Ziwen Xing

Simulation of discharge pressure pulsation within twin screw compressors

Abstract Even in the absence of self-acting valves, flow through the discharge port of a twin screw compressor is oscillatory in nature. This unsteady but periodic flow variation at the discharge port excites the pressure pulsation, which is the main source of noise and vibration. In this paper, a mathematic model based on the one-dimensional unsteady gas flow equations is established to describe the discharge pressure pulsation, which considers the effects of friction and heat transfer between the gas and the pipe. The boundary conditions of the discharge pressure pulsation model are given in detail. The two-step Lax—Wendroff scheme is applied to solve the one-dimensional unsteady gas flow equations. In order to verify the theoretical analysis, the discharge pressure pulsations under various working conditions are measured and compared. It is shown that the model established in this paper is a useful tool to obtain a better understanding of the behaviour of the pressure pulsation in discharge pipe. It is found that the most important factor that affects the discharge pressure pulsation is the pressure difference between the pressure upstream of the discharge port in the compression chamber and the pressure in the discharge line some way downstream. The minimum fluctuation occurs when the discharge pressure is equal to the pressure corresponding to the built-in volume ratio. For a constant discharge pressure, the pressure pulsation amplitude increases with the rotational speed.

Analysis of the working process in an oil-flooded screw compressor by means of an indicator diagram

Abstract This paper presents the results of an experimental investigation of the thermodynamic processes in an oil-flooded screw compressor. The pressure within the working chamber is measured with a small pressure sensor, embedded in the female rotor on the discharge side. The results so obtained were transformed into an indicator diagram. Based on the indicator diagrams at various operating conditions, the working process is analysed. Owing to oil restricting back-flow of the gas through the discharge port, constant-volume compression is not evident even at substantial under-compression conditions, thus making the compressor maintain a high efficiency over a wide range of pressure ratios. However, the additional power consumption resulting from over-compression is comparatively large at pressure ratios lower than that for which the compressor was designed. So a compressor with fixed volume ratio should be designed with the built-in volume ratio low enough to avoid this effect. At the end of the discharge process, the pressure rises steeply due to increased resistance to oil flow, and therefore, the design of a flow-guiding slot to assist oil discharge is recommended. Higher rotating speeds increase the pressure slightly, but the losses associated with this are compensated by the increased volumetric efficiency. Thus a screw compressor maintains its isentropic efficiency over a wide range of speeds. Larger oil:gas ratio presents a higher pressure level in the indicator diagram, offsetting the improved sealing effect, and test results show that the appropriate oil:gas mass ratio range is between 8:1 and 20:1.

Development and field test of a high-temperature heat pump used in crude oil heating

In this paper, a high-temperature heat pump (HTHP) is developed and manufactured to replace the traditional oil-fired boiler heater for crude oil heating. It extracts thermal energy from waste hot water separated from the crude oil to provide high-temperature hot water to heat the crude oil. A prototype of the HTHP system is installed in the Jinzhou oil treatment station in Liaoning, China and the field test is conducted for about 6000 h. A typical 144 h of field testing data is analyzed to evaluate the performance of the designed HTHP system. It is observed that the temperature of hot water provided by the HTHP unit varies from 86 ℃ to 95 ℃ throughout the whole operating period and is sufficient for the crude oil heating (80–90 ℃). The heating capacity and power consumption of the HTHP system varies from 1350 to 1785 kW, and 171 to 197 kW, respectively. The overall system coefficient of performance ranges from 3.5 to 4.4 with an average value of 3.8. Based on the experimental results, a primary energy ratio is introduced to evaluate and compare the economics of the studied HTHP system and oil-fired boiler heater. The comparison shows that the energy consumed by the HTHP unit is only 57% of that consumed by the oil-fired boiler heater. If all traditional oil-fired boiler heaters are replaced by the HTHPs in the Jinzhou oil treatment station, the total yearly energy saving is around 1.12 × 104 tons of equivalent coal which equates to 1.76 × 104 tons of CO2 emissions.

