md journeymans practice test

[josh32]

i am trying to find a formula to figure out a practice question for a study guide and the question is how many btus must be added to one pound of ice at 32 degrees fahrenheit at sea level to change the entire pound of ice to one pound of water at the same temp. I know there is a formula just cant remember it can anyone help.

[dlove]

check this out http://www.berg-group.com/refrigerat...atbehavior.htm

[man from trane]

Latent Heat of Fusion = 144 BTUs/Lb. The Latent Heat of Fusion refers to a change of state from water to ice, or vice versa. No temperature change.

[ckartson]

Josh, it's not a formula it's a constant that you are going to have to memorize. the magic number is 144 BTU/lb. Water to steam is 970 BTU/lb. Try to memorize these among the few dozen others that you are going to run into though out your career. Thats what's great about this sight is when you forget something either ask or check out the educational areas and it's in here somewhere.

[ckartson]

Figure 2-9 in the link that Dlove provides has a nice graphical chart! If you haven't seen it already.

[pressordrop1]

Ice changes to water at 32° F and atmospheric pressur. Heat of absorption to produce this change is 144 Btu/lb thats per lb .

The equation for changing ice to water is
Heat= wt. Of ice x sp. Ht. Capacity of ice x temperture change.

The heat will be in btu. The (wt) will be in pounds
Spcific heat will be in btu/lb. ( the specific heat of water is 1 btu/lb.
The specific heat of ice is 0.50 Btu/lb.)

[pressordrop1]

Example : how many btu will be absorbed in changing 25lb. Of ice at 5°F to water at 40°F.

Raise the temperture of ice 5°F to 32°F

Btu=wt. Of ice x sp. Ht. Capacity of ice x temperture change.

Btu= 25x0.50 x ( 32-5)
Btu= 25x 0. 50 x 27
Btu= 337.5 btu.

To melt ice at 32°F
Btu = wt. Of ice x latent heat of fusion of ice .
Btu= 25 x 144 .
Btu = 3600

To warm water from 32°F to 40°F .

Btu= wt. Of water x sp. Ht. Capacity of water x the temperture change

[pressordrop1]

Lets finish
Btu = 25x1 x (40- 32 )
Btu= 25 x 1 x 8
Btu= 200 Btu

Total heat required to change 25lb. Of ice to water at 40°F :
Btu= 337.5 + 3600 + 200 = 4137.5 Btu

Now this excersise should help you with your test good luck

[josh32]

Thanks for the help I was making the question harder then it had to be and that's why this site is great thanks again to all who answered my question

[Samshawn]

check this out http://www.berg-group.com/refrigerat...atbehavior.htm

Thank you for the website.