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Author Topic: Off Topic- Thermal Dynamics and hot baths  (Read 786 times)

Offline goozgog

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Off Topic- Thermal Dynamics and hot baths
« on: February 20, 2021, 10:14:28 AM »
   The cold and snowy weather has me thinking about
hot bath water. Nothing to do with fine model aircraft
but there are smarter people here than me.

  My wife likes extravagant hot baths so our 35 gallon
electric water heater gets regular workouts. Listening to
the furnace pumping heat into the house as I think of
those BTU's running down the drain, we now have a
habit of keeping the water in the tub until it cools to
ambient temperature.

  So here's the question. After the water is at room temp.
does it then become an energy liability or an asset, or
is it neutral ?

Sorry this has nothing to do with aircraft. I probably
need to build something to maintain sanity.

Cheers! - K.
Keith Morgan

Online Steve Berry

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #1 on: February 20, 2021, 10:19:40 AM »
Not an engineer, but my dad is (52 years as a mechanical engineer). I would say once the water has achieved room temperature, it would then start to become a liability as water is a terrific heat sink. Heat will always flow to lower temperatures in an effort to find equilibrium.

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Offline Scott Richlen

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #2 on: February 20, 2021, 10:44:56 AM »
Remember that the water will only cool to the ambient (bathroom) temperature.  It will give off heat until it reaches ambient.  Once it hits ambient it is neutral.  It would have to cool below ambient to be a heat sink.  And that is not possible since it is not in contact with a colder reservoir.  It is now in equilibrium with the room  temperature, so it is not a liability.

Online Dan McEntee

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #3 on: February 20, 2021, 10:54:45 AM »
   In cold weather, the benefit beyond any potential heat release is the humidity it adds to the air. The more humid the air is the warmer it feels. That is why they add humidifiers to furnaces, for comfort levels and to reduce static in the air. Once the water gets to a point where it doesn't add the humidity level, then it's just a puddle or water, I think. Warm air in the room is above it.
  Type at you later,
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Online Brett Buck

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #4 on: February 20, 2021, 12:29:33 PM »
  So here's the question. After the water is at room temp.
does it then become an energy liability or an asset, or
is it neutral ?

   Just sitting there at ambient?  Neutral, although is it also increasing the humidity. When you drain it out, then you lose the residual heat. Note that you are not recovering all the energy that went into it to heat it, either - the heater neither transforms all its energy to heat the water, and when its cooling down, the enemy of perpetual motion machines everywhere, entropy, loses more of it.

     Brett

Online Steve Dwyer

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #5 on: February 20, 2021, 01:09:27 PM »
We are more so we're into Thermodynamics here. The energy transferred (gained or lost) in a mass of water is determined by the amount of water, its specific gravity, and the difference between the two temperatures. Q (energy change) = M (gallons of water) x C (Specific Gravity of water) x the difference between T1 and T2. Once the tub water reaches room temperature (neutral) and the temperature of the room was allowed to drop further the amount of stored energy in the water begins to rise (asset). Further, the energy stored within the mass of tub water (system) or Enthalpy at any given time is determined by Enthalpy H = Internal stored energy E x P pressure x  V volume.

The reason we feel more comfortable in the winter season when a humidifier is operating is because the increased moisture in the air reduces the evaporation rate on our skin. Our bodies perspire all the time by expelling moisture through our skin as a means to cool us. The evaporation on our skin increases during the winter when the air is dryer and is reduced in the summer when the humidity of the air increases to the point where evaporation almost ceases. This is when we see sweat and feel uncomfortable. Stand in front of a fan in the summer and the evaporative cooling makes us feel better. Any kind of humidification equipment must be cleaned periodically since they are breeders of mold and bacteria.

Steve
 

Offline Chuck_Smith

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #6 on: February 20, 2021, 02:03:59 PM »
Heat only flows from hot to cold. The bath will never reach thermal equilibrium with the house unless the air in the house is at or below the dew point.

The hot bath will fall to the same temperature as the house, but as long as the water is evaporating from the tub it will take heat from it's surroundings. The heat of evaporation is lost until the water vapor condenses, at which time it is released. The heat required to make the phase change from liquid to gas is rather high for water.

Chuck
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Offline Howard Rush

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #7 on: February 20, 2021, 03:37:48 PM »
You're asking people who think that putting heat into castor oil keeps their engines cool? 

Looks like Steve found some thermo stuff on the World Wide Web.  Sure, Steve.

I do things around the house all the time based on sound thermodynamics, and my wife (a mechanical engineer) still yells at me about them.  My first thought is whether letting the water stay in the tub makes it harder to clean.  Next consideration would be animals.  Little kids--not allowed in my house-- could drown.  Large, woolly, water dogs, e.g. the late Chumley, might gleefully immerse themselves in the water, then shake it off in the kitchen, exacerbating the evaporation problem that so concerns Chuck.

