Liquid Ring Vacuum Pump Vacuum System Optimization

[SASC]

Gentlemen,
I am looking for insights into this problem:
Situation - I am running a Vacuum System composed of liquid ring vacuum pumps processing saturated air at 100 degree F, seal water at 100 degree F, and pumps averaging in the 80 - 90% efficiency range(based against their published performance curves). Seal water is what ist is and can't be found of a lower temperature here. Also, we can't add condensers ahead of the vacuum pumps....cost and water consumption prohibitive. This system provides for the development of marginal vacuum levels(about 16.5" Hg./13.4" Hg.A), both at the pumps and at the application points, and we seek to get higher levels(approaching 18 -20" Hg. range)to assist production. Speeding up pumps is not an option(power limited). Piping, both inlet and exhaust, is superb and not at all inhibitive. I have arrived at an idea...WHAT DO YOU THINK? Idea: The process will accept higher processing temperatures without detrimental results. So, given that latitude, elevate the process temperatures from 125 degree F to 140 degree F, so that the vacuum pumps receive ~115 degree F vapor and thus we gain an enhanced condensing effect and we can "synthetically" accentuate air flow development to gain system resistance that will provide a vacuum level increase.

Please tear apart this idea if you can, and show where the shortcomings are therein.

 

[ccfowler]

SASC,

The water temperature effectively sets the limit for the vacuum that the liquid ring pumps can draw, but you can augment this with jet ejectors discharging to the liquid ring pump suction pressure to get a deeper vacuum. You may want to check your liquid ring pump manufacturer's literature and product line for such arrangements. Compatibly sized components may be readily available.

 

[SASC]

CCFowler,
Thanks for the response. Yes, steam ejectors discharging into the vacuum pumps, or some system thereof, would produce the vacuum depth, but in this case we are seeking the collapse of the air volume through improved condensing and we don't have the budget to add capital items. The crux question is this: We know cooler water produces the effect when we mate it against warmer saturated vapor. However, do we know that, if the seal water temperature is fixed and we can't get cooler seal water, can we let the process temperature rise at the application points rise and thereby gain a "condensing" effect this way as a means to collapse more air volume and in turn produce, effectively, an efficiency enhancement for the system? Can the system be manipulated this way?

Regards

 

[quark]

SASC!

Your idea seems to be good. There is an increase of gas flow by 2% because of heating up, considering constant suction pressure. But the capacity of pump increases by 22% at 115deg.F. Actually for a typical liquid ring pump, the condensation correction factor at 13.4"Hg abs. is 1.09 for 100deg.F of gas temperature, and 1.11 for 115 deg.F (considering 100deg.F sealing fluid)

But the condition is that the gas should be essentially saturated at both the temperatures. At lower suction pressures the condensation correction factor increases further and I feel it is worth giving a trial provided your gas is saturated.

Regards,

 

[SASC]

Quark,
Thank you for your feedback! You nailed the essential question I had, which was "What has more weight within the system: 1) the volumetric expansion of the processed gas with the addition of heating-up of the gas or 2) the gain in the capacity of the vacuum pumps with the heating-up of the gas to provide an enhanced condensing effect? Certainly, if we are to achieve a net gain in capacity, we have to be firm in the idea that the condensing effect rate gain, with steady temperature seal water supply provided, has to be the larger of contributing effects versus the gas volume expansion rate gain brought on with the heat-up of the gas...for this idea to be valid, the following equation must be satisfied in positive disposition:
PUMP CAPACITY GAIN = CONDENSING GAIN - GAS VOLUME GAIN

Quark, the process gas is saturated air. Any other thoughts?

 

[SASC]

Quark,
Thank you for your feedback! You nailed the essential question I had, which was "What has more weight within the system: 1) the volumetric expansion of the processed gas with the addition of heating-up of the gas or 2) the gain in the capacity of the vacuum pumps with the heating-up of the gas to provide an enhanced condensing effect? Certainly, if we are to achieve a net gain in capacity, we have to be firm in the idea that the condensing effect rate gain, with steady temperature seal water supply provided, has to be the larger of contributing effects versus the gas volume expansion rate gain brought on with the heat-up of the gas...for this idea to be valid, the following equation must be satisfied in positive disposition:
PUMP CAPACITY GAIN = CONDENSING GAIN - GAS VOLUME GAIN

Quark, the process gas is saturated air. Any other thoughts?
Regards

 

[ccfowler]

SASC,

Relatively high pressure water can be used as the motive fluid in the jet pump stage used in series with the liquid ring vacuum pump.

What is it that you are seeking to optimize? Are you trying to evacuate a greater flow of non-condensible gases from the process, increase the volumetric flow rate of the saturated gas and water vapor mixture from the process, or reduce the pressure of the process?

 

[SASC]

CCFOWLER,
The net purpose we're trying to achieve here is to increase the vacuum level/differential pressure of the process by generating a higher air flow evacuation at the application point. The application point is a dewatering point of a product, which happens to be a porous membrane, so increasing the presented air flow passing through the product is the driver to elevate the vacuum level/differential pressure. My thoughts are that we can assist the development of higher vacuum levels by elevating the process temperature of the product at the application's dewatering point thus presenting saturated gas of a higher temperature to the liquid ring vacuum pumps so that the higher gas temperature presented to the seal water will yield a higher condensing rate and thus great suction air flow will be developed. Normally, in most applications, the simple fix is to lower the seal water temperature to get the enhanced condensing rate, but we don not have that capability with our system. We do, however, have the ability to elevate the process temperature at the product application point in order to get better condensing at the vacuum pumps.

CCFOWLER, do you have any thoughts about the potential of this idea? Please respond.

Regards,
SASC

 

[ccfowler]

SASC,

Your plan should improve the performance of your process since the elevated temperature will increase the removal of moisture from the product.

The net effect would present a somewhat greater burden on the vacuum pump, but that can be easily relieved by introducing a cooling mist spray of the 100 F seal water well upstream of the vacuum pump. This spray will serve to cool the flow and condense some of the water vapor. The combination of this spray flow and the condensed water can serve as part of the seal water flow required by the vacuum pump. By relieving some of the condensing duty from the vacuum pump, the flow of non-condensible gases through the vacuum pump can be increased even though the vacuum level remains essentially unchanged. This spray system will only provide some modest performance gain, but you are trying to get all the performance you can from the equipment that you have.

 

[Cormoran]

SASC
Can use another fluid in liquid ring vacuum pump?, (with less vapor pressure)
Alfredo

 

[SASC]

Cormoran,

No, have to stick to water as seal medium.

SASC