In the discussion her:

Someone named Guest insists that it is possible to boil 8.8 kg of water in
30 min with 700 W input. He give a set of reasons similar to the ones cited
here by Horace Heffner; i.e. the enthalpy of wet steam is much lower than
dry steam. If that were true, it would be possible to boil away water much
faster than the textbook heat of vaporization of water indicates, with a
method that produces wet steam.

I tested this assertion by boiling 500 ml of water in an electric frying
pan. I uploaded the results along with a photo.

Boiling Water Test

See attached photo [NOT HERE -- AT WEB SITE] of frying pan with boiling
water and orange indicator light and control set for top temperature. 500 ml
cup in foreground. Data:

2:11 Power on to highest level 425°F (218°C), nominal 1.5 kW
2:14 Orange indicator shows terminal temperature reached
2:14 500 ml room temperature water added
2:15 Indicator light on again. Pan partly covered to bring to boil
2:16 Boiling
2:17 Pot top removed
2:17 Lots of visible vapor, just above water. Very wet steam. Condensation
on top of pan.
2:21 Not too hot to hand 30 cm above pot. Tissues on table next to pot not
2:25 Take photo
2:29 Indicator light on steady the whole time; power is continuous
2:32 Tissues remain dry; water is not falling back into pot.
2:32 Close to bottom
2:36 Dry!

FIRST PHASE Water from room temperature to boiling took 2 min

2 min * 60 s =120 s
120 s * 1,500 W = 180,000 J = 180 kJ

Heat capacity of water (4.2 kJ/kgK) and
Temperature change nominally 80°C
Energy to bring water to 100°C: 80°C*4.2*0.5 kg = 168 kJ

~ 93% heat recovery but probably less because pot was pre-heated to 218°C

SECOND PHASE 20 min to boil dry

20 min * 60 s = 1,200 s
1,200 s * 1,500 W = 1,800,000 J = 1,800 kJ

Heat of vaporization of water (2260 kJ/kg)
Energy to vaporize 0.5 kg of water: 2260*0.5 = 1,130 kJ

It takes much more than the expected amount of heat to boil the water
because the pot poorly insulated, heat is lost from the bottom and sides of
the pot. Note that the Rossi device is also is also poorly insulated and
loses heat to other paths. The flowing water does not recover all the heat.

If this were dry steam, heat recovery would be 63% (1130/1800).

CONCLUSION Boiling water in a pot to produce wet steam gives a value
reasonably close to the textbook heat of vaporization of water (2260 kJ/kg).
It is not possible to boil away water with wet steam in far less time than
predicted by the heat of vaporization.

Website is
right, and Guest is wrong.

Reply via email to