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I'm having a hard time understanding the benefits of VCD (like Slingshot) over a conventional boiler in terms of volume of condensate produced per unit of energy input. I've found quite a few resources explaining (in too much detail for me to fully grasp) the operation of VCDs ([A], [B]), but I haven't come across (or understood maybe) any explaining why compressing the vapor is either necessary or more than marginally advantageous.

Here's what I think I understand: inlet water comes into the system, flows through a heat exchanger to be preheated by outflowing condensate, bringing the inlet water as close to the boiling temp as practical. It's then boiled by heat exchanged from the compressed vapor (above atmospheric pressure and normal boiling/condensing temp), producing low pressure vapor that feeds into the compressor. Let me know if I've got this wrong.

I can see how this would be a closed system and could be very efficient, but it seems to me (erroneously, I assume) that a non-compression boiler could be (very nearly) as efficient. Is the higher temperature difference between the compressed vapor and inlet water the main advantage?

I've seen several sources focus on how the compression stage raises the condensation point. Beyond increasing the temp difference across the heat exchangers, it seems like this would also push the point (or zone) where the vapor condenses further downstream in the heat exchanger and a larger share of its length would be filled with vapor than without compression. Is there an advantage to this? Seems like the lower mass (vapor vs. liquid) would transfer less total heat, even with the larger temp difference.

I'm sure I'm looking at this wrong, but I can't see where. Any thoughts?

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  • $\begingroup$ I think you're looking at it correctly. I'll take a closer look, but the main advantage I see is that this process does not require an additional cooling water stream. The compressor is used to pressurize and heat the water vapor which simultaneously allows the vapor to condense at a higher temperature and provides the driving force (temperature differential) to run the evaporator. It's not obvious to me how the overall energy balance works out compared to a traditional boiler though. I think it's a comparison of the cost of running the compressor vs. a cooling water system. $\endgroup$
    – Byron Wall
    Commented Jul 13, 2016 at 14:34

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No, no, no, guys. Vapor compression recovers the latent heat of condensation that is normally lost to cooling water in a condenser. It does this by taking vapors that would normally go to a condenser and compressing them, raising their temp and pressure, then reinjecting them into the process. Since the energy in latent heat for water/steam is 5 or 6 times as much energy as it takes to raise water from freezing to boiling, we are talking about a LOT of energy. Ideally, a vapor compression system can operate on about 15% of the energy required to run the same process with a boiler and condenser. The dirty trick is that the 15% is mechanical energy, not thermal, so it is penalized by the cost of converting thermal to mechanical.

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  • $\begingroup$ I'll just add here that vapor compression is by no means the only way (nor the most efficient way) of doing this. Most large scale thermal desalination uses multistage flash distillation (MSF) or multi-effect distillation (MED), both of which thermally recover the heat of condensation by using it to pre-heat the incoming salt water. $\endgroup$
    – Emily Conn
    Commented Oct 1 at 19:36

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