Understanding Psychrometrics - Soner Yeşilgöz - Home · PDF filePsychrometrics -...

PSYCHROMETRICS - BY SEKAR RAM

Understanding Psychrometrics


Psychrometric Chart & Its use

Goals for this Chapter

To understand the Psychrometric chart

To plot a standard cooling process on

the Psych chart


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


Variables

%100Sx

x

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.

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

x = Water vapour in g/kg

Xs = Water vapour for saturated air in g/kg

Expressed always in %

Temperature : The perceptible heat state of air - can be

measured using a thermometer Referred to as DRY

BULB TEMPERATURE (°C or °K or °F)


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 air-

conditioning systems and hold for:

Dry air : cP = 1.01 kJ/(kg*K) : Water vapor : cP = 1.86 kJ/(kg*K)

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.

Variables


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


Variables - Pressure


Variables

Flow : Volume flow : M3/s or l/s

Mass flow : Kg/s or Kg/h


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

-15° c 50° c

25°c

Maximum

Moisture that

air can hold at

25 C, DBT

SATURATION LINE

The chart

continues in

this region also


Psychrometrics

-15° c 50° c

25°c

20 G/KG

10 G/KG

Relative

Humidity

Lines

Relative

Humidity

Lines

Relative

Humidity

Lines

Wet Bulb Lines Wet Bulb Lines Wet Bulb Lines

SATURATION LINE


Psychrometrics


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


Dry Bulb and Wet bulb Temperature

Methods of Wet Bulb Temperature / Humidity Measurement

5

1

(tD °C)

2

5 5

3

(tH °C)

4

B22-4

Aspirating Thermometer

1-Thermometers

2 – Dry Bulb thermometer

3 - Wet Bulb Thermometer

4 – Fan

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’

Is a Direct Measure of

Heat Added

Depends on the

Moisture content of

air


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


Psychrometrics

PSYCHROMETRIC CHART – BY SEKAR RAM

1.00

0.95

0.90

0.85

0.80

0.75

0.70

0.65

0.60

0.55

PSYCHROMETRICS

ENTHALPY-50 KJ/KG

25 DEG C, DBT

WBT – 18 DEG C

ABS HUMIDITY = 10 GRAMS/KG

DRY AIR

DPT – 14 DEG C

25°C DBT & 50% RH

PSYCHROMETRICS - BY SEKAR RAM PSYCHROMETRIC CHART – BY SEKAR RAM

1.00

0.95

0.90

0.85

0.80

0.75

0.70

0.65

0.60

0.55

From Psychromteric Chart

identify the values of all

other important parameters

for condition of air defined

at 30 Deg C, 40% RH.

PSYCHROMETRICS

Moisture content (ABS HUM) : 10.5

grams / kg Dry air

WBT : 20 deg c

DPT : 14.9 deg c

Enthalpy : 57.5 Kj / KG


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

Sensible Heating

No Moisture Addition

Dry bulb Temperature Increases

Wet Bulb Increases

RH Decreases

Enthalpy Increases

Sensible Cooling

No Moisture Addition

Dry bulb Temperature Decreases

Wet Bulb Decreases

RH Increases

Enthalpy Decreases

Heating and Cooling





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)





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)



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)





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)

Tmix = (m1T1 + m2T2) / (m1+m2) For the mixing of 2 equal

substances (Specific Heat

being same)



PSYCHROMETRIC CHART – BY SEKAR RAM

PSYCHROMETRICS - PROCESSES

Sensible Heating Sensible Cooling

Cooling and De-humidification

Evaporative Cooling

Chemical Dehumidification

Steam Humidification


1.00

0.95

0.90

0.85

0.80

0.75

0.70

0.65

0.60

0.55

Initial Condition 10 C / 50% rh

Final Condition 30 C / 27 C WBT

Find

Final RH

Moisture Added

Enthalpy Added

PSYCHROMETRICS - PROCESSES

~ 80% RH

17.7 g/kg

~ 21.5 g/kg

~ 3.81g/kg



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

Mixed air temperature = (10+90)

= 27°C (10 * 45) + (90 * 25)

Mixing of Air


1.00

0.95

0.90

0.85

0.80

0.75

0.70

0.65

0.60

0.55

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


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) (Kw)

Psychrometrics – Important Equations


Psychrometrics -

DERIVATION OF AIR CONSTANTS

1.08 = 0.244 X 60

13.5

0.68 = 60 X 1076

13.5 7000

4.45 = 60

13.5

0.244 = specific heat of moist air at 70°F

db and 50%rh Btu/lb °F

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 -

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 -


Psychrometrics – Outside Airside conditions

35°C @ 80% RH

45°C @ 30% RH

35°C @ 20% RH

20°C @ 90% RH

45°C @ 30% RH

35°C @ 80% RH

20°C @ 90% RH

35°C @ 20% RH

35°C @ 80% RH

45°C @ 30% RH

35°C @ 20% RH

20°C @ 90% RH


Psychrometrics – Inside Airside conditions

45°C @ 30% RH

35°C @ 80% RH

20°C @ 90% RH

35°C @ 20% RH

Comfort condition,

20 to 24°C,

40 to 70% RH


Psychrometrics – Sample Project

Exposed

1.2

M X

1.5

M X

4 W

IND

OW

S

1.2M X 1.5M

1.2M X 1.5M’ X 6 WINDOWS

15.2

5M

30.5 M

9.1

M

Non-conditioned

space

Exposed

Exposed

Exposed

N Ht = 3.05M

1.2M X 1.5M X 4 WINDOWS



1 ACPH = 1 Air Change Per Hour

Rate of air circulation = Room Volume per unit time

1 ACPH in LPS = Room Volume in M3 / 3.6

1 ACPH in M3/s = Room Volume in M3 / 3600

= 30.5 x 15.25 x 3.05 / 3.6 = 394 lps

Inside Design Conditions = 23° C, 50 % RH

Room Sensible Heat = 55132 watts

Room Latent Heat = 7704 watts

Room Sensible Heat Factor = Sensible Heat / Total Heat

= 55132 / 62836 = 0.88

Assume Coil Leaving = 13° C, DBT


Psychrometrics – Sample Project – Cooling Process

Room Condition at 23 C / 50%

Reference Circle Sensible Heat = 1.23 x lps x t

55132 Watts = 1.23 x lps x (23-13)

Air flow required L/s = 4482 lps

Room Sensible Heat = 1.23 x lps x t = 55132 w

55132 Watts = 1.23 x 4300 x (t) t = 55132/(1.23*4300) = 10.4°

Room Temperature = 13+10.4 = 23.4°C

Cross check unit capacities with standard DX

unit of airflow Capacity 4300 l/s


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



OA – 46.1 DB / 29.4

WB

Tmix = (394x46.1)+(4690x23)

5084

= 24.8 C ADP = 11 C (WHERE ESHF –

SLOPE OF 0.85 CUTS

SATURATION CURVE).

Process through coil –

GSHF – Line

connecting “Air on”

temperature and ADP

“Air off coil” =

ADP + [(Air on-

ADP)xBF]

= 11+[(24.8-11)x0.2)

= 13.76 C

ROOM – 23 DB, 50%

RH

Changes in room –

along RSHF = 0.85


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


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

Psychrometrics – Summary

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