High efficiency steam ejector for desalination applications

Abstract

A high efficiency ejector for use in desalination applications includes a mixing duct, a boiler, a primary high pressure steam duct connected to a plurality of inner supersonic nozzles and a plurality of outer supersonic nozzles. The inner nozzles and the outer nozzles have axes that are canted radially with respect to each other, in an asymmetric arrangement or in a 2-dimensional geometric arrangement. The inner nozzles and the outer nozzles each communicates with the mixing duct. A secondary duct communicates upstream with the boiler, and downstream, the secondary duct communicates with steam exiting the inner nozzles and the outer nozzles upstream of the mixing duct.

Description

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a high efficiency steam ejector. More specifically, the present invention relates to a high efficiency steam ejector for use in water desalination by vapor compression distillation.

[0004] 2. Discussion of the Related Art

[0005] Vapor compression distillation systems have typically employed mechanical fans for low pressure systems, or either centrifugal or axial compressors for high pressure systems. Steam ejectors have also been used in high pressure systems of the “thermal compression” variety which normally use plant waste heat as the energy source. Mechanical compressors are costly to manufacture and involve close-tolerance seals that are both costly to manufacture and to maintain. Conventional steam ejectors that operate at high pressure are inefficient, relative to energy use, and particularly in terms of energy consumed per unit volume of fresh water produced in desalination applications.

[0006] Referring to FIG. 4, a conventional steam ejector 100 is illustrated. As high pressure steam flows through the “primary” convergent-divergent nozzle 102 its velocity increases to “local” sonic condition at the throat 104, and supersonic conditions in the divergent section 106, and its static pressure simultaneously decreases. This supersonic, low static pressure steam, enters the mixing chamber 108 and entrains the secondary stream entering from conduit 110, reducing its pressure by accelerating it to sonic conditions at the mixer entrance where a “sonic surface” or “sonic line” forms. This flow further accelerates to supersonic conditions within the mixer 108 itself. Through this pumping action arbitrarily low pressures can be obtained in the primary stream, upstream of the mixer entrance—which would be the boiler, or evaporator of the desalination distillation apparatus.

[0007] Within the mixer 108, the mixed, supersonic primary and secondary streams form a shock “train” and by a constant area, adiabatic, but not isentropic, process (Fanno Line) that is roughly equivalent to that occurring through a single, equivalent, “normal” shock, reach subsonic conditions. Total temperature is maintained constant while static pressure and temperature increase according to well-known thermodynamic rules. Beyond the constant area mixer, a divergent diffuser is used to increase the flow static pressure to either atmospheric, in many applications, or to the pressure needed to drive the condensation of the steam stream.

[0008] Pressure ratios, or the “compression ratio” of the ejector, of 3, 4 and higher can normally be achieved between the diffuser exit static and secondary entrance and pressure ratios (taken as the “total” pressure divided by the “static” pressure at the primary nozzle exit plane) of 10 or more are needed in the “primary,” or driver.

[0009] However, this type of ejector is not energy efficient, and therefore not suitable for energy-critical water desalination applications. This is so for two reasons:

[0010] The relatively high primary nozzle pressure ratio results in a highly supersonic combined stream in the mixer. This conditions result in relatively high total pressure losses (high entropy increase) through the shock train's process of transitioning from supersonic conditions. The result is that the “pressure recovery” (conversion of total pressure before the shock system to static pressure at the diffuser outlet) will be low due to energy losses to viscous (boundary layers and shocks are viscous phenomena) effects. However, it must be noted that such losses, manifested through pressure deficits, will also result in temperature increases above the ideal (loss-free or isentropic) predictions, and some of this temperature increase can be taken advantage of by proper design of the distillation cycle—by “regeneration,” or re-use of the energy “lost” from the (pressure) process.

[0011] A more important inefficiency issue is the characteristic of conventional ejectors to have a low mass flow ratio (primary-to-secondary). This results from the high primary pressure ratio (also measurable as the ratio of total pressure in the primary to static in the secondary entrance).

[0012] The steam ejector's energy consumption comes from the generation of steam needed for the primary or “driver,” first through vaporization of water to saturated steam (not useful for use in the primary, since increasing its speed and decreasing its pressure, and therefore temperature, would rapidly cause condensation and two-phase flow) and then superheating the steam to the primary nozzle exit design conditions. It is important to minimize energy consumption in the desalination process, therefore it is essential to design an ejector system that has a very low primary mass flow compared to the secondary, or very high mass flow ratio, which is achievable through a low pressure ratio.

[0013] Thus, there is a need in the art for an ejector having a low secondary stream pressure ratio, a low primary total pressure and a very low, although still supersonic, speed at the primary nozzle exit. Accordingly, it is an object of the present invention to provide such an ejector.

SUMMARY OF THE INVENTION

[0014] An ejector system includes a mixer section equipped with an exit diffuser, a primary or driver—duct and nozzle system, and a secondary duct. In the high efficiency ejector the primary includes a plurality of nozzles in either an asymmetric arrangement or a 2-dimensional one. In an asymmetric arrangement, the outer nozzles are canted radially inward and the inner nozzles are canted radially outward. In axial cross-sectional view, the inner nozzles are spaced between the outer ones. In a 2-dimensional arrangement the nozzles are similarly “interdigitated.”