So let's get quantitative.  Help me out here, Steve.  Google says the heat capacity of water is 4.18 J/g*C.  I guess C is degrees Celsius, and it's in the denominator.  Alexa tells me that the standard bathtub holds from 35 to 50 gallons of water.  Call it 35, assuming that the occupant was equivalent to much of the other 15 gallons. 35 gallons weighs 35 gallons* 3784 grams / gallon = 132,440 grams.  Alexa says 40C water is hot.  To what temperature would it cool?  Temperature would decay in pretty much e-kΔT, but what the heck is k, and what's the low temperature?  Use a thermometer to track water temperature.  If you have a setback thermostat, which you would if you are this concerned with energy recovery, check the temperature somewhere around dawn (or maybe next Thursday if you're in Texas).  I'll guess 65F = 18.33C.  So you'd recover 4.18 * 132,440 * (40 - 18.33) joules = 11,852,996 joules.  Alexa says 3.6 million joules = 1 kW-hour, so heat recovered = (11,852,996 / 3,600,000) kW-hr = 3.292  kW-hr.  My marginal cost of electricity is 11 cents/kW-hour, so you'd recover 36 cents worth of electricity.  Electric heat is 100% efficient, so that'd be worth 36 cents if you have electric heat.  Gas would be more complicated.  It would depend on your furnace efficiency. If you have a fancy geothermal heat pump like JCT Manor, it's even more complicated, but 36 cents is maybe an upper bound. 

Heat only flows from hot to cold. The bath will never reach thermal equilibrium with the house unless the air in the house is at or below the dew point.

The hot bath will fall to the same temperature as the house, but as long as the water is evaporating from the tub it will take heat from it's surroundings. The heat of evaporation is lost until the water vapor condenses, at which time it is released. The heat required to make the phase change from liquid to gas is rather high for water.

Folks decided above that the humidity is good, but Chuck points out that it's not free.  One could avoid evaporation by putting a sheet of Saran wrap or equivalent on the water surface (Marilou and I wonder why California doesn't do this with its aqueducts) or by pouring used motor oil on the water.  One probably changes his oil less frequently than he bathes, so a combination of the two techniques might be indicated.  I muse that the ratio of oil changing to bathing is possibly invariant with socioeconomic status within a certain range. 

Speaking of electric heat, my favorite thermodynamic thing is telling people who have electric heat that they can leave their lights on in the winter with impunity. 
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Online Brett Buck

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #8 on: February 20, 2021, 03:45:10 PM »

Speaking of electric heat, my favorite thermodynamic thing is telling people who have electric heat that they can leave their lights on in the winter with impunity.

   Maybe you can help me with my machine - I hooked a motor to a generator, and then use the generator to run the motor. Problem is, it just keeps going faster and faster!  What's the problem?

    Brett


Offline Scott Richlen

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #9 on: February 20, 2021, 04:18:56 PM »
Quote
Maybe you can help me with my machine - I hooked a motor to a generator, and then use the generator to run the motor. Problem is, it just keeps going faster and faster!  What's the problem?

    Brett

Uhhh, you talking about the Green New Deal here?   ;D

Online Brett Buck

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #10 on: February 20, 2021, 04:24:35 PM »
Uhhh, you talking about the Green New Deal here?   ;D

  Where's my bullet train to Australia?

    Brett

Online Steve Berry

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #11 on: February 20, 2021, 04:32:09 PM »
  Where's my bullet train to Australia?

    Brett
They had a station in Guam, but it tipped over.

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Online Brett Buck

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #12 on: February 20, 2021, 04:47:39 PM »
They had a station in Guam, but it tipped over.

   I think the word used was "capsize", he wanted to sound scientific.

   My sole contribution to the Green New Deal is my paper "The Hydrodynamic Properties of the Steam Locomotive". Look for it soon.

    Brett

Offline FLOYD CARTER

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #13 on: February 20, 2021, 05:59:01 PM »
A bathtub full of "neutral" water (has reached equilibrium) should not be wasted.

I would get several nice Koi and give them a nice place to live.
"Growing old is easy.
 Staying old is hard"
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Offline goozgog

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #14 on: February 21, 2021, 06:05:19 AM »
Thank you Gentlemen for your considered replies.

  I hadn't thought about the cool standing water
sucking in energy as it evaporates. The winter air
here is crackling dry. My cats are static generators.

  36 cents ! 
That does it.
My wife will have to shower instead !

   
  Wondering why California doesn't cover their
reservoirs with Saran Wrap?
They do.
Sort of....



Cheers!... and thanks - K.    y1
Keith Morgan

Offline Chuck_Smith

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #15 on: February 21, 2021, 06:23:41 AM »
All I know is I no longer need to run the wood stove in the man cave since I put the 60" plasma in there.
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Online Steve Dwyer

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #16 on: February 21, 2021, 08:48:11 AM »
Howard,

The only help I can offer you is to try grasping a better understanding of Vapor Pressure and how it relates to the thermodynamic equilibrium of the tub of water. It's all textbook stuff here so don't get too caught up in the weeds trying to comprehend the physics of mass and energy related to a tub of bathwater as it expels heat to the crapper room. And certainly don't believe everything Alexa tells you, you'd be far better listening to your wife. Glad she's one of us!

