1
CHAPTER 2
THE DEVELOPMENT OF THE PSYCHROMETRIC CHART AND
HOW ITS USE MAKES THE SEVEN PROCESSES IN AIR
CONDITIONING SO EASY TO ANALYSE
PAGE
LEARNING OUTCOMES 3
2.1 INTRODUCTION 3
2.2 PROPERTIES OF AIR 4
2.2.1 Composition of air 4
2.2.2 Barometric pressure 4
2.2.3 Dry bulb temperature 5
2.2.4 Wet bulb temperature 5
2.2.5 Effective temperature 6
2.2.6 Humidity ratio 6
2.2.7 Vapour pressure 7
2.2.8 Relative humidity 7
2.2.9 Dew point temperature 8
2.2.10 Enthalpy 9
2.2.11 Specific volume 9
2.2.12 Properties of water vapour 9
2.2.13 Steam tables 10
2.3 THE PSYCHROMETRIC CHART 13
2.3.1 Introduction 13
2.3.2 The state point 17
2.3.3 Reading the psychrometric chart 17
2.3.3.1 Vapour pressure 17
2.3.3.2 Dew point temperature 18
2.3.3.3 Enthalpy 19
2.4 AIR-CONDITIONING PROCESSES 19
2.1.1 Sensible heating or cooling 20
2.1.2 Humidification 22
2.12.1 Evaporative humidifiers (adiabatic humidifiers) 23
,2
2.1.2.2 Steam type humidifiers 24
2.1.3 Dehumidification 26
2.1.3.1 Dehumidification by means of cooling 26
2.1.3.2 Dehumidification by means of chemicals 27
2.1.4 Heating and humidification 28
2.1.5 Cooling and dehumidification 29
2.1.6 Evaporative cooling 31
2.1.7 Air mixing 34
2.2 CALCULATIONS OF AIR REQUIREMENTS 37
2.5.1 Removal of sensible heat 37
2.2.2 Removal of latent heat 38
2.2.3 The sensible heat factor 38
2.2.4 Removal of total heat RSHF 40
,3
LEARNING OUTCOMES
On completion of this module the learner will be able to do the following :
* understand the various thermodynamic properties of moist air ;
* use these properties to analyse conditions and processes involving moist air ;
* deal with psychrometric charts to establish the interrelationship between
these properties from the charts ;
* illustrate the various air conditioning processes on a psychrometric chart in order
to simplify their calculations ; and
* analyse important cooling, heating, humidification and dehumidification processes
on a psychrometric chart.
2.1 INTRODUCTION
Psychrometry can be defined as the science dealing with the behaviour of the mixture
of dry air and water vapour.
Having reviewed a number of basic principles of physics and their application to air-
conditioning, we now turn to the study of air itself. Air is the medium with which
buildings are cooled and heated and their occupants made comfortable in summer and
winter.
Before a building can be air-conditioned, the air itself must first be ‘conditioned’. It is
important to re-emphasise that air-conditioning is not simply a matter of heating or
cooling air to some desired temperature. The complete process of air-conditioning is
also concerned with the humidity of the air, its cleanliness and its distribution. Only
when all these parameters are controlled can we speak of complete air-conditioning.
RAC401M 3
, 4
A thorough understanding of the properties of moist air and the ability to analyse the
various processes involving air is absolutely essential for the air-conditioning technician
and engineer.
In 1911, Willis H Carrier made a significant contribution to the air-conditioning field
when he published his paper on psychrometry and a psychrometric chart. The formulae
in his paper have become fundamental to the industry.
2.2 PROPERTIES OF AIR
2.2.1 Composition Of Air
Air is a mixture of nitrogen, oxygen and small amounts of some other gases. Air in the
atmosphere normally contains some water vapour or moisture and is referred to as
atmospheric air. Air that contains no water is called dry air. The variation of the amount
of water vapour in the air or its ‘moisture content’ varies from about 0,1% to 3% by
mass.
In psychrometry, the concept ‘dry air’ in an air-conditioning process is a constant whilst
the amount of moisture does vary when humidification and dehumidification takes
place. Thus expressing the properties of the air per kg of dry air greatly simplifies
psychrometric calculations.
2.2.2 Barometric Pressure
The earth is surrounded by air up to a height of some 80 km. Since air has mass, and
as such is subject to the force of gravity, it exerts pressure.
This pressure is known as the atmospheric pressure and is measured with a barometer.
It is called the barometric pressure.
