Compressor Start-up Research Articles

Study on start-up characteristics of a heat pipe cooled reactor coupled with a supercritical CO2 Brayton cycle

The new megawatt nuclear power system coupling a heat pipe cooled reactor with a supercritical CO2 Brayton cycle is considered one of the most promising energy conversion systems for large Unmanned Undersea Vehicles (UUVs). This system combines the characteristics of a heat pipe cooled reactor and a S-CO2 Brayton cycle, such as heat pipe start-up limitations and significant thermal inertia. It is unclear whether existing start-up control strategies for either heat pipe cooled reactors or S-CO2 Brayton cycle are suitable for this new system. Therefore, the start-up characteristics and the control strategies of the new megawatt nuclear power system are investigated in this study. A simulation code is developed for the study of the start-up characteristics of this system, which includes models for the reactor, heat pipes, and the S-CO2 Brayton cycle. The validation of the code is conducted based on the experiments as well as the design values. Good agreement between the calculation results and the experimental values or the design values are obtained. And then, the control scheme and start-up strategy of the new system are proposed. The start-up process is divided into four sub-stages, that is, core start-up from cold to critical state, heat pipes start-up, compressor start-up and turbine preheating, and branch switching. Different control strategies are applied to each sub-stage. The results show that the shaft raising rate of 2000 rpm/min is effective in overcoming expansion resistance without causing excessive overshoot in mass flowrate. This control strategy can achieve stable start-up of the new system without exceeding a temperature change rate of 10 K/min. This research may serve as a reference for the conceptual design of this new megawatt nuclear power system.

A machine-learning method to accurately recognize the leakage pressure-drop signals in trunk natural gas pipelines

A block valve installed along a trunk natural gas pipeline can automatically shut down when it detects a pressure-drop rate that is over a threshold value for a specified duration, indicating that leakage accidents have occurred. However, current methods have difficulties on identifying the differences among the pressure-drop rate signals caused by small-hole leakage, compressor startup, and block valve emergency shut-down conditions. This usually leads to improperly shutting down the block valve. Based on an optimized support vector machine, a machine-learning method is proposed to recognize the pressure-drop rate signals through the eigenvalue of the pressure-drop curve instead of only using the pressure-drop signal and its duration. Firstly, the singular decomposition method and the support vector machine method are applied to initially classify the pressure-drop rate signals of three operating conditions: the gas leakage, compressor startup, and block valve emergency shut-down conditions. Secondly, a teaching-learning-based optimization method is applied to optimize the penalty factor and kernel function parameter in the support vector machine model. Particularly, the tent chaotic maps and adaptive inertia weight methods are applied to improve the teaching-learning-based optimization method to balance its local and global search performance. Thirdly, an improved teaching-learning-based support vector machine method is established. Finally, 960 sets of simulated pressure-drop signals taken from three trunk natural gas pipelines and pressure-drop rate signals collected from an actual pipeline are applied to verify the accuracy of the proposed model. The results show that the improved teaching-learning-based support vector machine model achieved high classification and recognition accuracy. Specifically, it achieved accuracy rates of 99.4%, 100%, 98.3%, and 100% for the pressure-drop rate signals generated by pipelines A, B, C, and Cangzhou branch in the presence of pipeline leakage (the leak hole diameter is from 25 to 125 mm). Additionally, it demonstrated an average recognition accuracy of 97.42% for the pressure-drop rate signals generated by pipelines under other operating conditions. Through cooperating with Supervisory Control And Data Acquisition system, this method provides a more relevant approach to determine the shutdown conditions of a block valve while preventing mistaken actions.

A novel experimental rig to investigate the effect of the refrigerant on the oil supply of a variable capacity reciprocating compressor

The oil pumping plays a key role in the reliability and efficiency of compressors adopted in refrigeration systems. However, the investigations available in the literature on this subject do not consider the presence of the refrigerant and its effects on the oil supply. This paper reports the development of a novel experimental rig to measure the flow rate provided by the oil pump of a hermetic variable capacity reciprocating compressor considering the presence of the refrigerant in contact with the lubricating oil. A visualization system was also developed to investigate the transient outgassing phenomenon that takes place in the oil supply system when a sudden depressurization occurs, such as during the compressor startup. Measurements were carried out with the AB ISO VG 5 oil and the R-600a as the refrigerant fluid under different pairs of evaporating and condensing temperatures and compressor speed. Considering that the AB ISO VG 5 oil and air are immiscible, tests were also performed with air to isolate the effect of the refrigerant fluid on the oil supply. The results show that the oil flow rate is decreased due to the presence of refrigerant, with this effect becoming slightly stronger as the evaporating temperature is increased. The visualization system revealed that the outgassing brought about by the solubility between oil and refrigerant fluid has the potential to temporarily interrupt the oil supply.

