Indoor Environment Control - MEL413

Topics

Introduction: Air conditioning, HVAC systems

Classifications

Applications

Indoor Air quality (IAQ)

Working substance

Methods for estimating properties of moist air

Gibbs-Dalton Law for a mixture of perfect gases

Important psychrometric properties

Psychrometric chart

Measurement of psychrometric properties

Calculation of psychrometric properties from p, DBT and WBT

Psychrometer

Thermodynamic wet-bulb temperature or adiabatic saturation temperature

Air conditioning

Air Conditioning refers to the treatment of air so as to simultaneously control its

Temperature

Moisture content

Cleanliness

Odor

Circulation

as required by occupants, a process, or products in the space.

Air conditioning

“The science and practice of creating a controlled climate in indoor spaces.”

HVAC systems

A complete heating, ventilating and air conditioning (HVAC) system has the

facility to

Heat

Cool

Humidify

Dehumidify

Clean

Distribute

the conditioned air into the room so as to meet the indoor year-round human

comfort or industrial applications.

Large buildings or in large installations like

shopping complexes, academic complexes, research laboratories and office

complexes

Different areas requiring different levels of

temperature, humidity and cleanliness

Hotel rooms with each room requiring a different level of temperature and humidity

The regions with different requirements are divided into different zones of similarrequirements and the system is integrated for energy optimization.

Conditioned air from a single unit cannot be economically transported to variouslocations of a far spread building complex .

Fan power requirement and the size of the duct to carry conditioned air andreturn air will occupy enormous space.

Remedy: Chilled water (summer air conditioning) and hot water (winter airconditioning) are generated at central place and transported in undergroundinsulated pipes to individual buildings.

Classifications

There are 3 major ways of classifying the air conditioning systems:

Based upon the arrangement of equipment Central HVAC system (beyond 20 tons)

Unitary or packaged systems (5 to 20 tons)

Window and Split air conditioner (up to 5 tons)

Combination systems

Based upon the major function Comfort air conditioning

Industrial air conditioning

Based upon the season Summer air conditioning systems

Winter air conditioning systems

Year-round air conditioning systems

Central HVAC Systems

components like compressor, condenser,

pumps, cooling tower, etc. are installed in a

central power plant room

This type of system is not really suitable for

hotel or office buildings where individual

room control is necessary

Also not suited, if the area to be served is

spread out or if it is a high-rise building

Capacity of the plant depends on the type of

compressor usedReciprocating compressor

Screw compressor

Centrifugal compressorFig. 1 Central HVAC system

Based upon the arrangement of equipment

Unitary or Packaged systems

These are factory assembled or packaged units

Can be mounted on floor or on rooftop

Different types of condensers usedAir cooled condenser for smaller capacity

Water cooled condenser for larger capacity

Unit may be hung from the ceiling

Very convenient for single storey buildings or

to serve a single floor of high-rise buildings.

Fig. 2 Unitary or packaged system

Based upon the arrangement of equipment

Combination systems

Central plant where the water is chilled in a

refrigeration system and there is a facility

such as a boiler or furnace to heat the water.

Ideally suited for large buildings, large

installations like shopping complex,

academic complex, research laboratories

and office complex, etc.

Various control strategies are used to carry

out energy optimization by microprocessors

Fig. 3 Combination system

Based upon the arrangement of equipment

Comfort air conditioning

To create indoor conditions conducive to human health, comfort and

efficiency

For the comfort of workers in offices and for the comfort of customers in

stores, restaurants, theatres, hospitals and schools

Industrial air conditioning

Provides a partial measure of comfort to workers in a hostile environment

To create an environment conducive to research and industrial operations

in order to maintain manufacturing tolerances in electronics, space and

computer industries and all high speed automated manufacturing

operations

Based upon the major function

Applications

Industrial Air Conditioning

Laboratories

Printing

Manufacture of Precision Parts

Textile Industry

Pharmaceutical Industries

Photographic Material

Farm Animals

Computer Rooms

Vehicular Air-conditioning

Comfort Air Conditioning

Residences

Offices

Shopping centers

Stores and super markets

Theatres

Auditorium

Operation theatres

Restaurants

Large buildings

Indoor Air Quality

Indoor air quality (IAQ)

Indoor Air Quality (IAQ) refers to the ways and means of reducing and maintaining the pollutants inside the occupied space within tolerable levels.

