Jet Pump Design

Jet design is normally handled by the vendor. However, the process engineer must specify the system into which the jets are incorporated. He must also supply the vendor with operating conditions which include :
1. Flows of all components to be purged from thesystem (often air plus water vapor).
2. Temperature and pressure entering the jets and pressureleaving if not atmospheric.
3. Temperature and pressure of steam available to drive the jets.
4. Temperature and quantity of cooling water available for the intercondensers. Also cooling water allowable pressure drop for the intercondensers.
In addition, the process engineer must be aware of good design practices for vacuum jets.


The vendor will convert the component flow data into an "air equivalent." Since jets are rated on air handling ability, he can then build up a system from his standard hardware. The vendor should provide air equivalent capability data with the equipment he supplies. Determination of air equivalent can be done with Equation 1.

(1)

Where

ER = Entrainment ratio (or air equivalent). It is the ratio of the weight of gas handled to the weight of air which would be handled by the same ejector operating under the same conditions.
MW = Gas mol. wt.
F = 1.00, for MW 1 - 30
F = 1.076 - 0.0026 (MW), for MW 3 1 - 140
Equation 1 will give results within 2% of the entrainment ratio curve.
The effect of temperature is shown by Equations 2 and 3.

ERTA = 1 .O 17 - 0.00024T (2)
ERTS = 1.023 - 0.00033T (3)

where
ERTA = The ratio of the weight of air at 70°F to the weight of air at a higher temperature that would be handled by the same ejector operating under the same conditions.
ERTS = Same as above for steam.
T = Gas temperature, "F

The vendor should also supply steam consumption data. However, for initial planning the process engineer needs to have an estimate. Use the following equations to calculate the horsepower required to compress noncondensing components from the jet inlet pressure and temperature to the outlet pressure.

Where

HP = Gas horsepower
W = Flow. lb/min
Hpoly = Polytropic head
HAD = Adiabatic head
EP = Polytropic efficiency
EA = Adiabatic efficiency



where
Z = Average compressibility factor; using 1 .O will
R = 1,544/mol. wt
TI = Suction temperature, R
K = Adiabatic exponent. Cp/Cv. yield conservative results
P1, P2 = Suction, discharge pressures, psia
N = Polytropic exponent.

For process water vapor handled by the jets with intercondensing, calculate horsepower for the first stage only. After the first stage the condenser will bring the system to the same equilibrium as would have occurred without the process water vapor. Use an adiabatic efficiency of 7% for cases with jet intercondensers and 4% for noncondensing cases. Estimate the steam consumption to be the theoretical amount which can deliver the previously calculated total horsepower using the jet system steam inlet and outlet conditions. These ballpark results can be used until vendor data arrive. This procedure will give conservative results for cases with high water vapor compared to the Ludwig’ curves for steam consumption.
Following are some general rules of thumb for jets:
1. To determine number of stages required, assume 7 : 1 compression ratio maximum per stage.
2. The supply steam conditions should not be allowed to vary greatly. Pressure below design can lower capacity. Pressure above design usually doesn’t increase capacity and can even lower capacity.
3. Use Stellite or other hard surface material in the jet nozzle. For example 316s/s is insufficient.
4. Always provide a suitable knockout pot ahead of the jets. Water droplets can quickly damage a jet. The steam should enter the pot tangentially. Any condensate leaves through a steam trap at the bottom. It is a good idea to provide a donut baffle near the top to knock back any water creeping up the vessel walls.
5. The jet barometric legs should go in a straight line to the seal tank. A 60”-90” slope from horizontal is best.


References :

1. Branan, C. R., “The Process Engineer’s Pocket Handbook”, Vol. 2, Gulf Publishing Co., 1983.
2. Brown, G.G., “Unit Operations”, John Wiley and Sons, Inc. , 1950.
3. Evans, E L., “Equipment Design Handbook For- Rejneries and Chemical Plants”, Vol. 1, 2nd Ed., Gulf Publishing Co., 1979.
4. GPSA Engineering Data Book, “Gas Processors Suppliers Association”, Vol. 1, 10th Ed.. 1987.
5. Kern, R., “How to Design Piping for Pump-Conditions,” Chemical Engineering, 1975.
6. Kirk, R.E. and Othmer, D.F., “Ensyclopedia of Chemical Technology”, Interscience Ensyclopedia, Inc. , 1951.
7. Ludwig, E. E., “Applied Process Design for Chemical and Petrochemical Plants” Vol. 1, Gulf Publishing Co., 1977.
8. Perry, R. H., and Chilton, C. H., “Chemical Engineers’ Handbook” New York: McGraw-Hill, Inc, 1973.
9. Standards for Steam Jet Ejectors, 3rd Ed., Heat Exchange Institute, New York, N.Y.
10. WALAS, Stanley M., “Chemical Process Equipment - Selection and Design”, (Butterworth-Heinemann Series in Chemical Engineering). Boston, MA: Butterworth-Heinemann, a division of Reed Publishing (USA) Inc., 1998.