Increasing the Stable Operating Range of a Fixed-Geometry Variable-Speed Centrifugal Compressor

Centrifugal compressors used on water-cooled chillers require stable operation over a wide range of flows at greatly varying pressure ratios. These operational requirements are dictated by variations in cooling demand and ambient conditions. Variable-speed centrifugal compressors are known to maintain their peak efficiency at the varying operating conditions much better than fixed-speed compressors. Replacing a fixed-speed centrifugal compressor with a variable speed one can reduce the annual energy consumption of a chiller by 40-45%. The majority of centrifugal chillers sold today are therefore inverter driven. Lower speed operation maintains and sometimes even increases compressor efficiency along a wide band of capacity and head combinations which fits quite naturally with most of the chiller operating requirements. However, the variable speed compressor will eventually surge when forced to operate at lower capacity while maintaining head. Some variable-geometry compressor features are necessary to enable stable compressor operation at these conditions. Variable-geometry inlet-guide-vanes and/or variable-geometry diffusers have to be added to variable speed centrifugal compressors to allow stable operation at all possible centrifugal chiller operating conditions. The inherent mechanical complexity of variable-geometry hardware has a negative effect on compressor cost and reliability. What is less appreciated is that compressor efficiency also suffers from variable geometry hardware. The inlet guide vanes introduce additional flow blockage and frictional losses at compressor inlet while the clearances needed for the movement of the variable geometry diffuser hardware introduce flow leakage passages resulting in parasitic flow leakage losses. Moreover, these losses affect compressor performance under all operating conditions, even those where variable speed control without variable geometry flow passage reduction results in stable compressor operation. This paper describes the application of the newly developed IntraFlowTM technology on a recently introduced two-stage variable-speed centrifugal refrigeration compressor. The concept will be explained in detail and test results will be shown. The compressor is stabilized and surge is postposed by injecting a small amount of flow upstream of the throat area of the vaned diffuser of the first stage compressor. The increase in stable operating range using this technique is substantially larger than what can be obtained with variable geometry inlet guide vanes. Using this technology the compressor also achieves higher efficiency due to the elimination of the blockage, friction and leakage losses that accompany the variable mechanical geometry surge/capacity control concepts. The amount of flow to be injected is controlled by an externally mounted flow control valve which increases reliability and serviceability

Comparing R1233zd And R245fa For Low Temperature ORC Applications

The majority of recently introduced ORC systems use R245fa as working fluid. The lower saturation pressures of R245fa versus R134a allowed the use of existing HVAC electric motors, compressors, evaporators and condensers after minor modifications as ORC electric generators, turbines/expanders, boilers and condensers. At elevated saturation temperatures R245fa turbine/expander equipment matches the power density of R134a HVAC compressors equipment. Refrigerants with still lower saturation pressures such as R123 and the newly developed low GWP fluid DR2 are excellent ORC working fluids but lack the synergy that exists in terms of power density between existing R134a compressors and their R245fa turbine derivatives. The use of these fluids in ORC applications prevents the use of existing HVAC compressor/motor hardware. Recently, a new refrigerant R1233zd has been introduced as a low GWP alternative for R245fa. This paper analyses the effect of this new fluid as a drop-in for R245fa into an existing 75 kW variable-speed, oil-free low temperature ORC system. The somewhat lower saturation pressure and vapor density of R1233zd allows a somewhat higher boiling temperature at maximum operating pressure, resulting in 8.7% higher cycle efficiency at equal capacity

Using Magnetic Bearing Orbit Information to Maximize Centrifugal Compressor Efficiency at Off-Design Conditions