IAQ involves specifying suitable levels of

fresh air supply (ventilation),

suitable air filters,

use of proper materials of construction,

furniture,

carpets,

draperies etc.

Sick building syndrome is very common in poorly designed air conditioned buildings due to inadequate ventilation and use of improper materials.

Sick building syndrome is characterized by the feeling of

nausea,

headache,

eye and throat irritation and the

general feeling of being uncomfortable with the indoor environment.

Working substance

• An important aspect in air conditioning is the study of working substance.

• Atmospheric air makes up the environment in almost every type of air conditioning

system.

• Thorough understanding of the properties of atmospheric air and the ability to

analyze various processes involving air is fundamental to air conditioning design.

• Psychrometry is the study of the properties of mixtures of air and water

vapour, i.e . moist air

• Atmospheric air is a mixture ofmany gases

water vapour and

a number of pollutants

After filtration

Moist air for conditioning

Fig. 4 Atmospheric air

• Above an altitude of about 10 km, atmospheric air consists of only dry air.

• Below this, the amount of water vapor and pollutants vary and reaches a

maximum near to the earth’s surface.

• Hence, the pollutants have to be filtered before processing the air.

• What we process is essentially a mixture of various gases that constitute air

and water vapour.

• This mixture is known as moist air.

• The moist air can be thought of as a mixture of dry air and moisture.

• For all practical purposes, the composition of dry air can be considered as

constant.

Constituent Molecular weight Mol fraction

Oxygen 32.000 0.2095

Nitrogen 28.016 0.7809

Argon 39.944 0.0093

Carbon dioxide 44.010 0.0003

Composition of standard air

The molecular weight of dry air is found to be 28.966 and the gas constant R is

287.035 J/kg.K.

The amount of water vapour present in the air may vary from zero to a maximum

depending upon the temperature and pressure of the mixture (dry air + water

vapour), keeping the dry air composition constant.

When the moisture content is maximum, then the air is known as saturated air.

Methods for estimating properties of moist air

• Estimating various properties of air are essential for performing air conditioning calculations.

• Exact property values of moist air is difficult to estimate as it is a mixture of several permanent gases and water vapour.

• However, moist air upto 3 atm. pressure is found to obey perfect gas law with accuracy sufficient for engineering calculations.

• Goff and Gratch tables (for higher accuracy).

• These tables are based on statistical mechanics that take into account the real gas behavior of dry air and water vapour

• Limitations: Goff and Gratch tables are valid only for a barometric pressure of 1 atm.

• Mixture models can be used to estimate moist air properties even for other pressures also, despite of quite complex calculations involved.

• However, since in most cases the pressures involved are low, one can apply the perfect gas model to estimate psychrometric properties.

18

Dry and Atmospheric Air

Atmospheric air: Air in the atmosphere containing some water vapor (or moisture).

Dry air: Air that contains no water vapor.

The cp of air can be assumed to

be constant at 1.005 kJ/kg·°C in

the temperature range 10 to

50°C with an error under 0.2%.

Taking 0°C as the reference temperature, the enthalpy and enthalpy change of dry

air can be determined from

Although the amount of water vapor in the air is small, it

plays a major role in human comfort. Therefore, it is an

important consideration in air-conditioning applications.

The temperature of air in air-conditioning applications

ranges from about 10 to about 50°C.

In this range, dry air can be treated as an ideal gas with

a constant cp value of 1.005 kJ/kg · K with negligible

error (under 0.2 percent).

19

Dry and Atmospheric Air

Therefore, water vapor in air behaves as if it existed alone and obeys the ideal-gas

relation Pv = RT.

Then the atmospheric air can be treated as an ideal-gas mixture:

Pa Partial pressure of dry air

Pv Partial pressure of vapor (vapor pressure)

It certainly would be very convenient to also treat the water vapor in the air as an

ideal gas and you would probably be willing to sacrifice some accuracy for such

convenience.

Well, it turns out that we can have the convenience without much sacrifice.

At 50°C, the saturation pressure of water is 12.3 kPa.

At pressures below this value, water vapor can be treated as an ideal gas with

negligible error (under 0.2 percent), even when it is a saturated vapor.

20

h = h(T) since water vapor is an ideal gas

For water

hg = 2500.9 kJ/kg at 0°C

cp,avg = 1.82 kJ/kg · °C at 10 to 50°C range

In the temperature range 10 to 50°C, the

hg of water can be determined from Eq. 1

with negligible error.