Correlation to predict conditions that lead to liquid-flooding at compressor start-up as a function of evaporator size and fluid properties

Refrigeration cycles with fixed speed compressors cycle on and off to modulate their time averaged cooling capacity. While the cycles are turned off, refrigerant can migrate from the condenser into the colder evaporator. The next start-up process can then pull liquid refrigerant out of the evaporator and into the compressor causing liquid flooding. This phenomenon results in a loss of latent heat for cooling, can undercool the lubricant, and may cause damage to the compression mechanism. The geometric and thermophysical conditions that lead to liquid flooding are poorly understood. Experimental data collected for four refrigerants and three heat exchangers was analyzed for flooding conditions. A correlation was fitted to the experimental data, predicting the experimentally determined charge threshold for flooding with an average deviation of 6.6% and a maximum deviation of 16.7%. Repeating the tests with other heat exchanger types and smaller or larger compressors is suggested as important future work.

Liquid Flooding From an Evaporator Upon Compressor Start-up in Microgravity

Liquid Flooding From an Evaporator Upon Compressor Start-up in Microgravity

Experimental Investigation on Intermittent Operation Characteristics of Dual-Temperature Refrigeration System Using Hydrocarbon Mixture

An experimental rig of a zeotropic mixture separation condensation-based dual-temperature refrigeration cycle is built and the mixture R290/R600a is used as the refrigerant. Compared with a conventional cycle, the proposed refrigeration system demonstrates its application advantages under an on–off operation mode. Furthermore, the on–off periodic operation behaviors of this refrigeration system are experimentally investigated. The influence of a different refrigerant charge, a refrigerant mass fraction, a throttling valve opening, and ambient temperature are explored to evaluate the cyclic operation characteristics. The results reveal that the compressor average power for the duration of the compressor startup increases and the compressor duty cycle first declines then increases with the rise of the refrigerant charge. The average compressor during an on-period decreases from 104.5 W to 79.2 W and meanwhile the compressor duty cycles fluctuates between 72.1% and 96.9% as the R600a-charged concentration increases from 30% to 70%. The average power of the compressor during the on-period and the duty cycle are also sensitive to the freezer valve opening variation. Thus, the minimum energy consumption of 1.60 kWh·24 h−1 is achieved at the refrigerant charge of 300 g, a R600a-charged mass fraction of 50%, and a freezer throttling valve opening of 10%. A higher ambient temperature deteriorates heat transfer during condensation and increases the cabinets’ heat load, the compressor duty cycle, and eventually affects the daily power consumption. Generally, the present study offers an in-depth insight of cyclic operation characteristics of a separation condensation-based hydrocarbon mixture dual-temperature refrigerator under two parallel evaporators’ concurrent cooling process.

Transient start-up of an electric swashplate refrigeration compressor

• An easy to implement transient swashplate compressor model is developed. • Mechanism inertia is included to evaluate motor torque loading during start-up. • The start-up behaviour is tracked experimentally using a high-speed data logger. • Inertia is found to have only a small effect on the compressor torque and power output. • Supply lines pressure are found to have the largest influence on the staring torque. The design of compact, high performance electric swashplate refrigeration compressors demands a clear understanding of different physical phenomena and their interactions taking place inside the compressor. The dynamic characteristics of the compressor are associated with the start-up transients of the swash-plate mechanism and the time variation of suction and discharge pressures. An experimentally validated, easy to implement transient swashplate compressor model has been developed that can capture the essential physics, including inertia of the pistons and swashplate to evaluate the electric motor torque loading during compressor start-up. The effects of moment of inertia, bearing torque, viscous resistance to piston motion, and suction and discharge pressures on the torque and compressor mechanical input power are investigated. For model validation, the start-up behavior is tracked experimentally using a high-speed data logger to monitor the changing phase currents of the brushless DC motor, capturing both the instantaneous power and rotational speed. Rotational mass moment of inertia is found to have only a small effect on the compressor torque and power output and can be made negligible by changing settings in the start-up algorithm for the electric motor controller. Suction and discharge pressures during start-up are found to have the largest influence on the required starting torque. More than 95% of torque is found to be because of the line pressures. Predictions are in good agreement with measurements and show that depending on the starting refrigerant pressures in the supply lines, the starting torque can be lower than the operating torque for the compressor. The original contribution of this work is in deriving a transient swash-plate compressor model that includes the inertia of the swash-plate mechanism and in clarifying the relative importance of inertia, line pressures, viscous losses and bearing resistance on the required start-up torque for this type of compressor.

