yukarıda da belirtildiği gibi maddenin faz değiştirirken ortaya çıkardığı ısıdır. bir de sensible heat
var ki bu da maddeninin sıcaklık değişimi dolayısıyla yaydığı ya da absorbe ettiği ısıdır. peki bu iki kavramın ne oldukları en iyi sıcak bir mantar çorbası üzerinden anlatılır mı? (bkz: a working guide to process equipment
) - norman lieberman
one day my mother served me a bowl of mushroom soup which i didn’t want to eat. i disliked mushroom soup, but i was a practical child. it would serve no purpose to tell my mother i hated the taste of mushrooms because she would say, “i’ve spent all day cooking. you’re not going outside till you eat that soup.” so i said, “mom, the soup is too hot. i’ll burn my tongue.” and she said, “norman, blow across the soup to cool it off.” while i knew this would cool the soup, i really didn’t like mushrooms. so i responded, “mom, why will blowing across the soup cool it off? how does that work?”
at this point your typical mother would slap the kid in the head and say, “children in europe are starving (this was in 1947; now european children are overweight). shut up and eat your soup.” but not my mother. “norman, blowing across the soup blows away the molecules of steam covering the top of the soup. this makes room for more molecules of water to escape from the surface of the soup in the form of steam. when the molecules of water are changed into molecules of steam, that takes a lot of heat. this heat is called latent heat. this latent heat does not come from your breath, which is colder than the soup. the heat to vaporize the soup comes from the hot soup itself. the temperature of the soup is called sensible heat. when you blow across the soup, you’re helping the sensible heat content of the soup to be converted to latent heat of evaporation of the soup. and that’s why the soup cools. but your breath simply acts as a carrier—to carry away the molecules of steam covering the surface of the soup.”
and i said, “what?”
and mom said, “norman, in effect, your breath is reducing the partial pressure of steam in contact with the soup. for every one weight percent of evaporation, the soup will cool by 10ºf.”
if my mother had served me a hydrocarbon soup, then for every one weight percent of evaporation, the soup would have cooled by 2ºf. then she would have said the carrier gas or stripping steam would be reducing the hydrocarbon partial pressure.
i have designed process equipment where the carrier medium is the air. sometimes we use nitrogen or hydrogen. but mainly we use steam because it’s cheap and condensable. we use steam:
-in the feed to towers.
- as the stripping medium in steam strippers.
- in evaporators.
the steam is used to promote vaporization of the product. but the heat of vaporization does not come from the steam, it comes mainly from the product itself. this is true even if the steam is superheated. as an adult, i grow my own mushrooms on logs and consume them quite happily. mom’s gone now, and i would give a lot for a bowl of her mushroom soup. but i still remember the lesson about the reduction in partial pressure and the conversion of sensible heat to latent heat.
en yakın zamanda çevirisini de koyayım, bu da kendime not olsun.