Building Services Engineering 5th Edition Handbook

Heating 97
6. Choose pipe sizes from a chart or data, using an estimate of pressure loss rate and maximum
allowable water velocity.
7. Calculate the pump head and water flow rate; compare with the pump manufacturer’s
stated performance curves and choose a suitable pump.
The temperature of water flowing through a heat emitter or along a pipe will drop from its
flow temperature t f ◦ C to its return temperature t r ◦ C and lose heat Q W. If there is one heater in a
room, Q will be equal to the room steady-state heat loss. Heat losses from distribution pipework
may initially be assumed to be 10% of the room heat loss, and the radiator water flow rate
should be increased by this amount.
The specific heat capacity (SHC) of water is 4.19 kJ/kgK. The heat balance equation is:
heat lost by water = radiator heat output Q + pipe heat loss
or
water flow rate q × SHC × temperature drop = Q + 10% × Q
water flow rate q =
1.1 Q
SHC(t f − t r ) kg/s
The radiator manufacturer’s heat output data will be for a fixed temperature difference
between the mean water temperature and the room air temperature. Usually a figure of 55 K
is used. Comparison of design conditions and literature data can be made using the following
equations:
radiator mean water temperature MWT = t f + t r
2
room heat loss
radiator heat output at 55 K difference =
temperature correction factor
where:
[ ] ((tf + t r )/2) − t 1.30 a
temperature correction factor =
55
and t a is the room air temperature ( ◦ C).
EXAMPLE 4.1
A double-panel radiator is to be installed in a room where the air temperature is 22 ◦ C
and the heat loss is 4250 W. Water flow and return temperatures are to be 82 ◦ C and
71 ◦ C respectively. An extract from a radiator manufacturer’s catalogue for a temperature
difference of 55 K is given in Table 4.2. Select a suitable radiator and find the water flow
rate through it.