Inter-stage Pressure Research Articles

Parameter Sensitivity Study for Typical Expander-Based Transcritical CO2 Refrigeration Cycles

A sensitivity study was conducted for three typical expander-based transcritical CO2 cycles with the developed simulation model, and the sensitivities of the maximum coefficient of performance (COP) to the key operating parameters, including the inlet pressure of gas cooler, the temperatures at evaporator inlet and gas cooler outlet, the inter-stage pressure and the isentropic efficiency of expander, were obtained. The results showed that the sensitivity to the gas cooler inlet pressure differs greatly before and after the optimal gas cooler inlet pressure. The sensitivity to the intercooler outlet temperature in the two-stage cycles increases sharply to near zero and then keeps almost constant at intercooler outlet temperature of higher than 45 °C. However, the sensitivity stabilizes near zero when the evaporator inlet temperature is very low of −26.1 °C. In two-stage compression with an intercooler and an expander assisting in driving the first-stage compressor (TEADFC) cycle, an abrupt change in the sensitivity of maximum COP to the inter-stage pressure was observed, but disappeared after intercooler outlet temperature exceeds 50 °C. The sensitivity of maximum COP to the expander isentropic efficiency increases almost linearly with the expander isentropic efficiency.

Performance evaluation and determination of minimum desorption temperature of a two-stage air cooled silica gel/water adsorption system

This paper presents an in-depth numerical and thermodynamic study of a two-stage, 2-bed silica gel/water adsorption system for simultaneous generation of cooling power and potable water. The system is air cooled where the ambient temperature remains constant at 36°C. The first part of this paper investigates the effect of cycle time, chilled water inlet and heat source temperature on system performance viz. specific cooling capacity (SCC), specific daily water production (SDWP) and coefficient of performance (COP). A significant outcome of this study is to show that decrease in heat source temperature not only reduces the specific throughput but also increases the optimum cycle time, whereas COP is relatively insensitive to such alterations. The second part of this paper discusses the estimation of the minimum desorption temperature from the simulated system throughput results as well as from fundamental thermodynamic analysis of a two-stage adsorption cycle. This thermodynamic analysis provides a theoretical limit for minimum desorption temperature and optimal inter-stage pressure for a two-stage adsorption cycle.

Specifying Carbon Dioxide Centrifugal Compressor

Summary This paper highlights the care to be taken by an engineer while specifying and selecting a centrifugal compressor for carbon dioxide (CO2) compression. With the increase in the levels of CO2 in the atmosphere, there is an increase in the popularity of capturing CO2 emitted from the large source points such as fossil-fuel power plants, steel mills, cement plants, and others before its release to the atmosphere and storing it in geological formations [also used for enhanced oil recovery (EOR) where possible]. The compressors used to transport and store the CO2 at such depths need to compress the gas from atmospheric pressure to the pressures on the order of 200 bar or more. The critical temperature of CO2 is only 31.1°C, so the CO2 is generally transported and stored in a supercritical state. The thermodynamic properties of supercritical CO2 are considerably different from those of the other real gases that are generally compressed. Further, to achieve this supercritical state, the critical point of CO2 is crossed somewhere in the compression stage. Near critical point, the ideal-gas law will not hold for CO2. Moreover, there is a reduction in the choke margin of the compressor caused by the reduction of the sound speed in CO2, particularly near the thermodynamic critical point. Also, the CO2 compressibility and specific heats are not linear near the critical pressure and temperature. The impurities in the CO2 will affect further the thermodynamic characteristics of the working fluid. Also, the water content in the CO2 makes it extremely corrosive. Considering all the aspects mentioned previously, specifying the CO2 compressor correctly in terms of the equation of state (EOS) to be used, the interstage pressures and temperatures to be maintained, suggesting the number of impellers per stage to maintain the desired flow coefficient, metallurgy to be selected and scheme of compressor dry gas seals, and others becomes all the more important and is described in the paper.

