Naked Science Forum
General Science => General Science => Topic started by: wolfekeeper on 21/08/2009 11:15:26
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So the question in 2007 was whether vacuum drying laundry would save energy:
http://www.thenakedscientists.com/HTML/content/questions/question/1487/
I thought about it a bit, and it looks like it would do. As the pressure goes down the rate of evaporation goes up. In particular if you go below about 0.5 psi then the boiling point goes below room temperature. That means that water should all boil off... eventually.
So the question is, does that save energy? Well, it depends on how much volume the pressure vessel has (lower is better) and how much water you get boiling off (since the water forms vapour the pump will need to extract it to minimise the pressure to keep it boiling.
If you ask how much energy it needs to boil off 1kg of water (for example), then the question becomes a lot easier. To use a conventional dryer you need to put in the latent heat of vapourisation for the water- that 2.2MJ per kg. However, if you use ambient heat to boil the water then that's only the pump energy- 1kg of water vapour at 1 atmosphere produces about 1 m^3 of volume which is only about 100 kJ of energy, neglecting losses.
Bottom line is, it looks like it's a lot less energy to vacuum dry clothes, since you get all the heat back from the boiling the water off (the water boils and recondenses again in the room, so you're recycling the vapourisation energy).
There is a slight catch though, but I'm not 100% sure how serious it is. When you pump the air out the water in the vacuum chamber will tend to cool below ambient and will tend to freeze. Even then the water will sublime, but it might be a bit slow (possibly days- I've never tried it.) However the external heat will radiate in and convect in (even below 0.5 psi you will get some convection) so it may not be *too* bad, it might only be a few hours.
I did wonder whether some sort of plastic bag might work as the pressure vessel, just suck out the air and leave it on the window ledge in the sun, but it may tend to crease the clothes. If you had a big pressure vessel to dry your clothes that will tend to get expensive and potentially dangerous and would cost more because you would have to pump the air out of it to start with (100 kJ/m^3).
Hope this helps
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I once argued with my friends until I was blue in the face that this is how dryers should be built. But there must be some reason why such a machine isn't already commercially widespread?
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Well, pressure vessels can be quite expensive, that's probably the main gotcha.
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I think it depends on how fast you want to dry your clothes, if you want to dry them very slowly, you don't need to pump the chamber down very far. In the extreme example, you don't need to pump it down at all - you can just leave it to dry. If you want to speed it up, which I guess is the point, you are going to have to reduce the pressure.
A 200W vacuum pump moves about 80l of air a minute to probably 50-100mBar so probably about 100 minutes to shift 1m3 of water vapour. so 6000s at 200W - about 1.2MJ
I am sure there are more efficient pumps, and you probably wouldn't have to pump to this sort of pressure, but the order of magnitude is similar to just evaporating it.
You could of course save a lot of energy with converntional drying by running a heat pump to provide the heat.
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I suppose in principle even if you don't pump it down in pressure if you used a regenerative heat exchanger system you could move the clothes into the dryer where they get heated by the exchanger, and then as you move them back out of the dryer the heat exchanger would scavenge the heat from the clothes and the hot air and condense the water vapour and recycle the heat for the next set of clothes. That way you need no energy at all, in principle, although in practice the heat exchanger will give you some losses.
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A 200W vacuum pump moves about 80l of air a minute to probably 50-100mBar so probably about 100 minutes to shift 1m3 of water vapour. so 6000s at 200W - about 1.2MJ
If you get the chamber down to 0.5 psi (about 30 mbar) then the boiling point reaches room temperature and the water comes off as vapour, if you then pump the vapour up just a little bit in pressure (which takes hardly any energy) then you could use a room temperature water loop to recondense it. That saves about 100kJ.
I am sure there are more efficient pumps, and you probably wouldn't have to pump to this sort of pressure, but the order of magnitude is similar to just evaporating it.
It seems to be a bit less. Whether it's economically viable I'm not sure; energy is fairly cheap compared to the cost of pressure vessels and pumps, but it might make sense for launderettes.
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I realise this post is two years old, but I am starting a project to build a prototype vacuum pump dryer! At the moment I cannot find any evidence for previous designs, has anybody came across any?
I am thinking as a point of departure that wielded aluminium could be used for the shell of the pressure vessel
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Have a look at this.
Once the water freezes the vapour pressure drops even further. It's a good way of getting the clothes cold, but not so good at getting them dry.
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That's the worst case though; it's got minimum surface area. You want maximum surface area.
For example, if you do it with the clothes pegged flat in a thick plastic bag wrapped around a window frame type thing, then it has the maximum surface area. You could probably melt it as you pump it down by pouring cold water on the bag to melt it through the bag.
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I think it depends on how fast you want to dry your clothes, if you want to dry them very slowly, you don't need to pump the chamber down very far. In the extreme example, you don't need to pump it down at all - you can just leave it to dry. If you want to speed it up, which I guess is the point, you are going to have to reduce the pressure.
