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Fluids:What happens to steam after a heat exchanger/condenser & Pump...

  1. Jan 5, 2016 #1
    I need some basic help as someone who has never done this before and have little understanding of what's going on:

    Suppose I have steam with mass flow rate of W [kg/s] at 100C and 1 bar going to a heat exchanger (secondary side at 30C, 5 bar, 5 kg/s).

    First Question)
    This steam should condense to about 30C right? What is its pressure? Is it still at 1 bar or at saturated P (T=30C) which is 0.042 bar?

    (on a sidenote, how would I calculate what the properties of the fluid after the condenser would be like? Is there a straight forward way other than going through time steps?)

    Second Question)

    If the fluid at the outlet of the condenser is going to be about 30C at 0.042bar, I would need a pump to get the liquid back to 1 bar right? How would I propose the specs of such a pump?

    i) Rated Pump Rotational Velocity [rad/s] : does it matter what I write here?
    ii) Rated Pump Flow [m3/s]: do I convert the W [kg/s] into volumetric flow rate here?
    iii) Rated Pump Torque : could I just arbitrarily use any number I find?
    iv) Rated Pump Motor Torque : also, could I just use any number I find on any pump?

    v) Rated Head : Now I think this is what's important. If 10m of head is equivalent to about a rise of 1 bar, would I need to specify a Rated Head of 9.58m?


    I understand that this pump may not exist at all in the real world.. but I would like to understand some of these parameters, and decide which real world pump would most closely match what I want to use.

    There aren't many pump tutorials around.

    Thanks everyone, I realize that this isn't very helpful question and it may be not a very smart question. But I would like to learn.

    Best regards
     
  2. jcsd
  3. Jan 5, 2016 #2
    I wish. But its not. Its a research I'm naively doing because I don't really understand these pumping parameters. If the answer to the first question is "no" then I could just skip the pump question.
     
  4. Jan 5, 2016 #3
    There are several things left out in your system description.

    First you need a source of steam, a boiler supplying the steam at pressure.

    Second you need a source of water to feed the boiler. In general the water needs to be treated to minimize or eliminate corrosion in the boiler and piping system. One such system is a Deaerator Tank which feeds pumps that increase the water pressure to the boiler outlet pressure. These are the Boiler Feed Pumps.

    Third the steam will flow to the heat exchanger, but a control valve is installed in the piping to control the steam going into the exchanger. Because of this control valve, the steam will condense to water at the lower pressure based on the saturation point of the lower temperature. Gravity will drain the water to the bottom of the exchanger where it will flow out.

    Fourth, Usually a trap will be installed at the outlet to separate the water at this exchanger from any other use points. The water will flow to a condensate collection tank, to be pumped back to the boiler.

    Fifth there is a condensate collection tank that receives the water from all the various collection tank pumps. Make-up water & treatment chemicals are added to that tank and the water is pumped from that tank to the Deaerator Tank.

    This is a simplified description of a typical steam supply system. I have not included such things as boiler blow-down, water treatment, piping design, vents, etc. But now there is enough description to answer some of your questions.
     
  5. Jan 5, 2016 #4
    Thank you for your reply..

    Unfortunately it doesn't quite help me. Yes I already have a steam supply system, some mirrors that reflect sunlight to turn water into steam at 100C.
    Now I need to turn the steam back to water by using a heat exchanger with a secondary side from a river.

    I want to know what happens to the steam once there is heat transfer between the steam and the river water through the heat exchanger pipes, would the pressure of the steam remain at atmospheric pressure, or would it reduce to saturated pressure at T=25-30C.

    If it goes below atmospheric, would I need a pump then to bring it back to atmospheric pressure?
     
  6. Jan 5, 2016 #5
    The steam will condense to water at the saturation pressure of the cooling water temperature.

    A pump will be needed to increase the water pressure to atmospheric.

    You will need some collection tanks and devices to make this work, you won't be able to just run piping to all the devices involved.
     
  7. Jan 5, 2016 #6
    Ok, thank you. Would it be possible at all to calculate on paper what the temperature of the condensed water after the heat exchanger would be, if I can measure the flow rate, and know the river temperature etc? I think it should be possible to prove the temperature and pressure would be below atmospheric after the heat exchanger.

    The pump to increase water pressure to atmospheric is also a concern, I suppose these things are available in the market. I should only care about Head and Rated Pump Flow rate right? Does it matter what the rotation per seconds and moment of inertias are?
     
  8. Jan 5, 2016 #7

    SteamKing

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    Homework Helper

    For a heat exchanger, you need to do a first law analysis on the primary and secondary sides of the unit. If the unit is perfectly insulated, total heat in = total heat out.

