Building Services Engineering 5th Edition Handbook

Hot- and cold-water supplies 191
The head loss rate to the bath is:
H 1
=
4m
EL 1 24.7 m
= 0.162 m head/m run
The head loss rate to the sink is:
H 2
=
8m
EL 2 31.2 m
= 0.2564 m head/m run
The pipe circuit to the bath has the lowest H/EL figure; this is the index circuit. H 1 /EL 1 is the
available pressure loss rate, which drives water through the upper part of the system. Branches
to other fittings on the same floor level can be sized from the same figure. All pipes below the
upper floor are sized using the value of H 2 /EL 2 that is appropriate to that circuit. Now,
H =
p
9807 m H 2O
where p is the pressure exerted by a water column of height H m. Therefore
and
p 1
EL 1
= H 1
EL 1
× 9807 N/m 3
= 0.162 × 9807 N/m 3
= 1589 N/m 3
p 2
EL 2
= 0.2564 × 9807 N/m 3
= 2514.5 N/m 3
These pressure loss rates are rounded and entered on Fig. 6.8. A different pressure loss rate
could be calculated for each pipe but sizing them all on the index values is sufficiently accurate
at this stage. Suitable pipe diameters are chosen from either Table 4.3 or Table 6.3, or the full
data tables in CIBSE (1986). Notice that the bath connection size has been used rather than the
15 mm pipe, which would satisfy the design data for much of the upper floor branch. The lower
water velocities produced will minimize noise generation in the pipework. The pressure loss rate
causing flow in the water main is:
Now,
H 3 head available for overcoming pipeline friction
=
EL 3 equivalent pipe length
H 3 = water main pressure − vertical lift − ball valve resistance
− water velocity pressure head at ball valve
= 20 − (4 + 1 + 3 + 1) − 2m
= 9 m water guage