psychrometrics rev

PSYCHROMETRICSPSYCHROMETRICS

J.ILANGUMARANJ.ILANGUMARAN

Psychrometrics

Psychrometrics

The The PsychrometricPsychrometric ChartChart

COMFORT

“We air condition buildings to provide a comfortabl e environment for the occupants”

Temperature and Humidity:24oC DB and 60% RH for summer and 21oC DB and 50% RH for winter is energy efficient.

Air Movement: The terminal velocity, at neck level, should be designed for approximately 0.2m/s.

Air Freshness: For body odour dilution of 8l/s of fresh air per person should be introduced.If smokers are present 16l/s of fresh air is required.

Air Cleanliness: Selection of an adequate filtration system is very important and some types of air conditioning system permit better air cleansing than others.

Noise Levels: Careful selection of equipment and consideration of both external and internal noise pollution is important. A certain level of noise is necessary to mask clarity of individual conversations etc.

Relative Humidity:A minimum RH of 35% is recommended to prevent dry eyes, nose and throat.A maximum RH of 70% is recommended to prevent surface condensation and mould etc.Electrostatic charges can be generated at RH less than 45%.

-5 0 5 10 15 20 25 30 35 40

Dry Bulb Temperature oC

DRY BULB TEMPERATURE

0.005

0.01

0.015

0.02

Mo

istu

re C

on

ten

t kg

/kg

(D

ry A

ir)

MOISTURE CONTENT(Specific Humidity)

The weight of water in the air

21oC DB and 50% RH = 0.007857 kg/kg

0.007857kg/kg 1.186kg/m3 (air density)

= 0.66%

100% Relative Humidty(Saturation Curve)90%80%70%60%50%40%30%20%10%

RELATIVE HUMIDITY

RH is actual vapour pressurecompared to the vapour pressure at saturation for the same dry bulbtemperature.

COOLING

HEATING

COOLING & HEATING

DEWPOINTDewpoint is the temperature where

moisture begins to condense.

• Cool to dewpoint (from A to B)no change in moisture content.

• Further cooling (to C) water vapour condenseslowering the moisture content.

MoistureContent0.01kg/kg

Dewpoint

22oCDry

Bulb

AB

14oCDry

Bulb

COOL

COOL

C

10oCDry

Bulb

MoistureContent0.075kg/kg

A

B

C

Saturation Curve

WET BULB TEMPERATUREThe sling psychrometer is used to determine

the moisture content of the air.

Wet BulbThermometer

Dry BulbThermometer

WettedCotton wick

Handle

If air is saturated none of the moisture on the wick will evaporate and the WB temperature will be the same as the DB.If the air is less than saturated moisture will evaporatefrom the wick cooling the WB thermometer.

The difference between the WB and DB is called the WB depression.

WB T

empe

ratu

re o C

WET BULB TEMPERATURE

-50

5

10

15

20

25

15oC Dry Bulb

On the saturation curve WB and DBtemperatures are the same

Saturation Curve

The inverse of density is the Specific Volumem3 /kgThe volume increases with temperature increase.The density decreases with temperature increase.

1.32 1.3 1.28 1.26 1.24 1.22 1.2 1.18 1.16 1.14 1.12

Density kg/m3

DENSITY (Specific Weight)

7570

656055

5045

05

1015

20 2530

3540

0 5 10 15 20 25 30 35 40

45

50

55

60

65

70

75

80

85

90

95

100

Total Heat Content kJ/kg

-5-10

TOTAL HEAT CONTENT (SPECIFIC ENTHALPY)

0 1000 2000 3000

1.0

0.9

0.8

0.7

Altitude Metres

Specific EnthalpyCorrection Factor

SPECIFIC ENTHALPY CORRECTION FACTORDUE TO ALTITUDE

1

2

3

4

5

5

6

7

PSYCHROMETRIC DATA

1 DB Temperature = 24oC2 WB Temperature = 17oC3 RH = 50%4 Moisture Content = 0.0093kg/kg5 Total Heat Content = 48kj/kg6 Dewpoint = 13oC7 Saturated Moisture

Content = 0.0188kg/kg

COOLING

HEATING

COOLING & HEATING

COOLING CURVES Due to Cooling Coil inefficiencythe air loses moisture before it reaches the saturation curve.The cooling process of a Cooling coil follows the Cooling Curves.

