PSYCHROMETRICS

Understanding Psychrometrics
PSYCHROMETRICS - BY SEKAR RAM
Psychrometric Chart & Its use
Goals for this Chapter
To understand the Psychrometric chart
To plot a standard cooling process on
the Psych chart
PSYCHROMETRICS - BY SEKAR RAM
Thermodynamics Fundamentals - Recap
The physical quantities used to describe air are
referred to as variables. The most important of these
variables are
Temperature
Humidity
Pressure
PSYCHROMETRICS - BY SEKAR RAM
Variables
Temperature : The perceptible heat state of air - can be
measured using a thermometer Referred to as DRY
BULB TEMPERATURE (°C or °K or °F)
Absolute Humidity (x): The amount of water in grams (g) per
kilogram (kg) of air. (g/kg or grains/ lb) [7000 grains make a
pound]
Relative Humidity () : Ratio of water vapour carried at a specified atmospheric
temperature and condition to the water vapour that can be carried by air at the
same temperature when saturated.
 = Relative Humidity
x
  100%
xS
x = Water vapour in g/kg
Xs = Water vapour for saturated air in g/kg
Expressed always in %
PSYCHROMETRICS - BY SEKAR RAM
Variables
Density (r) : Mass per unit volume, expressed in kg/M3
The density of dry air r  1.293 kg/m3
The density of water vapour r  0.804 kg/m3
Specifc Heat (c):The specific heat “c” of a solid, liquid or gaseous material is the
amount of heat required to heat up a mass of 1 kg of the material by 1 K.
Specific heat increases with increasing temperature of the material and for gases
also with increasing pressure. As a result, for gases, we distinguish between cP, the
specific heat at constant pressure and cV, the specific heat at constant volume.
Tables generally specify the values for cP at 20 °C and 1013 mbar air pressure.
These values are also suitable for calculations in heating, ventilation and airconditioning systems and hold for:
Dry air : cP = 1.01 kJ/(kg*K)
: Water vapor : cP = 1.86 kJ/(kg*K)
PSYCHROMETRICS - BY SEKAR RAM
Variables
Thermal capacity or enthalpy (h) Kj/kg. Absolutely dry air
having a theoretical water content of 0 g/kg at a temperature of
0°C has an enthalpy defined as h=0kj/kg
Differences in enthalpy h between the beginning and end
state of an air modification, can be depicted graphically on a
psychrometric chart. If we multiply the mass [kg] of the air to
be processed with the graphically-determined enthalpy
difference h, the result is the required quantity of heat for this
state change.
PSYCHROMETRICS - BY SEKAR RAM
Variables
Pressure: Mass per unit area. The weight of the air on
the surface of the earth is called atmosphere pressure.
At sea level the average value is 1.013 bar or 760 mm
Hg. The pressure unit in the international system of units
(Sl units) is:
1 Newton/m2 = 1 N/m2 = 1 Pa (Pascal)
1 bar = 1000 mbar (millibar) = 105 N/m2 = 105 Pa
PSYCHROMETRICS - BY SEKAR RAM
Variables - Pressure
PSYCHROMETRICS - BY SEKAR RAM
Variables
Flow :
Volume flow :
Mass flow :
M3/s or l/s
Kg/s or Kg/h
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics
Dry air exists only in theory. Atmospheric air is always a
mixture of dry air and water vapour.
To keep the air under desired, favourable conditions, air has to
be treated. To what levels this air has to be treated can be
calculated (in order to maintain desired, favourable conditions)
using the PSYCHROMETRIC CHART
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics
The chart
continues in
this region also
Maximum
Moisture that
air can hold at
25 C, DBT
SATURATION LINE
-15° c
50° c
25°c
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics
Relative
Humidity
Lines
SATURATION LINE
20 G/KG
10 G/KG
-15° c
50° c
25°c
Wet Bulb Lines
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics
Dew point Temperature
That temperature at which further cooling of air causes condensation.
If we have to de-humidify a water - vapour air mixture, we have to cool it
below the dew point temperature. The lower the temperature to which the
mixture is cooled, the greater is the dehumidifying effect.
Wet-bulb Temperature
The temperature measured by a wet wick thermometer while exposed to a
rapid flow of air.
