Air Condition Terms

There are two ways of heating air — directly and via moisture control.

It is the condition of water vapour that is the issue here.  The following are terms which reflect this, and these terms form essential vocabulary to anyone concerned with air conditions.

MOISTURE CONTENT

kg.kg^-1

The amount of water (as a vapour) present per kilogramme of dry air.

VAPOUR PRESSURE
N.m^-2 or Pascal

This is the partial pressure ( see Law of Dalton) exerted by the water vapour in atmospheric air.

The maximum value of Vapour Pressure (for a specific, given, temperature) is called the SATURATION VAPOUR PRESSURE (Pa).

RELATIVE HUMIDITY
%

The per cent ratio, at a specific temperature, of Vapour Pressure to Saturation Vapour Pressure.

100% x	(	Vapour Pressure	)
		———————————————-
		Saturation Vapour Pressure

When the Saturation Vapour Pressure equals the vapour Pressure, Relative Humidity = 100% so the water vapour condenses out — this is called DEW POINT TEMPERATURE.

SATURATION MOISTURE CONTENT

The amount of water vapour contained in atmospheric air at saturation (at DEW POINT).

PERCENTAGE SATURATION

%

The per cent ratio of the moisture content at some specific temperature, to the Saturation Moisture Content…

100% x	(	Moisture Content	)
		———————————————-
		Saturation Moisture Content

SPECIFIC VOLUME
m³ . kg^-1
The volume of atmospheric air which contains 1.kg dry air per kg of combined dry air and associated water vapour

SPECIFIC MASS
kg.m^-1
mass per volume
density
viscosity

SPECIFIC HEAT CAPACITY
kJ . kg^-1 . C^-1
The amount of heat an element will accept.
Experimentally determined and results tabulated.
The quantity of heat required to raise unit mass through unit change in temperature.

VELOCITY PRESSURE
N.m^-2
Pa
Pressure created by velocity of air flow at any point.
Always a POSITIVE value.

STATIC PRESSURE
N.m^-2
Pa
The radial (bursting) pressure at any point.
A positive value = bursting outward
A negative value = suction inward.

TOTAL PRESSURE
N.m^-2
Pa
Static Pressure plus Velocity Pressure.

WORKING PRESSURE
N.m^-2
Pa

This is the sum of the surrounding atmospheric pressure and the static pressure.

Fluids Constants

SPECIFIC HEAT CAPACITY OF DRY AIR
1.01 . kJ . kg^-1 . C^-1 da

SPECIFIC HEAT CAPACITY OF WATER VAPOUR
1.89 . kJ. kg^-1

SPECIFIC HEAT CAPACITY OF ATMOSPHERIC AIR (stp)
1.012 . kJ . kg^-1 . C^-1

MEAN SPECIFIC HEAT CAPACITY OF WATER
4.185 . kJ. kg^-1

LATENT HEAT OF WATER VAPOUR AT 273K (0C)
2500 . kW

SPECIFIC MASS OF WATER (stp)
1000 . kg . m^-3

SPECIFIC MASS OF ATMOSPHERIC AIR AT 293K (20C)
1.205 . kg . m^-3

VELOCITY OF SOUND IN AIR
330 . m . s^-1

ATMOSPHERIC PRESSURE

The standard atmosphere (1.atm) =

101 325 Pa  — or 101.325 kPa.

The following units are equivalent, but only to the number of decimal places shown:

• 760 mm Hg (Torr),
• 29.92 in Hg,
• 14.696 PSI,
• 1013.25 millibars.

One standard atmosphere is standard pressure used for pneumatic fluid power (ISO R554), and in the aerospace industry(ISO 2533) and petroleum industry(ISO 5024) .

Temperature Correcting

Because the specific mass of atmospheric air at 293 . K

= [1.205 . (293. K/293 . K) ] . kg . m^-3

= 1.205 . kg . m^-3

So the specific mass of atmospheric air at a temperature of 29.C

= [1.205 . (293 /273+29) ] . kg . m^-3

= [1.205 . (293 /302) ] . kg . m^-3

= 1.169 . kg . m^-3

which is the specific mass corrected for the higher temperature.