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Chapter 5 Air Control in Buildings

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  1. Chapter 5 Air Control in Buildings What air do we need ? —— fresh air —— Correct temperature —— Contains the right amount moisture and —— Is free from odours and pollutants. air supply of a building also affects * the thermal design *energy performance (consumption).

  2. the reasons and requirements for ventilation calculaterates of ventilation (通风换气量) for particular environments. ventilation and air-conditioning installations the nature and effects of moisture in the air Know the terms and measurements of humidity Use psychrometric charts(温湿图)to predict states of air What is condensation in buildings why condensation occurs in buildings how to control condensation calculate the risk of condensation in walls and roofs

  3. 5.1 Ventilation 通风 • Why do we need ventilation ? —— Supply of Oxygen —— Removal of carbon dioxide —— Control of humidity for human comfort —— Control of air velocity for human comfort —— Removal of odours —— Removal of heat ……….

  4. How much ventilation quantity do we need? • Two methods of specifying the ventilation quantity: Air supply to a space: such as 1.5 room air changes per hour Table 5.1 typical ventilation rate Air supply to a person: Such as 8 litres per second per person. table 5.2 typical ventilation standards for dwellings PSV=passive stack ventilation 被动竖井通风

  5. Worked example 5.1 • How to calculate the extraction rate (排气量)of an extract fan(排气扇)for ventilation aims? • Accommodation 住处,调节 • Urinal 小便池 • Ach:air change per hour

  6. ventilation can be divided into (1)natural ventilation What cause natural ventilation? Air pressure difference Stack effect (2)mechanical ventilation Table 5.4 features of mechanical ventilation

  7. Coil 盘管(盘起来的管子) Evaporator 蒸发器 Condenser 冷凝器 Cooling coil 冷却盘管 Heating coil 加热盘管 heater

  8. Diffusers 散流器

  9. Figure 5.1 schematic diagram of ventilation system

  10. Extract grilles 排风百叶

  11. Delivery grilles 送风百叶

  12. Air conditioning can also realize ventilation Figure 5.2 schematic diagram of air-conditioning installation

  13. 5.2 humidity 湿度 Why do we talk about humidity? —— can affect human comfort in buildings ——condensation in buildings ——the durability of materials ——the drying of materials ——the operation of industrial processes —— the growth of plants

  14. Say in brief : • For humans relative humidity below 25% feels uncomfortable dry. • Relative humidity above 60% feels uncomfortable wet. • Human comfort requires the relative humidity to be in the range 25-60% RH.

  15. how humidity affects the performance of buildings • Relative humidity above 70% may cause condensation on cold surfaces - causing mold, corrosion and moisture related deterioration(恶化、变质、退化). • To low relative humidity may cause problems with static electricity, cracking of paint and shrinkage of wood furniture and wood floors. Condensation in buildings 建筑内冷凝

  16. Where is the moisture from? • Most of the moisture is a result of evaporation from the sea which covers more than two-thirds of the Earth’s surface. • At any Particular place ,natural humidity is dependent on local weather conditions • Inside a building ,humidity is further affected by the thermal properties and the use of the building

  17. Some terms about humidity (1)Water vapour 水蒸气 Can be defined as a substance in the gaseous state which may be liquefied by compression.

  18. How is the Water vapour in the air formed? Water vapour is formed naturally in the space above liquid water which is left open to the air • This process of evaporation occurs because some liquid molecules gain enough energy, from chance collisions with other molecules, to escape from the liquid surface and become gas molecule. • The latent heat required for this change of state is taken from the other molecules of the liquid, which therefore becomes cooled.

  19. the difference between vapour, steam and mist Water vapour is invisible Steam and mist, which can be seen, are actually suspended droplets小滴of water liquid, not water vapour.

  20. (2)saturation 饱和 dry air moisture air Air unsaturated air 未饱和空气Saturated air 饱和空气 • If a sample of air contains the maximum amount of vapour possible at that temperature • Saturated vapour pressure饱和水蒸汽压力 SVP is found to increase with increase in temperature Warm air can hold more moisture than cold air • Question: Which one can hold more moisture, Warm air or cold air? And why? Which can be abbreviated to SVP

  21. Figure 5.3 saturated air

  22. (3)moisture content 含湿量 Moisture content Unit: kg/kg of dry air Moisture content is not usually measured directly but it can be obtained from other types of measurement. Moisture content values are needed in determining what quantity of an air-conditions plant needs to add or to extract from a sample of air

  23. (4)Vapour pressure 水蒸气(分)压力 • Vapour pressure is the partial pressure exerted by the molecules of a vapour. • Unit: pascal (Pa) • Vapour pressure are usually derived from other measurements • One of the uses of vapour pressure is to determine the rate which water vapour moves through materials

  24. (5)dew-point 露点 • The dew-point is the temperature at which a fixed sample of air becomes saturated. • Unit: ℃ or K • Dew point values can be measured directly or derived from other measurement

  25. (6)relative humidity 相对湿度is commonly abbreviated toRH • SVP is the short of saturated vapour pressure. • Unit : per cent RH , at a specified temperature • A convenient alternative formula for RH

  26. Heating the air lowers the RH And for the same reason, • Cooling the air increases the RH

  27. Worked example 5.2 • Show us how to calculate RH

  28. How to measure humidity? • Hygrometers 湿度计(psychrometers) (1)hair, paper hygrometer 毛发,纸张湿度计 Use the fact that animal hair or paper change their dimensions with change in moisture content. • these instruments can be made to give a direct reading of RH on their details, but they need calibration校准against another instrument.