The difference between the Dry bulb temperature and the wet bulb
temperature is known as the wet bulb depression. Drier the air, larger is the
wet bulb depression. When the rh of air approaches saturation values, the wet
bulb temperature approaches Dry bulb temperature.
At saturation, the air cannot hold any more moisture, the Dry bulb, the wet
bulb and the dew point temperature are the same
PSYCHROMETRICS - BY SEKAR RAM
Dry Bulb and Wet bulb Temperature
Methods of Wet Bulb Temperature / Humidity Measurement
1-Thermometers
4
5
2 – Dry Bulb thermometer
3 - Wet Bulb Thermometer
4 – Fan
1
(tD °C)
(t H °C)
Is a Direct
Measure of
Heat
Added
Depends
on the
Moisture
content of
air
3
2
5
5
B22-4
Aspirating Thermometer
5 - Air
3 – thermometer with bulb covered with
wet wick
Due to air movement, thermometer 3 is
cooled by the evaporation of moisture
The degree to which thermometer 3 can
be ‘cooled’ depends on the ‘dryness’ of the
air. Drier the air, greater is the
temperature difference between ‘2’ and ‘3’
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics
The state of a given atmosphere is
represented by a point on the chart,
known as the status point. If any two of
the three commonly available
characteristics DBT, WBT and RH are
known, the others can be read from the
chart
PSYCHROMETRICS - BY SEKAR RAM
PSYCHROMETRICS
Psychrometrics
25°C DBT & 50% RH
0.55
0.60
ENTHALPY-50 KJ/KG
0.65
0.70
WBT – 18 DEG C
0.75
DPT – 14 DEG C
ABS HUMIDITY = 10 GRAMS/KG
DRY AIR
25 DEG C, DBT
PSYCHROMETRIC CHART – BY SEKAR RAM
PSYCHROMETRICS - BY SEKAR RAM
0.80
0.85
0.90
0.95
1.00
PSYCHROMETRICS
From Psychromteric Chart
identify the values of all
other important parameters
for condition of air defined
at 30 Deg C, 40% RH.
Moisture content (ABS HUM) : 10.5
grams / kg Dry air
WBT : 20 deg c
DPT : 14.9 deg c
Enthalpy : 57.5 Kj / KG
0.55
0.60
0.65
0.70
0.75
0.80
0.85
0.90
0.95
1.00
PSYCHROMETRIC CHART – BY SEKAR RAM
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics - Processes
Psychrometric processes, ie, changes in the condition of the atmosphere,
can be represented by the movement of this status point in the following
ways
Heating and Cooling
Sensible Cooling
Sensible Heating
No Moisture Addition
Dry bulb Temperature Decreases
Wet Bulb Decreases
RH Increases
Enthalpy Decreases
No Moisture Addition
Dry bulb Temperature Increases
Wet Bulb Increases
RH Decreases
Enthalpy Increases
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics - Processes
Psychrometric processes, ie, changes in the condition of the atmosphere,
can be represented by the movement of this status point in the following
ways
Dehumidification by Cooling
At some stage, continuous
cooling causes the status
point to meet the saturation
line. The DBT corresponding
to this point is called as the
DEW POINT
TEMPERATURE
From hereon further cooling
causes condensation resulting in moisture being
pulled out from air (reduction
in absolute humidity –
dehumidification)
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics - Processes
Psychrometric processes, ie, changes in the condition of the atmosphere,
can be represented by the movement of this status point in the following
ways
Adiabatic Humidification (Evaporative Cooling)
Evaporative Cooling
Moisture vaporises to the surrounding
air without any addition or removal of
external heat
Latent heat required for this process
is taken from the surrounding air
thereby bringing down the DBT
temperature of air.
No change in enthalpy.
WBT remains same
Absolute Humidity increases
(moisture added)
RH goes up (air moves towards
saturation)
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics - Processes
Psychrometric processes, ie, changes in the condition of the atmosphere, can
be represented by the movement of this status point in the following ways
Adiabatic De-Humidification (Chemical dehumidification)
Sorbent (Chemical)
Dehumidification
Chemical having high affinity for
moisture (silica gel) absorbs moisture
form surrounding air.
