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Technical Information

Basic Principles for the Design of

Liquid Ring Vacuum Pumps and Compressors

Editor VTF - Dr. Jnemann Replaces No. ---

Date August 1998 No. 120.70004.52.21E

4.3 Gas/Vapour Mixtures in General

The condensation effect occurs with all gas/vapour mixtures.

The cited equations for calculating influence factors I, IIand IIIonly apply, however, forthe compression of dry or water-vapour saturated air with water as the service liquid.

5 Service Liquid and Service Liquid Flow Passages

The liquid ring vacuum pump requires a continuous supply of service liquid to maintain itsliquid ring. The gas which is drawn in and compressed expels part of the service liquidfrom the pumps discharge nozzle.

Since the intensive contact between the service liquid and pumped gas provides goodheat transfer, the liquid ring vacuum pump ensures virtually isothermal compression.

The drive power (coupling power) absorbed passes as compression heat into the serviceliquid.

Q Pcomp = 3600 (27)

QkJ

hcomp compression heat

P kW absorbed drive power

When vapour is drawn in, the service liquid also absorbs the corresponding compressionheat.

Q m rcond K= (28)

QkJ

hcond condensation heat flow rate

mkg

hK mass flow of condensing vapour

rkJ

kg evaporation heat

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Technical Information

Basic Principles for the Design of

Liquid Ring Vacuum Pumps and Compressors

Editor VTF - Dr. Jnemann Replaces No. ---

Date August 1998 No. 120.70004.52.22E

When hot gases or vapours are pumped, it is important to take account of cooling to theservice liquids outlet temperature.

( )Q m c t tcool A p A V= (29)

QkJ

hcool cooling heat flow rate

mkg

hA mass flow rate of aspirated gas

ckJ

kg Kp specific heat of the gas

tA C temperature of the aspirated gas

tV C outlet temperature of the service liquid

The cooling heat flow rate can generally be ignored as the example calculation (section8.5) shows.

If the aspirated gas is colder than the service liquid, Qcool becomes negative, i.e. heat is

extracted from the service liquid.

The situation is analogous if dry gas is drawn in and becomes saturated with the service

liquid. Qcond is then negative.

The heat balance for the liquid ring vacuum pump is:

Q Q Q Qin comp cond cool= + + (30)

The heat flow Qin is released when the pumped medium is drawn in and compressed

and must be discharged via the service liquid Qout .

Q Qout in= (31)

( )Q m c t tout B pB V B=

tB C temperature of the service liquid at the pumpinlet

tV C temperature of the service liquid at the outlet

mB kg/h mass flow rate of the service liquid

cpB kJkg K specific heat of the service liquid

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Technical Information

Basic Principles for the Design of

Liquid Ring Vacuum Pumps and Compressors

Editor VTF - Dr. Jnemann Replaces No. ---

Date August 1998 No. 120.70004.52.23E

The service liquid is generally fed to the pump under discharge pressure. As the suctionpressure decreases, the service liquid flow increases since the pump possesses anapproximately constant hydraulic resistance.

The catalogue sheets specify the volumetric flows for water as the service liquid as afunction of the suction pressure.

If the service liquid flow through the pump is known, the temperature of the service liquidat the outlet can be calculated. The medium being pumped exists the pump at the sametemperature.

Liquid ring vacuum pumps offer various options for handling the service liquid.

5.1 Once Through Liquid Operation

The simplest but most expensive form of handling the service liquid used in liquid ringvacuum pumps is once through liquid operation (Fig. 9).

Different versions are listed in the Vacuum Catalogue, Sheet A1, Figs. 1, 2 and 3.

Fig. 9: Once Through Liquid Operation

With this version, the temperature of the added service liquid tFis the same as that of theservice liquid at the inlet to the pump tB(tB= tF).

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Basic Principles for the Design of

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Date August 1998 No. 120.70004.52.24E

5.2 Combined Liquid Operation

Combined liquid operation (Fig. 10) is preferable to once through liquid operation since itsaves significant quantities of liquid. Various versions are included in the VacuumCatalogue, Sheet A1, Fig. 4, 5 and 6.

Fig. 10: Combined Liquid Operation

With this operating mode, part of the service liquid extracted from the gas in the separatoris routed back and mixed with cold service liquid in front of the pump. The service liquidtemperature tBis a mixture of the temperature at the discharge nozzle tVand the oncethrough liquid temperature tF.

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Technical Information

Basic Principles for the Design of

Liquid Ring Vacuum Pumps and Compressors

Editor VTF - Dr. Jnemann Replaces No. ---

Date August 1998 No. 120.70004.52.25E

The following apply:

V Vt t

t tF B

V B

V F

=

(32)

Vm

hF

3

once through liquid requirement in combined

operation

Vm

hB

3

service liquid flow rate of the pump

(FB in catalogue sheets)tF C temperature of the once through liquid

tB C temperature of the service liquid at the serviceliquid connection

tV C temperature of the service liquid at the dischargenozzle

5.3 Recirculation Flow Operation

Fig. 11 shows that the service liquid is routed around a closed circuit in recirculation flowoperation. After passing through the liquid ring vacuum pump, the service liquid travelsthrough a heat exchanger. 2 possible solutions for the system design are set out in the

Vacuum Catalogue, Sheet A1, Figs. 7 and 8.

Fig. 11: Recirculation Flow Operation

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Basic Principles for the Design of

Liquid Ring Vacuum Pumps and Compressors

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Date August 1998 No. 120.70004.52.26E

Depending on the circumstances, any condensate must either be removed from thecircuit or liquid which has evaporated through saturation of the pumped gases must bereplaced. The service liquid does not come into contact with the cooling medium. As aresult, this is a particularly environmentally-friendly operating mode for the liquid ringvacuum pump.

The heat exchanger must be of adequate design to discharge the heat of compressionand, where appropriate, the condensation and cooling heat.

Note: An additional liquid pump is required for the service liquid circuit if the flow

resistance of the heat exchanger, including the pipes, exceeds 0.2 bar.

6 Special Applications for Liquid Ring Vacuum Pumps

SIHI liquid ring vacuum pumps are robust and are designed to handle even the mostarduous operating conditions.

Liquid ring vacuum pumps need not necessarily pump at atmospheric pressure. The

discharge pressure can be smaller or, to some extent, even larger. Where dischargepressures are higher than atmospheric pressure, the shaft load will need to berecalculated (absorded power).

The range of application of liquid ring vacuum pumps can be extended to handle smallerpressures by connecting a gas ejector ahead of the pump.

SIHI gas ejector pumps can be used over a suction pressure range from 4 to 80 mbar.

SIHI lobular pumps can be used as an alternative to or in addition to the gas ejector pumpto extend the range further.SIHI lobular pump assemblies, consisting of e.g. liquid ring vacuum pump, gas ejectorpump and lobular pump(s) can achieve suction pressures of 100 to 10-3mbar.

Liquid ring vacuum pumps are particularly popular in chemical process engineeringapplications where they are used for a wide range of pumped gases and service liquids.The physical properties of these substances (e.g. density, viscosity, solubility and vapourpressure) must be taken into consideration when designing the pumps.