Compressor Discharge Pressure Research Articles

Dither extremum seeking control of a variable refrigerant flow system with equality constraint handling

In this paper, we apply an extremum-seeking control strategy to a variable refrigerant flow (VRF) system, where the total power consumption is minimized by adjusting the compressor discharge pressure setpoint and the outdoor unit fan mass flow rate, incorporating constraints on the zonal temperature setpoint regulation. A simulation study is conducted using a Modelica dynamic simulation model of a VRF system with one outdoor unit and four indoor units. Simulation results validate the effectiveness of the proposed method of power minimization with the constraint of zone-temperature regulation. The ESC strategy incorporates online penalty-weight estimation and loop-gain adaption, which are shown to improve convergence.

Performance and Optimization Study of R290 as Alternative Refrigerant for R22 in Low Temperature Heat Pump System

This paper mainly studies the replacement performance of R290 in R22 low temperature heat pump system from the experimental point of view. By comparing the performance differences under different working conditions, it is found that when R22 is directly extracted from the original system and filled with R290, the heat capacity and COP of the system are attenuated, and the compressor discharge temperature and pressure of the R290 system are higher than those of the original R22 system in low temperature environment. Through the analysis of the system components, it can be considered that the main reason for the above phenomenon is that the compressor displacement of the R22 system is too large and does not match the R290 system. Therefore, in order to meet the safety requirements of the system and improve the overall performance of R290 in the low temperature heat pump system at the same time, it is considered to replace the compressor with a smaller displacement which is more matched with R290 in the system. The experimental results show that the compressor displacement optimization of the R290 low temperature heat pump system can effectively reduce the exhaust temperature and pressure of the system and improve the overall performance of the system. The COP of the optimized R290 low temperature heat pump system is 6.5% higher than that of the original R22 system, and the exhaust temperature in the low temperature environment is reduced by 36% to below 80 C.

Alternative positions of internal heat exchanger for CO2 booster refrigeration system: Thermodynamic analysis and annual thermal performance evaluation

• Three CO2 booster systems with different locations of IHX are proposed. • Thermodynamics models of booster systems are developed and annual performance is studied. • S1 is the optimal solution system to improve COP, which is enhanced by 0.6% - 6.36%. • By adding IHX, the APF of CO2 booster system can be improved significantly. • IHX can improve annual performance in subtropical and tropical climatic regions. Refrigeration systems running on transcritical CO 2 cycle are considered an alternative to phase-down the use of hydrofluorocarbons (HFCs) in response to the Kigali Amendment. Responding to the need to improve the efficiency of CO 2 booster systems and to identify optimum system designs, thermodynamic models of three booster systems are proposed with the use of an internal heat exchanger (IHX). Findings from this study suggest that placing the IHX with a low-temperature fluid side at the suction line of high pressure stage compressor and high-temperature fluid side at the outlet of gas cooler represents the most optimal approach in improving the coefficient of performance (COP) of the booster system. When operating in the transcritical conditions, the COP values can be improved by 6.35% at the IHX thermal effectiveness of 0.8 and by 6.48% at the ratio of medium temperature load to low temperature load of 6. Using IHX can significantly reduce the compressor discharge pressure, which can be reduced by 0.55 MPa at the ambient temperature of 40 °C. Furthermore, by adding IHX, the annual performance factor of CO 2 system can be improved significantly by 1.68% and the annual total power consumption can be decreased by 6.48% in the tropical climate. It can be concluded that IHX can improve the COP values of a booster system when operating in the subtropical and tropical regions.

Influence of key parameters on the performance of a helium cryogenic system in refrigeration and liquefaction modes