HU

MID

IFY

ING

DE

HU

MID

IFY

ING

HUMIDIFYING To add moisture

DEHUMIDIFYING To remove moisture

SENSIBLE HEAT:A change in DB temperature. No change in moisture content.

LATENT HEAT:A change In moisture content . No change in DB temperature.

TOTAL HEAT (ENTHALPY):Sensible Heat + Latent Heat

Tota

lH

eat

TOTAL HEAT

Total

Heat

TOTAL COOLING

SensibleHeat

LatentHeat

Dehumidification

Humidification

Cooling & Dehumidification

Sensible Cooling Sensible Heating

Heating & HumidificationEvaporative Cooling

�TechYORK Production System

All rights Reserved

Copyright 1999 by York International Inc.Brian Fossey 03/99

Brian Fossey

�Tech

Winter

Summer

THE COMFORT ZONE

CIRCULATIONZONE

Floor Conduction Sensible Gain

Light SensibleGain

To RoomCeiling

SensibleGainTo Room

Occupants GainSensible & Latent

Miscellaneous Equipment Sensible Gain

PERIMETERZONE

INTERIORZONE

Glass Solar &Conduction Sensible Gainto Room

Infiltration Gain - Sensible & Latentto Room

Wall Solar & Conduction Sensible Gainto Room

PERIMETERZONE

INTERIORZONE

Roof Solar & Conduction Sensible Gain to Ceiling Void

Floor Sensible Gain

To Room

CIRCULATIONZONE

Wall Solar & Conduction Sensible Gainto Ceiling Void

ROOM SENSIBLE HEAT FACTOR=

SENSIBLE HEATSENSIBLE HEAT + LATENT HEAT

=SENSIBLE HEAT

TOTAL HEAT

SENSIBLE HEAT FACTOR (ESHF, GSHF, RSHF) LINES

1.) Calculate SHF.

2.) Plot line through SHF ratio and plotting point.

3.) Plot line parallel to item 2.)through point of room condition.

SHF Plotting Point

Room Condition

SHF

Brian Fossey

�Tech

Heat gain from fan

RSHF - Room Sensible Heat FactorRoom Sensible and Latent Heat Gainsfrom Walls, windows, infiltration,people, lights and miscellaneousequipment

Condition of airleaving cooling coil

FAN COIL UNITS

Condition of airentering cooling coil

Room Condition

A

BC

B

C

A

AIR HANDLING UNIT SUPPLYING 100% OUTSIDE AIR

Filt

er

Hea

tin

g C

oil

Co

olin

g C

oil

Su

pp

ly F

an

Ou

tsid

e A

ir L

ou

vre

Hu

mid

ifie

r

A

B

D

C

Supply AirCondition

Leaving CoolingCoil Condition

Fan Gain

Brian Fossey

�Tech

AIR HANDLING UNIT SUPPLYING 100%

OUTSIDE AIR

Outside Air Summerand

Entering CoolingCoil Condition

SUMMER

A

C

D

A

B

D

C

Supply AirCondition

Preheater for Steam Humidifier

Preheater for Spray Humidifier

Spray Humidifier

Ste

am

Ste

am H

um

idif

ier

WINTERAIR HANDLING UNIT

SUPPLYING 100% OUTSIDE AIR

Fan GainA

B

C

Outside Air Winterand

Entering HeatingCoil Condition

B

D

A

B

D

C

Air ConditionedRoom

SupplyAir

ReturnAir

AIR HANDLING UNIT WITH RECIRCULATED AIR

B

Filt

er

Hea

tin

g C

oil

Co

olin

g C

oil

SupplyAirFanO

uts

ide

Air

Lo

uvr

e

Outside

Air

Disharge

Air

ReturnAirFan

Ou

tsid

e A

ir D

amp

erD

isch

arg

e L

ou

vre

Dis

char

ge

Dam

per

Recirculated Air

A

C

D EG

Hu

mid

ifie

r

F

SaturationCurve

RoomCondition

RSHF

B

B

Maximum60% RH

SaturationCurve Return Air

Fan Gain

CRecirculated AirCondition

B

BC


Fan Gain


Outside AirConditionSummer

A

B

B

A

C

SUMMER COOLING


Fan Gain


A

B

Outside Air and Recirculated AirMixture Condition

D


% Outside Air(minimum outside air)

