5 replies to this topic

[whotobe]

Ladies and gents,

I've been designing a vacuum de-aerator for removing oxygen from seawater for a water injection system as part of a group project.

I'm a little stuck with the liquid ring pumps (compressors) and in particular how to obtain a power requirement for each.

I'm running a 2-stage vacuum tower (upper absorber at 40 Torr, lower absorber at 10 Torr).

I understand the liquid ring pump will operate much like a compressor, with the suction pressure being either 40 or 10 Torr and each obviously discharging to atmosphere (i.e. 750 Torr).

The temperature in the de-aerator can be varied - and I'm looking at either 5, 15 or 25 degrees C.

So I know P1, P2 and T1.

Would the power requirements be calculated much like a centrifugal compressor - i.e. by calculating the polytropic head deltaH, and then calculating ((deltaH x mass flow rate of vapour) / efficienccy)?

Any help would be greatly received - even if a methadology. I had a search on the forums and found a couple of useful docs people had posted - but none really discussed how the power requirements would be calculated. I'm not really fussed by the outlet temperature, I need to know how much power this system would draw from a generator.

Many Thanks
5th Year MEng Chemical Engineering Student

[breizh]

This resource may help you .

Breizh

[whotobe]

Thanks breizh, had a flick through that but it seems like it's a little too detailed.

I'm just needing a very rough estimate. Would using the below equation (expanded into polytropic head equation for integral of VdP) be feasible for the compression of gases that would occur in a liquid ring pump (compressor)?

Power = integral of (VdP) / efficicency

I could assume the efficiency was roughly 75 or 80%, I know P2, P1 and T1, the molecular weight of the combined gases I could estimate, I could estimate the average compressibilty (0.98?) and estimate the ratio of specific heats k.

Would that be a reasonable way to make a basic assumption of power requirements for the compression of the gases?

[breizh]

http://igs.nigc.ir/ips/pr/e-pr-745.pdf

Let you try this standard ( last pages) .You will have to reconsolidate the correlations !

Breizh

[Adrian Tan]

whotobe, would you be able to share your excel design spreadsheet on your vacuum design?

[ankur2061]

A quick estimate of "Liquid Ring (NASH)" type vacuum pump absorbed power or BkW can be done by the following equation:

 

BkW = 21.4*(Size Factor)^0.924

 

Size factor Range = 0.02 to 16

 

Size factor can be calculated using the following equation:

 

SF = Equivalent Air flow Rate in kg/h at 20°C (68°F), M_a / Evacuated System Operating Pressure in torr (mm Hg), P_sys

 

Equivalent Air flow rate @20°C can be calculated by the following equation:

 

M_a = G*√((273.15 + T)*28.96 / (293.15*MW))

 

where:

 

G = mass flow rate of suction gas to vacuum pump, kg/h

T = temperature of suction gas, °C

MW = molecular weight of suction gas, kg/k-mol

 

Regards, 

Ankur