木材干燥—— 空氣的濕度
空氣的濕度
空氣的濕度有兩種概念,即絕對(duì)濕度和相對(duì)濕度。
(1)絕對(duì)濕度 每m3濕空氣中所含水蒸氣的質(zhì)量,稱(chēng)為濕空氣的絕對(duì)濕度。在數(shù)值上等于水蒸氣在其分壓力υ和溫度t下的密度,用ρυ(kg/m3)表示。它只能表明濕空氣中實(shí)際含水蒸氣的多少。
(2)濕容量 飽和濕空氣的絕對(duì)濕度稱(chēng)為濕容量,用ρO表示。
濕容量表示濕空氣容納水蒸氣的能力,它與溫度有關(guān),隨著溫度升高明顯增加。
(3)相對(duì)濕度 濕空氣中水蒸氣的實(shí)際含量,與同溫同壓下的最大可能含量ρo的比值,稱(chēng)為相對(duì)濕度,即絕對(duì)濕度與濕容量之比,用φ表示:
φ= (1-3)
又由理想氣體狀態(tài)方程式得:
所以φ (1-4)
即相對(duì)濕度又可定義為濕空氣的水蒸氣分壓力υ與飽和濕空氣的水蒸氣分壓力(即飽和蒸氣壓) o之比。由于 o隨溫度升高而增大,故當(dāng)υ一定時(shí),相對(duì)濕度φ隨溫度升高而減少。
相對(duì)濕度可稱(chēng)作空氣的飽和度,表明一定溫度下空氣被水蒸氣飽和的程度,常用百分率表示。φ值越小,表明濕空氣繼續(xù)容納水蒸氣的能力越強(qiáng),當(dāng)φ=0時(shí),就是干空氣。相反,φ值越大表明它吸收水蒸氣的能力越小,當(dāng)φ=100%時(shí),就成為飽和濕空氣,失去了吸收水分的能力。
相對(duì)濕度既可用濕度計(jì)來(lái)測(cè)定,也可用經(jīng)驗(yàn)公式計(jì)算。
濕度計(jì)由兩支精度相同的溫度計(jì)組成。一支溫度計(jì)的溫包外裹著潔凈且吸水性好的紗布,紗布下端浸在清水中,使紗布保持濕潤(rùn)狀態(tài)。這支包裹著濕紗布的溫度計(jì)叫作濕球溫度計(jì),用它測(cè)得的溫度叫作濕球溫度tM 。另一支溫包,不包紗布的溫度計(jì)叫作干球溫度計(jì),用它測(cè)得的溫度叫作干球溫度t 。在不飽和空氣中,濕球紗布所含的水分向空氣中蒸發(fā);水分蒸發(fā)時(shí)從濕球吸取熱量,使得濕球溫度小于干球溫度(即空氣溫度)。干球溫度和濕球溫度之間的差值(Δt=t-tM)叫作濕度計(jì)差,或干濕球溫度差??諝庠礁?,濕度計(jì)差越大,濕物料的水分蒸發(fā)越快。反之,空氣越濕,濕度計(jì)差越小,濕物料的水分蒸發(fā)越慢。當(dāng)空氣被水蒸氣飽和時(shí)(φ=100%),濕度計(jì)差為零,此時(shí)濕物料的水分停止蒸發(fā)。
根據(jù)干球溫度和干濕球溫度差,查表1-1可求得相對(duì)濕度值。
相對(duì)濕度也可由卡銳爾—卡噴特爾(Carrier-Carpenter)1982年的經(jīng)驗(yàn)方程來(lái)計(jì)算:
表1-1 干燥介質(zhì)相對(duì)濕度表φ% 氣流速度為1.5~2.5m/s
干燥介質(zhì)
溫度t(℃)
|
干 濕 球 溫 度 差 Δt(℃)
|
干燥介質(zhì)
溫度t(℃)
|
|||||||||||||||||||||||||||||
0
|
1
|
2
|
3
|
4
|
5
|
6
|
7
|
8
|
9
|
10
|
11
|
12
|
13
|
14
|
15
|
16
|
17
|
18
|
19
|
20
|
22
|
24
|
26
|
28
|
30
|
32
|
34
|
36
|
38
|
||
30
|
100
|
93
|
87
|
79
|
73
|
66
|
60
|
55
|
50
|
44
|
39
|
34
|
30
|
25
|
20
|
16
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
30
|
32
|
100
|
93
|
87
|
80
|
73
|
67
|
62
|
57
|
52
|
46
|
41
|
36
|
32
|
28
|
23
|
19
|
16
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
32
|
34
|
100
|
94
|
87
|
81
|
74
|
68
|
63
|
58
|
54
|
48
|
43
|
38
|
34
|
30
|
26
|
22
|
19
|
15
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
34
|
36
|
100
|
94
|
88
|
81
|
75
|
69
|
64
|
59
|
55
|
50
|
45
|
40
|
36
|
32
|
28
|
25
|
21
|
18
|
14
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
36
|
38
|
100
|
94
|
88
|
82
|
76
|
70
|
65
|
60
|
56
|
51
|
46
|
42
|
38
|
34
|
30
|
27
|
24
|
20
|
17
|
14
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
38
|
40
|
100
|
94
|
88
|
82
|
76
|
71
|
66
|
61
|
57
|
53
|
48
|
44
|
40
|
36
|
32
