Lesson 25. Definition, properties of air vapour mixtures

Module 7. Psychrometry

Lesson 25

25.1 Introduction

Air-conditioning signifies the control of an atmospheric environment either for human or to carry out industrial or scientific process efficiently. The purpose of air-conditioning is to supply sufficient volume of clean air having a specific amount of water vapour and at a required temperature within a selected space. The space may be a small compartment such as a research laboratory, computer laboratory or big area like cinema hall, shopping centre, air port etc. Thus, air conditioning refers the control of temperature, relative humidity, quality of air and distribution of air depending upon the application of air conditioning. The environmental air conditioned in terms of temperature, humidity, purity of air results in greater comfort to occupants when applied to public places , offices and factories . Air conditioning designed for industrial purpose has several benefits including better control of product quality and efficiency.

25.2 Classification

The air-conditioning systems are broadly classified into two groups.

1. Comfort air-conditioning

2. Industrial air-conditioning.

Air-conditioning system for human comfort provides an environment which is comfortable to the majority of the occupants . The comfort air-conditioning systems are subdivided into three groups.

25.2.1 Summer air -conditioning

The initial quality of air decides the way of treatment of the air to condition the air for human comfort. The processes involved in treatment of air for air conditioning varies from place to place depending on the environmental quality of air. In many places, summer air-conditioning requires to reduce the s e ns ible heat and water vapour of the air by dehumidifying.

25.2.2 Winter air-conditioning

In winter, it is necessary to increase the sensible heat and water vapour content of the air by heating and humidification .

25.2.3 Year-round air-conditioning

This system provides the control of temperature and humidity of air throughout the year when the atmospheric conditions are changing a s per the season .

Indu s trial air-conditioning provides air at required temperature and relative humidity to perform a specific industrial process successfully . The design conditions for industrial air conditioning are not based on th e comfort feeling of the human beings but on the requirement of the industrial process .

To understand the fundamentals of air conditioning, the basic knowledge of psychrometry is very essential.

25.3 Psychrometry

The science dealing with properties of air and vapour mixture is known as psychrometry. The knowledge of fundamental laws of gaseous mixture is necessary for understand ing of psychrometry. The content of the water vapour in air is one of the factors responsible comfort air - conditioning. The subject of psychrometry is also important in many unit operations where air is used in the process. A few unit operations, where the knowledge of psychrometry is very essential for better understanding of the process, are listed below.

  • Air heating system for spray drying plant.
  • Performance evaluation of cooling tower.
  • Performance evaluation of evaporative condenser.
  • Evaporative cooling using water.
  • Energy analysis casein or other types of dryers.
  • The properties of mixtures of air and water vapour are discussed in this lesson.

25.4 Psychrometry Properties

The definitions of different psychrometric properties of air are given below

a) Dry air

The dry air is considered as a mixture of nitrogen and oxygen and small percentage s of other gases. The volumetric composition of air is 79% nitrogen and 21 % oxygen and the molecular weight of dry air is taken as 29 approximately .

b) Moist air

It is a mixture of dry air and water · vapour . The quantity of water vapour present in the air depend s upon the temperature of the air and its quantity may change from zero to maximum (the maximum amount depends on saturation condition).

c) Water vapour

The moisture present in the form of vapour is known as water vapour. The relative humidity of air is an important factor in all air-conditioning systems . .

The environmental (mixture of air and water vapour) is said to be saturated when it contains maximum amount of water vapour that it can hold at the prevailing temperature. If the temperature of mixture of air and water vapour is above the saturation temperature of the water vapour, the vapour is called superheated vapour .

d) Dry bulb temperature

The temper a ture of air measured by ordinary thermometer is known as dry bulb temperature (dbt).

e) Wet-bulb temperature

The temperature measured by the thermometer when its bulb is covered with wet cloth and is exposed to a current of moving air is known as wet bulb temperature (wbt) . The difference between the dry bulb and wet bulb temperature is known as wet bulb depression (wbd) . Wet bulb depression b e comes zero when the air is fully saturated.

f) Dew Point temperature

The temperature of the air is reduced by continuous cooling than the water vapour in the air will start condensing at a particular temperature. The temperature at which the· condensing starts is known as Dew , point temperature. Dew point temperature is equal to the steam table saturation temperature corresponding to the actual partial pressure of the water vapour in the air . The difference between dry bulb temperature and dew point temperature is known as dew point depression (dpd).

g) Specific humidity (humidity ratio)

It is defined as the mass of water yapour present per kg of dry air . It is expressed as g/kg dry air or kg/ kg dry air.

h) Absolute humidity

The weight of water vapour present in unit volume of air is known as absolute humidity.

i) Degree of Saturation

The degree of saturation is defined as the ratio of mass of water vapour associated with unit mass of dry air to mass of water vapour associated with unit mass of dry air saturated at the same temperature.

j) Relative humidity

The relative humidity is defined as the ratio of actual mass of water vapour in a given . volume to the mass of water vapour if the air is fully saturated at the same temperature .

k) Sensible heat of air

The quantity of heat which can be measured by measuring the dry bulb temperature of the air is known as sensible heat of the air.

l) Total heat of air

The total heat of the humid air is the sum of the sensible heat of the dry air and sensible and latent heat of water vapour associated with dry air.

m) Humid specific volume

The volume of the air per kg of dry air in the mixture is known as humid specific volume of the air. It is expressed as m3/kg dry air.

Last modified: Friday, 19 October 2012, 8:53 AM