# Energy cost of evaporating water at different pressures

What is the comparative energy cost of evaporating (boiling dry) a given volume of water by exposing it to a sufficiently hard vacuum so that it boils at standard temperature, as opposed to boiling it dry at standard pressure by the application of heat?

Understood that a lot of this will depend upon the efficiency of the heater and the vacuum pump apparatus, but what I’m looking for is either a ball-park comparison, or one of theoretical ideals.

Interestingly, the latent heat (heat of vaporization) increases with decreasing pressure. So you will actually need to put in a greater amount of energy at room temperature than you would at higher pressure to get water to boil. This siteshows that if you want to boil water at 24 deg C (around room temp), you’ll have to add 2445 kJ/kg, while at 1 bar (roughly standard atmospheric pressure), you’ll only have to add 2258 kJ/kg. It’s easy to confuse temperature with heat, but they’re not the same.

The advantage to using lowered pressure to evaporate water is that you can use a multi effect evaporator train. With a 6 effect evaporator you can evaporate 4.5 to 5 pounds of water for every pound of steam you put in the first vessel.

The key to the multi effect system is to have each evaporator operate at a lower pressure than the preceeding one. This way the vapor boiled off in one vessel heats the liquid going to the next one.

I’ve seen as many as seven effects in an evaporator train. This one was used to concentrate black liquor for firing in a recovery boiler. Six effects are more common. With the seven effect train we could bypass any one and operate at a slightly lower rate and efficiency.

When you say ‘add’ though, you’re not talking about applying a heat source are you? If the pressure is lowered to the point where water boils at room temperature, then the ambient temperature does the adding for you, doesn’t it? (I realise in a technical sense, this still isn’t getting free energy, but in a pay-the-gas-bill sense, it is).

He is talking about adding a heat source. In the case he mentioned at 24 deg C, you can draw the heat out of the ambient environment or from the water itself. It’s very common to evaporate water by heating it under pressure and then reducing the pressure to cause it to flash.

That’s pretty much what I thought. To put some flesh on the bones of this problem, what I’m actually trying to do is to work out the likely financial cost of reducing a litre of fruit juice to something like 50ml of syrup/concentrate by means of vacuum evaporation, as opposed to boiling it for quite an extended period over a low heat (there are aesthetic considerations too - as boiling at standard pressure causes other, less-desirable changes such as caramelisation of sugars and reduction of nutrient value)

That makes sense. Make sure you allow for boiling point rise as the concnetration of solutes increases. Multi effect evaporator trains are tailor made for this application. I think they were initially invented to concentrate sugar.