|
29
|
26
|
23
|
20
|
16
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
40
|
42
|
100
|
94
|
89
|
83
|
77
|
72
|
67
|
62
|
58
|
54
|
49
|
46
|
42
|
38
|
34
|
31
|
28
|
25
|
22
|
19
|
16
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
42
|
44
|
100
|
94
|
89
|
83
|
78
|
73
|
68
|
63
|
59
|
55
|
51
|
48
|
43
|
40
|
36
|
33
|
30
|
27
|
24
|
21
|
18
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
44
|
46
|
100
|
94
|
89
|
84
|
79
|
74
|
69
|
64
|
60
|
56
|
51
|
48
|
44
|
41
|
38
|
34
|
31
|
28
|
25
|
22
|
20
|
16
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
46
|
48
|
100
|
95
|
90
|
84
|
79
|
74
|
70
|
65
|
61
|
57
|
52
|
49
|
46
|
42
|
39
|
36
|
33
|
30
|
27
|
24
|
22
|
17
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
48
|
50
|
100
|
95
|
90
|
84
|
79
|
75
|
70
|
66
|
62
|
58
|
54
|
50
|
47
|
44
|
41
|
37
|
34
|
31
|
29
|
26
|
24
|
19
|
14
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
50
|
52
|
100
|
95
|
90
|
84
|
80
|
75
|
71
|
67
|
63
|
59
|
55
|
51
|
48
|
45
|
42
|
38
|
36
|
33
|
30
|
27
|
25
|
20
|
16
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
52
|
54
|
100
|
95
|
90
|
84
|
80
|
76
|
72
|
68
|
64
|
60
|
56
|
52
|
49
|
46
|
43
|
39
|
37
|
34
|
32
|
29
|
27
|
22
|
18
|
14
|
—
|
—
|
—
|
—
|
—
|
—
|
54
|
56
|
100
|
95
|
90
|
85
|
81
|
76
|
72
|
68
|
64
|
60
|
57
|
53
|
50
|
47
|
44
|
41
|
38
|
35
|
33
|
30
|
28
|
23
|
19
|
15
|
—
|
—
|
—
|
—
|
—
|
—
|
56
|
58
|
100
|
95
|
90
|
85
|
81
|
77
|
73
|
69
|
65
|
61
|
58
|
54
|
51
|
48
|
45
|
42
|
39
|
36
|
34
|
31
|
29
|
25
|
20
|
17
|
—
|
—
|
—
|
—
|
—
|
—
|
58
|
60
|
100
|
95
|
90
|
86
|
81
|
77
|
73
|
69
|
65
|
61
|
58
|
55
|
52
|
49
|
46
|
43
|
40
|
37
|
35
|
32
|
30
|
26
|
22
|
18
|
14
|
—
|
—
|
—
|
—
|
—
|
60
|
62
|
100
|
95
|
91
|
86
|
82
|
78
|
74
|
70
|
66
|
62
|
59
|
56
|
53
|
50
|
47
|
44
|
41
|
38
|
36
|
33
|
31
|
27
|
23
|
19
|
16
|
—
|
—
|
—
|
—
|
—
|
62
|
64
|
100
|
95
|
91
|
86
|
82
|
78
|
74
|
70
|
67
|
63
|
60
|
57
|
54
|
51
|
48
|
45
|
42
|
39
|
37
|
34
|
32
|
28
|
24
|
20
|
17
|
—
|
—
|
—
|
—
|
—
|
64
|
66
|
100
|
95
|
91
|
86
|
82
|
78
|
75
|
71
|
67
|
63
|
60
|
57
|
54
|
51
|
49
|
46
|
43
|
40
|
38
|
35
|
33
|
29
|
25
|
22
|
18
|
15
|
—
|
—
|
—
|
—
|
66
|
68
|
100
|
95
|
91
|
87
|
82
|
78
|
75
|
71
|
68
|
64
|
61
|
58
|
55
|
52
|
49
|
46
|
44
|
41
|
39
|
36
|
34
|
30
|
26
|
23
|
19
|
16
|
—
|
—
|
—
|
—
|
68
|
70
|
100
