Czech Technical University in Prague Department of Microenvironmental and Building Services Engineering http://tzb.fsv.cvut.cz 125BEE2 Ventilation systems. Moist air processes

1

Assessment of minimum supply air volume: According to:

- Pollutants production - Recommended air change

Pollutants production Mentioned major air pollutants available in an office building as: Thermal load

( )sias ttc

QV−

=..ρ

[m3.s-1]

Q – total thermal load of a space [kW] ca – specific heat of air [kJ.kg-1.K-1] r – density of air [ 1.2 kg.m-3] ti – indoor air temperature [°C] ts – supply air temperature [°C]

Moisture production

( )sis xx

GV−

=.ρ

[m3.s-1]

G – total moisture gain of a space [g.s-1] r – density of air [ 1.2 kg.m-3] xi – moisture content of indoor air [g.kg-1 d.a.] xs – moisture content of supply air [g.kg-1 d.a.]

Supply air volume Vs:

a) Supply air as a mixture of outside air Ve and circulating air Vc. The ratio of mixed circulated air depends on pollutants produced in the conditioned space.

Vs = Ve + Vc

b) Supply air as adapted outside air Ve. In the case of high production of pollutants in the

conditioned space, so exhausted air is contaminated and cannot be used as circulating air. Vs = Ve

c) Supply air as circulating air Vc. Only for a short time, until the concentration of pollutants in the air reach it’s ultimate level.

Vs = Vc Ve – outside air [m3/h] Vc – circulating air, exhausted air from conditioned space [m3/h] Assessment of minimum outside air volume: According to:

- Number of occupants Room with prohibited smoking 30 m3.h-1.person-1 Room with permitted smoking 60 m3.h-1.person-1 - Floor area of conditioned space Shop, store, etc (ground floor) 5.5 m3.h-1.person-1 Shop, store, etc (upper floors) 3.5 m3.h-1.person-1 Corridor 1.0 m3.h-1.person-1

The minimum of air volume of outside fresh air is 15% of total supply air.

Czech Technical University in Prague Department of Microenvironmental and Building Services Engineering http://tzb.fsv.cvut.cz 125BEE2 Ventilation systems. Moist air processes

2

Basic theory of the Mollier’s chart What is Mollier’s chart? - the tool for determining of isobaric psychrometric processes of moist air - suitable for steady conditions - similar to psychrometric chart used mainly in Anglo-Saxon literature - each chart is determined for specific pressure Description of the Mollier’s chart

Czech Technical University in Prague Department of Microenvironmental and Building Services Engineering http://tzb.fsv.cvut.cz 125BEE2 Ventilation systems. Moist air processes

3

Determination of the dew point temperature and the wet-bulb temperature dew point temperature td: The temperature of moist air saturated at the same pressure p and with the same moisture content x as the given state of moist air. thermodynamic wet-bulb temperature tw: The temperature at which water evaporating into moist air, can bring air to the adiabatic saturation. The lowest temperature of adiabatic cooling of air.

Training example for diagram use Air state: dry-bulb temperature t = 15°C moisture content x = 7,8 g/kg d.a. barometric pressure p = 100 kPa

Find other moist air properties: specific enthalpy, relative humidity, dew point temperature, wet-bulb temperature

Results: specific enthalpy h = 35 kJ/kg, relative humidity rh = 73 %, dew point temperature td = 10 °C, wet-bulb temperature tw = 12,5 °C Air state: specific enthalpy h = 55 kJ/kg relative humidity rh = 45 % barometric pressure p = 100 kPa

Find other moist air properties: dry-bulb temperature, moisture content, dew point temperature, wet-bulb temperature

Results: dry-bulb temperature td = 28 °C, moisture content x = 10,6 g/kg d.a., dew point temperature td = 15 °C, wet-bulb temperature tw = 19 °C

Czech Technical University in Prague Department of Microenvironmental and Building Services Engineering http://tzb.fsv.cvut.cz 125BEE2 Ventilation systems. Moist air processes

4

Basic psychrometric processes Introduction - any process has two heat components

- sensible heat S – moisture is constant - latent heat L – temperature is constant 1 – 2 is any process in Mollier’s chart total heat Q = L + S where: latent heat component L = ma.(h2 – h1’) sensible heat component S = ma.(h1’ – h1) sensible heat factor u: - ratio of sensible heat component included in total heat

