1. Book E3 Section 1.4 Air-conditioning
Chilled drinks
How an air-conditioner works
Check-point 9
Cooling capacity
Reusing heat
Check-point 10 1 2 3
Book E3 Section 1.4 Air-conditioning

2. Book E3 Section 1.4 Air-conditioning
Chilled drinks
cold drink
hotter surroundings
heat
 less cold
Is it possible to make the cold drink even colder in the hotter surroundings?
Yes, if we make use of a heat pump.
Book E3 Section 1.4 Air-conditioning

3. 1 How an air-conditioner works
Natural direction of heat flow:
high-temperature bodies  low-temperature ones
hot body
cold body
heat
temperature decreases
temperature increases
Heat pump: a device that transfers heat against its natural direction of flow
transfer heat from a cold body to a hot body
Book E3 Section 1.4 Air-conditioning

4. 1 How an air-conditioner works
Example of heat pump
Does work to transfer heat from a cold room to the hotter outside
Overall amount of heat released QH > Heat removed from the room QC
 The outside of buildings is particularly hot when air-conditioners are turned on indoors.
Book E3 Section 1.4 Air-conditioning

5. 1 How an air-conditioner works
An air-conditioner consists of a sealed loop of coils and tubes.
 The refrigerant (coolant) flows inside.
 Carries energy from the room to the outside
refrigerant
energy
Book E3 Section 1.4 Air-conditioning

6. 1 How an air-conditioner works
The liquid refrigerant (colder than the room) passes through the evaporator coil.
sealed loop
evaporator coil
room
outside
absorbs energy and evaporates
heat removed from the colder room
The air becomes cooler and drier.
Book E3 Section 1.4 Air-conditioning

7. 1 How an air-conditioner works
2. The compressor compresses the vapour of the refrigerant (does work on it).
sealed loop
evaporator coil
room
outside
pressure and temperature 
heat removed from the colder room
compressor
Book E3 Section 1.4 Air-conditioning

8. 1 How an air-conditioner works
3. The hot pressurized vapour passes the condensing coil.
sealed loop
evaporator coil
room
outside
releases heat and condenses back into a liquid
condensing coil
heat removed from the colder room
heat released to the hotter outside
compressor
Book E3 Section 1.4 Air-conditioning

9. 1 How an air-conditioner works
4. The liquid refrigerant then passes through the expansion valve.
sealed loop
evaporator coil
room
outside
expansion valve
expands rapidly and cools down
condensing coil
heat removed from the colder room
heat released to the hotter outside
compressor
Book E3 Section 1.4 Air-conditioning

10. 1 How an air-conditioner works
Basic structure of a typical window-type air-conditioner:
Simulation
1.3 The working principles of air-conditioners
Book E3 Section 1.4 Air-conditioning

11. Book E3 Section 1.4 Air-conditioning
Check-point 9 – Q1
1. Arrange the following temperatures T in ascending order:
p = T of the room
q = T of outside
r = T of refrigerant in 
s = T of refrigerant in  r p q s
___ < ___ < ___ < ___
Book E3 Section 1.4 Air-conditioning

12. change of T of the refrigerant Book E3 Section 1.4 Air-conditioning
Check-point 9 – Q2
change of T of the refrigerant    
increase
increase
decrease
decrease
Book E3 Section 1.4 Air-conditioning

13. Book E3 Section 1.4 Air-conditioning
2 Cooling capacity
Cooling capacity describes how much heat can be removed from a room by an air-conditioner.
The cooling capacity is the rate at which heat is removed.
Unit: kilowatts (kW)
Typically 2–3 times its power input
Example 9
Cooling capacity
Book E3 Section 1.4 Air-conditioning

14. Book E3 Section 1.4 Air-conditioning
Example 9
Cooling capacity
An air-conditioner (cooling capacity = 2.78 kW) is turned on for 8 hours
Find the max. amount of heat it can remove from the room.
Max. amount of heat removed
= 2780  8  3600
= 8.01  107 J
= 80.1 MJ
Book E3 Section 1.4 Air-conditioning

15. Book E3 Section 1.4 Air-conditioning
2 Cooling capacity
Different types of air-conditioner have different cooling capacities.
Type
Window
Split system
Chiller
Cooling capacity
1.5–2.5 kW
2.5–10 kW
~1000 kW
The cooling capacity depends on the room size, lighting, the number of windows and the building materials used.
Book E3 Section 1.4 Air-conditioning

16. Book E3 Section 1.4 Air-conditioning
3 Reusing heat
Heat extracted from a building
 Exhausted into the atmosphere directly
 pollution
 Can be used before being exhausted
Large central air-conditioning systems can use heat recovery chillers.
 Recover waste heat for preheating water in the domestic water main or space heating
 E.g. some hospitals and hotels
Marine Outer Waters District Headquarters
Book E3 Section 1.4 Air-conditioning

17. Book E3 Section 1.4 Air-conditioning
3 Reusing heat
Large water-cooled air-conditioning systems with suitable ground conditions can use geothermal heat pump to recover the energy.
 Store the energy of circulating water beneath the ground.
 Utilizes the energy for space or water heating in winters and cooling in summers.
 E.g. Hong Kong Wetland Park
Book E3 Section 1.4 Air-conditioning

18. Book E3 Section 1.4 Air-conditioning
Check-point 10 – Q1
With its max. power, an air-conditioner can remove 61.2 MJ of heat from a room in 5 hrs.
Estimate its cooling capacity.
Cooling capacity =
heat removed
time =
61.2  106
5  3600
= 3400 W
= 3.4 kW
Book E3 Section 1.4 Air-conditioning

19. Book E3 Section 1.4 Air-conditioning
Check-point 10 – Q2
Cooling capacity of an air-conditioner = 1.1 kW
What is the min. time for the air-conditioner to remove 15 MJ of heat from the room?
Min. time =
heat removed
cooling capacity =
15  106
1.1  103
= s
= 3.79 hours
Book E3 Section 1.4 Air-conditioning

20. Book E3 Section 1.4 Air-conditioning
The End
Book E3 Section 1.4 Air-conditioning