Pysch Chart- Setp 2006

PSYCHROMETRICS–Sekar RPSYCHROMETRICS–Sekar R

PSYCHROMETRICS–Sekar R

PSYCHROMETRICS

Goals for this Chapter

To understand the Psychrometric chartTo plot a standard cooling process on the Psych chart



Thermodynamics Fundamentals

The physical quantities used to describe air are referred to as variables. The most important of these variables are

Temperature

Humidity

Pressure



%100∗=Sxxϕ

Relative Humidity (ϕ) : 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.

Temperature : The perceptible heat state of air - can be measured using a thermometer Referred to as DRY BULB TEMPERATURE (°C or °K or °F)

VARIABLES

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]

ϕ = Relative Humidityx = Water vapour in g/kgXs = Water vapour for saturated air in g/kg

Expressed always in %




VARIABLESDensity (ρ) : Mass per unit volume, expressed in kg/M3The density of dry air ρ = 1.293 kg/m3The density of water vapour ρ = 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

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




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

Mass flow : Kg/s or Kg/h

VARIABLES




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


PSYCHROMETRICS

DRY BULB TEMPERATURE

-15° c 50° c

25°c

ABSO

LUTE

HU

MID

ITY

(KG

/KG

)

Maximum Moisture that air can hold at

25 C, DBT

SATURATION LINE

The chart continues in this

region also


PSYCHROMETRICS

DRY BULB TEMPERATURE

-15° c 50° c

25°c

ABSO

LUTE

HU

MID

ITY

(KG

/KG

)

SATURATION LINE 20 G/KG

10 G/KG50 % RH

Relative Relative Humidity Humidity

LinesLines


LinesLines


LinesLines

Wet Bulb LinesWet Bulb LinesWet Bulb LinesWet Bulb LinesWet Bulb LinesWet Bulb Lines

EnthalpyEnthalpy


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


Sensible & Latent HeatMethods 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 - Air3 – 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 Is a Direct Measure of Measure of

Heat Heat AddedAdded

Depends on Depends on the Moisture the Moisture content of aircontent of air


PSYCHROMETRICS - PROCESSES

The The state of a given atmosphere is state of a given atmosphere is represented by a point on the chart, represented by a point on the chart, known as the known as the status point.status point. If If any twoany two of of the three commonly available the three commonly available characteristics DBT, WBT and RH are characteristics DBT, WBT and RH are known, the others can be read from the known, the others can be read from the chartchart



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PSYCHROMETRICS

ENTHALPY-50 KJ/KG

25 DEG C, DBT

WBT – 18 DEG C

ABS HUMIDITY = 10 GRAMS/KG DRY AIR

DPT – 14 DEG C


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From Psychromteric Chart identify From Psychromteric Chart identify the values of all other important the values of all other important parameters for condition of air parameters for condition of air defined at 30 Deg C, 40% RH.defined at 30 Deg C, 40% RH.

PSYCHROMETRICS

Moisture content (ABS HUM) : Moisture content (ABS HUM) : 10.5 grams / kg Dry air10.5 grams / kg Dry airWBT : 20 deg cWBT : 20 deg cDPT : 14.9 deg cDPT : 14.9 deg cEnthalpy : 57.5 Enthalpy : 57.5 KjKj / KG/ KG



Psychrometric processes, Psychrometric processes, ieie, changes in the condition of the atmosphere, can , changes in the condition of the atmosphere, can be represented by the movement of this status point in the follobe represented by the movement of this status point in the following wayswing ways

Sensible HeatingSensible Heating

No Moisture AdditionNo Moisture AdditionDry bulb Temperature Dry bulb Temperature IncreasesIncreasesWet Bulb Wet Bulb IIncreasesncreasesRH DecreasesRH DecreasesEnthalpy IncreasesEnthalpy Increases

Sensible CoolingSensible Cooling

No Moisture AdditionNo Moisture AdditionDry bulb Temperature Dry bulb Temperature DecreasesDecreasesWet Bulb Wet Bulb DecreasesDecreasesRH IncreasesRH IncreasesEnthalpy DecreasesEnthalpy Decreases