Modeling study of two-stage, multi-bed air cooled silica gel + water adsorption cooling cum desalination system

This paper presents a transient numerical study of two-stage, air-cooled silica gel+water adsorption system producing cooling and potable water. The key aspect of the present study is an attempt to model the inter-stage pressure dynamics when two beds exchange mass in terms of vapors across the stages. This is an important improvement over prior models wherein the inter-stage pressure was assumed to be either constant or user input. Furthermore, the evaporator and condenser pressures are also allowed to vary in this study in contrast to the previous papers. The simulation results are compared with the experimental data for various cycle times and chilled water inlet temperatures in the range of 11.5–20.0°C, while having the heat source temperature fixed at 85°C and air temperature at 36°C. Depending on the chilled water inlet temperature and cycle time, the SCC is found to be between 2 and 7Rton/tonne of silica gel; SDWP ranges within 0.3–0.9m3/day/tonne of silica gel and COP varies between 0.05 and 0.26.

Effect of Inter-stage Pressure on the Performance of a Two Stage Refrigeration Cycle Using Inter Cooler

Abstract Effect of inter-stage pressure on the performance of a two stage refrigeration cycle using inter cooler is studied based on thermodynamic modelling. Six common refrigerants are selected for the analyses. These are R134a, R22 and R143a as synthetic refrigerants and propane, carbon dioxide and nitrous oxide as natural refrigerants. The designed input range is -50oC to -30oC for evaporator temperature and 40oC to 60oC for heat rejection temperature. Three classical relations for estimating inter-stage pressure in a two stage refrigeration system are selected and the performance is evaluated against optimized inter-stage pressure corresponding to the best COP. Results show that for trans-critical cycles, the deviation of optimized inter-stage pressure with the classical ones is considerable, while for sub-critical cycles, COP is very less sensitive to inter-stage pressure. Further, comprehensive analysis of CO2 based trans-critical refrigeration system is presented for a wider range covering -50oC to 10oC evaporation temperature and 35oC to 60oC gas cooler outlet temperature. A correlation for estimating optimum inter-stage and gas cooler pressure for the same is also formulated. With gradual increase in focus towards adoption of natural refrigerants like CO2, the effect of inter stage pressure is expected to be an important consideration in warmer climatic condition.

Analysis of performance of two-stage screw compressor under various operating conditions

The screw compressor has a promising application in the process industry due to its appropriate pressure and flow rate. In this paper, a mathematical model of a two-stage oil-free screw compressor is developed to simulate the working process under various operating conditions. The results show that the interstage pressure of the two-stage oil-free screw compressor is built automatically depending on the equal mass flow rate of each stage. It increases with the suction pressure, discharge pressure, and clearance, but decreases with rotating speed. Although the power consumption of the 2nd stage decreases with the suction pressure, the power consumption of the whole compressor always increases with the suction pressure, discharge pressure, rotating speed, and clearance. The volumetric efficiency increases with the suction pressure and rotating speed, but decreases with the discharge pressure and clearance for each stage. The indicated efficiency increases with the rotating speed but decreases with the clearance. A maximum indicated efficiency is found. The volumetric and indicated efficiency of the 2nd stage is more sensitive to the operating condition than that of the 1st stage.

Comparative study of cycle modification strategies for trans-critical CO2 refrigeration cycle for warm climatic conditions

This paper presents a comparative study of performance of six prominent modifications on the basic trans-critical CO2 refrigeration system to investigate their suitability to high ambient temperature application (35–55°C). To explore the application in chiller, domestic refrigeration and air cooling the evaporator temperature chosen are −10°C, 0°C and 10°C respectively. In general the cycle modifications have a positive effect on the overall COP of the system. However, to comprehend practicability of these modifications for three application areas, a few other parameters which affect design and operation are also included in the study. These are compressor discharge pressure and temperature, mass flow rate, interstage pressure for multi-stage operation and exergy destruction. Effect of real time constraints like approach temperature, pressure drop in gas cooler, compressors efficieny, degree of superheat, expanders efficiency and effectivenesss of intermediate heat exchanger are also incorporated. Interrelation between these parameters are brought out from the study.