Sensible heating and cooling - the process with no changes of moisture content x - the sensible cooling and the sensible heating are reversed processes - in the case of sensible cooling the surface temperature of cooler have to be higher than the dew point temperature of moist air Properties of states: x1 = x2 sensible cooling: t2 < t1 rh2 > rh1

htc

QS

ΔΔ

==.ϑ

Czech Technical University in Prague Department of Microenvironmental and Building Services Engineering http://tzb.fsv.cvut.cz 125BEE2 Ventilation systems. Moist air processes

5

h2 < h1 heating t2 > t1 rh2 < rh1 h2 > h1 Equation: The heat required for the process: sensible cooling: Q = ma.(h1 - h2) = ma.cd.(t1 – t2) [kJ/s] = [kW] heating Q = ma.(h2 – h1) = ma.cd.(t2 – t1) [kW]

Cooling - the process with change of moisture content x - the surface temperature of cooler is lower than the dew point temperature of moist air → the condensation occurs Properties of states: x1 > x2 t2 < t1 rh2 > rh1 h2 < h1 Equations: The heat required for the process: Q = ma.(h1 - h2) [kW] The rate of condensing moisture:

Czech Technical University in Prague Department of Microenvironmental and Building Services Engineering http://tzb.fsv.cvut.cz 125BEE2 Ventilation systems. Moist air processes

6

mw = ma.(x1 – x2) [kg/s] Mollier’s chart:

The different between sensible cooling and cooling - while the cooler’s surface temperature is lower than the dew point temperature condensation of water vapour occurs. → a part of total heat is used for change from gaseous state to liquid state

Czech Technical University in Prague Department of Microenvironmental and Building Services Engineering http://tzb.fsv.cvut.cz 125BEE2 Ventilation systems. Moist air processes

7

Humidification - the moisture content can be increased in two ways

a) the direct injection of steam into the air stream b) passing the air through a spray chamber containing small water droplets

a) Steam injection Definition the mass balance: ma1 = ma2 = ma steady-flow energy equations: ma.x1 + mv = ma.x2 ma.h1 + mv.hv = ma.h2 ⇒ mv.hv = ma.( h2 - h1) mv = ma.(x2 - x1)

Properties of states: x1 < x2 t2 = t1 rh2 > rh1 h2 > h1 Mollier’s chart: ratio d

where: hv – specific enthalpy of injected steam (or any liquid) hv varies about 2,6 kJ/kg

ABAB ttSSLLSQBLLSQA

hhhhmQ

22

21

0;)0;)

;.

>→<⇒≠+==+=−=ΔΔ=

procestheofheatspecificqhxxhh

v →==−−

→ 1212

12

vhxh=

ΔΔ

=δ

Czech Technical University in Prague Department of Microenvironmental and Building Services Engineering http://tzb.fsv.cvut.cz 125BEE2 Ventilation systems. Moist air processes

8

The process of steam injection in the air stream is taken with reasonable accuracy as isothermal. b) Humidification in spray chamber Definition

Energy balance:

ma.dh = ma.dx.hf hf of water at temperature 10 to 15 °C varies between 0 to 0,42 kJ/kg Properties of states: x1 < x2 t2 < t1 rh2 > rh1 h2 = h1

tionhumidificainjectionsteamtheinasequationsamethehdxdhhdxmdhm ffaa ;=→⋅⋅=⋅

Czech Technical University in Prague Department of Microenvironmental and Building Services Engineering http://tzb.fsv.cvut.cz 125BEE2 Ventilation systems. Moist air processes

9

Equations: The rate of water transported into the air stream: mw = ma.(x2 – x1) [kg/s]

Mollier’s chart: Humidification with water in spray chamber is taken with reasonable precision as adiabatic ⇒ specific enthalpy is constant Mixing Definition - mix of two air streams steady-flow energy equation: ma1.h1 + ma2.h2 = ma3.h3 mass balance equation: ma1 + ma2 = ma3 for dry air ma1.x1 + mva2.x2= ma3.x3 for water vapour from the equations ensues:

Mollier’s chart:

231331

31

23

23

31

23

31

23

2

1 δδ ==−−

=−−

⇒−−

=−−

=xxhh

xxhh

xxxx

hhhh

mm

a

a

Czech Technical University in Prague Department of Microenvironmental and Building Services Engineering http://tzb.fsv.cvut.cz 125BEE2 Ventilation systems. Moist air processes

10

In the case of result state is below saturation curve: state 3 is oversaturated air ⇒ air immediately changes to the saturated air according to the line of constant temperature, Dx is amount of condensed water steam.