By Marcus Wilson 17/07/2020 9


Yesterday we arrived back in Cambridge after a few days holiday in Auckland, being tourists.

We sampled such delights as the unheated hotel swimming pool,  the complicated and expensive process of getting on a bus (basically having to find somewhere from which to buy a HOP card, for a non-refundable $10 a card), the completely non-social distanced pedestrian rugby scrum while navigating the building site outside Britomart, and the incessant foggy-drizzle of Tuesday that rendered the beautiful view of the city from Devonport utterly invisible.

Returning on the now-open Huntly bypass gave me a view of some Waikato countryside that I hadn’t seen before. And passing wetland-after-wetland in the pouring rain reminded me that much of this wonderful region of Aotearoa is swamp. In winter, one cannot escape the fact that Waikato is damp.

We then face the usual dilemma for any family with young boys returning to Waikato from a winter holiday – how to dry your washing. We would normally hang it outside on the line, but with the rain being interspersed only with periods of 98% humidity, it will start growing lichen before it gets dry. I am loathed to unnecessarily add to NZ’s climate change disaster by getting a clothes dryer, and besides, the internal laundry in the house, which has a space for a dryer, has no vent for it, so all we’d do is fill the laundry and garage with moisture. Waiting for the weather to improve before doing washing risks accumulating a volcano of dirty laundry the size of Mt Eden. That leaves drying clothing on a rack inside, which is not recommended either, since it adds to the humidity inside the house and encourages growth of mould.

But knowing some physics does help. When it comes to heating, the moisture content of the air makes a significant difference. Basically, the water molecules, being made of three atoms, have more modes of vibration and rotation available to them than the simpler diatomic nitrogen and oxygen molecules of dry air. That means water molecules require more energy to warm up, and so moist air takes more energy to heat than dry air (its heat capacity is higher).  When it comes to dehumidifying, we can note that the process of taking water out of air (by condensing it on a surface as in a dehumidifier) is much more efficient when the air is warmer, because a litre of warm air holds more moisture than a litre of cold air. This second effect is very significant – for example at 15 Celsius air will hold 0.011 g of water per g of dry air, whereas at 20 Celsius is will hold around 0.015 g, an increase of about 35%. The overall interplay of heat and moisture is captured on a psychrometric chart – a complicated-looking series of lines that shows how the energy content, moisture content, relative humidity and temperature are related.

ArthurOgawa / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

This means that heating and dehumidifying together is a good idea – warming the air helps the dehumidifying process, and dehumidifying helps the heating process. And both help remove the moisture from the damp washing.

So, out comes the dehumidifier from the garage, and off we go. One day we’ll be back to hanging clothes outside, but it’s not going to be today. Tomorrow isn’t looking too good either.

And one day, Auckland might have a public transport system that actually encourages people to leave their cars at home. (And so, I should add, might Cambridge/Hamilton).

 

The post All-pervading Waikato dampness appeared first on Physics Stop.


9 Responses to “All-pervading Waikato dampness”

  • I feel you really are somewhat overstating the case in saying “When it comes to heating, the moisture content of the air makes a significant difference.” (depending on what is meant by “significant”)
    At 25 deg C and at 100% humidity the water vapour content of air is only about 2%.
    The heat capacity of dry air is 1.005 kJ/KgdegC.
    The heat capacity of water vapour is 1.82kJ/KgdegC.
    So the extra degrees of freedom of vapourised water molecules, compared with those of the oxygen and nitrogen adds about 82% to the heat capacity.,
    But because the water vapour is only a small fraction, even in highly humid air, the effect is not much.
    Thus a Kg of dry air takes 1.005kJ to heat up by 1deg.
    A Kg of saturated air at 25deg takes about 1.005 plus 1.82 * 2% = 1.041kJ, a difference of about 3.6%
    In practice we never encounter absolutely dry air, and 100% humidity is not all that common, so generally the effect will be around 1 or 2% at commonly encountered temperatures and humidities. Significant?
    Thankyou Marcus for provoking me in to some mental exercise.

    • Fair enough. But I’ll still take a 1% reduction in my heating bill if it’s offered. And our washing gets dry.

  • Someone has forgotten about the latent heat of vaporization. Required if you are dehumidifying. Look it up Ron. It’s a ridiculously large number and exceedingly useful for heating your expresso. You are right that warming ot cooling the air is negligible.

  • I forgot an important point. Heating the air lowers the relative humidity. But the lower rh means that moisture will evaporate faster. Thus if you heat and dehumidify then you will dry washing surprisingly faster.

  • Ross: But will you save any energy? ie. have a lower “heating bill”, to use Marcus’ term.
    How is the latent heat of vaporization (of water), even if it is a relatively large quantity, “exceedingly useful” for heating expresso?

  • Sorry for the delay in replying.
    The heat of vapourisation for water is 2260 j/g. The specific heat of water is 4.2 j/g. When the water is heated to create the steam that hisses and roars its way into your coffee it is arriving at the steam outlet near completely vapourised. It then hits the milk in the cup and instantly turns to liquid and dumps 2260 j /g of energy into the milk. Now if we assume there is 100mls of milk in the cup and the milk is at 20 deg and we want to heat the milk to a lip burning temp of (say) 70 deg C. A change of 50 deg C. This takes 50 x 4,2 x 100 joules = 21,000 joules. If we now look at how much water vapour needs to be condensed to heat the milk then divide the required energy by 2260 to get grams of water. 2100/2260=9.3g. So to heat your 100ml cup of milk to drinking temp only adds 9.3 g (close to 10mls of milk) to the cup. (I think my maths is right….! I have ignored the added (nearly) 9.3mls dropping heat into the milk as it “cools” to 70 deg C. It would mean less water would be added to your cup).
    Thus my term of “exceedingly useful” may express the efficiency of heating your espresso.
    Such a large dump of heat is the reason why steam burns are exceedingly dangerous.
    Water has one of the largest specific heat values and thus liquid water at 100deg causes exceedingly painful burns. Almost certainly both effects caused the burns on the victims of the Whakaari White Island eruption.

  • The point I was trying to make is that to obtain water vapour from liquid water you have to put in energy. Indeed, because your heating mechanism will never be 100% efficient it will take OVER ALL more energy than if you just put your cup of coffee in a microwave oven to get it to whatever temperature you might be wanting in your ‘expresso’ coffee.
    Getting back to Marcus’ idea of saving energy, and therefore cost, by using a dehumidifier to supposedly make drying clothes more energy-efficient,I remaain to be convinced that it works. Dehumidifiers take energy to run, and are actually very inefficient at the process. So it is not just a matter of putting in the latent heat of vapourisation of the water content of the clothes to be dried, but also to run the electric motor in the dehumidifier, etc. Even if his laundry has no actual vents installed, it is not air-tight, so the process will inadvertently be dehumidifying some outside air that leaks in to the laundry over the several hours his clothes are drying.

    • Making lattes is all well and good, but my original point was how do I dry my washing on a day when it’s cold, near 100% relative humidity and wind-free. I could buy a dryer, but then with the tempting convenience of having it sit there next to the washing machine, I would use it, even on a fine, breezy day. Perhaps I should have calculated the CO2 emissions from bundling all the wet washing into the car and driving into town where there is a laundrette with dryers before hanging it up inside and switching the dehumidifier on.