Introduction to Psychrometric charts

by

Salim Deramchi

Email: salim.deramchi@bsria.co.uk

1

What is a Psychrometric chart?

Psychrometric is the study of moist air

2

Atmospheric Air

A mix of "dry" air and water vapour

Dry Air is a mixture

Mainly nitrogen and oxygen

Traces of argon, krypton, neon, helium, xenon, carbon dioxide, methane and others

3

NATURAL PSYCHROMETRY

Rain Snow

Hail

Fog/Mist

Dew Ground Frost

4

Why psychrometric charts?

5

Thermal comfort

6

TO AVOID

CONDENSATION

SWEATING

7

Where can psychrometric chart be used

8

CENTRALISED

CAV & VAV

LOCAL

PARTIALLY

CENTRALISED

UNIT

+

Design of AIR-CONDITIONING

SYSTEMS

+

& SPLIT

9

10

Types of psychrometric charts

11

12

13

CIBSE GUIDE C Tables

14

The properties of the psychometric chart

15

The properties to be discussed

Dry bulb temperature Wet bulb temperature Moisture content (kg Water / kg Dry air)

Percentage of relative humidity (this is shown

on psychrometric charts as percentage saturation)

Specific enthalpy

16

Dry bulb

The temperature which can be measured by a thermometer

17

WET-BULB TEMPERATURE

Dry-bulb

Temperature

Wet-bulb

depression

Wet-bulb

Temperature

18

Used to infer the amount of water moisture in the air (Water content)

Specific enthalpy

Specific enthalpy is the amount of energy per kg of air at a specific

condition

19

So lets determine some parameters

20

Dry Bulb Vertical Lines

21

Dry bulb temperature 10 20 30

Wet Bulb Diagonal Lines

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6

11

16

21

Dew point Horizontal Lines

23

Dew

po

int

tem

per

atu

re

V

erti

cal L

ines

Relative humidity

It is the Ratio of water vapour compared to the maximum amount

the air can hold

24

Exercise

Determine all the parameters at the temperatures of 20Cdb and 20%RH

25

26

Exercise

Dry

bu

lb

Dew Point Moisture content

Dew Point temperature

Exercise

Determine all the parameters at the temperatures of 22Cdband 15 Cwb

27

47%

Dew-point Temperature = 10 C 22 C

15 C

Enthalpy

41.2 kJ/kg

Moisture Content 7.6 g/kg

28 28

COMFORT

ZONE

Arabian Desert

Hong

Kong

UK

Comfort

Zone

DESIGN CONDITIONS

Singapore

29

Basic Formulae

Sensible Heat = mass x specific heat x temperature change

Flow flow

qs = m x cp x Dt

30

Exercise

Heat 2m3/s of air entering at 10OCdb and 7OCwb

to 30O C & determine the heater duty

31

32

33

10

7

30

a b

Ha=22.5

Hb=42.5

Exercise

Determine coil duty to cool 3m3/s of air at 30OCdb and 20

OCwb to 20OCdb

34

35

30

20

a

b

Hb=47

Ha=57

20

END

Any questions?

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