Computational fluid dynamics and experimental study of inter-stage seal clearance on submersible well pump

In this article, a typical submersible well pump was investigated to study the effects of inter-stage leakage on the inner flow field and external characteristics. The whole flow field of the model pump with different seal clearances was simulated by computational fluid dynamics software. The inter-stage clearance leakage calculated by numerical simulation was compared with the values obtained by the empirical formula. The pressure values were recorded with the arrangement of monitor points in the inter-stage clearance, inlet region, and outlet region, which aimed to study the effects of the change of inter-stage clearance pressure on the pump performance. Meanwhile, a comparative analysis of numerical simulation and performance test of the pump was conducted further. The results showed that the leakage at the small clearance is close to the value calculated using empirical formula. But when the clearance is large, the discrepancy between the simulation result and empirical value inclines. Comparing the numerical results of three kinds of clearance leakage, we found that the clearance leakage could not be ignored in the simulation since it has a large effect on the prediction of pump efficiency and head.

Energy Efficiencies of Three Configurations of Two-Stage Vapor Compression Refrigeration Systems

A thermodynamic analysis is performed on three different configurations of R134a two-stage vapor compression refrigeration systems designed to equip cold store rooms. For economic and commercial reasons relating to agricultural activities in southern Tunisia, two seasonal food products, dates and poultry, are selected for storage at different temperatures. Performance optimization of the refrigeration systems is investigated under various operating conditions in order to choose the most efficient for a given situation. The three-stage vapor compression refrigeration systems have the same main components, but they differ in the arrangement of the flash separator. For the first considered system, the flash separator is installed before the first evaporator. In the second system, the flash separator is introduced before the second evaporator. For the third refrigeration system, the flash separator is placed before the two evaporators. An analytic study is performed in order to analyze the influence of the condensing temperature, the inter-stage pressure, the subcooling, and the superheat on the system performances. For the same operating conditions, each cycle is optimized with regard to energy performance, refrigeration efficiency, total compressor power consumption, and specific cooling capacity. A comparison between the obtained results for the three considered cycles is established. Optimum values of evaporators’ superheat and inter-stage pressure are determined.

Simulation study of a two-stage adsorber system

The present study simulates a two-stage silica gel + water adsorption desalination (AD) and chiller system. The adsorber system thermally compresses the low pressure steam generated in the evaporator to the condenser pressure in two stages. Unlike a standalone adsorption chiller unit which operates in a closed cycle the present system is an open cycle wherein the condensed desalinated water is not fed back to the evaporator. The mathematical relations formulated in the current study are based on conservation of mass and energy along with isotherm relation and kinetics for RD-type silica gel + water pair. Various constitutive relations for each component namely the evaporator, adsorber and condenser are integrated in the model. The dynamics of heat exchanger are modeled using LMTD method, and LDF model is used to predict the dynamic characteristic of the adsorber bed. The system performance indicators namely, specific cooling capacity (SCC), specific daily water production (SDWP) and coefficient of performance (COP) are used as objective functions to optimize the system. The novelty of the present work is in introduction of inter-stage pressure as a new parameter for optimizing the two-stage operation of AD chiller system.

Identification of optimum inter-stage pressure for two-stage transcritical carbon dioxide refrigeration cycles

The disadvantage of high discharge temperatures associated with transcritical CO 2 refrigeration cycles can be overcome by adopting two-stage compression with inter-cooling between the stages. This also helps in improving the overall volumetric efficiency and some power saving. This paper considers the criteria of minimum work of compression, equal discharge temperatures at the exit of each stage and minimum exergy loss during inter-stage and gas cooling for identifying inter-stage pressure. It is shown that the equality of stage discharge temperatures is a more useful criterion than the saving in power or improvement in COP because multi-staging has at best a marginal (<10%) effect on the latter. The criterion of minimal exergy loss in heat rejection also yields the same inter-stage pressures as the minimum work criterion. Optimum inter-stage pressure for each condition is identified for several evaporator and gas cooler outlet temperatures and expressed in terms of the inter-stage pressure index of the product of cycle operating pressure limits.

Thermodynamic analysis and optimization of a novel two-stage transcritical N 2O cycle

Thermodynamic (energy and exergy) analyses and optimization studies of two-stage transcritical N 2O and CO 2 cycles, incorporating compressor intercooling, are presented based on cycle simulation employing simultaneous optimization of intercooler pressure and gas cooler pressure. Further, performance comparisons with the basic single-stage cycles are also presented. The N 2O cycle exhibits higher cooling COP, lower optimum gas cooler pressure and discharge temperature and higher second law efficiency as compared to an equivalent CO 2 cycle. However, two-stage compression with intercooling yields lesser COP improvement for N 2O compared to CO 2. Based on the cycle simulations, correlations of optimum gas cooler pressure and inter-stage pressure in terms of gas cooler exit temperature and evaporator temperature are obtained. This is expected to be of help as a guideline in optimal design and operation of such systems.