% RecirculatedAir

B

A

C

D

SUMMER COOLING

SaturationCurve

A

B


D

C

Cooling Coil

B

A

C

D

SUMMER COOLING

SaturationCurve

A

B


D

C

Cooling Coil

Bypass Factornormally 10%of total coil

Leaving CoolingCoil Condition

E

ApparatusDewpoint

(ADP)

B

A

C

DE

SUMMER COOLING

SaturationCurve

A

B

D

C

Cooling CoilE

Supply AirFan Gain

Supply AirCondition G

B

A

C

DE

G

SUMMER COOLING

A

D

SaturationCurve

B C

E

Supply AirFan Gain


WINTER

B

A

C

DE

G

Minimum40% RH


% Outside Air(minimum outside air)

% RecirculatedAir

A

D

SaturationCurve

B C

F

Supply AirFan Gain


WINTER - SPRAY HUMIDIFIER

B

A

C

D G

F

Wet bulb Line

DMore than minimumoutside air is required

Spray Humidifier

In this situation a Preheater is not requiredfor daytime operation but would be neededfor morning warm-up.

A

D

SaturationCurve

B C

F

Supply AirFan Gain


WINTER - SPRAY HUMIDIFIER

B

A

C

D G

F

Wet bulb Line

Spray Humidifier

Preheat

Preheater required whenlow temperature or high %outside air

E

E

A

D

SaturationCurve

B C

F

Supply AirFan Gain


WINTER - STEAM HUMIDIFIER

B

A

C

D GF

DMore than minimumoutside air is required

Steam Humidifier

In this situation a Preheater is not requiredfor daytime operation but would be neededfor morning warm-up.

RoomCondition

Supply AirFan Gain


Return AirFan Gain

EnteringCooling CoilCondition

LeavingCooling CoilCondition

Supply AirCondition

% Return Air

RSHF

RSHF plotting

line

AIR HANDLING UNIT WITH RECIRCULATED AIROutside Air Condition Summer 32oC DB, 19oC WB.


All rights Reserved


Brian Fossey

�Tech

RoomCondition

Supply AirFan Gain

Return AirFan Gain


EnteringCooling CoilCondition

LeavingCooling CoilCondition

Supply AirCondition

RSHF RSHF plotting

line

AIR HANDLING UNIT WITH RECIRCULATED AIROutside Air Summer 28oC DB, 19oC WB.

PSYCHROMETRIC TERMS AND FORMULEA

ROOM CONDITIONSRSH = Room Sensible Heat (watts)

RLH = Room Latent Heat (watts)

RTH = Room Total Heat (watts) = RSH + RLH

RSHF = Room Sensible Heat Factor = RSH/RTH

OASH = Outside Air Sensible Heat - kW = OA volume m3/s X specific heat 1.02kj/kg oC X (OADB oC - Room DB oC) X density kg/m3

OUTSIDE AIR CONDITIONSOutside Air Volume m3/s = Number of occupants X 0.008 to 0.01m3/s

OATH = Outside Air Total Heat - kW = OA volume m3/s X (OA enthalpy kj/kg - Room enthalpy kj/kg) X OA density kg/m3

OALH = Outside Air Latent Heat - kW = OATH - OASH

TSH = Total Sensible Heat = RSH + OASH + *Air conditioning system gains* Includes supply air and return air fan gains, duct leakage sensible losses,return air gains from lights.

TOTAL CONDITIONS

TLH = Total Latent Heat = RLH + OALH + supply air duct leakage latent losses.

