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General Science / Air conditioning condensate is thrown away?

« on: 24/02/2021 17:15:11 »

Anyone happen to know why air conditioning condensate is usually thrown away?

I would have expected it to be sprayed over the condensing coil, to cool that off, and to evaporate the water away. Surely the lower the delta-t between the condensing coil and the evaporator coil, the better? Even if the air entering the condenser coil is at 100% relative humidity, it wouldn't be after having been heated, so there's always capacity for evaporation.

The physics is that you're spending (quite a bit) energy to condense that water (dehumidifying the room air), but if you evaporate it on the condenser, you get a lot of that heat of vaporization back by reducing the work load on the compressor.

Is the problem rust, or legionnaires disease or what?

Does anyone know?

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General Science / Can tea be made in a microwave successfully?

« on: 18/11/2020 04:03:42 »

Everyone knows that microwaving teabags in a mug is a crime against humanity, because it tastes disgusting, but recently Chinese scientists have said no this can be done successfully:

https://www.theguardian.com/food/shortcuts/2020/aug/05/tea-in-a-microwave-new-research-says-it-could-be-the-perfect-cuppa

The paper is here:

https://www.sciencedaily.com/releases/2020/08/200804111516.htm

Basically, they put a carefully designed metal lid on the glass which reflects the microwaves away from the top of the glass. Turns out that a cup full of water normally gets hotter in a microwave at the top than the bottom, but by shielding the top, the bottom gets the heating, and convection currents then even out the temperature.

But I figured that a suitable 'metal lid' can be trivially formed out of aluminium foil, which is traditionally, safely used in microwave cooking anyway, so I thought I'd try it out.

First I microwaved a 200g mug of water with a ceramic plate on top for a couple of minutes, and measured the temperature with an electronic meat thermometer probe. Turned out it was 95C at the top and 84C a the bottom. Bearing in mind the optimum temperature for tea depends on the type of tea, but is generally a much narrower range than that, these figures are disastrous for good tea!

(For optimum brewing temperatures see:
https://www.thespruceeats.com/tea-brewing-times-and-temperatures-1328730)

So then I fashioned a crude cover for the mug out of foil. I had to leave a gap for the handle but I figured it probably didn't matter that much, but otherwise it covered the top and went about 1/2 of the way down the side of the mug to reflect away microwaves from the top, so that the convection currents would instead mix everything up rather than stratifying. I then placed the ceramic lid on top to keep everything the same as much as possible and microwaved for the same time.

The results were: 77C at the top and 73C at the bottom.

Notably these figures were rather lower but also significantly closer together than the unshielded mug. The shielded mug clearly was heating somewhat slower.

To make up for the slower heating I microwaved for another couple of bursts and found:

91C at the top and 87C at the bottom. Again, much more consistent temperatures with the shielded mug.

Anyway, thought it was interesting. If you want to try this, the number of ways you could damage yourself or your microwave is surprisingly long. Among other things avoid superheating water at all costs- it's perfectly possible to get water above 100C in a microwave and then it can suddenly boil and scald you really, really badly. Oh yeah, and avoid getting the foil wrinkled or too close to the walls, itself or anything else metal to avoid arcing.

I also very successfully made tea this way, by just sticking the tea bags in the mug, covering it, and running it. Tasted consistently good.

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Plant Sciences, Zoology & Evolution / Can I keep my chilli plant fruiting over winter?

« on: 30/09/2015 01:54:40 »

I recently bought a small LED grow lamp for a 6 inch high chilli plant; my plant seems to love it, lots of chillis on it.

I've bought it inside and put it on a south-facing window ledge in the room the hot water tank is in, and there's a radiator as well, so it's reasonably warm, and gets sun and then I'm topping it up with the lamp when the sun doesn't shine as the nights draw in.

I've overwintered chillis before there, without the lamp, and they did fairly well but the lack of light in midwinter obviously didn't help. I've managed to get two sets of chillis off the plant in question this year.

What I was wondering is whether, if I keep the chilli warm and give it enough water, fertiliser and light from the LED and sunlight, whether it will carry on fruiting. Apparently you have to keep the temperature between 21C and 29C for the flowers to set, but what I don't know is whether the plant is keeping track somehow inside; has it got its own 'season'. I mean it's a tropical plant, so I wouldn't think it has seasons?

I'm just a bit suspicious, it seems a bit too easy.

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The Environment / Is VW likely to have killed many people in the UK with their diesel cars?

« on: 23/09/2015 15:41:32 »

At the moment, the VW story seems to be that lots of diesel Golfs worldwide are generating very high NOx pollution output, and diesels and golfs are pretty popular in the UK; and these cars would be punching way above their weight in terms of their NOx emissions, perhaps as much as what 10-40 other diesel cars produce.

That suggests that this might actually be a significant fraction of the NOx produced in the UK; significant percentages.

Air pollution is a pretty big killer in the UK. Does anyone have an estimate what proportion of UK's NOx that might represent, and how many people will likely have died from it?

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Technology / Why do we eat off wooden plates?

« on: 11/10/2013 01:14:54 »

Why do we eat off ceramic plates?

I noticed that wooden cutting boards are now favoured, apparently wood is naturally antiseptic and you only have to wipe it down with a wet cloth and leave it to dry, and it sterilises itself overnight.

I know it's a bit stoneage, but does this mean that we could potentially eat off wooden plates and they would be easier and take less energy to clean?

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General Science / Why does my clock run backwards?

« on: 10/03/2013 01:35:58 »

Following instructions from the internet I took a clock, and opened it up, took apart the mechanism, and just flipped over a tiny piece of metal inside, and then, with difficulty, reassembled it again, and it now runs backwards.

I thought this was fun.

But what I don't know is why flipping over this piece of metal has this effect.

What type of motor is this, and why does flipping over this piece of metal do that?

The piece of metal seemed to be soft iron, it certainly didn't seem to be an electrical connection.

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General Science / Why does my kettle go thunk?

« on: 19/11/2012 19:57:18 »

I've noticed that after you boil a kettle, they switch themselves off; which is good.

But quite a long while after that, maybe half an hour, there's suddenly a loud 'thunk' or 'click'.

Why?

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General Science / My mind is boiling with questions about .. boiling

« on: 03/03/2012 00:50:35 »

So when you cook stuff, there's apparently, according to the cookbooks, many different ways to boil.

You can 'boil vigorously' or 'simmer' or 'boil vigorously uncovered' etc.

Given that they're all boiling, and presumably at 100 degrees centigrade does that actually make much difference to how things cook?

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General Science / Can we use water to make energy?

« on: 21/06/2011 15:16:34 »

I noticed that water is concentrated by the weather, but it's constantly trying to evaporate again (except on very humid days).

I wondered if it's possible to use evaporation to generate energy; when I spray water (if it's not too humid) it cools the air down, and cool air contracts, so I think you could use the cooling to pull on a piston and make energy to power your house (or theoretically make a water powered car!)

Would this really work?

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Plant Sciences, Zoology & Evolution / Why do insects waste energy buzzing about in swarms?

« on: 06/11/2009 00:59:51 »

Particularly during the summer months I notice that there are little clouds of midges and other flying insects. They often seem to form above relatively high points on hedges and around plants or trees in little flocks of insects.

My question is, doesn't it take a lot of energy to be flying around so energetically, so why do they form in these little clouds like that?

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General Science / Is vacuum-drying of clothes feasible?

« on: 21/08/2009 11:15:26 »

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