RAC401M 4
CHAPTER 2
THE DEVELOPMENT OF THE PSYCHROMETRIC CHART AND
HOW ITS USE MAKES THE SEVEN PROCESSES IN AIR
CONDITIONING SO EASY TO ANALYSE
PAGE
LEARNING OUTCOMES 3
2.1 INTRODUCTION 3
2.2 PROPERTIES OF AIR 4
2.2.1 Composition of air 4
2.2.2 Barometric pressure 4
2.2.3 Dry bulb temperature 5
2.2.4 Wet bulb temperature 5
2.2.5 Effective temperature 6
2.2.6 Humidity ratio 6
2.2.7 Vapour pressure 7
2.2.8 Relative humidity 7
2.2.9 Dew point temperature 8
2.2.10 Enthalpy 9
2.2.11 Specific volume 9
2.2.12 Properties of water vapour 9
2.2.13 Steam tables 10
2.3 THE PSYCHROMETRIC CHART 13
2.3.1 Introduction 13
2.3.2 The state point 17
2.3.3 Reading the psychrometric chart 17
2.3.3.1 Vapour pressure 17
2.3.3.2 Dew point temperature 18
2.3.3.3 Enthalpy 19
2.4 AIR-CONDITIONING PROCESSES 19
2.1.1 Sensible heating or cooling 20
2.1.2 Humidification 22
2.12.1 Evaporative humidifiers (adiabatic humidifiers) 23
,2
2.1.2.2 Steam type humidifiers 24
2.1.3 Dehumidification 26
2.1.3.1 Dehumidification by means of cooling 26
2.1.3.2 Dehumidification by means of chemicals 27
2.1.4 Heating and humidification 28
2.1.5 Cooling and dehumidification 29
2.1.6 Evaporative cooling 31
2.1.7 Air mixing 34
2.2 CALCULATIONS OF AIR REQUIREMENTS 37
2.5.1 Removal of sensible heat 37
2.2.2 Removal of latent heat 38
2.2.3 The sensible heat factor 38
2.2.4 Removal of total heat RSHF 40
,3
LEARNING OUTCOMES
On completion of this module the learner will be able to do the following :
* understand the various thermodynamic properties of moist air ;
* use these properties to analyse conditions and processes involving moist air ;
* deal with psychrometric charts to establish the interrelationship between
these properties from the charts ;
* illustrate the various air conditioning processes on a psychrometric chart in order
to simplify their calculations ; and
* analyse important cooling, heating, humidification and dehumidification processes
on a psychrometric chart.
2.1 INTRODUCTION
Psychrometry can be defined as the science dealing with the behaviour of the mixture
of dry air and water vapour.
Having reviewed a number of basic principles of physics and their application to air-
conditioning, we now turn to the study of air itself. Air is the medium with which
buildings are cooled and heated and their occupants made comfortable in summer and
winter.
Before a building can be air-conditioned, the air itself must first be ‘conditioned’. It is
important to re-emphasise that air-conditioning is not simply a matter of heating or
cooling air to some desired temperature. The complete process of air-conditioning is
also concerned with the humidity of the air, its cleanliness and its distribution. Only
when all these parameters are controlled can we speak of complete air-conditioning.
RAC401M 3
, 4
A thorough understanding of the properties of moist air and the ability to analyse the
various processes involving air is absolutely essential for the air-conditioning technician
and engineer.
In 1911, Willis H Carrier made a significant contribution to the air-conditioning field
when he published his paper on psychrometry and a psychrometric chart. The formulae
in his paper have become fundamental to the industry.
2.2 PROPERTIES OF AIR
2.2.1 Composition Of Air
Air is a mixture of nitrogen, oxygen and small amounts of some other gases. Air in the
atmosphere normally contains some water vapour or moisture and is referred to as
atmospheric air. Air that contains no water is called dry air. The variation of the amount
of water vapour in the air or its ‘moisture content’ varies from about 0,1% to 3% by
mass.
In psychrometry, the concept ‘dry air’ in an air-conditioning process is a constant whilst
the amount of moisture does vary when humidification and dehumidification takes
place. Thus expressing the properties of the air per kg of dry air greatly simplifies
psychrometric calculations.
2.2.2 Barometric Pressure
The earth is surrounded by air up to a height of some 80 km. Since air has mass, and
as such is subject to the force of gravity, it exerts pressure.
This pressure is known as the atmospheric pressure and is measured with a barometer.
It is called the barometric pressure.
RAC401M 4
