Re: Ejector Design Calculation Software
Bernardine Batchelder
A steam ejector is a device that uses high-pressure steam to create a vacuum by entraining and compressing a lower-pressure gas or vapor. It is commonly used in industrial processes for evacuation, compression, and mixing of gases.
A steam ejector works on the principle of the venturi effect, where a high-speed jet of steam creates a low-pressure region by entraining and compressing the surrounding gas or vapor. This creates a vacuum that can be used for various purposes.
The key components of a steam ejector include a motive nozzle, suction chamber, and diffuser. The motive nozzle is where high-pressure steam enters and creates the jet, while the suction chamber is where the gas or vapor is entrained. The diffuser is where the gas or vapor is compressed and exits the ejector.
The selection of a steam ejector depends on various factors, including the required vacuum level, gas or vapor flow rate, and operating pressure. It is essential to consult with a steam ejector design expert to determine the most suitable ejector for your specific application.
There are various resources available for steam ejector design, including engineering textbooks, online articles and tutorials, and design software. Consulting with a steam ejector manufacturer or design engineer can also provide valuable resources and expertise for your design needs.
Semi-empirical equation for entrainment ratio based on curve fitting of design data obtained from reputed ejector manufacturers such as Schutte-Koerting, Croll-Reynolds and Graham. The curve fitting equation is fairly satisfactory for the entrainment ratio range between 0.2 & 1. The calculated entrainment ratio may then be used to calculate the mass flow rate of motive steam or entrained vapor given either one of them.
Ejectors, or jet pumps, utilize the pressure energy of a high-pressure fluid stream to boost the pressure and/or flow of a low-pressure source. They can operate with either incompressible or compressible fluids as the primary (driving) and secondary (driven) flows. The main features of an ejector are shown in Figure 1 . The figure also defines the subscripts used later for primary (1), secondary (2), etc.
The primary fluid is passed through a nozzle where the pressure energy is converted into kinetic energy. The high-velocity jet entrains the secondary fluid. The two streams mix in the mixing tube, leading to pressure recovery. Further static pressure is recovered in a narrow-angle diffuser downstream of the mixing tube.
Ejectors are generally inefficient devices. However, their simplicity and lack of moving parts make them worthy of consideration, particularly where a high-pressure stream of fluid is already available. Table 1 summarizes potential ejector applications.
The most comprehensive source of design information for ejectors can be found in a series of Engineering Sciences Data Unit (ESDU) data items, Nos. 85032 and 84029. These are available on subscription as part of the ESDU Internal Flow series.
where C is the density ratio (secondary to primary). The loss coefficients Kp, Ks, Km and Kd account for losses in the primary nozzle, secondary flow inlet, mixing chamber and diffuser, respectively. For high Reynolds number applications (above 2 105), values of 0.05,0.1,0.15 and 0.2 can be assumed for a well-designed jet pump.
For a well-designed nozzle, a value of 0.95 for discharge coefficient (CD) can be used. The nozzle should be conical with a half-angle of 5-10. A parallel section at the nozzle outlet (see Figure 1) is not critical to performance, but can improve the mechanical strength of the design.
Entry to the mixing tube needs to avoid large secondary flow losses: either a converging conical section or a bell-mouth entry should be used. A mixing tube length of 7-10 (mixing tube) diameters is recommended.
To reduce downstream pressure losses, the flow needs to be expanded downstream of the mixing tube to lessen flow velocities to a reasonable level. As this may involve a large area ratio, a narrow angle diffuser is required (typically 2-3 half angle).
For small pressure differences, gas-gas ejectors can be treated like liquid jet pumps. However, for higher pressure ratios, compressibility effects need to be taken into account. Above a critical pressure ratio between primary and secondary (around 1.8, depending on gas properties), flow in the primary nozzle reaches sonic velocity. Flow in the nozzle becomes independent of secondary pressure, and is given by;
where CD is the discharge coefficient, STH the throat area, γ the ratio of specific heat at constant pressure to the specific heat at constant volume and R, the specific gas constant. (See Critical Flow, Jets and Nozzles.)