Heating and Heating and CoolingCooling





Dehumidification byDehumidification by CoolingCoolingAt some stage, continuous At some stage, continuous cooling causes the status cooling causes the status point to meet the saturation point to meet the saturation line. The DBT corresponding line. The DBT corresponding to this point is called as the to this point is called as the DEW POINT TEMPERATUREDEW POINT TEMPERATURE

From hereon further cooling From hereon further cooling causes condensation causes condensation --resulting in moisture being resulting in moisture being pulled out from air (reduction pulled out from air (reduction in absolute humidity in absolute humidity ––dehumidification)dehumidification)





Adiabatic Humidification (EvaporativeAdiabatic Humidification (Evaporative Cooling)Cooling)

Evaporative CoolingEvaporative Cooling

Moisture vaporises to the surrounding Moisture vaporises to the surrounding air without any addition or removal of air without any addition or removal of external heatexternal heat

Latent heat required for this process is Latent heat required for this process is taken from the surrounding air thereby taken from the surrounding air thereby bringing down the DBT temperature of bringing down the DBT temperature of air.air.

No change in enthalpy.No change in enthalpy.WBT remains sameWBT remains sameAbsolute Humidity increases (moisture Absolute Humidity increases (moisture added)added)RH goes up (air moves towards RH goes up (air moves towards saturation)saturation)





Adiabatic DeAdiabatic De--Humidification (Chemical dehumidification)Humidification (Chemical dehumidification)

Sorbent Sorbent (Chemical) (Chemical) DehumidificationDehumidification

Chemical having high affinity for Chemical having high affinity for moisture (silica gel) absorbs moisture moisture (silica gel) absorbs moisture form surrounding air.form surrounding air.

Latent heat required for this process is Latent heat required for this process is released to the surrounding air thereby released to the surrounding air thereby increasing the DBT temperature of air.increasing the DBT temperature of air.

No change in enthalpy.No change in enthalpy.WBT remains sameWBT remains sameAbsolute Humidity decreases Absolute Humidity decreases (moisture removed)(moisture removed)RH comes down (air gets drier)RH comes down (air gets drier)





Mixing ProcessMixing ProcessMixingMixing

The status point of the final air mixture The status point of the final air mixture always lies on the line connecting the always lies on the line connecting the status points of the initial conditions of status points of the initial conditions of the two airstreamsthe two airstreams

The location of the final status point is The location of the final status point is inversely proportional to their masses. inversely proportional to their masses.

(If m1 is greater than m2, the final (If m1 is greater than m2, the final point is closer to m1) point is closer to m1)

Tmix = (m1T1 x m2T2) / (m1+m2)For the mixing of 2 equal substances (Specific Heat being same) PSYCHROMETRICS–Sekar R



Sensible HeatingSensible Cooling

Cooling and De-humidification

Evaporative Cooling

Chemical Dehumidification

Steam Humidification



Psychrometrics


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Initial Condition Initial Condition 10 C / 50% 10 C / 50% rhrhFinal Condition Final Condition 30 C / 27 C WBT30 C / 27 C WBT

FindFind

Final RHFinal RHMoisture AddedMoisture AddedEnthalpy AddedEnthalpy Added


~ 80% RH

17.7 g/kg

85-1

9.8=6

5.2 kj

/kg

~ 21.5 g/kg

~ 3.81g/kg


PSYCHROMETRICS -

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


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PSYCHROMETRICS - Mixing of AirMM11 = 45 deg C, DBT, 16 g/Kg= 45 deg C, DBT, 16 g/KgMM22 = 25 deg C, DBT, 50 % RH= 25 deg C, DBT, 50 % RH

Mixture ConditionsMixture Conditions

W (Moisture content) : ~ 10.5 g/KgW (Moisture content) : ~ 10.5 g/KgDBT : 27 deg CDBT : 27 deg CWBT : ~ 19 deg CWBT : ~ 19 deg CRH : ~ 47 %RH : ~ 47 %DPT : ~ 14.8 deg C DPT : ~ 14.8 deg C H : ~ 54.2 KJ/KGH : ~ 54.2 KJ/KG


