Расход воздуха, скорость газов и диаметр сушилки

где № — производительность сушилки по испарившейся влаге, кг/с; 5 — поперечное сечение сушилки, м2; х и л:* — рабочее и равновесное влагосодержание воздуха, кг влаги/кг сухого воздуха; Р—поверхность высушиваемого материала, м2; рсв — плот­ность сухого воздуха при средней температуре в сушилке, кг/м3.

1 При условии шарообразности частиц заменим поверхность высушиваемого мате­риала йР на с1Р= [6(1 — е)/а?:э;]5с?й, где Л—выс^а псевдоожиженного слоя, м. Раз­деляя переменные и интегрируя полученное выражение, при условии постоянства температур частиц по высоте слоя находим:

^1^1=ехрГ—й^;'»(*-'Ы-:                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     (9.36)

'     '                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        Х*—Хо                                                                                                                                                                                                                                         I      Шрер     '•••   С1Э         '"'}'''•

Равновесное содержание влаги в сушильном агенте лг* определяем по 1-х диа­грамме как абсциссу точки пересечения рабочей линии сушки с линией постоянной относительной влажности ф== 100 %. Величина лг== 0,0438 кг/кг. При этом левая часть

уравнения (9.36) равна:

(**—*2)/(х*: —*о) ^=(0,0438-0,035)/(0,0438-0,0092) =0,254.

Порозность псевдоожиженного слоя е при известном значении рабочей скорости может быть вычислена по формуле [4]:

е = '[(18Ре + 0,36Ке2)/Аг]°'21.

.,_,.•_--,«Критерий Рейнольдса

Ке = а;</,рср/Цср = 1,0-1,35-10~3.0,96/ (2,2-10~5) =58,9. Критерий Аг = 7,17-104 (см. выше). Тогда

е=[(18.58,9-4-0,36.58,92)/(7,17.104)1°-21=0,486 м3/м3.

Коэффициент массоотдачи $у определяют на основании эмпирических зависимостей; при испарении поверхностной влаги он может быть рассчитан с помощью уравнения [7] :

Ыи^ = 2-ЬО,51Ре0'52Рг^0'33,                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           (9.37)

где №и'у = $у(1э/О — диффузионный критерий Нуссельта; Ргу = ^/рО — диффузионный критерий Прандтля.

Коэффициент диффузии водяных паров в воздухе при средней температуре в сушилке Ъ (м2/с). равен:

1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   /> = />2о[(Г0-Нср)/Го]>^                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    (9.38)"

Коэффициент диффузии водяных паров в воздухе при 20 °С #2о = 21,9-10~6 м2/с [1]. Тогда                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       .

О = 21,9.10-6[(273+95)/273]3/2 = 3,44.10-5 м2/с; Рг^ = 2,2.10-5/(0,96-3,44.10~5) =0,67.

Коэффициент массоотдачи из уравнения (9.37) равен:

Э,=— (2 + 0,51 Не0-52 Рг^°'33),                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        (9.39)

.,-:-..                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      .,                                                                                                     ' Г"                                                                                                                                           ' •                                                                                                     .   ^Э                                                                                                                                                    .         •                                                                                                                                                                           /                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   .       '   '

р^3,44.10-^5 (2 + 0>5Г.68>90-52-0>670-33) =0,145 м/с. 1,35 • 10

Подставляя вычисленные значения в уравнение (9.36), определим высоту псевдо­ожиженного слоя высушиваемого материала Н:

а ос,         /    0.145   6 (1-0,486) Л °-254=еХР(-Т^%-   1,35.10-   *)'

откуда /1 = 4-10~3 м.

Проверим правильность определения величины Н по опытным данным для тепло­отдачи в псевдоожиженных слоях. Приравняем уравнение теплового баланса и урав­нение теплоотдачи:

^д = шрсрс5^ = а(/-/м)^/7,                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         (9.40)

где с — теплоемкость воздуха при средней температуре, равная 1000 Дж/(кг»К); а — коэффициент теплоотдачи, Вт/(м2-К); / — температура газа, °С; /„ — темпера­тура материала, °С.

Сделав приведенные выше преобразования, получим:

±^=ехрГ—^.6<'-е>л1.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  (9.4.)