|
96
|
91
|
87
|
83
|
79
|
76
|
72
|
68
|
64
|
61
|
58
|
55
|
52
|
50
|
47
|
44
|
41
|
39
|
37
|
35
|
31
|
27
|
24
|
20
|
17
|
—
|
—
|
—
|
—
|
70
|
72
|
100
|
96
|
91
|
87
|
83
|
79
|
76
|
72
|
69
|
65
|
62
|
59
|
56
|
53
|
50
|
47
|
45
|
42
|
40
|
38
|
36
|
32
|
28
|
25
|
21
|
18
|
—
|
—
|
—
|
—
|
72
|
74
|
100
|
96
|
92
|
87
|
84
|
80
|
76
|
72
|
69
|
65
|
63
|
60
|
56
|
53
|
51
|
48
|
46
|
43
|
41
|
39
|
37
|
33
|
29
|
26
|
22
|
19
|
14
|
—
|
—
|
—
|
74
|
76
|
100
|
96
|
92
|
87
|
84
|
80
|
77
|
73
|
70
|
66
|
64
|
61
|
57
|
54
|
52
|
49
|
47
|
44
|
42
|
40
|
38
|
34
|
30
|
27
|
23
|
20
|
15
|
—
|
—
|
—
|
76
|
78
|
100
|
96
|
92
|
88
|
84
|
80
|
77
|
73
|
70
|
66
|
64
|
61
|
58
|
55
|
53
|
50
|
48
|
45
|
42
|
40
|
38
|
34
|
31
|
27
|
24
|
21
|
16
|
—
|
—
|
—
|
78
|
80
|
100
|
96
|
92
|
88
|
84
|
80
|
77
|
73
|
70
|
66
|
64
|
61
|
58
|
55
|
53
|
50
|
48
|
45
|
43
|
41
|
39
|
35
|
31
|
28
|
25
|
22
|
17
|
—
|
—
|
—
|
80
|
82
|
100
|
96
|
92
|
88
|
84
|
80
|
77
|
74
|
71
|
67
|
65
|
62
|
59
|
56
|
54
|
51
|
49
|
46
|
44
|
42
|
40
|
36
|
32
|
29
|
26
|
23
|
18
|
—
|
—
|
—
|
82
|
84
|
100
|
96
|
92
|
88
|
84
|
80
|
77
|
74
|
71
|
38
|
65
|
62
|
59
|
56
|
54
|
51
|
49
|
46
|
44
|
42
|
40
|
36
|
32
|
29
|
26
|
23
|
19
|
14
|
—
|
—
|
84
|
86
|
100
|
96
|
92
|
88
|
84
|
80
|
78
|
75
|
72
|
69
|
66
|
63
|
60
|
57
|
55
|
52
|
50
|
47
|
45
|
43
|
41
|
37
|
33
|
30
|
27
|
24
|
20
|
15
|
—
|
—
|
86
|
88
|
100
|
96
|
92
|
89
|
85
|
81
|
78
|
75
|
72
|
69
|
66
|
63
|
60
|
57
|
55
|
52
|
50
|
48
|
46
|
44
|
42
|
38
|
34
|
31
|
28
|
25
|
21
|
16
|
—
|
—
|
88
|
90
|
100
|
97
|
93
|
89
|
85
|
81
|
79
|
75
|
72
|
69
|
66
|
63
|
61
|
58
|
56
|
53
|
51
|
49
|
47
|
45
|
43
|
39
|
35
|
32
|
29
|
26
|
22
|
18
|
—
|
—
|
90
|
92
|
100
|
97
|
93
|
90
|
86
|
82
|
79
|
76
|
73
|
70
|
67
|
64
|
62
|
59
|
57
|
54
|
52
|
50
|
47
|
45
|
43
|
39
|
36
|
33
|
30
|
26
|
22
|
19
|
16
|
—
|
92
|
94
|
100
|
97
|
93
|
90
|
86
|
82
|
79
|
76
|
73
|
70
|
67
|
65
|
62
|
60
|
57
|
54
|
82
|
50
|
48
|
46
|
44
|
40
|
37
|
33
|
30
|
27
|
23
|
20
|
17
|
—
|
94
|
96
|
100
|
97
|
93
|
90
|
87
|
83
|
80
|
76
|
73
|
70
|
68
|
65
|
62
|
60
|
58
|
55
|
53
|
51
|
48
|
46
|
44
|
41
|
37
|
34
|
31
|
28
|
24
|
21
|
18
|
—
|
96
|
98
|
100
|
97
|
93
|
90
|
87
|
83
|
80
|
77
|
74
|
71
|
68
|
65
|
63
|
60
|
58
|
55
|
53
|
51
|
49
|
47
|
45
|
41
|
38
|
34
|
31
|
28
|
25
|
22
|
19
|
16
|
98
|
100