GTH = Grand Total Heat = TSH + TLH

GSHF = TSH/GTH

EFFECTIVE CONDITIONS

ERSH = Effective Room Sensible Heat= RSH + (BF* X OASH) + **Air conditioning system gains* BF = Cooling coil bypass factor (approximately 0.1)

** Includes supply air and return air fan gains, duct leakage sensible losses,return air gains from lights.

ERLH = Effective Room Latent Heat= RLH + (BF* X OALH) + **Air conditioning system gains* BF = Cooling coil bypass factor (approximately 0.1)** Includes supply air and return air fan gains, duct leakage latent losses,return air gains from lights.

ERTH = Effective Room Total Heat= ERSH + ERLH

ESHF = Effective Sensible Heat Factor = ERSH / ERTH

AIR VOLUME CALCULATIONS

Dehumidified Supply Air Volume m3/s= ERSH / Specific Heat 1.02 kj/kgoC X (RoomoC DB - ADPoC DB) X Density kg/m3

PsychrometricPsychrometricCalculationCalculation

ExampleExample

CIRCULATIONZONE

Floor Transmission Sensible Gain

Light SensibleGain

To RoomCeiling

SensibleGainTo Room

Occupants GainSensible & Latent

Miscellaneous Equipment Sensible Gain

PERIMETERZONE

INTERIORZONE

Glass Solar & Transmission Sensible Gainto Room

Infiltration Gain - Sensible & Latentto Room

Wall Solar & Transmission Sensible Gainto Room

PERIMETERZONE

INTERIORZONE

Roof Solar & Transmission Sensible Gain to Ceiling Void

Floor Sensible Gain

To Room

CIRCULATIONZONE

Wall Solar & Transmission Sensible Gainto Ceiling Void

BUILDING COOLING LOADS ASSOCIATED WITH AIR HANDLING UNIT No 1

ROOM COOLING LOADSSENSIBLE HEAT GAINS RSH1. Conduction Heat Gains:

walls, roof, ceiling, floor.2. Solar Heat Gains-

windows, Walls,Roof.3. Heat gains from lighting

load to space.4. Miscellaneous equipment:

computers, printers etc.5. OccupantsTotal= 329.45kWInfiltration Gain= -5.079kWTotal Space Gains= 325kW

LATENT HEAT GAINS RLH3. Occupants= 31.9kW4. Infiltration= 11.64kWTotal = 44kW

AIR CONDITIONING SYSTEM SYSTEM SENSIBLE HEAT GAINS SSH5. Supply air fan and motor. = 28.59kW6. Supply air duct gain. = 23.53kW7. Supply air duct leakage to areas

other than the air conditioned space. = 0

8. Heat gain from lights to the return air plenum via the ceiling void. = 27kW

9. Minimum Outside Air Volume =4.84m3/sHeat gain from minimum outside air OASH.To be calculated using Psychrometric data.

10. Return air fan and motor. = 10kW

Total = 89kW

SYSTEM LATENT HEAT GAINS SLH10. Cooling required to offset supply air

duct leakage to areas other than the air conditioned space. = 0

11. Heat gain from minimum outside air OALH.To be calculated using Psychrometric data.

Load peaks at 9am July when outside air condition is 21oC DB, 19oC WB

SYSTEM DESIGN CONDITIONS

Outside Air Summer = 21oC DB, 19oC WB

Outside Air Winter = - 4oC DB, 50% RH

Room Summer = 23oC DB, 60% RH

Room Winter = 21oC DB, 50% RH

Night set back room temperature winter = 10oC DB

B

A

WB T

empe

ratu

re o C

10

15

20

10 15 20 25 30Dry Bulb Temperature oC

Room 23oC DB, 60% RHB

Outside Air21oC DB, 19oC WB

A

SYSTEM DESIGN CONDITIONS


All rights Reserved


Minimum Outside Air = 4.84m3/s

Outside Air Total Heat (OATH)= OA air volume X (OA enthalpy - Room enthalpy ) X OA densityThe data in blue italics can be derived from the Psychrometric chart or from tables

= OA m3/s X (OA kj/kg - Room kj/kg) X OA kg/m3

= 4.84 X (54 - 50) X 1.19 = 23kW

Outside Air Sensible Heat (OASH)= OA air volume X specific heat X density X (OADB oC - Room DB oC)= OA m3/s X kj/kg oC X kg/m3 X (OADB oC - Room DB oC)= 4.84 X 1.02 X 1.16 X (21 - 23) = -11kW