Stay cool !!!

Steve

Offline Motorman

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #17 on: February 21, 2021, 09:05:42 AM »
Are you Scottish by any chance?


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Offline goozgog

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #18 on: February 21, 2021, 10:38:44 AM »
If you mean me Motorman,
Glaswegian grandmother.
Hard ass civil engineer father. ( IBM )
How could you tell?

 #^
Keith Morgan

Offline Paul Smith

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #19 on: February 21, 2021, 11:03:06 AM »
   The cold and snowy weather has me thinking about
hot bath water. Nothing to do with fine model aircraft
but there are smarter people here than me.

  My wife likes extravagant hot baths so our 35 gallon
electric water heater gets regular workouts. Listening to
the furnace pumping heat into the house as I think of
those BTU's running down the drain, we now have a
habit of keeping the water in the tub until it cools to
ambient temperature.

  So here's the question. After the water is at room temp.
does it then become an energy liability or an asset, or
is it neutral ?

Sorry this has nothing to do with aircraft. I probably
need to build something to maintain sanity.

Cheers! - K.

The energy loss due to warm water going down the drain is a keen observation.  That's a double whammy, both water and fuel bills.  But it's fair price to pay vs the backlash of attacking your wife's showers.

Humidity is good because it reduces skin evaporation which is what makes you feel cold even in a warm dry house.

Back in the summer of '73 we were stationed in the Merced California.  At 100 degrees F we got out of the swimming pool and actually shivered as the moisture evaporated.  The heat loss due to evaporation is the real thing. 
« Last Edit: February 21, 2021, 11:53:12 AM by Paul Smith »
Paul Smith

Offline Paul Smith

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #20 on: February 21, 2021, 11:54:47 AM »
   The cold and snowy weather has me thinking about
hot bath water. Nothing to do with fine model aircraft
but there are smarter people here than me.

  My wife likes extravagant hot baths so our 35 gallon
electric water heater gets regular workouts. Listening to
the furnace pumping heat into the house as I think of
those BTU's running down the drain, we now have a
habit of keeping the water in the tub until it cools to
ambient temperature.

  So here's the question. After the water is at room temp.
does it then become an energy liability or an asset, or
is it neutral ?

Sorry this has nothing to do with aircraft. I probably
need to build something to maintain sanity.

Cheers! - K.

I treat my wife better.  She has a 50 gallon water heater.  The steam helps humidify the house.  Mothers' Day is coming.
Paul Smith

Online Brett Buck

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #21 on: February 21, 2021, 12:22:08 PM »
You're asking people who think that putting heat into castor oil keeps their engines cool? 

      See, here I was thinking that the clear viscous fluid coming out of the exhaust was what was burning my hand. Good thing we have science to keep us straight.

    Brett

Offline Mike Griffin

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #22 on: February 21, 2021, 01:03:29 PM »
GOD,  please end this pandemic soon. AMEN

Mike

Offline Howard Rush

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #23 on: February 21, 2021, 02:24:31 PM »
      See, here I was thinking that the clear viscous fluid coming out of the exhaust was what was burning my hand. Good thing we have science to keep us straight.

    Brett

There's an empirical observation for you.  Reminds me of the guy who told me that fiberglass cloth and silk were the same thing. The proof was that they behave the same when you hold a match to them.

I posted something awhile back showing the amount of heat castor oil can absorb.  You can calculated it yourself.  It's the same idea as the bathtub example above.  That's why God gave you a sliderule.

I just now asked Alexa the heat capacity of castor oil.  She said she couldn't find an answer, but she'd notify me when she did.  She's getting pretty clever.  She can open and close the north gate at JCT Manor now.  Don't ask her to open the pod bay door, though.  She's really tired of hearing that and gives a snotty response. 
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Offline Howard Rush

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #24 on: February 21, 2021, 02:29:54 PM »
Back in the summer of '73 we were stationed in the Merced California.  At 100 degrees F we got out of the swimming pool and actually shivered as the moisture evaporated.  The heat loss due to evaporation is the real thing. 

We see a similar phenomenon driving to the Nats.  When the water stops evaporating from your hands on the the way back to the car at a rest stop, it means you're within a day of Muncie. 
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Online Brett Buck

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #25 on: February 21, 2021, 04:29:34 PM »
I posted something awhile back showing the amount of heat castor oil can absorb.  You can calculated it yourself.  It's the same idea as the bathtub example above.  That's why God gave you a sliderule.

   So, it does carry heat away. Just not very much.

   Brett

Offline Dennis Nunes

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #26 on: February 21, 2021, 04:39:06 PM »
...She's really tired of hearing that and gives a snotty response.
Probably because you didn't ask nicely?  LL~

Dennis
« Last Edit: February 21, 2021, 06:45:58 PM by Dennis Nunes »
If at first you don't succeed ---- let someone else try it!  ;)

Offline Dan Berry

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Re: Off Topic- Thermal Dynamics and hot baths
« Reply #27 on: February 21, 2021, 04:41:36 PM »
I'm just happy that no one has made any mention of the mythical 'hot water heater' .


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