Theoretical and experimental study on conveying behavior of a twin-screw multiphase pump

In this paper, the conveying behavior of a twin-screw multiphase pump is investigated when it pumps either pure water or gas–liquid mixtures with gas void fractions varying from 20% to 90%. A prototype of the twin-screw multiphase pump is developed and set up in a laboratory for this purpose. A theoretical model is established to evaluate backflow rates in the twin-screw pump clearances, total pump volume flow rates, and power consumptions at various pressure differences and gas void fractions. Results show that the predictions from the model agree well with experimental data. For pumping pure water, the power consumption increases by 45.3% when the pressure difference between inlet and outlet of the twin-screw multiphase pump increases from 0.6 to 1.0 MPa. However, the effect of the pressure difference on the total pump flow rate is negligible. For pumping air–water mixtures, the pressure difference has a significant effect on both power consumption and total volume flow rate of the multiphase pump. The pump power consumption increases by more than 40% when the pressure difference increases from 0.4 to 1.0 MPa. On the contrary, the pump volume flow rate decreases between 10% and 30% varying with the gas void fraction value. However, at the fixed pressure differences of 0.4 MPa and 1.0 MPa, the pump power consumption does not show much difference with the change of gas void fractions from 20% to 90% although the total pump volume flow rate reduces by 18.7% and 25.7%, respectively.

Experimental Analysis of Mass Composition of R417A in Presence of Leak/Recharge in a Heat Pump Water Heater

As a substitute for R22, an environmentally friendly zeotropic mixture R417A is often employed. The large glide temperature of the zeotropic mixture R417A could cause capacity and efficiency reductions due to changes in its composition after an isothermal vapor leak/recharge process. It is, therefore, necessary to predict the composition change in R417A under all leak conditions. This paper presents the experimental analysis of the mass composition of R417A in the presence of a 0–50% isothermal vapor leak/recharge in a heat pump water heater. Based on the experimental data, a simple model is proposed to describe the composition change in R417A for various leaks and recharge scenarios. The effect of composition change in the performance of a heat pump water heater is also dealt with. Experimental results show that the composition changes in R417A increase as ambient temperature decreases. Isothermal vapor leaks have a great effect on the composition of R417A and the performance of a heat pump water heater at lower temperatures. It is also found that when the ambient temperature is higher than 7°C with a 10% leak and R417A recharge, there is almost no difference in the performance of the plant with respect to the original state except that the compressor discharge temperature is lower.

Investigation of characteristics of discharge pressure pulsation in a twin-screw refrigeration compressor

In this paper, characteristics of discharge pressure pulsation in a twin-screw refrigeration compressor are investigated. A thermodynamic model is developed and validated using data from a comprehensive experimental study. This validated model is then applied to investigate effects of key parameters including condensing temperature, compressor rotational speed, super-feed pressure, part-load operation, and design parameters on the discharge pressure pulsation. The results showed that the discharge pressure pulsation was mainly due to periodic variations of mass and energy flow from the working volumes to the discharge chamber. As the condensing temperature increased or decreased from the design condition, the compressor was in either over- or undercompression leading to an increase in the amplitude of the pressure pulsation. The gas super-feed pressure could increase the pressure pulsation at condensing temperatures below the design value and reduce the pressure pulsation at condensing temperatures above the design value. The analysis also demonstrated that the pressure pulsation was lower at part-load conditions. However, the compressor rotational speed increased both the amplitude and frequency of the pressure pulsation. Theoretical analysis of design parameters indicated that a large discharge volume with a high number of lobes could lower the pressure pulsation. These analyses provide useful information for the compressor design and optimization.

Structural study on a swing compressor with no valves for air compression

This paper presents a novel swing compressor with no valves employed in air compression. The design and work principle of this swing compressor are introduced. And the effects of discharge port, exhaust cavity, and crank speed on the performances of this compressor are analyzed. Under the same operating conditions and dimensions, the size of exhaust cavity mainly depends on the discharge port width. Additionally, the exhaust cavity angle has a direct impact on the compression process, whether it is over-compression or under-compression. And the over-compression loss can be designed below 2%. Overall, the assessment of the structure shows that this novel swing compressor with no valves has the advantages of simple component geometries, light weight, and high efficiency, which makes it well suited to the requirements of air compression.