Outside Air Latent Heat (OALH)= OATH - OASH = 23 - -11 = 34kW

MINIMUM OUTSIDE AIR

EFFECTIVE ROOM SENSIBLE HEAT FACTOR

Effective Room Sensible Heat (ERSH)= RSH + (Bypass factor BF X OASH) + SSHBF assume cooling coil is 10% efficient = 0.1

= 325 + (0.1 X -11) + 89 = 413kW

Effective Room Latent Heat (ERLH)= RLH + (Bypass factor BF X OALH) + SLH= 44 + (0.1 X 34) + 0 = 47kW

Effective Room Total Heat (ERTH)= ERSH + ERLH = 413 + 47 = 460kW

Effective Room Sensible Heat Factor= ERSH/ERTH = 413/460 = 0.9

SENSIBLE HEAT FACTOR (ESHF, GSHF, RSHF) LINES

1.) Calculate SHF.

2.) Plot line through SHF ratio and plotting point.

3.) Plot line parallel to item 2.)through point of room condition.

SHF Plotting Point

Room Condition

SHF

WB T

empe

ratu

re o C

10

15

20


B

A

B

ESHF = 0.9

EFFECTIVE ROOM SENSIBLE HEAT FACTOR

ERSHF

Room 23oC DB, 60% RH

A



All rights Reserved


APPARATUS DEWPOINT ADP

The ADP is at the point where the ESHF intersects the Saturation Curve.

ADP = 14.2oC DB and WB

Total Sensible Heat (TSH)= RSH + OASH + SSH = 325 + (-11) + 89 = 403kW

Total Latent Heat (TLH)= RLH + OALH + SLH = 44 + 33 + 0 = 77kW

Grand Total Heat (GTH)= TSH + TLH = 403 + 77 = 480kW

Grand Sensible Heat Factor

= TSH/GTH = 403/480 = 0.84

GRAND SENSIBLE HEAT FACTOR

SUPPLY AIR VOLUME

Supply Air Volume m3/s

= ERSH/Specific Heat X (Room oC DB - ADP oC DB) X Supply Air Density*

= 413kW/ 1.02kj/kgoC X (23 - 14.2) x 1.215kg/m3

Supply Air Volume = 38 m3/s

RETURN AIR SYSTEM GAINS

Return Air System Heat Gain = (Return Air Fan + Lighting gain to return air)= 10+27 = 37kW

Temperature rise due to return air system gain= (Return air System gain) - divided bySpecific heat X Supply Air Volume X (1 - % pressurisation air remaining in the building)* X Density of room air= 37kW/ 1.02kj/kgoC X 38m3/s X (1- 0.1) X 1.19kg/m3 = 0.9oC

Recirculated Air = 23 + 0.9 = 24oC

* Assume that 10% of the total air remains in the space to pressurise the building to offset some of the effects of infiltration and for makeup air to toilet exhaust.

PERCENTAGE OUTSIDE AIR

The percentage of Minimum Outside Air to Supply Air Volume = Minimum Outside Air Volume / Supply air Volume= 4.84m3/s divided by 38m3/s

4.84/38 = 12.74%

Check the location of the mixed air condition

WB T

empe

ratu

re o C

10

15

20


B

A

C

D GF

ESHF = 0.9ADP = 14.2oC DB and WB

MIXED AIR CONDITIONCHECK

GSHF = 0.84

A


12.74%100%

87.26%Mixed Air Condition

Recirculated Air Condition


All rights Reserved


BYPASS FACTOR BF

Due to the cooling coil inefficiency assume thatthe air bypassing the coil is 10% of the total air

COOLING COIL

Entering Cooling Coil condition = 23.8oC DB, 18.2oC WB

Leaving Cooling Coil condition = 15.5oC DB, 14.8oC WB

D

F

ROOM SENSIBLE HEAT FACTOR RSHF

Room Sensible Heat RSH = 325kW

Room Latent Heat RLH = 44kW

Room Total Heat RTH = 325 + 44 = 369kW

Room Sensible Heat Factor = 325/369 = 0.88

RSHF = 0.88

SUPPLY AIR SYSTEM GAINS

Temperature rise due to SUPPLY air system gain

= Supply air System gaindivided by

Density of supply air XSpecific heat X Supply Air Volume

= 52kW 1.215kg/m3 X 1.02kj/kgoC X 38m3/s= 1.1oCFinal Supply Air condition = 15.5 + 1.1 = 16.6oC DB

SYSTEM SENSIBLE HEAT GAINS SSHSupply air fan and motor.Supply air duct heat gain.Cooling required to offset supply airduct leakage to areas other than the air conditioned space.