A 200W vacuum pump moves about 80l of air a minute to probably 50-100mBar so probably about 100 minutes to shift 1m3 of water vapour. so 6000s at 200W - about 1.2MJ
Actually, since the vapour is going to be cold, you only have to pump the water up a little bit of pressure before it condenses. So the pump is working a lot less hard than this; you don't have to pump the water vapour all the way up to room pressure, so the power is a lot less; (power is volume/second * pressure) you could shift a lot more vapour than that with 200W; you're not power limited.
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Once the bag is squashed flat there's a serious restriction to the flow of vapour. If, on the other hand you use a rigid container then you have to transfer heat across the vacuum filled space to melt the ice.
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How did the discussion get to ice?
Food is freeze-dried to preserve flavor. You don't have to preserve the flavor in clothes.
I did some winter line drying of clothes in St. Louis. Days below freezing definitely slowed down the process, although with patience, I could get them dry after a few days.
I've seen a discussion about using a vacuum to augment the drying of biodiesel as well as the distillation of methanol.
However, you need to determine where your water vapor is going. I.E. Do you want to build a condenser to drip out your water vapor. Or, do you wish to suck it through the pump.
If the plan is for the vapor to be sucked through the pump, then you need to create a certain amount of flow to the pump. I.E. Intentionally have air entering the system. Even with a condenser, you may choose to force some air circulation through the condenser.
I suppose the question is.
In the summer, would you be better off with a one liter per minute vacuum pump.
Or, a one liter per second fan, with high air circulation, no heat.
What about in the winter?
As far as the vacuum bag idea.
You would have nothing if you restrict the opening to your pump.
I think there are bags that have essentially a permeable layer, and an impermeable layer. Perhaps used for vacuum bag moulding of carbon fiber.
If you have very few clothes, for example doing one shirt at a time, then that might work. If you have a big wad of clothes, you still need access to whatever is in the middle. And, thus some kind of a tumbler design would be preferable.
If I was designing a drum system, I'd probably do a nested system. The outer drum would be pressurized and not rotating. The inner drum would be porous and rotating. Perhaps have an electric motor designed as an integral part into the inner drum itself.
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"How did the discussion get to ice?"
Because some git posted a video of what happens when you connect a vacuum pump to a container of water.
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Thanks for the quick replies its given me some very useful things to think about. The aim of my project is to look at the feasibility of using a vacuum pump to reduce the peak load drawn in tumble dryers, which typically use a 3kW heater and consume 4kWh/cycle. I think it will be necessary in the proposed dryer to have an auxiliary heating element, but if this could be downgraded to say 1.5kW there could be sufficient benefits of using a vacuum pump as long as the cycle time wasn't compromised by too much.
CliffordK, I think using a condenser to extract the water vapor would make sense. What I really need to do is better understand the thermodynamics involved, I will report back here once I have
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By the way I found this on the net
http://www.yankodesign.com/2010/07/29/tumble-dry-in-vacuum/?utm_source=twitterfeed&utm_medium=twitter&utm_campaign=Feed%3A+yankodesign+%28Yanko+Design+-+Form+Beyond+Function%29
I have emailed the designer to inquire how his product development is going. Unfortunately nowhere to be seen are any specifics about how it works, only pretty pictures
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Once the bag is squashed flat there's a serious restriction to the flow of vapour.
I don't *think* so; the material should permit the vapour to flow. Gases like water vapour have a very low viscosity and need only very small gaps and there's gaps in the fabric weave anyway. You'd need to make sure that the outflow was touching the fabric though.
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I don't think you have ever looked at the design of vacuum systems.
They use big pipes simply because there's not much pressure to push the gas through them so the flow would otherwise be small.
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I realise this post is two years old, but I am starting a project to build a prototype vacuum pump dryer! At the moment I cannot find any evidence for previous designs, has anybody came across any?
I am thinking as a point of departure that wielded aluminium could be used for the shell of the pressure vessel
The reason you don't see clothes drying units is largely that you need quite a bit of strength in the container to resist an implosion which could be quite dangerous. Secondly it is a superior way to do some drying. In the case of common laundry it should work well but you will find that some items are sort of scratchy feeling when dried as it is the tumbling action in a dryer that softens things like towels and wash cloths.
An air conditioning repairman's vacuum pump would work quickly enough. The rotary vane types are very quick but do not provide as absolute a vacuum as the piston types. I do wonder if residual, liquid soaps or lint would tend to screw up the pumps. You may need some sort of inline filtration to protect the pump. A copper coil in a bucket of ice on the exhaust side would recapture all lost liquids from the drying process. It may be a superior method but maybe not for the average house wife.
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last night i came up with this idea of drying clothes with vacuum, i thought i m the first one to come up with this idea... but omg u guy thought about it 4 years back
i thing its possible, it just needs some designing tricks
but have anyone of you tried making it?
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Note for clothes, but I used to use a vacuum desiccator to dry out wristwatches, and indeed anything else that needs drying without the application of heat. After all, that's what they are made for. Best way of drying clothes is to hang them up in the fresh air.