[0015] The multiple primary nozzles' jets are interdigitated such that enhanced mixing, or “hypermix” effect, occurs by two mechanisms:

[0016] 1. Viscous interaction between the (contact surfaces, greatly increased by using multiple small nozzles, compared to use of one large one) of the primary and secondary streams, and

[0017] 2. Vortices formed between the adjacent jets by the shear components of their relative flow velocities. The vortices promote rapid mixing of the primary and secondary streams and reduce mixer length and viscous losses due to friction with the wall.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0018] The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of a specific embodiment therefor, especially when taken in conjunction with the accompanying drawings wherein the reference figures are utilized to designate like components, and wherein:

[0019] FIG. 1 is a schematic side view of an ejector system in accordance with the present invention;

[0020] FIG. 2 is a cross-sectional view taken along lines 2-2 of FIG. 1 and looking in the direction of the arrows;

[0021] FIG. 3 is a schematic side view of the ejector system according to FIG. 1, showing the source of the driver steam; and

[0022] FIG. 4 is a schematic side view of an ejector system in accordance with prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0023] Referring now to FIGS. 1-3, a high efficiency ejector system 10 in accordance with the present invention is illustrated. System 10 includes a source of high pressure, superheated steam 12 and a source 14 of steam (or other gas) to be evacuated or pumped down. High pressure steam 12 enters the primary duct 18 which is connected to inner primary nozzles 22, and outer ones 24 through another duct 20. The driver steam 12 enters the mixer through a plurality of interdigitated primary nozzles 22, where the outer nozzles 24 are canted radially inward, and inner nozzles 22 are canted radially out. The inner nozzles 22 and outer nozzles 24 are canted radially with respect to each other in an asymmetric arrangement (e.g., a cylindrical pipe) or in a 2-dimensional geometric arrangement (e.g., a rectangular channel or between flat walls). The steam from both nozzles 22, 24 mix by a combination of viscous and jet interaction effects, in a mixer 32, with the evaporator's entrained steam that composes the secondary flow 26. Secondary flow 26 is fluid that is typically entrained from a boiler or evaporator where seawater is converted to saturated steam and brine. Brine is pumped out as waste. The primary flow is the pumping flow that has the motive power for pumping secondary fluid.

[0024] In the illustrated embodiment five inner and five outer primary nozzles are shown. More specifically, or fewer nozzles may be used depending on system requirements.

[0025] Mixer 32 is a constant cross-section conduit within which the supersonic primary and secondary streams mix and shock down to subsonic conditions. Mixer 32 typically has a length equivalent to six to seven hydraulic diameters. The mixed primary and secondary flow streams enter the diverging diffuser section (which preferably has a divergence half-angle of not more than 7-10° to prevent flow separation) at subsonic speed where the static pressure is increased and the speed is reduced. Also, a parabolic section may be used to achieve a constant pressure gradient.

[0026] Thus, while there have been shown, described, and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions, substitutions, and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit and scope of the invention. For example, it is expressly intended that all combinations of those elements and/or steps which perform substantially the same function, in substantially the same way, to achieve the same results are within the scope of the invention. Substitutions of elements from one described embodiment to another are also fully intended and contemplated. It is also to be understood that the drawings are not necessarily drawn to scale, but that they are merely conceptual in nature. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto. All patents, patent applications, procedures, and publications cited throughout this application are hereby incorporated by reference in their entireties.

Claims

1. A high efficiency ejector for use in desalination applications, comprising: a mixing duct; a boiler; a primary high pressure steam duct connected to a plurality of inner supersonic nozzles and a plurality of outer supersonic nozzles, said inner nozzles and said outer nozzles having axes that are canted radially with respect to each other in an asymmetric arrangement, said inner nozzles and said outer nozzles each communicating with said mixing duct; and a secondary duct communicating upstream with the boiler, said secondary duct communicating downstream with steam exiting said inner nozzles and said outer nozzles upstream of said mixing duct.

2. The high efficiency ejector in accordance with claim 1, wherein said mixing duct communicates downstream with a condenser.

3. The high efficiency ejector in accordance with claim 1, wherein said inner nozzles are canted radially outwardly and said outer nozzles are canted radially inwardly.

4. A high efficiency ejector for use in desalination applications, comprising: a mixing duct; a boiler; a primary high pressure steam duct connected to a plurality of inner supersonic nozzles and a plurality of outer supersonic nozzles, said inner nozzles and said outer nozzles having axes that are canted radially with respect to each other in a 2-dimensional geometric arrangement, said inner nozzles and said outer nozzles each communicating with the mixing duct; and a secondary duct communicating upstream with the boiler, said secondary duct communicating downstream with steam exiting said inner nozzles and said outer nozzles upstream of said mixing duct.

5. The high efficiency ejector in accordance with claim 4, wherein said mixing duct communicates downstream with a condenser.

6. The high efficiency ejector in accordance with claim 4, wherein said inner nozzles are canted radially outwardly and said outer nozzles are canted radially inwardly.