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).

the reasons and requirements for ventilation calculate rates 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

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

……….

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

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

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

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

Diffusers 散流器

Figure 5.1 schematic diagram of ventilation system

Extract grilles 排风百叶

Delivery grilles 送风百叶

Air conditioning can also realize ventilation

Figure 5.2

schematic diagram of air-conditioning installation

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

Say in brief ： For humans relative humidity below 25% feels uncomfort

able dry.

Relative humidity above 60% feels uncomfortable wet.

Human comfort requires the relative humidity to be in the

range 25-60% RH.

how humidity affects the performance of buildings

Relative humidity above 70% may cause condensation o

n cold surfaces - causing mold, corrosion and moisture r

elated deterioration （恶化、变质、退化） .

To low relative humidity may cause problems with static

electricity, cracking of paint and shrinkage of wood furnit

ure and wood floors.

Condensation in buildings 建筑内冷凝

Where is the moisture from?

Most Most of the moisture is a result of evaporation from t

he sea which covers more than two-thirds of the Eart

h’s surface.

At any Particular place Particular place ,natural humidity is depende

nt on local weather conditions

Inside a buildingInside a building ,humidity is further affected by the

thermal properties and the use of the building

Some terms about humidity

（ 1 ） Water vapour 水蒸气 Can be defined as a substance in the gaseou

s state which may be liquefied by compression.

How is the Water vapour in the air formed?

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.

Water vapour is formed naturally in the

space above liquid water which is left o

pen to the air

the difference between vapour, steam and mist

Water vapour is invisibleinvisible

Steam and mist, which can be seencan be seen, are actually

suspended droplets 小滴 of water liquid, not water vapour.

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

dry air

moisture air

Air （ 2 ） saturation 饱和

Figure 5.3 saturated air

（ 3 ） moisture content 含湿量

Moisture content

Unit: kg/kg of dry air

Moisture content is not usually measured directly but it c

an be obtained from other types of measurement.

Moisture content values are needed in determining wh

at quantity of an air-conditions plant needs to add or to

extract from a sample of air

dryair when that of mass

sampleair in our water vapof mass

（ 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 determi

ne the rate which water vapour moves through materials

（ 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

（ 6 ） relative humidity 相对湿度 is commonly abbreviated to RH

SVP is the short of saturated vapour pressure. Unit :Unit : per cent RH , at a specified temperature A convenient alternative formula for RH

100 temp.sameat sample of SVP

sample of pressure vapour humidity relative

temp.roomat SVP

point dewat SVP humidity relative

Heating the air lowers the RH

And for the same reason, Cooling the air increases the RH

100 temp.sameat sample of SVP

sample of pressure vapour humidity relative

Worked example 5.2

Show us how to calculate RH

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 the

ir 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.

（ 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

（ 3 ） Wet-and-dry-bulb hygrometer 干湿球湿度计

Figure 4.4 Whirling sling hygrometer

Whirling 涡流sling hygrometer 通风温湿度计

Question:

Dry air and moist air , which one has a lower wet

bulb temperature? And why?

In what case, dry bulb temperature equals wet b

ulb temperature?

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.

Psychrometric chart 温湿表

how to use the psychrometric chart ？Worked worked example 5.2

External air at 0 and 80%RH is heated to 18 . Use the psy℃ ℃chrometric 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 cond

ense.

Initial conditions: dry bulb =0 , RH=80%, so moisture content℃= 0.003kg/kg

a) For the heated air a) For the heated air moisture content =0.003kg/kg dry bulb= 18 ℃ So , reading from chart RH=23%

b) For the moistened airb) For the moistened air moisture content =0.003+0.005=0.008kg/kg dry bulb= 18 ℃ So , reading from chart RH=62%

c) For condensationc) For condensation RH=100% moisture content =0.003+0.005=0.008kg/kg So , reading from chart dew point =10.8 gives condensation℃

5.3 Condensation in Buildings 5.3 Condensation in Buildings 建筑内冷凝建筑内冷凝

Relative humidity above 70% may cause condensation o

n cold surfaces - causing mold, corrosion and moisture r

elated deterioration （恶化、变质、退化） .

This mould growth on

contaminated wallpaper

represents a potential

environmental health hazard to

those occupying the building.

5.3.1 Principles of condensation 5.3.1 Principles of condensation 冷凝的原理冷凝的原理

Condensation in buildings occurs whenever

The general requirement for condensation are

moist air + cold surface.

Warm moisture air

Surfaces that are at or below the dew point of that air

meets

Condensation can be divided into :Condensation can be divided into :

（ 1 ） surface condensation 表面冷凝（ 2 ） interstitial condensation 内部冷凝

Figure 4.6 Interstitial condensation

5.3.2 Cause of condensation 5.3.2 Cause of condensation 冷凝的原因冷凝的原因

（ 1 ） Moisture sources 湿源（ 2 ） temperature

（ 3 ） Ventilation

（ 4 ） Use of buildings

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

An exhaust duct from an ext

ractor fan in a shower unit in

stalled in an 18th century co

untry house, is shown disch

arging into 排入 the roof voi

d and providing the conditio

ns for condensation, mould

growth and insect decay 腐烂 in roof timbers. It also sh

ows an uninsulated cold wat

er tank which resulted in col

d bridge condensation of mo

isture air.

