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Quantities of Water Demand
Chapter 4
EXAMPLE 4.6 ESTIMATION OF THE RATIOS OF MAXIMUM, WEEK, DAY, AND HOUR DEMAND IN THE
ABSENCE OF ACTUAL WATER DEMAND DATA
Determine the “percent of annual average water demand” for (a) daily average in maximum month; (b) daily average in maximum
week; (c) maximum day in a year; and (d) peak hour within a day assuming the peak hour demand is distributed over 2 hours, all
using the following equation:
P = 180 (t)
where P = percent of the annual average day water demand for time t and t = time in days, which varies from 2/24 to 365.
Solution:
a. For the daily average in maximum month, t = 30 days. −0.1 (4.23)
P = 180(t) −0.1 = 180(30) −0.1 = 180 × 0.7117 = 128.
Daily average water demand in maximum month = 1.28 Q ave-day .
b. For the daily average in maximum week, t = 7days.
P = 180(t) −0.1 = 180(7) −0.1 = 180 × 0.8232 = 148.
Daily average water demand in maximum week = 1.48 Q ave-day .
c. For the maximum day in a year, t = 1day.

P = 180(t) −0.1 = 180(1) −0.1 = 180 × 1 = 180.
Maximum day water demand in a year = 1.8 Q .
ave-day
d. For the peak hour within a day assuming the peak hour demand is distributed over 2 hours, t = 2/24 days.

P = 180(t) −0.1 = 180(2/24) −0.1 = 180 × 1.2821 = 231.
Peak hour demand within a day = 2.31 Q ave-day .

4.5 DEMAND AND DRAINAGE LOADS OF
BUILDINGS

The demand load of a building depends on the number
1. Demands for service sinks are ignored in calculating
and kinds of fixtures and the probability of their simulta-
the total fixture demand.
neous operation. Different fixtures are furnished with water
2. Demands of supply outlets, such as sill cocks, hose
at different rates as a matter of convenience and purpose.
3
Expressed in cubic feet per minute (ft /min), or liter per connections, and air conditioners, through which
water flows more or less continuously for a consid-
second (L/s) for fixture units, these rates become whole
erable length of time, are added to the probable flow
numbers of small size. Common demand rates are shown in
rather than the fixture demand.
Table 4.15.
It is quite unlikely that all fixtures in a building system 3. Fixtures supplied with both hot and cold water exert
will draw water or discharge it at the same time. A probabil- reduced demands on main hot-water and cold-water
ity study of draft demands leads to the relationships plotted branches (not fixture branches). An allowance of
in Fig. 4.5. In practice, the values shown are modified as three-fourths of the demand shown in Table 4.15 for
follows: individual fixtures is suggested.