Additional Math Pages & Resources

Friday, July 2, 2010

Blowing off steam

Yesterday we focused on making hot water. Today, we look at the energy required to get water heated up to drinkable temperature.

We heat water by adding energy to it. Water has the ability to absorb lots of energy before it changes state into a vapor (boils and becomes steam).  This is not a science class, just a list of ways to warm up water:
  1. gas-burning water heater
  2. pot on gas stove
  3. electric resistance heater
  4. heat pump that takes heat from the air or ground and transfers it to water
  5. kettle with resistance elements
  6. electric shower heater
  7. microwave oven
  8. solar water heater outside the house (short video)
  9. pan on wood or coal fire
  10. heat collector attached to fireplace or wood stove
In researching this I found some fascinating references and articles. Here's one. What they say is, Don't heat water with your wood stove if you want to remain safe, insured, and clean. If you get something wrong, all that energy you put into the water will come right back at you!

Here's an example of the power of steam - it blew this wood stove apart. I found another story about a guy out in the country who drew water from a well into a firebox heated with his wood stove. When steam pressure built up, it exceeded his expectations, went backwards down his system and blew up the well.

Not this much steam, but you get the idea!

Back to the energy. Yesterday Chris heated 400 ml of water to a boil. He said that required .05 kwhr of electricity.

Q1. How many calories does it take to heat 400 ml of water to 80 degrees C?
You need 1 calorie to raise 1 gm of water 1 degree C. One milliliter of water is one gram.

Chris' 400 ml of water requires 400 calories to warm up 1 degree C.

Assuming his tap water is about 16 degrees C, we raise the water 64 degrees to make his green tea with 80 degrees C water.  (For a detailed explanation of heating water, see this site)

A1. 400 x 64 = 25,600 calories of heat must be added to the water to get it to 80 degrees C.

 My tea mug is filled almost to the brim with 400 ml of cold water.

Q2. How many more calories will it take to get the 400 ml water to 100 degrees (boiling)?

A2. 400 x 20 = 8000 more calories must be added to get water from 80 to 100 degrees C.

Q3. How many calories does it take to turn that amount of at-the-boil water into steam?

It takes a huge amount of energy! You need to add 540 calories per gram.

A3.  400 x 540 = 216,000 calories to turn 400 ml of water into steam (after it is boiling).
All that energy is itching to get out and do some work, so be careful!

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