(c) DR.A.M.SURENDRA KUMAR 11/22/2014 1

• The name “ Psychrometry ” is given to the study

of the properties of air-water vapour mixtures.

• Atmospheric air is considered to be a mixture of

dry air and water vapour.

PSYCHROMETRY

• Dry Bulb Temperature : DBT( tdb) the actual temperature of gas or

mixture of gases indicated by an error-free temperature measuring

device.

• Wet bulb temperature : WBT(twb) It is the temperature obtained by

an accurate thermometer having a wick moistened with distilled

water .

• Dew point temperature: DPT(tdp) It is the temperature at which the

liquid droplets just appear when the moist air is cooled continuously.

• The equipment used for measuring the dry bulb temperature and

wet bulb temperature simultaneously is known as Psychrometer.

TEMPERATURES

WET BULB THERMOMETER

PSYCHROMETER

PSYCHROMETRIC CHART

• Absolute humidity :

• Humidity ratio (Relative humidity)

• Degree of saturation :

MIXING PROCESS

[a]Adiabatic Mixing of Air streams [b] Mixing Process on

Psychrometric chart

• Simplifying the expression :

3

2a21a13

m m

am

ttt

BASIC PROCESSES IN CONDITIONING OF AIR

1. Sensible heating – Process OA

2. Sensible cooling – Process OB

3. Humidifying Process - Process OC

4. Dehumidifying Process- Process OD

5. Heating and Humidifying– Process OE

6. Cooling and Dehumidifying – Process OF

7. Cooling and Humidifying – Process OG

8. Heating and Dehumidifying– Process OH

SENSIBLE HEAT PROCESS –

HEATING OR COOLING

Heating or cooling of air without addition or subtraction of

moisture is termed as SENSIBLE HEATING OR COOLING .

Heating can be achieved by passing the air over heating coil like

electric resistance heating coils or steam coils.

Sensible cooling can be achieved by passing the air over

cooling coli like evaporating coil of the refrigerant cycle or

secondary brine coil.

HEATING COILS

• Sensible heating : Air flows over a heating coil

• Surface temperature ts higher than tdb of air

• Heating medium flowing through the coil – steam, hot gases from furnace

Methods employed for winter heating of air:

• 1. Hot water or steam coils

• 2. direct-fired furnace gases coils

• 3. finned eclectic strip heaters

SENSIBLE HEAT PROCESS – HEATING OR COOLING

SENSIBLE HEAT PROCESS – HEATING OR COOLING

• This heat denoted by S is called Sensible heat.

• If a building to be air conditioned receives or losses heat due to transmission or other reasons, it is supposed to have sensible heat load

• For the purpose of calculations, std air is taken at 200C

• and 50% RH.

• The density of std air is 1.2 kg/m3.

• The value of Cp is taken as 1.0216kJ/kgK

•

LATENT HEAT PROCESS- HUMIDIFICATION/DEHUMIDIFICATION

• When the state of air is altered along t= constant moisture in the form of vapour has to be transferred to change in humidity ratio of air

• Latent heat transfer

• If a building gains or lose moisture, it is supposed to have Latent

heat.

• A gain of moisture will require the condensation of moisture

(Dehumidification ) – Hence cooling load

• Loss of moisture – require evaporation of water (Humidification) –

Hence Heating load

• Where hfg0 =2501, we obtain

kW w50(cmm)

60

(2501)(cmm)(1.2)

wQL

SENSIBLE HEAT FACTOR [SHF]

• The ratio of the sensible heat transfer to the total heat transfer is sensible heat factor

• Pont B divides hC-hA - SHF: 1-SHF

• Sensible heat - along AB

• Latent heat along BC

• The process line AC is called Sensible Heat factor line or Process or condition line

• Sensible factor – unity- No latent heat transfer- SHF line is

horizontal in psychrometric chart

• Zero SHF line – vertical on psychrometric chart – No Sensible heat

• SHF = 0.75 to 0.8 common in Air conditioning practice – at normal

dry climate.

• A lower value of SHF such as 0.65 – implies a high latent heat load

– common in a humid climate

• Where tc, wc at the conditions at C

• tADP is known as Apparatus dew point temperature

• It is the temperature at point S where the SHF- line CA cuts the

saturation curve when extended.

)(50)(0204.0

)0.0204(t c

ADPcADPc

ADP

wwtt

tSHF

PSYCHROMETRIC PROCESSES IN AIR CONDITIONING EQUIPMENT

• Eight basic parametric processes together cannot be achieved in practice in air conditioning equipment.

• Different types of equipments are used for this purpose.

• By pass factor is a vital parameter signifying the performance of A/C equipment

1. COOLING & DEHUMIDIFICATION

• Industrial applications: air conditioning, cold storages, paper

industry, photography.

• Lowering the temperature & reducing the moisture content in air.

• Process used in summer air conditioning : air passes over cooling

coil or through cold spray.

COOLING & DEHUMIDIFICATION

• The removal of water vapour from the air is termed as dehumidification of

air. The dehumidification of air is only possible if the air is cooled below

the dew point temperature of the air..

• It is necessary to maintain the coil surface temperature of air for effective

dehumidification.

