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ME340A: Prof. Sameer Khandekarhttp://home.iitk.ac.in/~samkhan

Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur India 1

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ME340A: Refrigeration and Air ConditioningInstructor: Prof. Sameer KhandekarTel: 7038; e-mail: [email protected]

Department of Mechanical EngineeringIndian Institute of Technology KanpurKanpur 208016 India

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Introduction to Psychrometry

Sameer KhandekarSir M. Visvesvaraya Chair ProfessorDepartment of Mechanical EngineeringIndian Institute of Technology KanpurKanpur (UP) 208016 INDIAWebpage: home.iitk.ac.in/~samkhan/

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In this lecture...

◉ Weather, Climate, Local conditions

◉ Heat exchange processes: Built environment and humans

◉ Air-humidity interaction

◉ Psychrometry

◉ Psychrometric processes



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In this lecture...

◉ Properties of Air and Water Vapour mixture○ Total Pressure○ Dry Bulb Temperature, Wet Bulb Temperature and Dew Point○ Absolute humidity and Relative humidity

◉ Psychrometric Chart○ Fundamentals○ How to read it ○ Locating a point○ Some important points to note

◉ Human Comfort○ Definition ○ Factors affecting Human Comfort

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Air-humidity interactions and implications

Psychrometry: Thermodynamocs of Air and Water Vapour Mixture1



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Total Pressure

◉ Both dry air and water vapour in air generally follow the ideal gas law.

So, the total pressure, P = Pa + Pw as per Dalton’s Law of Partial pressure.Here, both Pa and Pw are partial pressures

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Psychometry

Psychrometrics, psychrometry, and hygrometry are names for the field of engineering concerned with the physical and thermodynamic properties of gas-vapor mixtures.



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Dry air Water Vapour

EquilibriumIn an isolated system when there is no change in the macroscopic property of the system like entropy,

internal energy etc, it is said to be in Thermodynamic equilibrium.

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heat

Low Temperature High TemperatureLow Kinetic Energy High Kinetic energy

T1° T2°



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Air

Water

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SaturationEquilibrium



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There is a limit to the number the water molecules which the dry air can accommodate, at a given

Temperature and Pressure.

Saturation point Humidity

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Dew Point

◉ The temperature at which the air becomes saturated with water when it is cooled at constant pressure is called Dew Point.

Tdp = Tsat @ Pv

◉ Examples:

Dew on leavesWater droplets on cold can



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Absolute and relative humidity

◉ Absolute humidity: The mass of water vapor present in a unit mass of dry air. It is also called specific humidity or humidity ratioSpecific Humidity, 𝜔 = 𝑚𝑣/𝑚𝑎

It can also be expressed as,

◉ Relative Humidity: 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).

Relative Humidity, φ = =

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Absolute humidity is the total mass of water vapourpresent in a given volume or mass of air.

Absolute Humidity



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It is the ratio of the Absolute Humidity to the Saturation point Humidity.

RH = AH / SH × 100

Relative Humidity

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Evaporation is a type of phase-change that occurs on the surface of a liquid as it changes into the gaseous phase before reaching its boiling point.

Evaporation

Latent heat of evaporation? Where does it come from?

Latent Heat

Box/Dry air/Environment



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T1°

Scenario#2

T2°

T2<T1

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Scenario#1

T1°

Scenario#2

T2°

Dry bulb Thermometer

Wet bulb Thermometer



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Dry bulb Thermometer

Wet bulb Thermometer

The dry-bulb temperature (DBT) is the temperature of air measured by a thermometer freely exposed to the air, but shielded from radiation and moisture

DBT is the temperature that is usually thought of as air temperature, and it is the true thermodynamic temperature.

At 100% relative humidity, the wet-bulb temperature is equal to the air temperature (dry-bulb temperature) and is lower at lower humidity.

Th WBT indicates the minimum temperature which can be achieved through evaporative cooling.

The wet-bulb temperature is the temperature read by a thermometer covered in water-soaked cloth (wet-bulb thermometer) over which air is passed

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Dry Bulb Temperature & Wet bulb temperature

◉ DBT: Regular temperature measured by a thermometer of the static air around the bulb.

◉ WBT: Temperature sensed by a thermometer, whose bulb is wrapped with a wick that is water-soaked, placed in fast moving air stream.

◉ When “less than saturated” air passes over water at same temperature, some of the water will evaporate due to difference in vapor pressure.

◉ This latent heat is provided by sensible heat from water which leads to a drop in temperature of water.



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Wet Bulb Temperature implications

◉ It is an indirect way to measure the humidity.

