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Near East University

Faculty of Engineering

Department of Mechanical EngineeringGraduation Project

Bright Benjamin (20113313)

Topic: PSYCHROMETRICS

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Outline1. Psychrometrics 2. Psychrometric Charts3. Advantages and uses of the chart4. How to read a psychrometric chart 5. Psychrometric properties 6. Psychrometric Processes 7. Conclusion

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1. PSYCHROMETRICS• Definition: This a term used to describe the field of engineering

concerned with the determination of the physical and thermodynamic properties of gas-vapor mixtures.• The term is derived from the greek “Psuchron” meaning “cold”And “Metron” meaning “means of measurement”.

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Psychrometric Charts • This is simply a graphical means of representing the thermodynamics

properties of moist air at constant pressure mostly at sea level.• For a given barometric pressure, all psychrometric properties of moist air are

determined by two independent variables.

• Below is an ASHRAE style psychrometric chart.

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Advantages and uses of the chart1. If all the values are calculated manually using the formula, it takes lots of time, but with a psychrometric chart these values can be found within seconds or minutes.2. With psychrometric chart we can easily find out the present and the final condition of the air. Say if the air is cooled from 100F to 75°F, all we have to do is to draw the horizontal line to locate the initial and final condition of the air.3. A psychrometric chart is very useful in carrying out heat load or cooling load calculations.

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How to read a psychrometric chartLets take for example we were given the values of dry bulb and wet bulb temperature which are 78°F and 65°F respectively. We will find out the values of other psychrometric properties using these two given properties. i. On the psych chart locate the value 78°F on the dry bulb temperature

scale located at the bottom of the chart .ii. Locate the wet bulb temperature of 65°F on the saturation curve scale.iii. Extend the vertical line which is the dry bulb temperature form 78°F and

the diagonal wet bulb line form 65°F and get the point of intersectioniv. Find out the values of various parameters at this point.

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Psychrometric Properties 1. Dry bulb temperature2. Wet bulb temperature3. Dew point temperature 4. Relative humidity5. Humidity ratio6. Specific volume 7. Enthalpy

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1. Dry bulb temperatureThis is the temperature indicated by a thermometer exposed to the air in a place sheltered from direct solar radiation.

The term dry bulb is customarily added to temperature to distinguish it from wet bulb and dew point temperature.

The dry bulb temperature is measured in degree Celsius/Fahrenheit. And denoted with a straight vertical line on the psych chart .

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2. Wet bulb temperatureThe wet bulb temperature is the temperature recorded by a thermometer when the bulb is enveloped by a cotton wick saturated with water.

The accuracy of a simple wet bulb thermometer depends on how fast air passes over the bulb and how well the thermometer is shielded from the radiant temperature of its surroundings.

The wet bulb temperature is also measured in degree Celsius/Fahrenheit and denoted by a straight diagonal lines on the psych chart .

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3. Dew point temperature• The temperature at which moisture starts to condense out of air is

called dew point.

• Dew point is also known a saturation temperature.

• Dew point temperature is determined by moving from a state point horizontally to the left along lines of constant humidity ratio until the upper, curved, saturation boundary temperature is reached.

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Relative humidity• Relative humidity (RH) is the ratio of the partial pressure of water vapor to the

equilibrium vapor pressure of water at a given temperature. Relative humidity depends on temperature and the pressure of the system of interest. It requires less water vapor to attain high relative humidity at low temperatures; more water vapor is required to attain high relative humidity in warm or hot air.

• Relative humidity is dimensionless, and is usually expressed as a percentage. 100% RH indicates the air is saturated and cannot hold any more moisture. Preferred values of comfort for people are between 35% and 60%.

• Lines of constant relative humidity are shown as exponential lines on the psychrometric chart

• RH%= Amount of moisture that a given amount of air is holding Amount of moisture that a given amount of air can hold

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Humidity ratio• Actual weight of water in an air to water vapor mixture

• Amount of moisture per unit of dry air

• Can be defined as W=m/G

• It is dimensionless

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6. Specific volume• In thermodynamics, the specific volume of a substance is the ratio of

the substance's volume to its mass. It is the reciprocal of density and an intrinsic property of matter as well. Specific volume is defined as the number of cubic meters occupied by one kilogram of a particular substance. The standard unit is the cubic meter per kilogram.• Specific volume for an ideal gas is also equal to the gas constant (R)

multiplied by the temperature and then divided by the pressure multiplied by molar mass of that ideal gas. • This is shown as diagonal lines on the chart.

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7. Enthalpy• Enthalpy is usually designated as h. It is the total amount of heat

energy of the moist air and therefore includes the amount of heat of the dry air and water vapor in the air. In the approximation of ideal gasses, lines of constant enthalpy are parallel to lines of constant wet bulb temperature. Thus the enthalpy is indicated by diagonal lines on the chart.• And its measured in KJ/Kg or BTU/lb

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Psychrometric processes 1. Sensible cooling of air2. Sensible heating of air3. Humidification a. Cooling and humidificationb. Heating and humidification4. Dehumidification c. Cooling and dehumidificationd. Heating and dehumidification5. Adiabatic mixing of two moist air streams.

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1. Sensible cooling of air• This is the removal of heat from moist air without the removal of

moisture. It also follows a constant humidity ratio on the psych chart.

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2. Sensible heating • It is the addition of heat to moist air without addition of moisture.• It follows a constant humidity ratio line on the psych chart.

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3. Humidification• This is the addition of moisture to moist air without the addition or

removal of heat.

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3a. Cooling and humidification• In this process the moisture is added to the air by passing it over the

stream or spray of water which is at temperature lower than the dry bulb temperature of the air.

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3b. Heating and humidification• It is the addition of heat and moisture to moist air, it also involves

sensible and latent heat transfer.

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4. Dehumidification • It is the removal of moisture from moist air without removing or

adding heat.

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4a. Cooling and dehumidification• This is the removal of heat and moisture from air, it also involves

sensible and latent heat transfer.

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4b. Heating and dehumidification• It is the addition of heat and removal of moisture from moist air.

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5. Adiabatic mixing of moist air streams• First adiabatic or evaporative cooling is a psych process which involves

the cooling without heat loss or gain. • Sensible heat loss by air is converted to latent heat in the added water

vapor.

• Adiabatic mixing of moist air streams is a psych process that involves no net heat loss or gain during the mixing of two air streams.

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Conclusion• This aspect of psychrometrics is very important when it comes to

analyzing indoor conditions as temperatures and humidity levels within our environments are very dynamic. In air-conditioning applications the knowledge of psychrometrics is essential to design the system for optimal thermal comfort. Thermal comfort is that condition of mind that expresses satisfaction with the thermal environment.

• A solid understanding of psychrometrics can prevent costly mistakes in the design and operation of commercial, residential, industrial cooling, dehumidification, drying and humidification systems.

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