COOLING & DEHUMIDIFICATION

• Air may be cooled and dehumidified by

• 1. By placing the evaporator coil across the air flow

• 2. By circulating chilled water in a tube placed across the air flow

• 3. By spraying chilled water to air in the form of fine mist to expose a

large surface area.

[b] by circulating chilled water in [c] By spraying chilled water to air in a tube across the air flow the form of fine mist

• In winter air conditioning , heating and humidification are very

common.

• System consists of a coil for sensible heating of air from state 1 to 3

then along the wet bulb temperature line through state 2. then

humidification along wet bulb line until the required moisture is

added to reach state 2.

HEATING AND HUMIDIFICATION

HEATING AND HUMIDIFICATION

• For large commercial systems, adopt simpler and economical

technique.

• Part of air should be cooled to such a low temperature that the

desired mass of moisture is removed.

• Air at state 1 is at much higher temperature than at state 2, it would

work as heating of the cooled air. Hence no extra energy for heating

and cooling is needed.

• This type of cooling and dehumidification is accomplished using

• By-pass as shown in fig.

• A part of the inlet air is by passed without being cooled and rest air

is cooled over the coil.

• X- By pass factor, 1-X amount of air cooled over the coil from state 1

to temp ts of the coil surface, is mixed with the rest air to get it at

state 2, a desired value.

• This desired lowest temperature is called apparatus dew point ADP

BYPASS FACTOR & CONTACT FACTOR

BYPASS FACTOR & CONTACT FACTOR

COOLING & DEHUMIDIFYING COILS –ADP OF COIL

AIR WASHER

• Air washer – flow of air through a spray of water.

• During the course of flow, the air may be cooled or heated or

dehumidified, depending on the mean surface temperature of water.

• Water accordingly, externally cooled or heated or simply

recalculated by a pump.

• Eliminator plates are provided to minimize the loss of water

• Droplets of water act as wetted surface, both sensible and latent

heat transfer take place.

• Different possible processes:

• 1. Process 1-2 A : Heating & Humidification[ ts > t1] ,

where ts is the mean surface temperature of water droplets. Mean

surface temperature (ts) greater than the dry bulb temperature. Here

water is externally heated.

2. Process 1-2 B : Humidification [ts =t1]

Enthalpy of air increases. Water is required to be externally heated.

3. Process 1-2 C: Cooling & Humidification[ t1 < ts < t1]

where is t1 wet temperature. Air is cooled, enthalpy increases as a

result of humidification. Water is required to be externally heated.

4. Process 1-2 D Adiabatic saturation [ t1 = ts ]

where is t1 wet temperature .Pumped circulation of water. Without

any external heating or cooling.

5. Process 1-2 E : Cooling & Humidification[ td < ts < t1 1]

Similar to 1-2C with difference that the enthalpy of air decreases.

Water is required to be externally cooled.

6. Process 1-2 F : Cooling [ts = td] where td is the dew point

temperature. . Water is required to be externally cooled.

7. Process 1-2 G: Cooling & Dehumidification[ts < td ] air is

simultaneously cooled and dehumidified.

• Conclusions

• 1. If the spray water is cooled external to the washer,

the WBT of air decreases.

• 2. If the spray water is heated external to the washer,

the WBT of air increases.

• 3. If the spray water is neither heated nor cooled external

to washer, the WBT of air is not changed.

SIMPLE AIR CONDITIONING SYSTEM

WINTER AIR CONDITIONING

• The winter conditions are say 150C & 20% R.H.

• Required comfort conditions are 240C & 60%.

• In general,

• Air conditioning apparatus for winter air conditioning for

comfort – involves heating and humidifying.

In winter – Building loss –sensible heat – partially

compensated by solar heat gains, internal heat gains – from

occupancy, lighting etc.

Latent heat loss – due low outside air humidity –

compensated by latent heat gains from occupancy.

In winter – heating load is less than cooling load in summer.

Actual situation – depends – swing of the outside

temperature and humidity w.r.to inside conditions.

SUMMER AIR CONDITIONING SYSTEMS

• Example: Summer A/C system for Hot & Dry Outdoor

conditions:

• Such systems are used places like Nagpur, Delhi,

Bhopal etc.

• Required comfort conditions are 240C & 60%..

• Atmospheric conditions in summer are 400C DBT &

20%.

YEAR ROUND AIR CONDITIONING

SUMMER AIR CONDITIONING SYSTEM WITH VENTILATION AIR – ZERO BY PASS FACTOR

• 0 - represents the outside state

• i- Inside air state

• 1- is the state of air after the mixing of re4circulated room air with

ventilation air.

Mixture entering the conditioning apparatus comprises- recalculates air

mai + ma0

Total load on the Air conditioning apparatus = Q

• BREAK UP OF THE LOAD • Room Load: • Sensible RSH • Latent RLH • ----------------------------------------------------- • Total RTH = RSH + RLH

• Ventilation Load : • Sensible OASH • Latent OALH • -------------------------------------------------------- • Total OATH = OASH + OALH

• Air Conditioning Equipment load:

• Total Sensible TSH = RSH + OASH • Total latent TLH = RLH + OALH • ------------------------------------------------------------ • Grant Total GTH = TSH + TLH

SUMMER AIR CONDITIONING SYSTEM WITH VENTILATION AIR –BY PASS FACTOR