At present, medium-scale helium cryogenic system is commonly based on modified Claude cycle with two expanders in series. In this study, considering the minimum allowable heat exchanger temperature difference (MHTDamd), the influence of key parameters (including compressor discharge pressure (Pcd) , temperature at the inlet of expander1 (Tin-EXP1) and temperature at the inlet of Joule-Thomson valve (Tin-JT)) on the system performance (including refrigeration capacity (RC)/liquefaction rate (LR), specific energy consumption (SEC) and total UA of the heat exchangers (UAtotal)) of the helium cryogenic system is investigated and analyzed in refrigeration and liquefaction modes, respectively. The results show that in refrigeration and liquefaction modes, the optimal ranges of Pcd, Tin-EXP1, and Tin-JT are within 8 ∼ 10 bar, 30 ∼ 45 K and 5.2 K ∼ 7.2 K, respectively. Furthermore, the constraint relation between UAtotal and SEC is obtained. By using genetic algorithm (GA), it is found that keeping constraint SEC within 100%∼115% of the chosen SEC from the optimal range can achieve a relatively economic trade-offs between SEC and UAtotal. In the end, a comparison of T-S diagram obtained from design and experimental values in refrigeration and liquefaction modes is carried out, respectively. The maximum temperature deviation is within 5%, which verifies the accuracy of the calculation models. The optimized results in this study are also applicable to the same processes with mass flow rate of compressor lower than 100 g/s.

Multi-objective thermo-economic optimization of Collins cycle

Many large scientific projects are emerging worldwide with the development of high energy physics nowadays. The indispensable subsystem, helium cryo-plants for such projects require extremely high energy consumption and total annual cost (TAC). In order to reduce energy consumption and TAC, multi-objective thermo-economic optimization that adopted non-dominated sorting genetic algorithm-II (NSGA-II) for helium cryo-plants is proposed in this work. As a typical helium liquefaction cycle, Collins cycle is selected as the optimization object. Both the exergetic efficiency (ηEx) and the TAC are set as objective parameters. The decision parameters, which include discharge pressure of compressor, flow fraction of expanders and effectiveness of heat exchangers, are optimized simultaneously. Linear programming technology of multidimensional analysis preference (LINMAP) is utilized as an example to select the optimal solution from the multi-objective optimization results. Compared to the maximum ηEx result, the LINMAP result reduces the TAC by 23.95% at the cost of 9.03% reduction in ηEx. In the proposed approach, eight decision parameters can be optimized simultaneously and the designed cryo-plant considers both economic and thermodynamic criteria, which is feasible for practical engineering project. Besides, alternative solution under limited investment situation is provided.

Experimental investigation of Al2O3 nanosuspension in vapor compression refrigeration system using tetrafluoroethane and iso-butane refrigerants

The indirect emissions due to energy consumption by refrigeration system has contributed potentially to global warming. The infusion of nanoparticle-based suspension (nanofluid) have shown significant improvement in heat transfer for refrigeration application with the noteworthy reduction in energy consumption. Experimental work was carried out with an aim to reduce indirect emissions from the refrigeration system. Thus, the present work consist of the performance investigation of aluminum oxide (Al2O3) nanoparticle suspension in a vapor compression refrigeration system with varying refrigerant mass charges. The refrigerants used for the experiment are tetrafluoroethane (R134a) which is high in GWP (Global Warming Potential) and isobutane (R600a) which has low GWP. Mineral oil (MO) and POE (Polyolester Oil) were used as the base fluid for the synthesis of Al2O3 based nanofluid with mass fractions of 0.02%, 0.04%,0.07%, and 0.1%.Varying mass charges of 150 gm, 180 gm, and 200 gm of R134a and R600a refrigerant were employed in the experiment to work with each mass fraction of nanofluid. The refrigeration system is designed to work within the cycle range of −6 °C to 30 °C for R134a and R600a refrigerant. Experimental results showed improvement in COP by 41.8%, reduction in power consumption 31.85%, increment in compressor discharge pressure by 12.56%, reduction in evaporator pressure by 3.94%, and reduction in pull-down time by 23.5% by using 200 gm of R600a-MO with 0.1 wt% of Al2O3 based-suspension. Also, there is a significant reduction in exergy destruction observed in a different component of the refrigeration system by using the nanosuspension compared to the pure refrigerant.

