Sharp JX-9400 Technické informace stáhnout pdf (Strana 101)

Figure 5.1, a psychrometric chart for air, shows several characteristics of

humid air. The curves show the water content of air as a function of its

temperature for various relative humidities. The water content cannot be

greater than that shown by the saturation curve. Air with a relative humidity

0 <’<1 has a water content ’ times that of the saturated air.

Energy is needed for heating or cooling the air as well as for evaporating

water in it or condensing water for drying it. Taking as a reference dry air at

08C, the speciﬁc enthalpy or energy needed to hea t and humidify 1 kg of air

to reach the temperature  and humidity ratio x is:

h ¼ c

 þðL þ c

Þx ð5:1Þ

where:

is the speciﬁc heat capacity of dry air, about 1006 J/(kgK);

is the speciﬁc heat capacity of water vapour, about 1805 J/(kgK);

L is the latent heat of evaporation, i.e. the heat required to evaporate 1 kg of

water, about 2,501,000 J/kg;

x is the humidity ratio, i.e. the mass of water vapour per kilogram of dry

air.

This humidity ratio, x, is related to the water content, !, which is the mass

concentration of water vapour in moist air of Figure 5.1, by:

x ¼

1  !

and ! ¼

1 þ x

ð5:2Þ

Water content [g/kg]

Sat.

90%

80%

70%

60%

50%

40%

30%

20%

10%

0 10 20 30 ºC

90 kJ/kg

Figure 5.1 Psychrometric chart with constant relative humidity curves

and constant enthalpy lines

Note: It is shown that air at 208C and 50 per cent relative humidity contains about

7.5 g of water vapour per kilogram. Its enthalpy is 39 kJ/kg and its dew point is close

to 108C.

80 Ventilation and Airﬂow in Buildings

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