They offer a more comprehensive method of computation than the tables although their precision is not so great as the original formula from which they are derived. For the majority of cases, however, the nomograms provide probably the most useful method of calculation, especially as one of them enables cal- culations for roof lights to be made.
Nomograms I and II are designed on the following assumptions (a) that the reflection factors of the ceil- ing and floor are 70 and 15% re- spectively and (b) that the external obstructions are horizontal and con- tinuous in relation to the window and have a luminance one-tenth the average sky luminance. If the ob- structions are not horizontal and continuous the equivalent horizontal obstruction should be assumed.
To use the nomograms it is neces- sary to know the ratio of window area to total surface area (scale A); the average reflection factor of all the interior surfaces (scale B); and where applicable. the angle of the external horizontal obstruction or its
reflection factor of internal sur-
faces:
----
Ceiling 70% Walls - 40% Floor = 15%
Net glass area in window = 162
sq. ft.
Angle of external obstruction (as measured from centre of win- dow) = 40°
Total area of internal surfaces
(including window) =
2(30 x 12 + 20 × 12 + 30 X 20) 2400 sq. ft. window area 162
Ratio,
=
total area 2400 Total wall area =
2(30 × 12 + 20 × 12)
wall area
Ratio,
total area
0.067
=
1200
1200
= 0.50
2400
The average reflection factor of room, from the table in Nomogram la is seen to be mid-way between 36 and 44 40%. Lay a straight- edge across Nomogram I from the graduation of 0.067 on scale A
TABLE 5
-
Use of Nomogram III for rooflights sloping at 30° to the horizontal (see Fig. 1)
Variation of function K with angles of obstruction from centre of rooflight (degrees above horizontal)
Obstruction on one
side of rooflight
0°
ly reflected component for the room with a 40° external obstruction.
Nomogram III for roof lights is similar to the other nomograms but introduces the factor K. given in Tables 4 7, to allow for various angles of obstruction. The following example explains the procedure to be followed using Nomogram III.
Consider the shed roof building shown in Fig. 1, 40 ft x 30 ft x 14 ft high (average).
Reflection factor of internal sur
faces:
30%
Walls = Roof = 70%
-
Floor 15%
Net area of roof glazing 214
sq. ft
Slope of glazing = 30° (approx) Angle of obstruction facing glazing = 20° Angle of
of obstruction opposing glazing
= 35° Total area of internal surfaces, in-
cluding roof light = 2(40 × 14 + 30 × 14)
+ (40 × 30) + 2(40 × 17)
= 1950 + 1200 + 1300 = 4520 sq. ft.
Ratio
roof glazed area
214
total area
4520
= 0.0473
Total wall area = 1960 sq. ft
wall area
total area
1960
= 0.43
4520
Average reflection factor = 36.5% K value, from Table 5 = 78
Obstruction opposed to
slope of glazing
0°-30°
40°
50°
60°
70°
80°
Ratio,
82
82
81
79
74
69
10°
81
80
80
77
73
67
20°
78
78
77
74
70
64
30°
74
73
72
70
66
60
4.0°
68
68
67
64
60
54
50°
61
61
60
58
53
48
60°
54
54
53
50
46
40
70°
46
45
45
42
38
32
80°
37
37
36
1
33
29
23
equivalent measured from the centre of the window. The average reflec- tion factor can be estimated in a number of cases from the table drawn on Nomogram I (strictly ap- plicable only where the ceiling and floor reflection factors are 70 and 15% respectively and the ratio of window area to total surface area is 0.05). The following example il- lustrates the ose of Nomogram I.
106
Assume a room 30 ft x 20 ft x 12 ft high.
(ratio of window area: total surface area) to the graduation of 40% on scale B (average reflection factor). Note where the line intersects scale C. in this case 1.2%. If there is no external obstruction, this figure re- presents the average internally re- flected component. As there is an external obstruction of 40°, place the straight-edge across the gradua- tion of 1.2% on scale D. The straight-edge intersects scale E at 0.67% which is the average internal-
Lay a straight-edge across Nomo- gram II between the graduation 0.0473 on scale A (ratio of roof glazing total surface area) to the graduation of 36.5% on scale B (average reflection factor of room); mark the point where the straight- edge intersects scale C in this case 5.75 then run the straight-edge from this point to the value 78 on scale D (value K. Table 5). The straight- edge intersects scale E at 2.1%, which is the average internally re flected component of the daylight factor.
ADDITIONAL CORRECTIONS
Having estimated the three com- ponents of daylight factor, the se- parate values are simply added together. Corrections may need to
THE HONG KONG & FAR EAST BUILDER-VOLUME 19, NUMBER 4