A methodology for identifying the most suitable measurements for engine level and component level gas path diagnostics of a micro gas turbine

Recently, the utilization of micro gas turbines in smart grids are rising that makes the part-load operation principal situation of the engine service. This leads to faster life consumption that increases the importance of the diagnostics process. Gas path analysis is an effective method for gas turbine diagnostics. Complex dynamics of gas turbine induces challenging conditions to perform applicable gas path analysis. This study aims to facilitate MGT gas path diagnostics through reducing the number of monitoring parameters and preparation a pattern for engine level and component level health assessment in both full and part load operation of a recuperated micro gas turbine. To attain this goal a model is proposed to simulate MGT off-design performance which is validated against experimental data in healthy and degraded operation modes. Fouling in compressor, turbine and recuperator and erosion in compressor and turbine as the most common degradations in the gas turbine are considered. The fault simulation is performed by changing the health parameters of gas path components. According to the result investigation, a matrix comprises deviation contours of four parameters, Power, fuel flow, compressor discharge pressure, and exhaust gas temperature is presented and analyzed. The analysis shows that monitoring these parameters makes it possible to perform engine level and component level diagnostics through evaluating a binary code (generated by mentioned parameter variations) against the fault effects pattern in different load fractions and fault severities. The simulation also showed that the most power drop occurred under the compressor fouling by about 8.7% while the most reduction in thermal efficiency is observed under recuperator fouling by about 7.84%. Furthermore, the investigation showed the maximum decrease in the surge margin induced by the compressor fouling during the lower part-load operation by about 45.7% while in the higher loads created by the turbine fouling by about 14%.

Experimental Investigation of a Split Air Conditioning Using Condensate as Direct Evaporative Cooling

Split air conditioning systems produce reasonable amount of condensate which is usually not utilized and thrown away to the environment. On the other hand, it consumes a lot of energy during operation. The aim of this study is to investigate the improvement of air conditioning systems performance utilizing condensate. A direct evaporative cooling using condensate is incorporated on a 0.74 ton-cooling capacity of split air conditioning to decrease the air temperature before entering the condenser. Performances of the split air conditioning with and without direct evaporative cooling are compared and presented in this paper. The results show that the use of direct evaporative cooling using condensate into the air before passing through the condenser reduces the compressor discharge pressure. The decrease of the condenser pressure led to 4.7% and 7% reduction of power consumption for air conditioner without cooling load and air conditioner with 2000 W cooling load, respectively. The cooling effect and coefficient of performance (COP) increase with the decrease of compressor power. The use of direct evaporative cooling with condensate into the air before entering the condensing system can enhance the system performance and protect the environment.

The effects of the opening of the electronic expansion valve in the high-stage cycle on the performance of a cascade heat pump water heater

The opening of the electronic expansion valve (EEV) in the high-stage cycle of a cascade heat pump not only affects the suction and discharge statuses of the compressor, but also regulates the intermediate temperature, thereby, determines the system performance. In this study, we conducted experimental analyses on a cascade heat pump water heater that was equipped with an EEV in the high-stage cycle (high-stage EEV) and an inverter compressor in the low-stage cycle (low-stage compressor) to investigate the effects of the opening of the high-stage EEV on the suction and discharge statuses of the high-stage compressor and the system performance. Our results indicated that the optimum opening of the high-stage EEV maximized the coefficient of performance, and the increase in the frequency of the low-stage compressor from 150 to 225 Hz increased the optimum opening of the EEV from 37.5% to 50%. Moreover, the decrease in the opening of the high-stage EEV and the increase in the frequency of the low-stage compressor increased the heating capacity of the system by 4.88W/% and 30.14W/Hz, respectively. Furthermore, the opening of the high-stage EEV had little influence on the suction superheat of the high-stage compressor, specifically, the suction superheats ranged from 6.0 to 9.5 °C at all of the EEV openings (32.5–75.0%). Nevertheless, low EEV openings increased the discharge pressure of the high-stage compressor approaching the safe operating limit of the compressor. Hence, the control on the opening of the high-stage EEV should be aimed to optimize the system performance and reduce the discharge pressure of the high-stage compressor.

Performance modeling of Allam cycle integrated with a cryogenic air separation process

The performance of an integrated Allam cycle and a cryogenic air separation process is numerically simulated. In contrast to previous research which has primarily focused on the design of the cycle alone, this work accounts for the operational co-dependence of the cycle and a co-located air separation unit (ASU). Heat integration between the cycle and ASU includes adiabatic heat of the main air compressor (MAC), and booster compressor of the ASU overlooked in past studies. The discharge pressure of the recycled CO2 compressor is determined based on its saturation pressure at cycle minimum temperature. The cycle efficiency exhibits a maximum at a MAC discharge pressure of 4.2 bar whereas it consistently decreases with discharge pressure of the booster. A genetic algorithm optimization is applied to estimate an optimum operational point. A maximum efficiency of 59.7% is obtained at turbine inlet temperature, inlet pressure, and outlet pressure of 1500 K, 305.5 bar, and 28.1 bar, respectively.