Simplified approach for simulating hermetic compressor startup regime

It is presented a hermetic compressor startup simulation procedure considering a simplified approach to obtain the transient data of different compressor parameters. The main numerical technique is a four order Runge-Kutta procedure to numerically integrate the systems of ordinary differential equations, which describe the variables’ variation with time in each part of the compressor. The simulations were performed in the Windali software. The results show how the oil viscosity influences the electrical motor torque, the compressor load and crankshaft angular velocity with time for two electric motor input voltage values. The increase of oil viscosity by a factor of 2, retards the compressor startup for more than one second. The presented simple model could be valuable for studying the geometrical and operational influence on compressor startup.

Применение метода анализа «компрессор-сеть» для системы с поршневым компрессором и линией рециркуляции

The subject of research was an opposed reciprocating compressor (RC) providing gas injection into the underground gas storage. Issues concerning implementation of compressor start-up conditions to recirculating line have been considered. As a tool, the compressor-network analysis method is used, which consists in determining the system operating points through matching characteristics of the reciprocating compressor and the network, for which in this case the recirculating line with the control valve is considered. Recommendations have been received to select a control valve type for the reciprocating compressor operating in both single-stage and two-stage modes.

Economically Optimal Control of a Cold Room Using an Artificial Neural Network and Dynamic Programming

We consider the economically optimal control of a cold store with a single cold room. The thermal inertia of a cold room acts as an energy storage and can therefore be used for economic optimization in the presence of a dynamic electricity price, under a bounding constraint on the internal temperature of the cold room. However, a high number of frost production startups may induce premature wear of the cold store’s compressors. Since the thermal losses are a function of the internal temperature of the cold room, conventional inventory management solving techniques are not suited for this problem. In this paper, we use an artificial neural network as temperature forecast. A dynamic programming algorithm is used to solve the model that includes the non-linear artificial neural network temperature forecast and a fixed cost at each compressor startup. This allows us to solve industrial instances of the problem optimally and within reasonable time. We show the interest of solving the problem optimally as opposed to using a conventional hysteresis-based control method, and discuss the opportunity of using an dynamic hourly price based on the electricity market instead of a traditional contracted price.

Experimental performance assessment of a hermetic reciprocating compressor operating in a household refrigerator under on–off cycling conditions

This paper reports a study on the performance of a hermetic reciprocating compressor operating in a household refrigerator. The refrigerator was provided with sensors to measure the mass flow rate and evaporating and condensing pressures, while the compressor was instrumented with thermocouples in selected components and pressure transducers to measure the p-V diagram. Tests were conducted at ambient temperatures of 32 °C, 25 °C and 16 °C with different freezer cut-off temperatures (−10 °C, −16 °C and −19 °C). The phenomena that affect the mass flow rate and power consumption were evaluated during typical on-periods. It was observed that the thermodynamic efficiency of the compressor varied by 5–10% during the first few minutes after the compressor start-up, but remained virtually constant throughout the remainder of the on-period. The results showed that the volumetric and thermodynamic efficiencies tend to increase at lower ambient temperatures, while the overall efficiency of the compressor remains practically constant. Additionally, the compressor average temperatures and efficiencies collected from the application tests were compared to data obtained with calorimeters, revealing that the two sets of data do not differ much from each other.