SSH = 28.59 + 23.53 = 52kW

WB T

empe

ratu

re o C

10

15

20


B

A

C

D GF

B

ESHF = 0.9


SUPPLY AIR HEAT GAIN


GSHF = 0.88

Recirculated Air Condition

Mixed Air Condition(Coil Entering Condition)


RSHF = 0.89

Supply Air Condition

F

G

Coil LeavingCondition

D

AOutside Air21oC DB, 19oC WB

Mixed Air Condition

�YORK Production System

All rights Reserved


SUMMARY - AIR HANDLING UNIT SUMMER CONDITIONS

FANS

Supply Fan Air Volume = 38m3/s

Return Air Fan Air Volume = 38 X 0.9* = 34.2m3/s* 10% air remains in the air conditioned space to o ffsettoilet exhaust and for pressurisation against infil tration.

COOLING COIL

Entering Cooling Coil condition = 23.8oC DB, 18.2oC WB

Leaving Cooling Coil condition = 15.5oC DB, 14.8oC WB

Supply Air condition = 16.6oC DB, 15.2oC WB

WINTER MIXED AIR CONDITION

Outside Air DB oC = - 4

Recirculated Air DB oC = 21 + 1(Return Air Gain) = 22

Minimum Outside Air = 12.74%

0.1274 X - 4 = -0.510.8726 X 22 = 19.2Mixed Air DB oC = 18.69

20

15

10

5

0

-5

-5 0 5 10 15 20 25

WB Temperature o C


Room Winter21oC DB, 50% RH

RSHF = 0.89

B

A

C

D GF

Mixed Air Conditionwith minimum Outside Air

RecirculatedAir Condition

B

C

DIncreased Outside Air Volume

SteamHumidifier

D

FG

STEAM HUMIDIFIER

Increase Moisture Content by0.0025kg/kg

Outside Air Winter

20

15

10

5

0

-5

-5 0 5 10 15 20 25

WB Temperature o C



RSHF = 0.89

B

A

C

D GF



B

C

D

Increased Outside Air Volume

SprayHumidifier

D

FG

SPRAY HUMIDIFIER


Outside Air Winter

HUMIDIFIER SIZING

Increase moisture content by 0.0025kg/kg

Density = 1.24 kg/m3

Steam Volume = 38m3/s X 3600 (m3/hour) X 0.0025kg/kg X 1.24kg/m3

= 424kg/Hour

STEAM HUMIDIFIER

SPRAY HUMIDIFIER

Increase moisture content by 0.0015kg/kg

Density = 1.24 kg/m3

20

15

10

5

0

-5

-5 0 5 10 15 20 25

WB Temperature o C



RSHF = 0.89

B

A

C

D GF


B

C

D

SteamHumidifier

D

FG

STEAM HUMIDIFIERWITH PREHEAT


Outside Air Winter

Mixed Air Conditionwith high percentageof minimum Outside Air

Preheater

20

15

10

5

0

-5

-5 0 5 10 15 20 25

WB Temperature o C



RSHF = 0.89

B

A

C

D GF

Mixed Air Conditionwith high percentage ofminimum Outside Air


B

C

SprayHumidifier

D

FG

SPRAY HUMIDIFIERWITH PREHEAT


Outside Air Winter

Preheater

20

15

10

5

0

-5

-5 0 5 10 15 20 25

WB Temperature o C



RSHF = 0.89

A

C

DGF



B

C

SprayHumidifier

D

FG

REHEAT


Outside Air Winter

Preheater

Reheater

B

H

H

psychrometrics rev

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