Second law optimization of two-stage transcritical CO2 refrigeration cycles in the cooling mode operation

Second law optimization studies of two-stage transcritical CO2 (TRCC) refrigeration cycles, incorporating options such as a new ejector-expansion with internal heat exchanger (IHE) and intercooler (IC), flash gas bypass, flash gas intercooling, compressor intercooling with IHE, are presented based on cycle simulation. To validate the simulations, the available numerical data in open literature are used. It is found that the coefficient of performance (COP) and second law efficiency of the new two-stage TRCC cycle are on average 16.5, 18.4, and 28.4 per cent higher than that of the two-stage TRCC with IHE and IC, the two-stage TRCC with flash gas bypass, and the two-stage TRCC with flash gas intercooling cycles, respectively. Hence, the new two-stage refrigeration cycle is a promising refrigeration cycle from the thermodynamic point of view. It is also concluded that for cases of the flash gas bypass and flash gas intercooling the optimum inter-stage pressure deviates significantly from the geometric mean pressure of the gas cooler and evaporator pressure. While for the new two-stage TRCC and the two-stage TRCC with IHE and IC, the optimum inter-stage pressure is approximately equal to geometric mean pressure. Finally, a regression analysis was employed in terms of evaporator and gas cooler exit temperatures to develop mathematical expressions for maximum COP, optimum discharge, and inter-stage pressures and entrainment ratio.

Performance analysis and optimization of a new two-stage ejector-expansion transcritical CO 2 refrigeration cycle

In this paper, a new two-stage configuration of ejector-expansion transcritical CO 2 (TRCC) refrigeration cycle is presented, which uses an internal heat exchanger and intercooler to enhance the performance of the new cycle. The theoretical analysis on the performance characteristics was carried out for the new cycle based on the first and second laws of thermodynamics. Based on the simulation results, it is found that, compared with the conventional two-stage transcritical CO 2 cycle, the COP and second law efficiency of the new two-stage cycle are about 12.5–21% higher than that of conventional two-stage cycle. It is also concluded that, the performance of the new two-stage transcritical CO 2 refrigeration can be significantly improved based on the presented new two-stage cycle. Hence the new two-stage refrigeration cycle is a promising refrigeration cycle from the thermodynamically and technical point of views. A regression analysis in terms of evaporator and gas cooler exit temperatures has been used, in order to develop mathematical expressions for maximum COP, optimum discharge and inter-stage pressures and entrainment ratio.

Second law analysis of two-stage compression transcritical CO2heat pump cycle

Because of the global warming impact of hydro fluorocarbons, the uses of natural refrigerants in automotive and HVAC industries have received worldwide attention. CO2 is the most promising refrigerant in these industries, especially the transcritical CO2 refrigeration cycle. The objective of this work is to identify the main factors that affect two-stage compression transcritical CO2 system efficiency. A second law of thermodynamic analysis on the entire two-stage CO2 cycle is conducted so that the exergy destruction of each system component can be deduced and ranked, allowing future efforts to focus on improving the components that have the highest potential for advancement. The inter-stage pressure is used as a variable parameter in the analysis study. The second law efficiency, coefficient of cooling performance and total exergy destruction of the system variations with the inter-stage pressure are presented graphically. It was concluded that there is an optimum inter-stage pressure that maximizes both first law and second law efficiencies. Copyright © 2008 John Wiley & Sons, Ltd.

Experimental Energetic Analysis of the Liquid Injection Effect in a Two-Stage Refrigeration Facility Using a Compound Compressor

The objective of this paper is to detail an experimental analysis of the energetic implications of using a liquid injection system to produce desuperheating between the compression stages in a two-stage refrigeration facility. The analysis is carried out by comparing the experimental performance of the two-stage cycle working with and without the liquid injection system. The experimental analysis of the two-stage refrigeration cycle is performed in an evaporating temperature range between −36°C and −20°C and in a condensing temperature range between 30°C and 47°C using R-404A as working fluid. The discussion compares the cycle working with liquid injection to the two-stage configuration without this intermediate system by using experimental data from condensing and evaporating pressure variation tests in a two-stage refrigerating plant driven by a compound compressor and working with one of the fluids most widely-used in Europe for low-temperature applications, R-404A. Both experimental and theoretical results show that the use of the liquid injection to produce desuperheating between compression stages produces an increment of the interstage pressure in the cycle, the consequences of which are variations of the cooling capacity and the absorbed power consumption of the two-stage cycle.

