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HEATINC VENTILATING AIR CONDITIONING GUIDE 1942
Had the tank been set 10 ft higher, the head available for friction, while still giving
the same pressure at the top fixtures, would have been 0.43 lb X 10 ft or 4.3 lb greater
and this, with the 1 lb drop used previously, would give a total allowable drop of 1 lb
+ 4.3 lb = 5-3 lb which, divided by the 600 ft equivalent run gives a drop per 100 ft of
5.3 X 100 600
0.9 lb.
With this drop, the sizes according to the chart (Fig. 2) are 6 in., 5 in., and 4 in., respectively. If the run is reduced to 200 ft instead of 600 ft, the allowable drop will
Table 6. Typical Calculation of Pipe Sizes on Down-Feed Rises with Flush Valve Water-Closets and Urinals
(Riser No. 2. Fig. 4)
Floor op
Bldg. let 2nd
3rd
4th
5th
6th
7th
8th
Fixtures on
Floor
1 W. C.
21 Wu.. a
1 Lav.
4 W. C.
2 U.
3 Lav.
4 W. C.
2 U.
3 Lav.
6 W. C. 4 Lav.
'
6 W. C. 4 Lav.
6 W. C. 4 Lav.
6 W. C. 4 Lav. '
Gpm PER Fixture
45
45
30 3
45 30 3
45 30 3
45 3
45 3
45 3
45 3
Maximum Gpm ON
Floor
Maximum Gpm
on Riser
Usage (per cent)
Probable Flow in
Riser Gpm
Allowable Drop Lb per
100 Ft
45 45 100
45
30
00
30 3
123 168 58 98
30
180 60
9
249 417
31 130
30
180
60
9
249 666
24 160
30
270
12
282 948
19 180
30
270
12
282 1230
270
12
282 1512
16 196 - 30
14 211
30
270
12
282 1794
12 215
2
Pipe Size
In.
IK
IK
2
2
2
2K 2M 4
L 5.3 lb X 100 200
2.7 lb per 100 ft. This gives 5 in., 4 in., and 3 in., respectively,
for the flows of 400, 200, and 100 gpm.
From Example 4 it is evident that, while the down-feed system possesses certain economies in size for the riser portion, it is quite likely to involve large distribution main sizes, especially when the tank is not elevated to a considerable degree.
Example 6. Fig. 4 shows a typical down-feed layout with three risers extending eight stories and with the fixtures noted on each floor. This will be solved assuming that the level of the water in the house tank is 30 ft above the fixtures on the top floor, that the
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CHAPTER 46. WATER SUPPLY PIPING AND WATER HEATING
length of run from the tank to the farthest fixture is 200 ft, equivalent length of fittings 100 ft, and the pressure required at the fixture is 7 lb.
The 30-ft head is equal to a static pressure of 0.43 X 30 or 12.9 lb per square inch and to maintain a pressure of 7 lb at the highest fixtures the drop allowable in pressure is 12.9 -- 7.0 lb or 5.9 lb. As the total equivalent run is 300 ft, this is a drop per 100 ft of 1.97 lb, or practically 2 lb. Therefore, all risers and mains from the top floor back to the tank must be sized on the basis of a drop of 2 lb per 100 ft. Tables 5,6, 7 and 8 show the schedule for Risers Nos. 1, 2 and 3 with the maximum flow taken from Table 1, the percentage of use at the peak taken from Fig. 1, and the probable flow at the peak worked
Table 7. Typical Calculation of Pipe Sizes on Down-Feed Riser with Flush Valve Water-Closets and Urinals
(Riser No. S. Fig. 4)
Floor op
Bldg. 1st 2nd
3rd 4th
5th 6th 7th 8th
Fixtures on
Floor
1 S. S. 3 W. C.
1 Lav.
2 Lav.
3 W. C.
1 Lav.
1 S. S.
1 s. s. 1 s. s. 1 s. s. 1 s. s.
Gpm PER Fixture
4 45
3
3 45
3
4
4 4 4 4
Maximum Gpm on
Floor
Maximum Gpm ON Riser
Usage (per cent)
Probable Flow in
Riser Gpm
Allowable Drop
Lb per 100 Ft
4 4 100 4 30
135
3
138 142 63 89
30
6 148 61 90
30
135
3
4
142 290 4 294 4 298 4 302 4 306
41 119
41 120 40 120 40 121 40 122
30 30 30 30
2
Pipe Size
In.
K
IK IK
2
2
2
2
3
Table 8. Size of Distribution Main for Down-Feed Systems (See Fig. 4)
Riser No.
1 2 3
Maximum Gpm Riser
1038 1794
306
Maximum Gpm Main
1038 2832 3138
Usage (per cent)
18 9 9
Probable Gpm
187 255 282
Allowable Drop
Lb per 100 Ft
Size op Main In.
2 \4 24 25
out for each portion of the riser. Riser sizes are taken from Fig. 2, using a drop of 30 lb per 100 ft except on the riser from the top story back to the tank where 2 lb per 100 ft is the allowable limit.
Since down-feed risers are nearly always sized for a pressure loss of 30 lb per 100 ft, it is possible to arrange useful sizing data in tabular form. Fig. 5 shows a typical down-feed riser for a 20 story building. Table 9 may be used for sizing such a riser of any height and for any probable flow up to 250 gpm.
It should be noted that the two top floors in Fig. 5 are sized for less than 30 lb per 100 ft. Regardless of the height of the riser being sized, the two top floors of it should be sized from values given for the top floors of Fig. 5.
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