Active magnetic bearings used on oil-free centrifugal refrigeration compressors have lower stiffness than conventional oil-lubricated journal or rolling element bearings. The lower stiffness of these bearings makes them sensitive to internal flow instabilities that are precursors of rotating stall or compressor surge. At operating conditions far away from surge the internal flow is very stable and the magnetic bearings keep the shaft centered, resulting in a minimal bearing orbit. The internal flow instabilities that arise when the compressor approaches the surge limit result in some radially fluctuating forces on the shaft. The active magnetic bearings correct for these fluctuating radial forces on the shaft. The bearing orbit increases with the size of these radial forces. Optimum compressor efficiency occurs close to surge at incipient stall conditions when maximum internal flow pressure recovery occurs. The positional feedback system of the active magnetic bearing control loop system indicates the bearing orbit which relates to compressor efficiency. At high flow conditions capacity is controlled by compressor speed for the imposed pressure ratio. At low flow conditions a combination of variable speed and a means of range extension (e.g. inlet guide vanes, variable geometry diffuser, or flow recirculation) are required to control capacity at the imposed pressure ratio and guarantee stable compressor operation. The bearing orbit signal can be used to determine which speed/geometry combination gives the highest compressor efficiency. If the bearing orbit is below a minimum value the compressor runs too close to choke and a reduction in speed combined with an opening of the diffuser throat area would increase compressor efficiency. If, on the other hand, the bearing orbit is above a maximum value the compressor runs too close to surge and an increase in speed combined with a closing of the diffuser throat area is required to increase compressor efficiency and guarantee surge-free compressor. Experience with this control scheme will be illustrated for a newly developed 350 ton two-stage centrifugal compressor where the variable geometry hardware is replaced with a controlled internal flow recirculation

Early-time free-surface flow driven by a deforming boundary

AbstractWhen a solid boundary deforms rapidly into a quiescent liquid layer, a flow is induced that can lead to jet formation. An asymptotic analytical solution is presented for this flow, driven by a solid boundary deforming with dimensionless vertical velocity $V_{b}(x,t)={\it\epsilon}(1+\cos x)\,f(t)$, where the amplitude ${\it\epsilon}$ is small relative to the wavelength and the time dependence $f(t)$ approaches 0 for large $t$. Initially, the flow is directed outwards from the crest of the deformation and slows with the slowing of the boundary motion. A domain-perturbation method is used to reveal that, when the boundary stops moving, nonlinear interactions with the free surface leave a remnant momentum directed back towards the crest, and this momentum can be a precursor to jet formation. This scenario arises in a laser-induced printing technique in which an expanding blister imparts momentum into a liquid film to form a jet. The analysis provides insight into the physics underlying the interaction between the deforming boundary and free surface, in particular, the dependence of the remnant flow on the thickness of the liquid layer and the deformation amplitude and wavelength. Numerical simulations are used to show the range of validity of the analytical results, and the domain-perturbation solution is extended to an axisymmetric domain with a Gaussian boundary deformation to compare with previous numerical simulations of blister-actuated laser-induced forward transfer.The authors gratefully acknowledge financial support for this research from the Na- tional Science Foundation MRSEC program through the Princeton Center for Complex Materials (grant DMR-0819860).This is the accepted manuscript. The final published version is available from CUP at http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=9571518&fileId=S0022112015000749

Een leer voor het openbaar bestuur gevraagd

The Concept for a new Constitution’ gives deplorably few guarantees for the organizational unity of administration, which is so greatly needed in our country. From an organizational point of view, it is a fatal misunderstanding to put together under one title ‘Legislation’ and ‘Administration’, two such totally different public institutions as parliament and government. We need a clear, separate regulation of our administration, which would enable us to use modern and more efficient administrative techniques. Especially the hierarchical principle should be followed more consistently than it has been up till now: one officeholder, i.e. the prime minister, should be constitutionally responsible for the good operation of our administration. A separate chapter of the constitution should be called ‘Administration’; the first articles of this chapter could be: 1. the prime minister is head of the administration; 2. the organization and operation of the administration are established by law. The author sees no need to have the general principles of administrative law established by law, as proposed, in addition to general rules concerning the operation of the administration. There is a desperate need for a doctrine for the modern democratic administration, based on law, administration studies and practice