Online educational resource on achieving indoor environmental quality with radiant based HVAC systems
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Table of Contents

Background Preparation
Units & Dimensions
Psychrometric Chart
Plotting On The Chart
Psychrometric Analysis
 

 

  

Definitions
by Peyush Agarwal
Absolute Humidity (dv):

Absolute humidity (alternatively, water vapor density) dv is the ratio of the mass of water vapor to the total volume of the sample of air:

dv = Mw / V where,
Mw = mass of water vapor of the sample of air
V = total volume of sample air


Bio-climatic Design:

The design of a habitat which reflects the consideration and analysis of environmental variables like dry bulb temperature, relative humidity, air movement and radiation to human perception and response, can be called Bio-climatic Design.


Comfort Zone:

It is an area plotted on the psychrometric chart that pertains to those conditions of dry-bulb temperature, wet-bulb temperature, wind speeds etc. in which most people wearing specified cloths and involved in specific activity will feel comfortable, ie, neither too cold nor too warm.


Density (rho):

The density of a moist air mixture is the ratio of the total mass to the total volume.
rho = (Ma + Mw) / V = (1 / v)(1 + W) where,
Mw = mass of water vapor of the sample of air
Ma = mass of dry air contained in the sample
V = total volume of sample air
v = the moist air specific volume, ft³ / lb (dry air)


Dew-point temperature (td):

Dew-point is the temperature of moist air saturated at the same pressure p, with the same humidity ratio W as that of the given sample of moist air. It is defined as the solution td(p,W) of the equation.
Ws (p, td) = W
The terms derives from the phenomenon of the formation of dew-drops, which are formed when the air at a high temperature and absolute humidity cools down so far that it can no longer hold the moisture as vapor and has to relinquish the excess moisture for that temperature.


Dry Bulb Temperature (DBT):

It is the air temperature taken in shade by an ordinary thermometer.

Enthalpy:

The enthalpy of a mixture of perfect gases equals the sum of the individual parital enthalpies of the components. Therefore, the enthalpy of moist air can be:
h = ha + Whg where,
ha = specific enthalpy for dry air
hg = specific enthalpy for saturated water vapor at the temperature of the mixture.


Humidity Ratio:

Humidity ratio (W) of a given moist air sample is defined as teh ratio of the mass of water vapor to the mass of dry air contained in the sample:
W = Mw / Ma where,
Mw = mass of water vapor of the sample of air
Ma = mass of dry air contained in the sample.


Psychrometry:

Psychrometry deals with thermodynamics properties of moist air and uses these properties to analyze conditions and processes involving moist air.


Relative Humidity (phi):

The humidity of air in any condition can be expressed relative to the amount of moisture the air could support at that temperature, ie, as a percentage of the saturation humidity. This is the relative humidty
RH = (AH/SH) x 100 (%) , where
RH = Relative humidity
AH = Absolute humidty
SH = Saturation humidity


Specific Humidity (q):

Specific humidity is the ratio of the mass of water vapor to the total mass of the moist air sample:
q = Mw / (Mw + Ma) where,
Mw = mass of water vapor of the sample of air.
Ma = mass of dry air contained in the sample.


Specific volume (v):

The specific volume v of a moist air mixture is expressed in terms of a unit mass of dry air, i.e:
v = V / Ma where,
V = total volume of the mixture
Ma = total mass of dry air


Thermal Comfort:

Thermal comfort is that condition of mind that expresses satisfaction with the thermal environment.


Wet-Bulb Temperature (DBT):

The psychrometer consists of two thermometers; one thermometer's bulb is covered by a wick that has been thoroughly wetted with water and the other is dry. When the wet bulb is placed in an airstream, water evaporates from the wick, eventually reaching an equilibrium temperature called the wet-bulb temperature. This temperature is usually below the DBT. It is a measure of the moisture in the air since no water would evaporate in 100% relative humidity and there would be no difference between WBT and DBT.


Click to lookup units and conversions.


 
 

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