FACTORS THAT AFFECT THE LOAD IN A BUILDING

EXTERNALEXTERNAL

••Temperature difference between Outside and SpaceTemperature difference between Outside and Space••Thermal resistance / conductance / thickness of material separatThermal resistance / conductance / thickness of material separating two zones ing two zones ••Solar RadiationSolar Radiation••OrientationOrientation••Geographical location of area to be conditionedGeographical location of area to be conditioned••Adjacent Buildings and shading available Adjacent Buildings and shading available

INTERNALINTERNAL

••Number of People / activity of PeopleNumber of People / activity of People••Equipment and nature of heat dissipation (dry heat / moisture)Equipment and nature of heat dissipation (dry heat / moisture)••MachinesMachines••LeakagesLeakages

OTHEROTHER

••Ventilation RequirementVentilation Requirement••Process RequirementProcess Requirement


SAMPLE - HEAT LOAD CALCULATIONS

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) (Kw)



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/m31.006 specific heat of dry air kj/kg°Kw = 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.


DERIVATION OF CONSTANTS



PSYCHROMETRICS

DERIVATION OF AIR CONSTANTS

1.08 = 0.244 X 6013.5

0.68 = 60 X 107613.5 7000

4.45 = 6013.5

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


SAMPLE PROJECT FOR PSYCHROMETRICS

Exposed

3.8

M

3.8 M

Conditioned space

Exp

osed

Conditioned space

N

Ht = 3.2M

2 M (W) x 2.4 M (H)


SAMPLE PROJECT FOR PSYCHROMETRICS1 ACPH = 1 Air Change Per Hour

Rate of air circulation = Room Volume per unit time

1 ACPH in LPS = Room Volume in M3 / 3.61 ACPH in M3/s = Room Volume in M3 / 3600

= 3.8 x 3.8 x 3.2 / 3.6 = 12.84 lps

Consider 12 ACPH as total circulation Room Air circulation = 12.84 x 12 = 154 lps

Inside Design Conditions = 23 deg C, 50 % RH

Sensible Heat = 1845 wattsLatent Heat = 231 watts

Sensible Heat Factor = Sensible Heat / Total Heat= 1845 / 2076 = 0.89


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The Cooling Process on the PSYCH Chart

Room Condition at 23 C / 50%

Coil Entering Condition = 23.8 deg C

Reference Circle

Coil Leaving Temperature = 13.26 deg C

Q Sensible(watts) = 1.23 x lps x ∆t∆t in room = 1845/(1.23*154)

= 9.74 deg CTemp of air entering room = 23 – 9.74

= 13.26 deg C

Enthalpy of air Entering room = 34.8 kj/kgRoom Design Enthalpy = 45.8 kj/kgEnthalpy gain in room = 1.2 x 154 x (45.8-34.8)

= 2033 watts

RSHF = 0.89


The Cooling Process on the PSYCH Chart



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The Cooling Process on the PSYCH ChartExample with – Addition of 1 ACPH Fresh Air for Ventilation

The mixed condition of air lies somewhere along this line

OA

Mixture Temperature (air entering coil) = 24.7 deg C

14 deg CAir leaving coil (entering room) = 14 deg C

Process through Coil

He = 49.2 kj/kg

Hleave = 34.8 kj/kg

Room Gain(dehumidified raise)

ADD 1 ACPH OA FORVENTILATION


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He = 49.2 kj/kg

Hleave = 34.8 kj/kg

H = 46 kj/kg

Sensible Heat removed by coil : 1.2 x 167 x (46 - 34.8) = 2244

Latent Heat removed by coil : 1.2 x 167 x (49.2 - 46) = 641

Coil GSHF = 2244 / (2244 + 641) = 0.79


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Apparatus Dew Point Temp (ADP) = 9.9 deg C

Cross verify Air Leaving Temperature ={(24.7 – 9.9) x 0.277} + 9.9 =

14 deg C (matches exactly With our earlier values)

The ratio of air that does not make contact with cooling coil (bypass).In our example this is approximately 27.7%


IMPORTANT POINTSA 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.


Summary Summary –– Psychrometrics & Heat Load CalculationPsychrometrics & Heat Load Calculation

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

Pysch Chart- Setp 2006

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