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Thread: vacuum pump

  1. #1 vacuum pump 
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    Hi,

    I am trying to fit a vacuum pump for a small evaporative desalination plant, but I have no idea which pump should I install (type, HP power...) . Can you please help me with your suggestions? The size of the vessel is around 600 US gallons.


    I would really appreciate your help...


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  3. #2 vacuumpump 
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    on the top of my head?
    the size of the vessel doesn't defines the size of the required pump
    desalination plants I know off have a water supply pump to feed the desalinator unit (pump-SW condensor-brine ejector) and an fresh water extraction pump


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  4. #3  
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    3s thanks a lot for your reply.


    I do have a pump to spray the water in the vessel, but I need a vacuum pump to reduce the pressure below the atmospheric pressure. The water temperature is around 60-75 oC, so I would need a vacuum pump to reduce the pressure from 3-5 psi.

    I looked up online and I could not distinguish between the too many types of vacuum pumps available in the market. Also, I don't have enough steam to use an ejector.

    Thanks,,
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  5. #4  
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    It's not so much the type of pump as it is the construction and capacity. You need a vacuum source that can handle the volume/rate of water vapor you wish to remove which of course is based on mass balance of the volume/rate of incoming brine and the concentration. You also need to be certain its material make up and construction is compatible with water vapor and the physical conditions you want to operate it at. Finally you want to use a power source that is available to you. You mentioned that you don't have steam, so obviously a steam ejector is out. Small compressed air ejectors can generate 10-14 psi of vacuum if extraction rates are low. Do you have a compressor?
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  6. #5  
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    cypress thanks for the reply,


    NO, I don't have a compressor. I think a compressor would need some kind of protection so no water can reach it, or it would damage the compressor.


    Is your idea to use the compressor backward, so it would create a vacuum?

    By the way, this is a homemade project thing. I have a farm, but it is so close to the ocean. Obviously, the water table is so low with a TDS (total dissolved solids) of 35,000. I have an RO (reverse osmosis) system installed, but it is a headache. The production dropped by 50% and I canít afford that. SO, I decided to design my own Multi-Stage Flashing system.

    If you are interested I can provide you with more details..

    Thanks again.. 
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  7. #6  
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    Quote Originally Posted by tunaiji
    cypress thanks for the reply,


    NO, I don't have a compressor. I think a compressor would need some kind of protection so no water can reach it, or it would damage the compressor.


    Is your idea to use the compressor backward, so it would create a vacuum?
    No, compressed air can be blown through an ejector to create a vacuum. An ejector is a nozzle in a pipe that creates a very low pressure "slip stream" just up stream of the nozzle which then provides the vacuum source. The ejector gets "wet" but the compressor never is in contact with the water vapor. It is not very energy efficient though.

    By the way, this is a homemade project thing. I have a farm, but it is so close to the ocean. Obviously, the water table is so low with a TDS (total dissolved solids) of 35,000. I have an RO (reverse osmosis) system installed, but it is a headache. The production dropped by 50% and I canít afford that. SO, I decided to design my own Multi-Stage Flashing system.

    If you are interested I can provide you with more details..

    Thanks again..
    Yes the RO membranes get damaged by the sulfates generated by bacteria in the water plus iron is hard on the membranes too. And any organisms plug the membranes badly too.

    I understand now. Its for potable water I guess. Then the compressed air ejector is not ideal but could work. I thought you were trying to make concentrated brine. So you want to do vacuum distillation.... A surplus diaphragm vacuum pump might be a good choice to reduce pressure in the still along with a water chiller and circulating pump for the condenser. A converted refrigerator would work ok if you don't need a lot of water. We have some of these commercial distillation units on our older offshore platforms. Let me have a look at the design and get back to you.
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  8. #7  
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    i don't know how to attach a sketch, so I try it with words:

    first you have a sea water pump, which feeds the fresh water condensor (mounted inside the vacuum bowl,typically it is a tube heat exchanger with a half open shell
    (to collect the condensed water).the sea water goes through the tubes.Before the SW enters the condensor,part of the stream is branched of, to feed the vacuum bowl.This feed enters the evaporator (heater),typically a vertical tube stack on the bottom of the vacuum bowl, the tubes fed on the bottom with the feed, on top they are open to the vacuum bowl.
    The shell side of the evaporator is connected to your heat source (steam,hot water @80 degC).The feed is adjusted in such way that the pipestack overflows into the vacuum bowl(brine).
    The sea water coming out of the tube stack of the fresh water condensor is used to drive the ejector.this ejector keeps the unit under vacuum condition and evacuates the brine.
    Now you only need a fresh water pump to evacuate the produced water. the flow is adjusted so that the suction of the pump is always submerged with fresh water
    1/4 fresh water and 3/4 of brine gives the sea water consumption rate. evaporation is done at 60degC, so this defines the size of the evaporator and condensor pipe stack.
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  9. #8  
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    A liquid ring vacuum pump should be reliable and pretty much maintenance free. Gardner Denver has a range of small ones.
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  10. #9 hi 
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    Thanks a lot for your help..

    I decided to use a 59ft stack to get the vacuum inside the vessel. It will not give me a sufficient vacuum, but I think circulating the water in the vessel would do the job. My idea is to use a pump and an electric heater to shower back the water. It is much cheaper.


    Once I am done I will try to post the results..
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