    If the coolant condenses, then the pressure will equal that for the saturated liquid. You'll probably have to use the steam tables to find the properties of the condensate once you have worked out what its enthalpy is.
    Most pumps are specified to deliver a certain flow at a maximum output head. The designer is usually interested in the pump power. I don't think I've ever seen anyone want pump torque figures, which can be easily derived from the pump power and RPM, if necessary.

    Why do you think this? There are all kinds of pumps rated with output heads larger than 10 m. How do you think feed water gets into the steam drum of a boiler rated at 1000 psi?
    There are some tutorials around. You must make an effort to find them, though. A good place to start is a pump manufacturer's web site.
     
  9. Jan 5, 2016 #8
    Thank you SteamKing, I'll find out if I know someone who can do the enthalpy analysis.
    I did go through a couple of pump manufacturers website. There I learned why people talk about Head instead of Pressure. But first I needed to know if I needed a pump at all (I now realize yes I do because the steam condenses to below-atmospheric pressure unfortunately!), and then try to decide what are the important parameters when trying to look for a pump.

    I saw some specs about rotational speed, pump torque, etc. I wasn't sure if these were important.
     
  10. Jan 6, 2016 #9
    It is possible to build a system that does not need a pump. In the old days, homes would have a gravity steam heating system. Steam vapor would feed from the boiler through a pipe to a radiator. The steam would condense in the radiator and the condensate would drain back to the boiler through the same pipe via gravity.

    The same thing can happen with your system, put the cooling heat exchanger higher than the solar boiler so the condensate can drain back by gravity. This system will be at the boiler pressure throughout, even in the exchanger. The piping must be sized to allow condensate flow against the steam flow. No pump is needed but initial water supply must be carefully Fede into the system.
     
  11. Jan 6, 2016 #10

    russ_watters

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    Note however that if there is no pipe, the pressure must be the same everywhere in the system, minus the gravitational head. In either case, the pressure does not change in the boil and condenser, contrary to some of what has been posted here. If the inlet of the heat exchanger is 1 ATM, the outlet is at 1 ATM (not including minor losses due to friction).
     
  12. Jan 6, 2016 #11
    Vedward ; is that what's called the natural circulation system? It sounds really good but I won't attempt that for this first version, but will look into that if this one works.

    Russ_watters ; yes the whole system would be a closed loop system, so the inlet of the heat exchanger is 1 atmospheric pressure and I want the outlet to be 1 atmospheric pressure as well and I think I need to use a pump (unless I go with the gravity system).

    Can I ask you guys something.. if the pressure at the condenser outlet is 0.042 bar, what pump would I need to get it to 1 bar?
    SteamKing mentioned that I should look for a pump with a specific pump power. What other parameters should I look for? I should also consider the pump head? Or the pump power is enough?

    And how would I determine the pump power that I need, if I have a pipe of area A and a flow rate of W? I would like to understand some equations if there are any, rather than just say "Some guys at an internet forum said I need a pump of 5 KW".. I mean I would like some basis to say why I need 5 KW.

    Would anyone be able to spend some time to assist with this?
     
  13. Jan 6, 2016 #12

    russ_watters

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    Your description in the OP was confusing and you've got several different pressures you are referring to. IE:
    By "secondary side" are you saying the condensate flows out at 5 bar? So which is it, 1 bar, 5 bar or 0.042 bar?

    In any case, the pressure on the inlet and outlet of the condenser should be about the same and 1 bar steam condenses at 100C, not 30C.
    Steamking said you want to look at the flow and head. Those are the input data for the selection. He said you would be "interested" in the power, but it is not a primary selection parameter (though you do want to maximize efficiency). Just flow and head.
    You have to fully design the system and figure out what flow and head you need, then you just pick a pump from a catalog that meets those demands. There are no equations for the pump selection -- though there are for the system design (heat transfer, pressure loss, flow, pipe sizing, etc.)
    We are, but so far you are being fairly vague about what what you are doing. This isn't that easy of a task. You probably need to start at the beginning and define the purpose of this system, sketch it and start designing the components. Frankly, the pump is probably the easiest part and should come last.

    Anyway, I don't know where any of your numbers came from, but 5 kg/sec is a pretty powerful steam system: about 11 megawatts. That's big (a boiler the size of a tanker truck), powerful (such as the heating system of a high-rise apartment building), expensive (half a million dollars) and dangerous - not something that an amateur should be working on.
     
    Last edited: Jan 6, 2016
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