Automated identification of a complex storage model and hardware implementation of a model-predictive controller for a cooling system with ice storage

Future sustainable energy systems could increase the share of energy converted from fluctuating renewable energy sources by intelligent model-based predictive control of cooling systems with thermal energy storage. This study investigated an experimental cooling system comprising a compression chiller and an ice storage. A runtime-efficient predictive model for partial charge and discharge of ice storage was derived. In addition, techniques for automatic model determination and adaptation were introduced and examined. The experimental setup involved the development and implementation of a model-predictive controller (MPC) to minimize operating expenses under dynamic electricity pricing based on a forward dynamic programming algorithm. The objective function included energy charges, compressor start-up costs, and terminal costs that depended on the state of charge and state of the chiller at the end of the optimization horizon. Three examples of cases validated and compared the advantages of the MPC over an open-loop (day ahead) optimal control concept. The cases examined the influence of temperature and load forecast inaccuracy, and investigated the coping mechanism of the system to sudden updates involving price and temperature predictions. The findings illustrated that the MPC achieved significant savings of operating expenses when compared with the open-loop optimal control concept.

Generic Approach of Using Dynamic Simulation for Industrial Emission Reduction under Abnormal Operations: Scenario Study of an Ethylene Plant Start-up

Flare minimization under normal and abnormal operating conditions in large-scale industrial processes, especially for refineries and petrochemical plants, is a double-win practice, which simultaneously benefits industrial and environmental sustainability. Unfortunately, proactive and cost-effective flare minimization (PCFM) approaches under abnormal situations are still lacking. In this study, a PCFM approach for an ethylene plant is presented for its start-up operations. This approach employs rigorous steady-state and dynamic models of a front-end deethanizer ethylene plant to serve as a foundation to explore flare root causes during plant start-ups and subsequently a test bed to support both design and operational strategies for start-up flare minimizations. It has been demonstrated that the charge gas compressor (CGC) start-up is the most critical operation, which results in the largest amount of flaring sources. Several start-up strategies at different CGC feed rates and compositions, including scenarios recycling off-spec C 2s and C 3s from downstream recovery units to the CGC inlet as a substitute of a portion of charge gas, are virtually examined to identify the least flaring one. This work contributes to evaluating new start-up strategies, predicting abnormal process dynamic behaviors, and acquiring more precise estimation of flare emission sources. The study shows that the plant flaring can be significantly reduced, while the start-up time is shortened.

Popping noise in household refrigerators: Fundamentals and practical solutions

An intermittent noise that occurs around 30 s after compressor start-up has been disturbing customers of different domestic refrigerator manufacturers over the past 10 years. This noise is very similar to that produced by popcorn popping, being referred to as popping noise. The most probable cause of this strange and unpleasant noise is thought to be the CIS (Condensation Induced Shock) phenomenon. The aim of this study was firstly to check the possible occurrence of CIS in a specific household refrigerator and, secondly, to identify ways to avert it without affecting the product performance. To this end some changes were made to the refrigerator: i) the diameter of the filter dryer was altered; ii) the slope of the filter dryer was altered; iii) the compressor was replaced; and iv) an additional internal heat exchanger was installed. The noise was effectively eliminated only by placing the filter dryer horizontally or by installing an additional internal heat exchanger. Visualization studies were also carried out, making it clear that the occurrence of the popping noise is closely related to the flow pattern at the capillary tube inlet.

Dynamics of gas bubble growth in oil-refrigerant mixtures under isothermal depressurization

This paper proposes a numerical model to predict the growth of gaseous refrigerant bubbles in oil-refrigerant mixtures with high contents of oil subjected to isothermal depressurization. The model considers an Elementary Cell (EC) in which a spherical bubble is surrounded by a concentric and spherical liquid layer containing a finite amount of dissolved liquid refrigerant. The pressure reduction in the EC generates a concentration gradient at the bubble interface and the refrigerant is transported to the bubble by molecular diffusion. After a sufficiently long time, the concentration gradient in the liquid layer and the bubble internal pressure reach equilibrium and the bubble stops growing, having attained its stable radius. The equations of momentum and chemical species conservation for the liquid layer, and the material balance at the bubble interface are solved via a coupled finite difference procedure to determine the bubble internal pressure, the refrigerant radial concentration distribution and the bubble growth rate. Numerical results obtained for a mixture of ISO VG10 polyolester oil and refrigerant HFC-134a showed that the bubble growth dynamics depends on model parameters such as the initial bubble and liquid layer radii, the initial refrigerant concentration in the liquid layer, the initial pressure in the liquid phase, the decompression rate and the EC temperature. Despite its simplicity, the model demonstrated to be a potential tool for predicting bubble growth and foaming that may occur as a result of cavitation in oil-lubricated bearings and refrigerant degassing from the oil sump during compressor start-up. Keywords: refrigeration compressor, oil-refrigerant mixtures, bubble growth, numerical modeling