Performance investigation of vapor compression refrigeration system using R134a and R600a refrigerants and Al2O3 nanoparticle based suspension

The improvement in performance parameters of refrigeration system using nanofluid has been investigated intensively for many years. The compatibility, stability and feasibility of nanofluid in refrigeration system have been quite challenging. The present experimental work involves the performance of Al2O3 nanoparticle based nanofluid in a vapor compression refrigeration system. The average diameter of 35 nm of Al2O3 nanoparticle with near-spherical morphology used for the experimentation. The lubricant oil of compressors has taken as the base fluid for the synthesis of Al2O3 nanofluid. The nanofluid has synthesized in the experiment of mass fractions of 0.02 wt%, 0.04 wt%, 0.07 wt%, and 0.1 wt% of Al2O3 nanoparticles. Two different types of refrigerant which are synthetic refrigerant (R134a) and hydrocarbon refrigerant (R600a) have used separately for the investigation. The polyester oil (POE) for R134a refrigerant and mineral oil for R600a refrigerant is incorporated in the experimental test rig. The coefficient of performance (C.O.P.), power consumption, compressor discharge pressure, evaporator pressure, and pull-down time were considered as performance parameters for the investigation of the vapor compression refrigeration test rig. The experimental results showed that by using 0.1 wt% of R600a-MO-Al2O3 mixture improves the C.O.P. by 37.2%, reduction in power consumption by 28.7%, increment in compressor discharge pressure by 8.9%, lowered evaporator pressure by 24.7%, and lowered the pull-down time by 17.6% compared to the conventional refrigeration system.

Inter-Stage Dynamic Performance of an Axial Compressor of a Twin-Shaft Industrial Gas Turbine

In this study, the inter-stage dynamic performance of a multistage axial compressor is simulated through a semi-empirical model constructed in the Matlab Simulink environment. A semi-empirical 1-D compressor model developed in a previous study has been integrated with a 0-D twin-shaft gas turbine model developed in the Simulink environment. Inter-stage performance data generated through a high-fidelity design tool and based on throughflow analysis are considered for the development of the inter-stage modeling framework. Inter-stage performance data comprise pressure ratio at various speeds with nominal variable stator guide vane (VGV) positions and with hypothetical offsets to them with respect to the gas generator speed (GGS). Compressor discharge pressure, fuel flow demand, GGS and power turbine speed measured during the operation of a twin-shaft industrial gas turbine are considered for the dynamic model validation. The dynamic performance of the axial-compressor, simulated by the developed modeling framework, is represented on the overall compressor map and individual stage characteristic maps. The effect of extracting air through the bleed port in the engine center-casing on transient performance represented on overall compressor map and stage performance maps is also presented. In addition, the dynamic performance of the axial-compressor with an offset in VGV position is represented on the overall compressor map and individual stage characteristic maps. The study couples the fundamental principles of axial compressors and a semi-empirical modeling architecture in a complementary manner. The developed modeling framework can provide a deeper understanding of the factors that affect the dynamic performance of axial compressors.

Experimental Assessment of the Thermal Performance of a Heat Pump Dryer System Based on the Variations in Compressor Discharge Pressure on Oregano Drying

The current study shows an empirical analysis to establish the effects of the variations in compressor discharge pressure on the drying performance of aromatic herbs, in terms of the coefficient of performance (CoP), moisture content (MC), specific moisture extraction rate (SMER), drying temperature, drying time and energy consumption. In conducting the research, a heat pump drying system was utilized as a mechanism for dehydrating herbs, seeds, and fruits. It was used thanks to its benefits like higher efficiency and its low power consumption. Three levels of discharge pressure were considered, 1380 kPa, 1100 kPa, and 827 kPa, using 1,1,1,2-tetrafluoroethane (R134a) as a refrigerant and oregano leaves as the main product. The findings show that, concerning the same oregano moisture sample, the lower the compressor discharge pressure, the lower drying temperature, also, the higher drying time was obtained. Despite the fact that the CoP decreased with the compressor discharge pressure, in comparison with the baseline case, it remained essentially the same for the other two cases.

