PSYCHROMETRIC PROPERTIES & PSYCHROMETRIC CHART CLASS NOTES

PSYCHROMETRIC PROPERTIES

& PSYCHROMETRIC CHART CLASS

NOTES

Psychrometry is the study of the

properties of moist air (dry air + water

vapor). It includes the study of

psychrometric chart and

 Psychrometric processes. This chart

contains psychrometric (moist air)

properties. Use psychrometric

processes to achieve desired conditions

from the existing room conditions.

It is quite easy to learn. In summer, it is

cooling and dehumidification. In winter,

it is heating & humidification.

1. Dry Air

Mixture of gases (mainly oxygen and nitrogen) which does not contain water vapor.

2. Moist air

Moist air is a mixture of dry air and water vapor.

3. Saturated air

Saturated air contains maximum water vapors at a particular temperature.

For saturated air,

Dry bulb temperature= wet bulb temperature= dew point temperatures

Relative humidity is 100 %.

4. Dry air temperature

Temperature of dry air is the temperature of atmospheric air (moist air). A thermometer measures dry bulb temperature.

5. Wet bulb temperature

Temperature of fully saturated air. Wet bulb thermometer measures it. Sling Psychrometer has the wet bulb thermometer..

6. Sling Psychrometer

Sling psychrometer measures dry and wet bulb temperatures.

7. Dew point temperature

At dew point temperature, first condensation of water vapor takes place on sensible cooling of moist air. Dry bulb temperature is greater or equal to wet bulb temperature. Wet bulb temperature is greater or equal to dew point temperature.

9. Specific humidity 

Specific humidity (w) is the amount of water vapor per kg of dry air. Mathematically

w = 0.622 p_wv/(p_t–p_wv)

10. Relative humidity

Relative humidity is at a particular temperature on volume basis.

mass of  H₂O vapor in moist air/ mass of H₂O vapor in saturated air

Expressed as ‰ .

Its symbol is ‘Ф’.

Mathematically

Ф = p_v/p_vs

Where p_v is pressure of water vapors. Read from steam tables at the dry bulb temperature.

p_vs  pressure of water vapors in saturated air

Read from steam tables at the wet bulb temperature.

11. Degree of saturation

Degree of saturation is at a particular temperature.

It is on mass basis.

Its symbol is ‘ μ’.

μ = mass of H₂O vapor/mass of H₂O vapor in saturated air

It has no units.

Example, its value is like 0.56

12. Enthalpy of moist air

Enthalpy of moist air = enthalpy of dry air + enthalpy of water vapor portion in 1 kg of dry air

Let w be the mass of water vapor in 1 kg of moist air. Mathematically

Enthalpy of moist air/kg of dry air, h_ma

h_ma = 1 c_p dt + w [h_g +c_pv (t_db -t_dp)]

 

PSYCHROMETRIC CHART

Construction

Refer Fig. is a Psychrometric chart.

Fig. Psychrometric Chart

• Dry bulb temperature is the abscissa (along x-axis).  Specific humidity is the ordinate (along y-axis) at 1 atmospheric pressure. The various details are as under.
• Vertical lines constant dry bulb temperatures
• Horizontal lines towards right for specific humidity
• Horizontal lines towards left at saturation curve  for dew point temperature.
• Relative humidity lines are curves. On the extreme left is 100 % relative humidity line. On the right of it is 90 %, 80%.
• AB, CD, EF Constant enthalpy lines and constant wet bulb temperatures are slant (diagonal ) lines and coincide.
• GH, IJ, KL Constant volume lines
• Wet bulb temperatures are available on the saturation curve (100 % RH).
• dew point temperatures are available on the saturation curve (100 % RH)
• Enthalpy lines represent the enthalpy of moist air per kg of dry air.

Psychrometric properties

1. Dry bulb temperature, t_db
2. Wet bulb temperature, t_wb
3. Dew point temperature, t_dp
4. Specific humidity, w
5. Relative humidity, Ф
6. Enthalpy of moist air, h
7. Specific volume of moist air/kg of dry air

 Utility of the psychrometric chart

Given two properties, find the remaining five.

Say known are dry bulb and wet bulb temperatures.

Draw a vertical line as per given dry bulb temperature. Move along the constant enthalpy line as per given wet bulb temperature. It will meet the dry bulb vertical line.  We get a point on the chart.

(i) To find the dew point temperature

Move horizontally towards left up to the saturation line. It gives the dew point temperature.

(ii) to  find the specific humidity, move horizontally towards right extreme and read the value on the vertical line.

(iii)To find the relative humidity, find the position of the point in between two relative humidity lines.  Interpolate the required value.

(iv) To find the enthalpy, locate the constant enthalpy line passing through the point. Read its value from the enthalpy values marked on the chart.

(v)To find the specific volume, locate the two specific volume lines in between which the point lies. Interpolate the value of specific volume at the point

Use of psychrometric chart

we can know the condition of air present in a room, veranda outside Or anywhere. Measure  the dry and wet bulb temperatures using sling Psychrometer . With the help t_db and t_wb, first point can be located on the psychrometric chart. We know the comfortable conditions. Mark second point on the Psychrometric chart. Now to reach from first point to second point, move horizontal, vertical lines and along the saturated curve. Fig. shows the psychrometric processes.

PSYCHROMETRIC PROCESSES

 

Fig. PSYCHROMETRIC PROCESSES ON PSYCHROMETRIC CHART

• OA=Sensible heating moving horizontally towards right of the point.
• OB= Sensible cooling moving horizontally towards left of the point.
• OC =Humidification moving vertically upwards of the point
• OD =Dehumidification moving vertically downwards of the point
• OE =Heating and humidification
• OF =Cooling and dehumidification
• OG =Cooling and humidification
• OH = Heating and dehumidification

ADIABATIC MIXING OF TWO AIR STREAMS

In this there are two air streams. One is recirculation of room air. The other is the fresh air. These mix with each other.

Let the flow rates of recirculated air be m^._re = m^.₁

Flow of fresh air be  m^._fr = m^.₂ .

Let the mixture condition is point ‘3’.

Find the mixture condition using the moisture and enthalpy balance.

Moisture balance

m^.₁w₁ + m^.₂w₂ = (m^.₁ + m^.₂)w₃

Energy balance

m^.₁h₁ + m^.₂h₂ = (m^.₁ + m^.₂)h₃

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