Component 154: Steam Jet Vacuum Pump

Specification

Line Connections
1	Suction Port
2	Discharge Port
3	Motive Steam Inlet

General User Input Values Physics Used Displays Example

General

Component 154 (Steam Jet Vacuum Pump) represents a device used to create and maintain vacuum, and to remove gasses from the steam cycle, as for instance in the de-aeration of turbine condensers. Through acceleration in the nozzle, the steam vacuum pump uses the expansion work of the motive steam to generate a vacuum. In the suction chamber, the jet entrains the suction medium and accelerates its velocity. The pressure of the resulting mixture is raised when passing through the throat and the final diffuser section of the steam vacuum pump. Single stage steam jet vacuum pumps achieve a compression ratio P2/P1 of up to 10 given a sufficiently high expansion ratio of P3/P1. For higher compression ratios (or lower vacuum), multi-stage vacuum pumps are used which can be modelled by a series of steam jet vacuum pump units. If the suction medium is a gas, intercoolers may be used to condense the motive steam, which increases efficiency by reducing the amount of suction flow in the subsequent stages.

User Input Values

FMODE	Flag to set the calculation mode Design / Off-design Like in Parent Profile (Sub Profile option only) Expression =0: Global =1: Local off-design (i.e. always off-design mode, even when a design calculation has been done globally) =-1: Local design (i.e. always design mode, even when a off-design calculation has been done globally)
FSPECX	Method for specification of material equation handling (i.e. defining the two streams that are used for calculating the composition of the third) Like in Parent Profile (Sub Profile option only) Expression =0: Any given =1: Pins 1 and (2 or 3) given =2: Pins 2 and (1 or 3) given =3: Pins 3 and (1 or 2) given =12: Pins 1 and 2 given =23: Pins 2 and 3 given =13: Pins 1 and 3 given
FMETHOD	Flag to set the calculation method Like in Parent Profile (Sub Profile option only) Expression =0: Use correlations published by GEA =1: Use GEA correlations with carline correction =2: Use GEA correlations with matrix corrections
FTARGETOD	Flag to set the target value of the off-design calculation; (function parameters have to be defined on connecting streams) Like in Parent Profile (Sub Profile option only) Expression =0: P1 = f(M1, P3) =1: M1 = f(P1, P3) =2: P3 = f(M1, P1)
ENAN	Nominal effective nozzle area

Physics Used

The mass flow of motive steam M3 that is needed for entraining the suction flow M1 depends on the type and the conditions (P1, T1) of the suction medium as well as on the required exit pressure P2 of the steam jet pump.

The motive steam flow is determined from the correlation described in the graph below (picture source: GEA Group) as the specific steam consumption b that describes the amount of motive steam per kg of equivalent water vapour suction flow (steam at 150°C).

The parameters of this graph are the expansion ratio E=P3/P1 and the compression ratio K=P2/P1. The actual mass flow at the suction port is converted to the equivalent suction flow utilizing the graph below (picture source: GEA Group) which is valid for pure gases or gas mixtures (top x-axis) or steam flows (bottom x-axis) with the mean molecular mass of the suction fluid as parameter: M1_eq= f1/f2 * M1. The factor f1 equals 0.73537 for steam at 150°C used for the determination of the specific steam consumption b.

With the results of the two graphs the motive steam flow can be determined as M3 = b(E,K) * 0.73537 / f2 * M1.

In order to adjust the model to vendor data, this correlation can be extended with the method FMETHOD to include correction factors:

with FMETHOD = 0: the original equation as per GEA brochure shown above will be used , or
with FMETHOD = 1: the equation will be extended by curve-based correction factors M3 = CFP2P1(P2/P1) * CFLNP3P1(ln(P3/P1)) * b(E,K) * 0.73537 / f2 * M1, or
with FMETHOD = 2: the equation will be extended by a matrix-based correction factor M3 = MXCFM3M1(P2/P1, ln(P3/P1)) * b(E,K) * 0.73537 / f2 * M1.

Design Calculations

In design mode P1, M1, and P3 have to be specified, and the nozzle area of the steam jet is sized based on the speed of sound for the resulting flow of motive steam. Since the flow in the nozzle is always at the speed of sound, so that this value determines the flow through the nozzle as a function of the motive pressure P3 also under off-design conditions.

Off-Design Correlations

In off-design calculation mode, the nozzle area of the steam jet vacuum pump is fixed. For a given pump discharge pressure P2, the off-design calculation can either determine

with FTARGETOD = 0: the suction pressure P1 that will be generated for a specified suction flow M1 and a given motive steam pressure P3, or
with FTARGETOD = 1: the suction flow M1 that will be generated from a given suction pressure P1 and a given motive steam pressure P3, or
with FTARGETOD = 2: the motive pressure P3 that is necessary to produce a given suction flow M1 at given suction pressure P1.

In all three cases the solution will be determined through an iteration that uses the correlations of the two graphs above and the speed of sound for the motive steam which is a function of the effective nozzle area, P3 and M3.

Characteristics Lines

Characteristic line 1 (CFP2P1): correction factor for motive steam flow = f (P2/P1)

Characteristic line 1: flow correction factor for motive steam flow

     X-Axis        1          P2/P1                    1^st point
                        2          P2/P1                    2^nd point
                        .
                      N         P2/P1                    last point

Y-Axis 1 correction factor Flow (CFP2P1) 1^st point
2 correction factor Flow (CFP2P1) 2^nd point

.
N correction factor Flow (CFP2P1) last point

Characteristic line 2 (CFLNP3P1): correction factor for motive steam flow = f (ln(P3/P1)) - Attention: logarithmic scale!

Characteristic line 2: flow correction factor

     X-Axis        1          ln(P3/P1)                    1^st point
                        2          ln(P3/P1)                    2^nd point
                        .
                      N         ln(P3/P1)                    last point

Y-Axis 1 correction factor Flow (CFLNP3P1) 1^st point
2 correction factor Flow (CFLNP3P1) 2^nd point

.
N correction factor Flow (CFLNP3P1) last point

Specification Matrices

MXCFM3M1: correction factor Flow CF = f ((P2/P1, ln(P3/P1))

Correction factor for motive steam flow as a function of compression ratio (P2/P1) and the natural logarithm of the expansion ratio ln(P3/P1) - Attention: logarithmic scale!

Literature References

GEA Wiegand GmbH, Product Catalogue Jet Pumps, www.gea.com

HEI Standards for Steam Jet Vacuum Systems, 6th Edition, Heat Exchange Institute, Inc., 2007

DIN 28430, Messregeln für Dampfstrahlvakuumpumpen und Dampfstrahlkompressoren, Beuth Verlag, 2017

Component Displays

Display Option 1

Example

Click here >> Component 154 Demo << to load an example.

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