A Mathematical Model of Intermittent Gas Lift in Elevation-Production Operation with Line-Pack and Line-Drafting Phenomena in a Gas Line

This paper discusses a transient model of the intermittent gas lift technique in an oil well. The model is developed in the gas line, in the tubing-casing annulus, and the tubing. The line-pack and line-drafting phenomena in the gas line are considered in the model. A numerical approach will be used to solve the mathematical model that represents fluid flow during intermittent gas lift injection. The dynamics of important variables in the intermittent gas lift are investigated and analyzed to determine the best production strategy for intermittent gas lift. The variables are film thickness and velocity, slug height and velocity, and gas height and velocity. The relationships between surface injection control parameters (gas injection pressure and gas injection rate) and the velocity and height of film, gas, and liquid are shown in one cycle of the gas lift intermittent process. The higher the gas injection pressure, the faster the gas injection velocity, and the thinner the film thickness in the tubing. In order to obtain clean tubing from film thickness, the gas injection pressure needs to be optimized, which will lead to maintaining compressor discharge pressure availability. Detailed observation of the dynamic performance inside the tubing production well will give the optimum oil production rate for oil wells under a gas lift intermittent production strategy for field application.

Investigation of performance improvement of a household freezer by using natural circulation loop

Growing number of household refrigerators and freezers also increase the overall energy consumption and number of household refrigerators have grown nearly 3% per year. On the other hand, investigations showed that household refrigerators have a significant technical energy saving potential of up to 30–40%. In this study, a single-phase natural circulation loop is considered as a suction line heat exchanger between the high and low pressure sides of conventional refrigeration cycle of a household chest freezer. The potential heat transfer by the single-phase natural circulation loop is studied by a CFD analysis, considering the possible temperature range for refrigeration cycle and geometrical parameters that affect the loop performance. The performance map of this refrigeration cycle is presented for different compressor suction and discharge pressures (LP/HP), discharge temperatures (T2) and different compressor speeds. The results indicate that the highest amount of heat transfer is obtained for the single-phase natural circulation loop with aspect ratio of 0.8 and pipe diameter of 6.53 mm. Moreover, the single-phase natural circulation loop provides 11.5% efficiency improvement for the household freezer for maximum compressor speed at lowest T2 and LP/HP. Considering reduction in required compressor work, SPNCmL assisted refrigeration system has a wider operating range.

Energetic and exergetic studies of modified CO2 transcritical refrigeration cycles

Abstract Thermodynamic analysis including energetic and exergetic analysis is carried out employing Engineering Equation Solver for the five modified cycles: dual expansion cycle, internal heat exchanger cycle, work recovery cycle, work recovery with internal heat exchanger cycle and vortex tube expansion cycle. Contours are developed to study the effect of gas cooler temperatures and evaporator temperatures on the system performance and optimum gas cooler pressure. The modified cycle with work recovery turbine offers relatively higher COP and higher exergetic efficiency with lower compressor discharge pressure. The exergy loss in compressor, gas cooler, throttle valve and vortex tube (VT) are considerably higher than that in internal heat exchanger (IHX), evaporator and turbine. It is observed that COP of modified cycle with VT is slightly less than that with IHX, whereas the cycle with work recovery turbine brings the highest COP with the improvement of 25% at the gas cooler exit temperature of 305 K and evaporator temperature of 248 K.