Latent heat required for this process
is released to the surrounding air
thereby increasing the DBT
temperature of air.
No change in enthalpy.
WBT remains same
Absolute Humidity decreases
(moisture removed)
RH comes down (air gets drier)
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics - Processes
Psychrometric processes, ie, changes in the condition of the atmosphere,
can be represented by the movement of this status point in the following
ways
Mixing Process
Mixing
The status point of the final air mixture
always lies on the line connecting the
status points of the initial conditions of
the two airstreams
The location of the final status point is
inversely proportional to their masses.
(If m1 is greater than m2, the final
point is closer to m1)
For the mixing of 2 equal
substances (Specific Heat
being same)
Tmix = (m1T1 + m2T2) / (m1+m2)
PSYCHROMETRICS - BY SEKAR RAM
PSYCHROMETRICS
- PROCESSES
Psychrometrics
- Processes
Steam Humidification
Evaporative Cooling
Sensible Cooling
Cooling and De-humidification
Sensible Heating
Chemical Dehumidification
PSYCHROMETRIC CHART – BY SEKAR RAM
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics - Processes
PSYCHROMETRICS - BY SEKAR RAM
PSYCHROMETRICS - PROCESSES
Initial Condition  10 C / 50% rh
Final Condition  30 C / 27 C WBT
~ 21.5 g/kg
Find
Final RH
Moisture Added
Enthalpy Added
~ 80% RH
0.55
0.60
0.65
0.70
17.7 g/kg
0.75
0.80
0.85
0.90
0.95
1.00
~ 3.81g/kg
PSYCHROMETRIC CHART – BY SEKAR RAM
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics - Processes
Mixing of Air
Mixing of air
10 % of Outside air @ 45°C DBT and 16 g/kg mixes with 90% re-circulated
air @ 25°C and 50 % rh
(10 * 45) + (90 * 25)
Mixed air temperature =
= 27°C
(10+90)
PSYCHROMETRICS - BY SEKAR RAM
PSYCHROMETRICS - Mixing of Air
M1 = 45 deg C, DBT, 16 g/Kg
M2 = 25 deg C, DBT, 50 % RH
Mixture Conditions
W (Moisture content) : ~ 10.5 g/Kg
DBT : 27 deg C
WBT : ~ 19 deg C
RH : ~ 47 %
DPT : ~ 14.8 deg C
H : ~ 54.2 KJ/KG
0.55
0.60
0.65
0.70
0.75
0.80
0.85
0.90
0.95
1.00
PSYCHROMETRIC CHART – BY SEKAR RAM
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics – Important Equations
SENSIBLE HEAT : 1.08 X CFM X T(°F)
1.23 X M3/S X T(°C)
LATENT HEAT : 0.68 X CFM X W (gr/LB)
3010 X M3/S X W(kg/kg)
TOTAL HEAT : 4.45 X CFM X H (BTU/LB)
1.20 X M3/S X H(KJ/KG)
(Btu/Hr)
PSYCHROMETRICS - BY SEKAR RAM
(Kw)
Psychrometrics -
DERIVATION OF AIR CONSTANTS 0.244 = specific heat of moist air at 70°F
db and 50%rh Btu/lb °F
1.08 = 0.244 X 60
13.5
0.68 = 60
X 1076
13.5
7000
4.45 = 60
13.5
60 = min/hour
13.5 = specific volume of moist air at
70°F db and 50%rh (Ft3/lb)
1076 = average heat removal required to
condense one pound of water vapour
from the room air (Btu/lb)
7000 = grains / pound
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics -
DERIVATION OF AIR CONSTANTS
Sensible Heat = 1.20 (1.006+1.84W) x m3/s x t(°C)
~ 1.23 x M3/S x t(°C)
Latent heat = 3010 x m3/s x W (kg/kg)
Total Heat = 1.20 x m3/s x H(kj/kg)
1.2 = density of air in kg/m3
1.006 specific heat of dry air
kj/kg°K
w = absolute humidity = kg/kg
(=0.01kg/kg for most A/C
applications)
1.84 = specific heat of water
vapour kj/kg°K
3010 kj/kg = 1.2 x 2500, where
2500 is the approximate heat
content of 50% rh vapour at 24°C,
less the heat content of water at
10°C. 50%rh / 24°C is a common
design point for conditioned space,
and 10°C is normal condensate
temperature for cooling &
dehumidifying coils.