  29. (2)Dew air hygrometer露点湿度计 • The dew point temperature and the room temperature can be used to obtain an RH value from tables or charts

  30. (3)Wet-and-dry-bulb hygrometer干湿球湿度计 Whirling 涡流 sling hygrometer 通风温湿度计 Figure 4.4 Whirling sling hygrometer

  31. Question: • Dry air and moist air , which one has a lower wet bulb temperature? And why? • In what case, dry bulb temperature equals wet bulb temperature?

  32. Answer: • Dry air and moist air , which one has a lower wet bulb temperature? And why? • Dry air with a low RH causes rapid evaporation and produces a greater wet bulb depression than moist air. • In what case, dry bulb temperature equals wet bulb temperature? • Saturated air at 100 per cent RH causes no net evaporation and the dry bulb and wet bulb thermometers then record the same temperature.

  33. Psychrometric chart 温湿表

  34. how to use the psychrometric chart?Worked worked example 5.2 External air at 0 ℃ and 80%RH is heated to 18 ℃. Use the psychrometric chart to determine the following information. a) The RH of the heated air b) The RH of the heated air if 0.005kg/kg of moisture is added c) The temperature at which this moistened air would first condense. Initial conditions: dry bulb =0 ℃, RH=80%, so moisture content= 0.003kg/kg

  35. a) For the heated air moisture content =0.003kg/kg dry bulb= 18 ℃ So , reading from chart RH=23% b) For the moistened air moisture content =0.003+0.005=0.008kg/kg dry bulb= 18 ℃ So , reading from chart RH=62% c) For condensation RH=100% moisture content =0.003+0.005=0.008kg/kg So , reading from chart dew point =10.8 ℃ gives condensation

  36. 5.3 Condensation in Buildings 建筑内冷凝 • Relative humidity above 70% may cause condensation on cold surfaces - causing mold, corrosion and moisture related deterioration(恶化、变质、退化).

  37. This mould growth on contaminated wallpaper represents a potential environmental health hazard to those occupying the building.

  38. Condensation in buildings occurs whenever The general requirement for condensation are moist air + cold surface. 5.3.1 Principles of condensation 冷凝的原理 Surfaces that are at or below the dew point of that air meets Warm moisture air

  39. Condensation can be divided into : (1)surface condensation 表面冷凝 (2)interstitial condensation 内部冷凝 Figure 4.6 Interstitial condensation

  40. 5.3.2 Cause of condensation 冷凝的原因 (1)Moisture sources 湿源 (2) temperature (3)Ventilation (4)Use of buildings

  41. Mould growth can occur in carpet and underlay in both bathrooms and showers with inadequate ventilation.

  42. An exhaust duct from an extractor fan in a shower unit installed in an 18th century country house, is shown discharging into 排入the roof void and providing the conditions for condensation, mould growth and insect decay 腐烂 in roof timbers. It also shows an uninsulated cold water tank which resulted in cold bridge condensation of moisture air.

  43. 5.3.3 Remedies for condensation 冷凝的补救 (1)ventilation (2)heating (3)insulation

  44. Chapter 5 Air Control in Buildings 5.1 Ventilation Why do we need ventilation? How to specify the quantity of ventilation? How to realize ventilation ? 5.2 Humidity Some specification of humidity: moisture content; vapour pressure; dew-point; relative humidity; How to measure humidity : hair,paper hygrometer; dew-point hygrometer; wet-and-dry-bulb hygrometer Psychrometric chart 5.3 Condensation in buildings The cause ,What results ,How to remedy for condensation? 5.4 Condensation conditions 冷凝条件

  45. 5.4.1 Vapour transfer 湿传递 areas of higher vapour pressure • Because the occupants of a building and their activities add moisture to the air, the vapour pressure of the inside air is usually greater than that of the outside air • In general ,Water vapour passes through structures from inside to outside Vapour transfer from areas of lower vapour pressure

  46. Page114 • a similar formula

  47. Vapour resistance 蒸汽热阻(Rv) of that layer Vapour resistivity 蒸汽比热阻 (rv) Vapour resistance 蒸汽热阻Rv Total vapour resistance 总蒸汽热阻(RvT) of a compound structure

  48. Worked example 5.3 plot the structural temperature gradients and the dew-point gradients. R in m2K/W, are: 0.123+0.06+0.75+0.105 +0.055 rV, in MN s/g m, 50 100 30 RV in MN s/g: 0.5 2.5 4.5