Proposal of a new parabolic solar collector assisted power-refrigeration system integrated with thermoelectric generator using 3E analyses: Energy, exergy, and exergo-economic

In this study, a new parabolic solar collector integrated combined power and refrigeration system with the thermoelectric generator (CPR-PTSC with TEG) was proposed. The suggested system was evaluated through energy, exergy, and exergo-economic analyses, and the effect of the TEG on the power generation performance was investigated. The influences of significant parameters such as pressure ratio of supercritical Brayton cycle (BC), length of parabolic solar collector, turbine inlet pressure (TIP) of transcritical Rankine cycle (RC), compressor discharge pressure (CDP) of vapor compression refrigeration cycle (VCRC) were determined. According to the results, it was found that the integration of the thermoelectric units to the system improved the exergy performance of the system with an exergy destruction rate of 803.4 kW, where it was calculated as 821.38 kW for the system without TEG. The result of parametric analyses represented that the various system parameters have the same effects on the performance of both CPR-PTSC with and without TEG. Furthermore, with the exergo-economic analysis of CPR-PTSC with TEG, the cost of generated power for TEG I and II were determined to be 0.6311 US$/h and 2.202 US$/h, respectively, and the cooling costs were calculated as 0.994 US$/h and 0.7462 US$/kWh.

FRIB helium refrigeration system commissioning and performance test results

At its maximum capacity condition, the 4.5 K refrigerator system for the Facility for Rare Isotope Beams (FRIB) accelerator supports a 180 g/s 30 K 1.16 bar cold compressor return flow, a 14 g/s 4.5 K liquefaction load, a 4 kW 4.5 K refrigeration load, and a 20 kW 35-55 K shield load. Five additional design conditions, ranging from liquefaction to refrigeration and a stand-by/reduced load state, were specified for the sizing and selection of its components. The cold box system is comprised of a 300-60 K vertical cold box that incorporates a liquid nitrogen pre-cooler and a 60-4.5 K horizontal cold box housing seven turbines that are configured in four expansion stages including one Joule-Thompson expander. This cold box system, operates using the Ganni-Floating pressure process, automatically adjusting to the linear accelerator (Linac) load with the cold box supply (and compressor discharge) pressure varying from 6 to 21 bar, without introducing additional (artificial) loads or throttling turbine inlet valves (or other exergy loss mechanisms), and with minimal liquid nitrogen usage. This paper will briefly review the salient 4.5 K system design features and discuss the recent commissioning results.

Experimental study on the performance of air source heat pump system with multiple parallel outdoor units

This paper presents a novel air source heat pump for heating of buildings named air source heat pump with multiple parallel outdoor units (ASHPMO). Multiple outdoor units were connected in parallel with the aim of realising alternate defrosting and uninterrupted heating simultaneously. An experimental apparatus of the ASHPMO system was developed. The defrosting performance was experimentally investigated under different outdoor air temperatures, outdoor air relative humidity, and condensation temperatures, among other factors. The test results showed that the novel ASHPMO system could provide continuous heating when defrosting even under an outdoor air temperature of −10°C. Variations in compressor vapour suction and discharge pressure and temperature were observed. The minimum heating capacity could still reach 60% of that without defrosting. Under the defrosting condition with outdoor air temperature −10°C, both the heating coefficient of performance (heating COP) and total energy efficiency ratio (EER) of the system can reach to 2.0 and 2.32, respectively.

Thermo-economic assessment of molten carbonate fuel cell hybrid system combined between individual sCO2 power cycle and district heating

A molten carbonate fuel cell (MCFC) is a high-performance electric power generation system, but much research is still required to develop technologies to enhance its power efficiency, because the exhaust gases of an MCFC contain sensible energy. The high level of heat energy in the exhaust gases leads to a poor performance, with an increased electricity rate and decreased efficiency. This study investigated the supercritical carbon dioxide cycle with three different configurations as a bottoming cycle using the exhaust gases of an MCFC stand-alone system and compared the supercritical carbon dioxide cycle with a combined heat and power system in terms of economical operational strategies. The thermodynamic characteristics of each cycle were determined by varying the turbine inlet pressure and isentropic efficiency of the turbomachinery. The results of an analysis found that optimal discharge pressure of compressor, are remarkable parameter for supercritical carbon dioxide cycle. Furthermore, the net power efficiency was increased (3.41%–4.6%) compared to that of the MCFC stand-alone system. In addition, the levelized cost of electricity values for the supercritical carbon dioxide cycle and combined heat and power system were calculated, and sensitivity analyses were conducted for factors such as the heating cost and major equipment cost. The results showed that using the supercritical carbon dioxide cycle as the MCFC bottoming cycle was more economical when the heating cost was less than $28/Gcal and the printed circuit heat exchanger cost was less than $100/kW.