Building Services Engineering 5th Edition Handbook

Ventilation and air conditioning 147
Specific enthalpy of the air entering the coil is 68 kJ/kg and is 40 kJ/kg on leaving. The specific
volume of the air entering the coil is 0.874 m 3 /kg.
Refrigeration capacity = 10.25 m3
s
= 328.4 kW(r)
kg
kJ
× × (68 − 40)
0.874 m3 kg × kWs
kJ
= 328.4 kW(r) × 1 ton(r)
3.517 kW(r)
= 93.4 ton(r)
Refrigeration capacity is commonly rated in ton(r) as this is the energy needed to freeze a US
tonne of ice over a period of 24 h,
1TR =
2000 lb
24 h
= 3.517 kW(r)
×
144 Btu
lb
× 1 kWh
3412 Btu
EXAMPLE 5.9
A6m 3 /s recirculated room air at 22 ◦ C d.b., 50% saturation is mixed with 1.5 m 3 /s of
incoming fresh air at 10 ◦ C d.b., 6 ◦ C w.b. Calculate the mixed air dry-bulb temperature.
Plot the process on a psychrometric chart and find the mixed air moisture content.
Q 1 = 6m 3 /s, Q 2 = 1.5 m 3 /s, Q = supply air flow 7.5 m 3 /s
Using the equation:
t = Q 1
Q t 1 + Q 2
Q t 2
t = 6 1.5
× 22 +
7.5 7.5 × 10
= 19.6 ◦ C
From the chart,
g = 0.0076 kg H 2 O/kg air
Air-conditioning systems
Single duct
The single-duct system (Fig. 5.11) is used for a large room such as an atrium, a banking hall, a
swimming pool, or a lecture, entertainment or operating theatre. It can be applied to groups of
rooms with a similar demand for air conditioning, such as offices facing the same side of the