Unformatted text preview: transfer areas:
7,388 ( 958.8 )
Q1
A1 =
=
= 377 ft 2
U1 ( Ts − T1 )
400 ( 47 ) A2 = (16,860 − 11, 380 ) ( 988.5) = 292 ft 2
Q2
=
U 2 ( 273 − T2 )
350 ( 53) Because the areas are far apart, make another iteration. To do this, adjust the ∆Ts, as follows:
Assume the heat duties from above will still hold. That is, Q1 = 7,388(958.8) = 7,084,000 and
Q2 = 5480(988.5) = 5,417,000, both in Btu/h. Then, for equal Areas, A:
A(400)(230 – T1) = 7,084,000 and A(350)(T1 – 130) = 5,417,000
Solving these two equations, T1 = 176.6oF, compared to 183oF in the first iteration. Iteration 2:
Solve (4) for the new temperatures:
(5)
(16,860 − m1 ) ( 992.4 ) = ( m1 − 5, 620 ) (1117.7 ) + 5, 620 ( −17 )  m1 ( 82 )
Solving, m1 = 11,400 lb/h, which is very close to the value of 11,380 lb/h
Thus, essentially no change in these flow rates from iteration 1 and the problem is converged.
Now, solve for the heattransfer areas:
7,388 ( 958.8 )
Q1
A1 =
=
= 332 ft 2
U1 ( Ts − T1 ) 400 ( 230 − 176.6 )
A2 = (16,860 − 11, 400 ) ( 988.5) = 327 ft 2
Q2
=
350 (176.6 − 130 )
U 2 ( 273 − T2 ) (c) The area of each evaporator = 330 ft2
(a) The pressure in the first effect is that corresponding to 176.6oF. From steam tables, 7 psia.
(b) Evaporation in the Effect 1 = 16,860 – 11,400 = 5,460 lb/h of heating steam.
Therefore, percent of total evaporation in the first effect = 5,560/11,240 = 0.495 or 49.5 %
(e) Heating steam required for Effect 1 is obtained from a revision of (3),
ms(958.8) = (16,860 – m1)(1136.8) + m1(82) – 16,860(6)
Solving, ms = 7,340 lb/h
Economy = 11,240/7,340 = 1.53 or 153%. Exercise 18.1
Subject: Selection of dryer for removal of surface moisture from 0.5 mm crystals of NaCl
without breakage.
Find: Suitable type continuous, directheat dryers. Highest gas feed temperature.
Analysis: Applicable dryers are tunnel dryer or belt dryer.
Inlet gas temperature can be high, to say 1400oF, because of the high melting point of NaCl. Exercise 18.2
Subject: Batch drying of 100 kg/h of a toxic, temperaturesensitive material of 350 mm size.
Find: Suitable dryers.
Analysis: Select a rotating, doublecone vacuum dryer. Exercise 18.3
Subject: Drying of a thin, milklike liquid to produce a powder
Find: Suitable continuous, directheat dryer that will restrict temperature to less than 200oF.
Analysis: Select a spray dryer. Exercise 18.4
Subject: Selection of dryers from conditions of the feed, temperature sensitivity, and form of
the dried product
Find: Suitable batch or continuous dryers for different cases.
Analysis: (a) A temperatureinsensitive paste that must be maintained in slab form.
Select a batch tray dryer or a continuous belt dryer.
(b) A temperatureinsensitive paste that can be extruded.
Select a continuous throughcirculation belt or band dryer, or a batch,
agitated pan dryer.
(c) A temperatureinsensitive slurry.
Select a continuous spray dryer.
(d) A thin liquid from which flakes are to be produced.
Select a continuous drum dryer.
(e) Pieces of lum...
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 Spring '11
 Levicky
 The Land

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