◉ The drop in temperature (Wet Bulb Depression or WBD) due to the air flow would quantify the amount of water that went into the air stream determining the amount the air could absorb.

◉ If the flowing air is saturated, there will be no drop in temperature. So, DBT and WBT will be same.

◉ Thus, the difference in DBT and WBT measures the level of unsaturation of the flowing air.

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Some other properties/definitions

◉ Percentage saturation: Amount of water vapor in the air expressed as a percentage of the max. amount of water vapour it can hold.

Percentage Saturation = 100 x mv/mg

◉ Moisture content: Same as specific humidity

◉ Specific Enthalpy: The enthalpy of mixture per unit weight of DRY air

◉ Specific Volume: NOT the reciprocal of density as the name suggests. It is the volume of mixture per unit weight of DRY air.

All the properties discussed till now may be tabulated, but can be displayed more effectively in a graphical form Psychrometric Chart



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Psychrometric Chart2

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Climatic Conditions and their effect on human comfort

◉ The adjacent figure shows the maximum climatic conditions in different areas of the world

◉ The humid tropical zones have high humidity but the DBT rarely exceeds 35°C.

◉ The deserts have an arid climate, with higher DBT.

◉ The colder places have low DBT. Use of thicker insulation (in the form of clothing) required to prevent heat losses from body!



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Psychrometric Chart

◉ Psychrometric chart is a concise chart which can be readily looked up for relevant information such as all the properties discussed previously in this slide.

◉ The basic properties shown are DBT, WBT, moisture content, RH and specific enthalpy.

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Reading a Psychrometric Chart



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Psychrometric Chart – Locating a point

◉ Given two independent properties, we can locate the condition of air on this chart, as they will intersect.

◉ Note that, the properties must be independent. For example, if we know the Dew point and the moisture content, we cannot locate the point as the constant Dew point and constant lines are parallel.

◉ Note that the properties are continuous in reality but the chart is only useable if the iso-lines are spaced at some distance (to reduce clutter). Hence, as true for most charts, the use of the psychrometric chart will also involve interpolation whenever needed.

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Psychrometric Chart – A word of caution

◉ The chart does not contain any more information than can be obtained from the saturated steam table and the DBT and WBT.

◉ The charts are made for standard conditions, i.e., 1 atm pressure and typical DBT range of 0- 50 deg C. But in cases where this assumption is NOT valid, we need to make another modified chart or use the steam tables for air conditioningcalculations.



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Psychrometric Chart

◉ Before proceeding further to various air conditioning processes and design calculations, we must first realise the need for them. For this, we need to understand Human Comfort

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Human Comfort3Need and requirements



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Human Comfort

◉ The human body can be viewed as a heat engine whose energy input is food. The body generates waste heat that must be rejected to the environment if it is to continue operating.

◉ The rate of heat dissipation depends on the level of activity (see next slide).

◉ A body feels comfortable when it can freely dissipate its waste heat, and no more.

◉ As engineers, it is our duty to help people feel comfortable! We cannot change weather in an area. However, we can control the weather conditions in a confined space such as a house or a workplace.

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Human Comfort

◉ Heat emission from body at different levels of activity (2 m2)



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Human Comfort

◉ The comfort of the human body depends primarily on three factors: the (dry-bulb) temperature, relative humidity, and air motion

◉ The temperature is the most important index of comfort. Most people feel comfortable when the environment temperature is between 22° and 27°C (72° and 80°F).

◉ High relative humidity slows down heat rejection by evaporation, and low relative humidity speeds it up. Most people prefer a relative humidity of 40 to 60%.

◉ Air motion removes the warm, moist air that builds up around the body and replaces it with fresh air. Therefore, air motion improves heat rejection by both convection and evaporation. Most people feel comfortable at an airspeed of about 15 m/min.

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Other factors affecting human comfort

◉ Constant supply of fresh air and removal of respired CO2 is also essential for comfort in closed spaces like offices. It is generally recommended to be between 6-8 litres/s per occupant.

◉ It should be kept in mind that human comfort can vary from person to person. However, an acceptable comfort level would be one where majority feel comfortable.



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Warm and Humid Climate

Mumbai

• DBT: 35°C• RH : 60%

DehumidificationLatent Load

Reducing DBTSensible Load

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Warm and Humid Climate

Mumbai

• DBT: 35°C• RH : 60%

Increasing DBT

Reducing DBTSensible Load



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Cold and Dry Climate

Laddakh

• DBT: 10°C• RH : 10%

HumidificationLatent Load

Increasing DBTSensible Load

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Any questions ?

You can write to me

◉ [email protected]

Thanks!

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