Building Services Engineering 5th Edition Handbook

146 Ventilation and air conditioning
Outside air
O
Air leaving
cooling coil
Cooling and
dehumidification
M Mixed air
R Room air
C
S Supply air
Air heating
5.10 Summer psychrometric cycle for a single-duct system.
EXAMPLE 5.7
Outside air at −5 ◦ C d.b., 80% saturation enters a preheater coil and leaves at 24 ◦ C d.b.
The air volume flow rate is 6.5 m 3 /s. Find (a) the outdoor air wet-bulb temperature and
specific volume, (b) the heated air moisture content and percentage saturation, and (c) the
heating coil power.
From the CIBSE psychrometric chart:
(a) −5.9 ◦ C and 0.7615 m 3 /kg;
(b) 0.00198 kg H 2 O/kg air and 10%;
(c) SE 2 at 24 ◦ C d.b. and 0.00198 kg H 2 O/kg is 29.0 kJ/kg
SE 1 at −5 ◦ C d.b. and 0.00198 kg H 2 O/kg is −0.073 kJ/kg
Heater duty = 6.5 m3
s
= 248.2 kW
kg
kJ
×
× [29 − (−0.073)]
0.7615 m3 kg × kWs
kJ
EXAMPLE 5.8
A cooling coil has water passing through it at a mean temperature of 10 ◦ C, an air flow of
10.25 m 3 /s air enters the coil at 28 ◦ C d.b., 23 ◦ C w.b. and leaves at 15 ◦ C d.b. Find the
leaving air wet-bulb temperature and percentage saturation. Calculate the refrigeration
capacity of the coil in equivalent tonnes refrigeration capacity, ton(r), when 1 ton(r) is
equal to 3.527 kW(r).
Plot the cooling and dehumidification line on the psychrometric chart in the manner shown
in Fig. 5.5 with a target point of 10 ◦ C on the saturation curve. The air leaves the cooling coil at
15 ◦ C d.b., 14.2 ◦ C w.b. and 91% saturation.