Two-body missile separation dynamics

The dynamics of the separation of a two-body missile has been investigated. This missile consists of an outer boost vehicle and an inner center body vehicle. Upon completion of the boost phase the two bodies separate as a result of differing aerodynamic loads applied to each body and the thrust from a sustainer motor contained within the center body. Through simulation, the separation event was evaluated for interaction between the two bodies as a function of flight-path-angle, interstage pressure, sustainer motor thrusting force, thrust misalignment angles, and radial clearance. The impact and friction forces created by any interaction were evaluated and their effects on the separation event were quantified. The flight characteristics of the center body after the separation event were also calculated for use in determination of the missile flight profile. Finally, recommendations are made regarding the design and performance conditions for the specific two-body missile addressed in this study.

Studies on a two-stage transcritical carbon dioxide heat pump cycle with flash intercooling

Simulation studies on a two-stage flash intercooling transcritical carbon dioxide heat pump cycle are presented. Sub-critical and super-critical thermodynamic and transport properties of carbon dioxide are calculated employing an exclusive precision property code based on recently published correlations. Results exhibit that flash intercooling technique is not economical with CO 2 refrigerant unlike NH 3 as the refrigerant. COP is considerably lower than that of the single cycle for a given gas cooler and evaporator temperature. There is no optimum inter-stage pressure as well. However, a marginal increase in COP occurs as inter-stage pressure decreases from the classical estimate of geometric mean of gas cooler and evaporator pressure. It is observed that incorporation of desuperheating of vapour in the intercooler almost doubles the mass flow rate in the second stage which can be attributed to the large flashing that occurs in the intercooler; this increase depends on the discharge temperature from the first stage and mass flow rate of refrigerant flow in the evaporator. Compressor isentropic efficiency shows marginal influence on system performance.

Optimization of two-stage transcritical carbon dioxide heat pump cycles

Optimization studies of two-stage transcritical carbon dioxide heat pump cycles, incorporating options such as flash gas bypass, flash intercooling and compressor intercooling, are presented based on cycle simulation. Sub-critical and super-critical thermodynamic and transport properties of carbon dioxide coded and then integrated with the simulation code for further analyses. Results exhibit improvement in performance by adopting optimal operating conditions. The optimum interstage pressure, thus obtained, deviate from the classical estimate of geometric mean of gas cooler and evaporator pressure. It is observed that the flash gas bypass system yields the best performance among the three two stage cycles analyzed. Internal heat exchanger effectiveness and compressor isentropic efficiency shows marginal influence on the system performance. Internal heat exchanger effectiveness shows marginal influence on the system performance while compressor isentropic efficiency shows an about 10% variation in COP. However, optimum gas cooler pressure and optimum intermediate pressure are only marginally affected. Based on the cycle simulations, correlations of optimum gas cooler pressure and inter-stage pressure in terms of gas cooler temperature and evaporator temperature are obtained. This would be useful as a guideline in design of such systems.

Exergy analysis of a two-stage refrigeration cycle using two natural substitutes of HCFC22

The aim of the present paper is to carry out a detailed exergy analysis of a two-stage vapour compression cycle by calculating its components exergetic losses. The exergy equations have been developed using refrigerant thermodynamic properties computed by means of a simple model of local equations of states. The results of the exergy analysis of a two-stage refrigeration system operating between a constant evaporating temperature of -30°C and condensation temperatures of 30, 40, 50 and 60°C with two natural substitutes of HCFC22, namely, propane (R290) and ammonia (R717) as working fluids, are presented. It is found that the most significant losses occur in the compressors, expansion valves and condenser. Furthermore, it is shown that the optimum inter-stage pressure for a two-stage refrigeration system is very close to the saturation pressure corresponding to the arithmetical mean of the refrigerant condensation and evaporation temperatures.