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics -
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics – Outside Airside conditions
35°C @ 80% RH
45°C @ 30% RH
35°C @ 20% RH
20°C @ 90% RH
35°C @ 80% RH
45°C @ 30% RH
20°C @ 90% RH
35°C @ 20% RH
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics – Inside Airside conditions
35°C @ 80% RH
Comfort condition,
20 to 24°C,
40 to 70% RH
45°C @ 30% RH
20°C @ 90% RH
35°C @ 20% RH
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics – Sample Project
N
Ht = 3.05M
1.2M X 1.5M X 4 WINDOWS
Non-conditioned
space
1.2M X 1.5M
9.1M
1.2M X 1.5M X 4 WINDOWS
15.25M Exposed
Exposed
1.2M X 1.5M’ X 6 WINDOWS
30.5 M
Exposed
PSYCHROMETRICS - BY SEKAR RAM
Exposed
Psychrometrics – Sample Project
1 ACPH = 1 Air Change Per Hour
Rate of air circulation = Room Volume per unit time
1 ACPH in LPS
1 ACPH in M3/s
= Room Volume in M3 / 3.6
= Room Volume in M3 / 3600
= 30.5 x 15.25 x 3.05 / 3.6 = 394 lps
Inside Design Conditions
= 23° C, 50 % RH
Assume Coil Leaving
= 13° C, DBT
Room Sensible Heat
Room Latent Heat
= 55132 watts
= 7704 watts
Room Sensible Heat Factor
= Sensible Heat / Total Heat
= 55132 / 62836 = 0.88
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics – Sample Project
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics – Sample Project – Cooling Process
Sensible Heat
55132 Watts
Air flow required L/s
= 1.23 x lps x t
= 1.23 x lps x (23-13)
= 4482 lps
Reference Circle
Cross check unit capacities with standard DX
unit of airflow Capacity 4300 l/s
Room Sensible Heat
= 1.23 x lps x t = 55132 w
55132 Watts
= 1.23 x 4300 x (t)
t
Room Temperature
= 55132/(1.23*4300) = 10.4°
= 13+10.4 = 23.4°C
Room Condition at 23 C / 50%
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics – Sample Project – Cooling Process – Add OA
Ventilation
Requirement
1 ACPH in
M3/sec =
Volume in
M3/3600
L/s = Volume
in M3/3.6
In this
example we
choose
Direct L/s
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics – Sample Project – Cooling Process – Add OA
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics – Sample Project – Cooling Process – Add OA
Process through coil –
GSHF – Line
connecting “Air on”
temperature and ADP
OA – 46.1 DB / 29.4
WB
“Air off coil” =
ADP + [(Air onADP)xBF]
= 11+[(24.8-11)x0.2)
= 13.76 C
ROOM – 23 DB, 50%
RH
ADP = 11 C (WHERE ESHF –
SLOPE OF 0.85 CUTS
SATURATION CURVE).
Changes in room –
along RSHF = 0.85
PSYCHROMETRICS - BY SEKAR RAM
Tmix = (394x46.1)+(4690x23)
5084
= 24.8 C
Psychrometrics – Important Points
A High Sensible Load application often requires large airflow
quantity to off set the loads
Increasing the number of rows of cooling coil decreases bypass
factor and increases contact over the coil. High latent loads require
more rows of cooling coil.
SHF approaches 1, when the sensible heat gets higher and higher
Greater the Latent Load, larger is the deviation of SHF from Unity.
High Latent Load Application often results in a low ADP. It may not
be possible to achieve such a low ADP with a normal chilled water
application. We have to select an ADP within permissible levels. In
doing so, the equipment must be selected with a re-heat coil as well
as additional air quantity to offset this reheat.
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics
PSYCHROMETRICS - BY SEKAR RAM
Psychrometrics – Summary
Representation of the condition of air in a
psych chart
Representation of various processes in the
Psych Chart
What is cooling capacity and Air Quantity
Effect of Varying Bypass Factor
Plotting the sample project on the Psych
Chart
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