Building Services Engineering 5th Edition Handbook

Ventilation and air conditioning 163
θ = 16 h for the time of 1600 h
t ao = t max − t (
)
max − t min
θπ − 9π
× 1 − sin
2
12
(
)
26 − 12
16π − 9π
= 26 − × 1 − sin
2
12
= 26 − 7 × (1 − sin 1.833)
The 1.833 is in radians and not degrees. Switch on the radians mode of the calculator then press
the SIN X key to find the answer of sin 1.833 = 0.966. Alternatively, in degree mode, multiply
1.833 by 360 and then divide the answer by (2π). This produces an angle of 105 ◦ . Press the SIN
key and find:
sin 105 = 0.966.
There are 2π radians in 360 ◦ .
t ao = 26 − 7 × (1 − 0.966)
= 25.8 ◦ C d.b.
indoor air temperature t ai = 25.8 + 1.5 ◦ C d.b.
= 27.3 ◦ C d.b.
The calculated indoor air temperature exceeds that recommended. While this may prove be
acceptable for reasonable thermal comfort, there is solar radiation and possible glare during the
afternoon in summer. External solar shading or interior blinds are recommended if seating cannot
be relocated away from direct solar glare. Sedentary office workers would benefit from measures
to increase the throughput of cooler air from the north side of the building or by means of a
mechanical cooling system.
Questions
1. A banking ball is cooled in summer by an air-conditioning system that provides an air flow
rate of 5 m 3 /s to remove sensible heat gains of 50 kW. Room air temperature is maintained
at 23 ◦ C. Derive the formula for calculating the supply air temperature and find its value.
2. A room has a sensible heat gain of 10 kW and a supply air temperature of 10 ◦ C d.b. Find
the supply air rate required to keep the room air down to 20 ◦ C d.b.
3. Figure 5.22 shows a west-facing window in a warm climate at a latitude of 35 ◦ south around
midday. Explain how solar control is being achieved here and how and if it may be applied
to commercial buildings in other latitudes.
4. Ten people occupy an office and each produces 50 W of latent heat. The supply air flow rate
is 0.5 m 3 /s and its temperature is 12 ◦ C d.b. If the room is to be maintained at 21 ◦ C d.b.
and 50% saturation, calculate the supply air moisture content.
5. The cooling coil of a packaged air conditioner in a hotel bedroom has refrigerant in it at a
temperature of 16 ◦ C. Room air enters the coil at 31 ◦ C d.b. and 40% saturation and leaves
at 20 ◦ C d.b. at a rate of 0.5 m 3 /s.
(a)
(b)
(c)
Is the room air dehumidified by the conditioner?
Find the room air wet-bulb temperature and specific volume.
Calculate the total cooling load in the room.