How can we get clothes out of a vacuum chamber without condensing water on them?

[russ_watters]

I was so curious if someone else had made a vacuum dryer before. I have found so many pages saying it was not possible, but it seems that it has been done.

I don't know what sources you are looking at, but vacuum drying is a pretty standard industrial process:
https://en.wikipedia.org/wiki/Vacuum_drying

 

[sophiecentaur]

It is still drying the clothes with hot air, just at a lower pressure.

It's an expensive operation but a Condensing Drier reduces the running costs a lot. You take the hot, damp exhaust air and run it though a heat exchanger to heat up the incoming air (and, at the same time, the exhaust water is condensed and runs into a drain). So the energy input required is 'useful' as it evaporates the water but much less energy is required to warm up the air inside. We are very impressed with our new one, particularly as the exiting air is actually dry so it contributes usefully to heating the house without wet windows.
I'm not sure how well a condenser system would work at the reduced pressure and temperatures of a 'vacuum' system. The exhaust air would be cooler than the inlet air - that seems the wrong way round for efficiency. (Opinions??)
PS I have not trawled through all the pages of this old thread so I may be repeating what's been said already.

 

[jrmichler]

Drying is a huge deal in the paper industry. I remember calculating that the paper mill where I once worked evaporated 300,000 gallons of water per day in the paper drying process.

Most paper drying is done with dryer cans, which are steam heated cast iron cylinders typically 4 to 6 feet diameter or 10 to 20 feet for Yankee dryers. They also had the idea for assisting the process by using a vacuum. The Minton Vacuum Dryer was built and sold in the 1920's and into the 1930's. The vacuum increased drying rates, which allowed smaller dryer sections. Maintenance was an issue.

 

[sophiecentaur]

Drying clothes in the home took a knock when they went from front loaders to top loading washing machines. The drum diameter and spin speed had to be reduced and that has meant the clothes are not as dry when they're put in the tumbler for a start. But kitchen / utility areas are less these days so front loading has been the only option, I think. Shame - mechanical extraction is better value than heating.

 

[russ_watters]

The exhaust air would be cooler than the inlet air - that seems the wrong way round for efficiency. (Opinions??)

I don't understand why this would need to be true. Normal heat recovery on drivers and furnaces used the hot exhaust to preheat the incoming [combustion] air.

Note: A condensing driver is an apt description for a desalination plant. And they can indeed be run at reduced pressure to reduce heating energy input in addition to heat recovery from the condensation.

 

[sophiecentaur]

I don't understand why this would need to be true. Normal heat recovery on drivers and furnaces used the hot exhaust to preheat the incoming [combustion] air.

Note: A condensing driver is an apt description for a desalination plant. And they can indeed be run at reduced pressure to reduce heating energy input in addition to heat recovery from the condensation.

The situation in a clothes drier gives you hot air out, which can warm up the input air so the only energy needed will be Latent Heat, which you can't get round. The exit air in a vacuum system will be cooler than ambient. This ain't what you want if you are to avoid freezing. So I am sure it's not suitable.

I see what you mean about a basic desalination plant but I do not understand how reduced pressure is an improvement. If the system operates at a low temperature, I think the latent heat actually increases so higher temperature operation would possibly be better.

 

[russ_watters]

The exit air in a vacuum system will be cooler than ambient.

Why? I don't see that that needs to be true. Are you thinking the boiling would take place below ambient temperature? I suppose it could, but it doesn't need to. But then again, f it did, it wouldn't need any active heat input; it could pull heat from the environment.

I see what you mean about a basic desalination plant but I do not understand how reduced pressure is an improvement.

It reduces the boiling point. Desalination is an essentially zero efficiency process because it is completely reversible; your starting and ending states are nearly the same (not including the small change in boiling point due to the water). So all of your energy use is loss. Anything you can do to reduce loss improves efficiency. That's my understanding of the benefit.

Here's an article, about it though there is an unfortunate typo in it...
Vacuum desalination is a process by which water is vaporized at a lower temperature when subjected to vacuum pressure. The heat energy requirement for desalination using a distillation process can be brought down by reducing the boiling temperature.
https://www.sciencedirect.com/science/article/pii/S001191649600104X
When I was in college, one of my thermo profs had us analyze a system I thought was being used by the Army. Thing is, it might have been in a prototype stage and he might have been the inventor...that wasn't made clear.

 

[sophiecentaur]

If you can obtain enough heat from the surroundings then the vacuum system can be prevented from freezing up but that would not be ‘free’ energy in a domestic situation and a suitable heat exchanger would be large (assuming the ‘drying power’ is significantly higher than a conventional system).
The weight of a unit that could sustain even a moderate vacuum would also be a consideration. Standard clothes driers are made of bean can gauge steel.
Drying speed would / could make it worth while though.

 

[Sherwood Botsford]

When you freeze dry food you don't heat it. Indeed, you freeze it first, otherwise the initial evaporation/cooling leads to a crust collapse on the surface.

So you have the vacuum pump chugging away. Is this what provides the energy to allow the water to sublime?

Note to one of the comments above about how fast it is, with a vacuum capable of N liters /minute. As the pressure drops, the mass of the water in that volume of gas declines. The vapour pressure of water at 0C is 1/130 of an atm. So your molar volume goes up by a factor of 130. It drops by another factor of 2 at -8C

It isn't clear at all to me how fast a substance will reach equilibrium with it's vapour pressure.

 

[sophiecentaur]

It reduces the boiling point. Desalination is an essentially zero efficiency process because it is completely reversible; your starting and ending states are nearly the same (not including the small change in boiling point due to the water). So all of your energy use is loss. Anything you can do to reduce loss improves efficiency. That's my understanding of the benefit.

I've read through what you have been saying and I now get your point about near-reversibility and efficiency. I now realize that the latent heat of vaporisation from the clothes can be returned to the system in the condenser (bloomin' obvious really!). The thermal pro's and con's of the vacuum system are probable added speed to the process but limits imposed by possible freezing when the process is too quick. The extra cost of a vacuum pump and the extra strength required are probably factors that would militate against the added complexity.

On the same lines, I did look into buying a tumbler with a heat pump and they claim significant electricity savings but they cost even more than you would expect because of the premium on 'new technology' in white goods.
The cost / benefit situation is different for occasional domestic use and continuous commercial use.