(1)

Therefore, the enthalpy of water vapor in air can be taken to be equal to the

enthalpy of saturated vapor at the same temperature.

Below 50°C, the h = const. lines coincide with the T =

const. lines in the superheated vapor region of water.

21

Specific and Relative Humidity of AirAbsolute or specific humidity (humidity ratio): The

mass of water vapor present in a unit mass of dry air.

For saturated air, the vapor

pressure is equal to the saturation

pressure of water.

Saturated air: The air saturated with moisture.

The amount of water vapor in saturated air at a

specified temperature and pressure can be determined

using above relation by replacing Pv by Pg, the

saturation pressure of water at that temperature

Relative humidity: The ratio of the amount of moisture

the air holds (mv) to the maximum amount of moisture

the air can hold at the same temperature (mg).

The difference between specific

and relative humidities.

=18.015

461.52 J/kg.K

28.966

287.035J/kg.K

v

v

a

a

M

R

M

R

22

What is the relative humidity

of dry air and saturated air?

In most practical applications, the amount of dry

air in the air–water-vapor mixture remains

constant, but the amount of water vapor changes.

Therefore, the enthalpy of atmospheric air is

expressed per unit mass of dry air.

The enthalpy of moist (atmospheric)

air is expressed per unit mass of dry

air, not per unit mass of moist air.

Atmospheric air is a mixture of dry air and water vapor, and thus the enthalpy of air

is expressed in terms of the enthalpies of the dry air and the water vapor.

23

Dry bulb temperature simply tells us the temperature of dry air, but the comfort also

depends on humidity i.e. water vapour present in the air.

Wet bulb temperature indicates the humidity in the air. Because the evaporation of

water in the bulb wick depends on the relative humidity of surrounding air

Saturated vapour pressure (psat) is the saturated

partial pressure of water vapour at the dry bulb

temperature.

Or

The pressure at which a pure substance changes

phase at a given temperature.

This is readily available in thermodynamic tables

and charts.

Saturation temperature Tsat: The temperature at

which a pure substance changes phase at a given

pressure.

Dry-bulb temperature: The ordinary temperature of atmospheric air.

24

Problem 1: A 5-m 5-m 3-m room shown in below figure contains air at 25°C and

100 kPa at a relative humidity of 75 percent. Determine (a) the partial pressure of dry

air, (b) the specific humidity, (c) the enthalpy per unit mass of the dry air, and (d ) the

masses of the dry air and water vapor in the room.

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Dew-point Temperature

Constant-presssure cooling of moist air and the dew-

point temperature on the T-s diagram of water.

Dew-point temperature Tdp: The temperature at which condensation begins when

the air is cooled at constant pressure (i.e., the saturation temperature of water

corresponding to the vapor pressure.)

When the temperature of a cold drink

is below the dew-point temperature of

the surrounding air, it “sweats.”

The air remains saturated during the condensation process and thus follows a path

of 100 percent relative humidity (the saturated vapor line).

The ordinary temperature and the dew-point temperature of saturated air are

identical.

26

Problem 2: In cold weather, condensation frequently occurs on the inner surfaces

of the windows due to the lower air temperatures near the window surface.

Consider a house, shown in below figure, that contains air at 20°C and 75 percent

relative humidity. At what window temperature will the moisture in the air start

condensing on the inner surfaces of the windows?

Degree of saturation (µ):

where Wv and Ws are specific humidity of air and saturated air respectively.

ps (or pg) is the partial pressure of water vapor when air is saturated

pv is the partial pressure of water vapor in a moist air

p is the total pressure of moist air

The degree of saturation represents the capacity of moist air to absorb water

vapor

Relative humidity, RH(Φ) = pv/ps = 0 when moist air is totally dry, i.e. which does not

contain water vapor.

If the moist air is saturated, then pv = ps, then RH = 1 and µ =1.

Degree of saturation varies from 0 to 1.

)

)

v

s

v

v sv

s s v

s

humidity ratio of moist air at temperature T W

humidity ratio of saturated air at the same temperature T W

p0.622

p (p - pp - p =

p p (p - p0.622p - p

Measurement of psychrometric properties

Based on Gibbs’ phase rule, the thermodynamic state of moist air is uniquely

fixed if the barometric pressure and two other independent properties are

known.

At a given barometric pressure, the state of moist air can be determined by

measuring any two independent properties.