Numerical modeling of startup and shutdown transients in reciprocating compressors

Several phenomena that affect energy consumption, noise level and reliability of compressors are associated with transient effects that occur during the compressor startup and shutdown. This paper presents a simulation methodology, experimentally validated, developed to analyze the compressor in such transients. Because the time scale associated with the compressor thermal behavior is much larger than that related to the mechanical physics, a thermal equilibrium condition has been considered herein for convenience. Results for valve displacement, piston motion, pressure in the compression chamber and resistive torques are provided to illustrate the application of the methodology and to assist an understanding of the physical aspects that affect the compressor performance throughout the startup and shutdown. It is observed that the dynamics of suction and discharge valves are greatly affected. Moreover, the model is employed to estimate the minimum voltage required for the compressor startup as a function of the equalized pressure and the auxiliary coil actuation time.

Analysis of oil pumping in a reciprocating compressor

The immediate provision of lubricant oil to the radial and thrust bearings and to the piston–cylinder gap in reciprocating refrigeration compressors is a crucial reliability issue following compressor start-up. The aim of this paper is to present a hydrodynamic analysis of the lubricant oil pumping system of a reciprocating compressor using computational fluid dynamics (CFD). In the present system, oil is pumped from the sump through a reed-type centrifugal pump located at the bottom end of the shaft. The oil flows initially as a climbing film on the internal surface of the shaft before it is directed to the external surface where it flows along a helix channel carved on the shaft wall. The model was implemented on the commercial package Fluent using the volume of fluid (VOF) method to resolve the free-surface oil flow. Parameters regarding the oil behavior during start-up, such as the time required to reach steady-state operation and the associated oil mass flow rate are explored in the manuscript.

Problems of normalizing the vibration state of gas pumping plant shafting for booster compressor stations

Work is presented and analyzed for vibration diagnostics and vibration adjustment using the example of the GPA-10DKS-04K Ural unit, developed and manufactured by NPO Iskra, on the site of DKS-3 Orenburggazprom. Recommendations are given for provision of a normal vibration state for shafting in the planning stage for units and compressor station equipment, factory tests, start-up and operation.

Refrigerant mass charge distribution in a domestic refrigerator, Part I: Transient conditions

Experimental results are presented of the refrigerant mass charge distribution in a steadily cycling domestic refrigerator. In detail it is shown how the charge is displaced at compressor start-up and shut-down. At start-up it was found that the charge was temporarily displaced towards the condenser before returning to a steady state distribution in the latter part of the on-period. As a result, initially the evaporator was starved with a lowered evaporation temperature and a peak 10 °C superheat. The superheat disappeared within 3 min as the evaporator was gradually refilled with refrigerant. At shut-down the pressure equalised within 3 min as refrigerant was pushed into the evaporator from the condenser. The losses due to charge displacements were estimated to 11% (capacity) and 9% (efficiency). Possible ways to reduce the losses are discussed.

Real time dynamic simulation using networked computers

Simulation of chemical plants in “real time” or faster, especially for training purposes, is a topic of increasing interest in the process industries.This is inevitably a computationally demanding task, and one where requirements for detail and realism are likely to increase indefinitely to just beyond the limits of cheaply available hardware. In principle, the use of distributed or networked low-cost personal computers or workstations offers, through economies of scale, the greatest computing power for unit hardware cost. Such an approach is also incrementally expandable. However, the effective use of the power available depends crucially on appropriate decomposition of the simulation problem. This must consider not just the computational load per machine, but also the communications load, the need for coordination and synchronization and the effect of these on the stability of the equation solution. Our experience with three diverse case studies: LPG processing, pressure swing adsorption and large compressor startup indicates that different strategies may be necessary in different circumstances. However, our conclusion is that the overall approach is feasible and can have some significant and unexpected advantages.