In some ejector designs, a converging-diverging nozzle is utilized to accomodate the expanding jet. An 'on-design' condition can be defined where the static pressures of primary and secondary flows are equal at the nozzle exit. However, work by Ashton, Green and Reade (1993) suggests that the use of a diverging section is not necessary for effective operation, at least at moderate pressure ratios. Performance can be then calculated by considering conservation of mass, momentum and energy in the mixing tube and diffuser. Owing to the complexity of the equations, these cannot be solved directly. A complex graphical method is available in ESDU 84029.
As with a jet pump, the key geometric factor in the design is the mixing tube diameter. Performance increases by reducing mixing tube diameter up to a point where the expanding supersonic primary jet almost fills the mixing tube before mixing can take place and choking occurs. A further decrease in mixing tube diameter (or any attempt to increase secondary flow) causes performance to decrease rapidly.
These ejectors utilize liquid as the primary fluid: generally, gas-driving liquid is not a very effective arrangement due to differences in density (and hence momentum) between the streams. Performance can be generally characterized by an equation of the form
where G and L refer to the gas and liquid, respectively and VLmin is a minimum value of liquid flow below which no gas flow will occur. An expression for VLmin has been derived by Henzler (1980), given by:
Given the different densities of the two streams, mixing duty tends to be more arduous than in a single-phase ejector, and generally longer mixing tubes than those for single-phase ejectors are used (typically 20-30 diameters). However, where mixing does occur, this is very intensive (the 'mixing shock'). This tends to result in significant energy losses. A spinner upstream of the primary nozzle is sometimes used to help disintegrate the jet and induce early mixing.
Gas-liquid ejectors can be very effective devices for mass transfer applications. They can be used either as stand-alone devices, or in combination with a contact vessel. They have the combined benefits of being able to draw in gas without the need for compression, and providing a very fine dispersion in the mixing tube.
Ejectors are characterized by cocurrent plug flow in the mixing tube, with very high energy dissipation rates (typically in the range 100-1,000 W/kg), but short residence times (less than 1 second). This leads to mass transfer coefficients typically 2-3 orders of magnitude greater than a typical stirred tank, making them particularly suited to absorption with rapid, competing chemical reactions where fast mixing is required to reduce byproduct formation.
Where longer residence time is required, ejectors are often combined with a contact vessel. In such cases, the ejector provides rapid initial mixing, a fine bubble dispersion and, if properly designed, good liquid mixing in the vessel. Gas can either be recycled, by using the suction characteristics of the ejector to draw gas back from the headspace of the vessel, or operated in once-through mode. The 'Buss Reactor,' successfully used for hydroginations, sulphonations, animations, etc. is designed on the former principle [e.g., see van Dierendonck and Leuteritz (1988)].
Water jet ejectors perform in a wide range of industrial fields with their simplicity in design, functionally in applications and low maintenance requirement in operations [1]. These devices work effectively in various areas, such as shipbuilding industries, power stations and fire extinguishers units.
Although most of the studies focus only on numerical solutions, a few experimentally validated works can be found for different fluid applications in the literature. In this study, the effects of dimensionless geometrical parameters of ejector on the ejector suction capacity have been examined, and optimum design intervals have been determined by using the design of experiment method. The effect of design parameters on the suction capacity of the water jet ejectors have been investigated through design of experiment (DoE) and the data obtained from the DoE has been used for developing suction capacity of the available water jet ejector. The improved two novel designs are used to manufacture two new bronze water jet ejectors for the naval ship systems.
A water jet ejector transfers the water via vacuuming from the desired vessel or system efficiently. It consists of five different parts, which are pump and suction parts, nozzle, mixing chamber and diffuser part as given in Fig. 1. The Pressure energy created from the centrifugal pump is converted into kinetic energy in the nozzle of the water jet ejector. The pressure energy created by the pump is converted into velocity energy at the nozzle. Due to the pressure drop of the fluid coming from the pump, a low-pressure zone is formed between at nozzle outlet and at the mixing chamber inlet. The created low-pressure zone enables vacuuming the fluid and mix with the activated fluid by the pump in the mixing chamber. The mixed liquid is discharged from the system by entering the diffuser part of the ejector where the velocity energy is converted into pressure energy. The basic principle of ejector pumps is based on the momentum transfer at the ejector zones as given in Fig. 1. Ejector theory considers friction, impact and pressure losses which can be used for incompressible and steady state flow.
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