One of them could be the dry-bulb temperature (DBT), which is fairly simple and

accurate to measure.

The accurate measurement of other independent parameters such as humidity

ratio is very difficult in practice.

Since measurement of temperatures is easier, it would be convenient if the

other independent parameter is also a temperature.

Of course, this could be the dew-point temperature (DPT), but it is observed

that accurate measurement of dew-point temperature is difficult.

In this context, a new independent temperature parameter called the wet-bulb

temperature (WBT) is defined. Compared to DPT, it is easier to measure the

wet-bulb temperature of moist air.

It is possible to find the other properties of moist air by knowing the dry-bulb

and wet-bulb temperatures from measurements.

Calculation of psychrometric properties from p, DBT and WBT

For a given barometric pressure, knowing the dry bulb and wet bulb temperatures, all

other properties can be easily calculated from the psychrometric equations.

The following are the empirical relations for the vapor pressure of water in moist air

i) Modified Apjohn equation:

pv = pw - 1.8p(Tdb – Twb )/2700

ii) Modified Ferrel equation:

pv = pw - 0.00066p(Tdb – Twb )[1+(1.8Tdb )/1571]

iii) Carrier equation:

pv = pw- 1.8(p - pw) (Tdb – Twb )/(2800-1.3(1.8 Tdb+ 32)

pw saturation pressure of water vapor corresponding to wet bulb temperature (from

steam tables)

30

Adiabatic Saturation and Wet-bulb Temperatures

The adiabatic saturation process and its

representation on a T-s diagram of water.

The specific humidity (and relative humidity) of air can

be determined from these equations by measuring the

pressure and temperature of air at the inlet and the exit

of an adiabatic saturator.

31

A simple arrangement to

measure the wet-bulb

temperature.

The adiabatic saturation process is not practical. To determine the absolute and

relative humidity of air, a more practical approach is to use a thermometer whose

bulb is covered with a cotton wick saturated with water and to blow air over the wick.

For air–water vapor mixtures at atmospheric pressure, Twb is approximately equal to

the adiabatic saturation temperature.

Sling psychrometer

The temperature measured is the

wet-bulb temperature Twb and it is

commonly used in AC applications.

Unlike thermodynamic WBT, the

WBT of wet bulb thermometer is

not a thermodynamic property as it

depends upon the rates of heat

and mass transfer between the

wick and air.

Thus, in psychrometric equations and

psychrometric charts where the wet

bulb temperature appears, it is

always the thermodynamics wet

bulb temperature that is considered.

Psychrometer

Any instrument capable of measuring the psychrometric state of air is called a

psychrometer.

In order to measure the psychrometric state of air, it is required to measure three

independent parameters.

Generally two of these are the barometric pressure and air dry-bulb temperature as

they can be measured easily and with good accuracy.

1) sling psychrometer

2) aspirated psychrometer: the thermometers remain stationary, and a small fan,

blower or syringe moves the air across the thermometer bulbs.

33

Problem 3: The dry- and the wet-bulb temperatures of atmospheric air at 1 atm

(101.325 kPa) pressure are measured with a sling psychrometer and determined to

be 25 and 15°C, respectively. Determine (a) the specific humidity, (b) the relative

humidity, and (c) the enthalpy of the air.

Psychrometric chart

A Psychrometric chart graphically represents the thermodynamic properties of

moist air.

Standard psychrometric charts are bounded by the dry-bulb temperature line

(abscissa) and the vapour pressure or humidity ratio (ordinate).

The Left Hand Side of the psychrometric chart is bounded by the saturation line.

Psychrometric charts are readily available for standard barometric pressure of

101.325 kPa at sea level and for normal temperatures (0-50oC).

DBT lines

WBT lines

RH lines

Specific humidity lines

DPT lines

Specific volume lines

Specific enthalpy lines

References

RAC – C.P. Arora

RAC – Stoecker and Jones

RAC – R.C. Arora

RAC – Ahmadul Ameen

RAC – S.N.Sapali

37

Gibbs’ Phase rule:

F = C – P + 2

where P is the number of phases in thermodynamic equilibrium with each other and

C is the number of components.

F is the number of degrees of freedom, which means the number of intensive properties such as temperature or pressure, which are independent of other intensive variables

Typical phases are solids, liquids and gases.

A system involving one pure chemical is an example of a one-component system. Two-component systems, such as mixtures of water and ethanol, have two chemically independent components..