5.3.3 Remedies for condensation 5.3.3 Remedies for condensation 冷凝的补冷凝的补救救

（ 1 ） ventilation

（ 2 ） heating

（ 3 ） insulation

Chapter 5 Air Control in BuildingsChapter 5 Air Control in Buildings

5.1 Ventilation5.1 Ventilation

Why do we need ventilation?

How to specify the quantity of ventilation?

How to realize ventilation ?

5.2 Humidity5.2 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 buildings5.3 Condensation in buildings

The cause ,What results ,How to remedy for condensation?

5.4 Condensation conditions 5.4 Condensation conditions 冷凝条件冷凝条件

5.4.1 Vapour transfer 5.4.1 Vapour transfer 湿传递湿传递

Because the occupants of a building and their activitie

s add moisture to the air, the vapour pressure of the in

side air is usually greater than that of the outside air

In general ,Water vapour passes through structures fro

m inside to outside

areas of higher vapour pressure

Vapour transfer from

areas of lower vapour pressure

Page114

a similar formula

TTR

R

TVT

V PR

R P

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 structu

re

TVT

V PR

R P

dyresistivitrkd )(/R

TTR

R

Lrdr VV VR

etcRRR VVV ....R 321VT

Worked example 5.3

R in m2K/W, are:R in m2K/W, are: 0.123+0.06+0.75+0.105 +0.055

rrVV, in MN s/g m,, in MN s/g m, 50 100 30

RRVV in MN s/g: in MN s/g: 0.5 2.5 4.5

plot the structural temperature gradients and the dew-point gradients.

TTR

R

TVT

V PR

R P

5.4.2 Dew-point gradients 5.4.2 Dew-point gradients 露点梯度露点梯度

Figure 5.7 Prediction of condensation

5.4.3 Condensation risk5.4.3 Condensation risk

Surface condensation 表面冷凝Occurs on the surfaces of the walls, windows, ceilings and f

loors. Interstitial condensation 内部冷凝Occurs within the construction of a building as shown in

Surface condensation 表面冷凝 Mould growth

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

Interstitial condensation 内部冷凝

Can damage important structural materials ,

Such as steelwork,

Can make insulating material less effective.

How to reduce the risk of interstitial condensation ？5.4.4 vapour barrier 5.4.4 vapour barrier ，， vapour check vapour check 防潮层防潮层

has a high resistance ,Can be classified into ： Liquid films 液膜 :

bituminous solutions 沥青 , rubberised 橡胶 or siliconised 硅化paints 涂料 , gloss paints 光泽涂料 ,

Pre-formed membranes 预制薄膜 :

aluminium foil board 铝箔板 , polythene-backed board 聚乙烯背面板 , polythene sheet 聚乙烯板 ,bituminous felt 沥青油毛毡 ,

vinyl paper 乙烯板 Check 阻止 , 控制 , 阻止物 , 支票 , 检讫的记号 , 饭馆的帐单

For a vapour barrier to be effective,

it must block the passage of water vapour before the vap

our meets an environment below the dew-point temperat

ure.

Vapour barriers must be installed on the warm side of th

e insulation layer.

Some examples ：（ 1 ） Wall vapour barriers 墙体防潮层

Vapour barriers should be installed on the warm

side of the insulation layer in a wall

（ 2 ） Roof vapour barriers 屋顶防潮

Figure 5.9 Vapour barriers in roofs

Warm roofs ：绝热层在屋顶板防水材料之下

A warm roof has its thermal insulation placed immediately b

eneath the waterproof covering and then protected with a v

apour barrier.

waterproof covering

inverted roof 倒置屋顶 “ upside-down roof ”

 Is a variation of warm roof design resulting from the installation of thermal insulating boards above the water proof layer of the roof.

The insulator must:　　 low-k ，而蒸汽渗透性大，材料内部不存在毛细管，处于反复冻融的条件下材质不变，吸水率小，抗压强度高

waterproof covering

That’s all for chapter 5.Today’s homework

Exercise 5 on Page120

Please copy down the exercise

If you have questions, you can ask me.

Class practice

1 saturated vapour pressure is found to increase with ( )

A increase with pressure

B increase with temp

C increase with volume

D increase with mass

2 wet-bulb temp may ( )

A be greater than dry-bulb temp

B be equal to dry-bulb temp

C be less than dry-bulb temp

D the above three all are possible

3 what can cause condensation in buildings( )

A moisture sources

B temperature

C ventilation

D use of buildings

4 how to remedy for condensation? ( )

A ventilation

B heating

C cooling

D insulation

5 RV is the signal of ( )

A thermal resistance

B thermal resistivity

C vapour resistance

D vapour resistivity

6 the following roof is ( )

A cold roof B warm roof

C inverted roof D upside-down roof