|
100
|
97
|
93
|
90
|
87
|
83
|
80
|
77
|
74
|
71
|
68
|
66
|
63
|
61
|
59
|
56
|
54
|
52
|
49
|
47
|
45
|
42
|
38
|
35
|
32
|
29
|
26
|
23
|
20
|
17
|
100
|
102
|
—
|
—
|
94
|
91
|
88
|
84
|
81
|
78
|
75
|
72
|
69
|
67
|
64
|
62
|
59
|
56
|
54
|
52
|
50
|
48
|
46
|
42
|
38
|
35
|
32
|
29
|
26
|
23
|
21
|
18
|
102
|
104
|
—
|
—
|
—
|
—
|
88
|
84
|
81
|
78
|
75
|
72
|
69
|
67
|
64
|
62
|
60
|
57
|
55
|
53
|
50
|
48
|
46
|
42
|
39
|
35
|
32
|
30
|
27
|
24
|
22
|
19
|
104
|
106
|
—
|
—
|
—
|
—
|
—
|
—
|
81
|
78
|
75
|
72
|
69
|
67
|
64
|
62
|
60
|
57
|
55
|
53
|
50
|
48
|
46
|
43
|
39
|
36
|
33
|
30
|
27
|
24
|
22
|
20
|
106
|
108
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
75
|
72
|
69
|
67
|
64
|
62
|
60
|
57
|
55
|
54
|
51
|
49
|
46
|
43
|
40
|
36
|
33
|
31
|
28
|
25
|
23
|
21
|
108
|
110
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
69
|
67
|
65
|
63
|
61
|
58
|
56
|
54
|
51
|
49
|
46
|
43
|
41
|
37
|
34
|
32
|
29
|
26
|
24
|
21
|
110
|
112
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
65
|
63
|
61
|
58
|
56
|
54
|
52
|
50
|
47
|
44
|
42
|
38
|
35
|
33
|
30
|
27
|
24
|
22
|
112
|
114
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
61
|
58
|
56
|
54
|
52
|
50
|
48
|
45
|
42
|
38
|
35
|
33
|
30
|
27
|
25
|
22
|
114
|
116
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
57
|
55
|
53
|
51
|
49
|
46
|
43
|
39
|
36
|
34
|
31
|
28
|
25
|
23
|
116
|
118
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
53
|
51
|
50
|
46
|
43
|
40
|
37
|
34
|
32
|
29
|
26
|
23
|
118
|
120
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
50
|
47
|
44
|
41
|
38
|
35
|
32
|
29
|
26
|
24
|
120
|
125
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
41
|
38
|
35
|
33
|
30
|
27
|
25
|
125
|
130
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
—
|
35
|
33
|
31
|
28
|
26
|
130
|
(自Союзнаучдрепром,1985)
由于相對(duì)濕度φ
式中: o—— 干球溫度下的飽和蒸氣壓,kpa,可由表1-2查得;
υ—— 濕空氣的水蒸氣分壓力,kpa,可用卡銳爾一卡噴特爾方程計(jì)算;
(1-5)
式中: ow —— 濕球溫度下的飽和蒸氣壓,kpa,查表1-2;
—— 大氣壓,kpa,隨海拔高度而異,查表1-3;
C和CW —— 分別為干球和濕球溫度,℃。
[例] 已知在海拔高度750m處的空氣干球溫度20℃,濕球溫度10℃,求相對(duì)濕度。
[解] 由表1-2查得: o=2.337kPa, ow=1.227kPa,由表1-3查得,在海拔750m處,大氣壓 =92.5kpa,以上數(shù)值代入公式(1-5)得:
=0.63kPa
相對(duì)濕度φ×100%=27%
用卡銳爾一卡噴特爾方程計(jì)算相對(duì)濕度,對(duì)不同海拔高度處的相對(duì)濕度求解很有效,也足夠精確。而根據(jù)干濕球溫度查表(1-1)法,只能求得接近海平面處的相對(duì)濕度,若海拔高度較大,則查表(1-1)法,不夠精確。
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