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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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 = 13 636 s = 3.79 hours Book E3 Section 1.4 Air-conditioning

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