation of the performance of a commercially available vapor absorption refrigeration (VAR) system is described. The natural gas-fired VAR system uses aqua-ammonia solution with ammonia as the refrigerant and water as the absorbent and has a rated cooling capacity of 10 kW. The unit was extensively modified to allow fluid pressures and temperatures to be measured at strategic points in the system. The mass flow rates of refrigerant, weak solution, and strong solution were also measured. The system as supplied incorporates air-cooled condenser and absorber units. Water-cooled absorber and condenser units were fitted to extend the VAR unit's range of operating conditions by varying the cooling water inlet temperature and/or flow rates to these units. The response of the refrigeration system to variations in chilled water inlet temperature, chilled water level in the evaporator drum, chilled water flow rate, and variable heat input are presented.     

Performance assessment of HC-290 as a drop-in substitute to HCFC-22 in a window air conditioner

As per the Montreal Protocol, CFCs and HCFCs are being phased out. HCFC-22 is used in window air conditioners. This paper presents the experimental performance study of a window air conditioner with propane (HC-290), a natural refrigerant, as a drop-in substitute to HCFC-22. Experimental results showed that HC-290 had 6.6% lower cooling capacity for the lower operating conditions and 9.7% lower for the higher operating conditions with respect to HCFC-22. The coefficient of performance for HC-290 was 7.9% higher for the lower operating conditions and 2.8% higher for the higher operating conditions. The energy consumption of the unit with HC-290 was lower in the range 12.4–13.5% than HCFC-22. The discharge pressures for HC-290 were lower in the range 13.7–18.2% than HCFC-22. For HC-290, the pressure drop was lower than HCFC-22 in both heat exchangers.This paper also presents simulation results for the heat exchangers of an HCFC-22 window air conditioner with HC-290 as a drop-in substitute. The simulation has been carried out using EVAP-COND, a heat exchanger model developed by NIST [National Institute of Standards and Technology. EVAP-COND: simulation models for finned-tube heat exchangers, Maryland, USA (2003). http://www2.bfrl.nist.gov/software/evap-cond/ [18]]. The simulated evaporator capacities are within ±4% of the experimentally measured cooling capacities for both refrigerants. Simulation results for HC-290 and HCFC-22 are compared. The exit temperatures of HC-290 are lower by 0.3–1.2 °C in the condenser and are higher by 2.1–2.4 °C in the evaporator than HCFC-22. Evaporating pressures of HC-290 are lower by 2.1–3.3% as compared to HCFC-22. The pressure drops of HC-290 are lower in both the evaporator and the condenser as compared to HCFC-22. The outlet temperatures of air for HCFC-22 and HC-290 in both heat exchangers are nearly the same.     

Modelling of the heat exchangers of a small capacity, hot water driven, air-cooled H2O–LiBr absorption cooling machine

General models for the design of the heat exchangers (absorber, generator, condenser and evaporator) of a prototype of an air-cooled absorption chiller of 2 kW for air-conditioning using the pair H₂O–LiBr have been developed. An absorption machine of such characteristics has been constructed to be used as a test facility for validating the results obtained from the mathematical models developed. The discrepancies considering the heat exchanged between numerical results and experimental data are under 15% in most cases for all these components except the condenser, where the discrepancies are higher. The conclusions reported will lead to: (i) future improvements of the mathematical simulation models and (ii) improvements in the experimental infrastructure.     

Numerical optimisation of a two-stage ejector refrigeration plant

Jet-refrigeration cycles seem to provide an interesting solution to the increasing interest in environment protection and the need for energy saving due to their low plant costs, reliability and possibility to use water as operating fluid. A steam/steam ejector cycle refrigerator is investigated introducing a two-stage ejector with annular primary at the second stage. The steady_state refrigerator, exchanging heat with the water streams at inlet fixed temperatures at the three shell and tube heat exchangers, evaporator, condenser and generator, is considered as an open system. Heat transfer irreversibilities in the heat exchangers and external friction losses in the water streams are considered, ignoring the internal pressure drop of the vapor. A simulation program numerically searches the maximum COP at given external inlet fluid temperatures as a function of mass flows, dimensions and temperature differences in the heat exchangers. The code gives the ejector and heat exchangers design parameters.     

Absorption chiller crystallization control strategies for integrated cooling heating and power systems

The concept of an air-cooled absorption chiller system is attractive because the cooling tower and the associated installation and maintenance issues can be avoided. However, crystallization of the LiBr–H₂O solution then becomes the main challenge in the operation of the chiller, since the air-cooled absorber tends to operate at a higher temperature and concentration level than the water-cooled absorber due to the relative heat transfer characteristics of the coolant. This leads to crystallization of the working fluid. The paper focuses on the crystallization issues and control strategies in LiBr–H₂O air-cooled absorption chillers. As a result a novel application opportunity is proposed for the integration of absorption chillers into cooling, heating and power (CHP) systems. This new methodology allows for air cooler operation while avoiding crystallization.     

Detailed simulation of fluted tube water heating condensers

Fluted tube-in-tube condensers are key components in advanced energy efficient water heating heat pumps. Therefore, there exists a need for a computer design tool that incorporates all the essential features of these heat exchangers. This paper describes the development of a detailed model to simulate fluted tube refrigerant-to-water condensers. The model allows the surface area to be divided into any number of sections for which all the refrigerant and water properties can be evaluated. This allows for the extension of the model to simulate heat exchangers for cycles employing zeotropic refrigerant mixtures. For the waterside existing empirical equations are used for both friction and heat transfer. However, on the refrigerant side no correlations are available in the literature to calculate friction and heat transfer coefficients. The approach followed in this paper is therefore to use existing smooth tube correlations combined with enhancement ratios based on correlations available for helical coils as well as enhancement factors based on empirical data for fluted tube condensers. The model is validated with the aid of results from independent tests on two commercial fluted tube heat exchangers. The average difference between the simulated and measured pressure drops is 7.27% and the average difference for the log mean temperature difference (LMTD) is 4.41%.