Psychrometrics

KChatterjea, 2008 1

Psychrometrics&

Air Conditioning Design

Kalyan Chatterjea

KChatterjea, 2008 2

Chatterjea - Psychrometrics & Air Conditioning Design

Introduction to Comfort Introduction to Comfort Air-conditioningAir-conditioningIntroduction to Comfort Introduction to Comfort Air-conditioningAir-conditioning

KChatterjea, 2008 3


Introduction - PsychrometryIntroduction - PsychrometryIntroduction - PsychrometryIntroduction - Psychrometry

The air conditioning processes are best understood by studying Psychrometry.

Psychrometry is the study of atmospheric air and its associated water vapour.

A psychrometric chart presents physical and thermal properties of moist air in a graphical form.

KChatterjea, 2008 4


Psychrometry – Dry Bulb TempPsychrometry – Dry Bulb TempPsychrometry – Dry Bulb TempPsychrometry – Dry Bulb Temp

Dry-bulb temperature (DBT) is the commonly measured temperature from a thermometer.

KChatterjea, 2008 5


Psychrometry – Wet Bulb TempPsychrometry – Wet Bulb TempPsychrometry – Wet Bulb TempPsychrometry – Wet Bulb Temp

Wet Bulb Temperature (WBT) is determined when air is circulated past a wetted sensor tip.

Intersection of DBT and WBT provides the status point.

KChatterjea, 2008 6


Psychrometry – Saturation LinePsychrometry – Saturation LinePsychrometry – Saturation LinePsychrometry – Saturation Line

Saturation line is a temperature condition at which water will begin to condense out of moist air.

It is also called the dew point temperature line.

KChatterjea, 2008 7


Psychrometry – Relative HumidityPsychrometry – Relative HumidityPsychrometry – Relative HumidityPsychrometry – Relative Humidity

Relative Humidity (RH) is a ratio between actual moisture content of the air compared with the moisture content of the air required for saturation at the same temperature.

RH is affects human comfort significantly.

Sensible heat removalwith increase in RH

KChatterjea, 2008 8


Psychrometry – Specific VolumePsychrometry – Specific VolumePsychrometry – Specific VolumePsychrometry – Specific Volume

Specific Volume indicates the space occupied by air.

This is useful in converting volumetric flow to mass-flow rate calculation.

KChatterjea, 2008 9


Psychrometry - EnthalpyPsychrometry - EnthalpyPsychrometry - EnthalpyPsychrometry - Enthalpy

Enthalpy is the energy content of dry air & moisture expressed in kJ/kg of dry air.

The information is useful is sizing up heating & cooling coils.

KChatterjea, 2008 10


Psychrometry – Sensible Heat Psychrometry – Sensible Heat ProcessesProcessesPsychrometry – Sensible Heat Psychrometry – Sensible Heat ProcessesProcesses

Heating & Cooling process – addition or removal of sensible heat without change in absolute moisture content.

But RH changes as temperature changes.



Psychrometry - DehumidificationPsychrometry - DehumidificationPsychrometry - DehumidificationPsychrometry - Dehumidification

Dehumidification by cooling – in cooling coils temp of air reduces and the saturation point (dew point) is reached.

Further cooling results in reduction of absolute humidity.



Psychrometry – Comfort ZonePsychrometry – Comfort ZonePsychrometry – Comfort ZonePsychrometry – Comfort Zone

Comfort Zone – statistical studies have been performed on large numbers of subjects of all ages, sexes and nationalities to arrive at a quantitative description of human comfort.



Comfort zone … comfort is a very subjective term!

Statistically, majority of human beings are comfortable when temp is between 18-220C and RH is 40-65%

Psychrometry – Comfort ZonePsychrometry – Comfort ZonePsychrometry – Comfort ZonePsychrometry – Comfort Zone



Psychrometry – Climate ClassificationPsychrometry – Climate ClassificationPsychrometry – Climate ClassificationPsychrometry – Climate Classification

Climate Classification shows the various approximate zones of temperature and humidity on the psychrometric chart.



Psychrometry – Heating at SeaPsychrometry – Heating at SeaPsychrometry – Heating at SeaPsychrometry – Heating at Sea

Heating without humidification at sea.

Sensible heat addition with decrease in RH…away from comfort zone



Psychrometry – Heating at SeaPsychrometry – Heating at SeaPsychrometry – Heating at SeaPsychrometry – Heating at Sea

Heating with humidification at sea.

Heat addition with humidification resulting in more comfortable RH



Heating Process in DetailHeating Process in DetailHeating Process in DetailHeating Process in Detail

Process showing sensible heat addition.



Heating Process in DetailHeating Process in DetailHeating Process in DetailHeating Process in Detail

Applying the SFEE

hgzvmWQ Air2

2

1

hgzvmWQ Air2

2

1

12 hhmQ Air h-values can be directly

read from the psychrometricchart



Process showing sensible heat cooling, followed by

Further cooling, which leads to precipitation.

Cooling & Dehumidification in DetailCooling & Dehumidification in DetailCooling & Dehumidification in DetailCooling & Dehumidification in Detail



Cooling & Dehumidification in DetailCooling & Dehumidification in DetailCooling & Dehumidification in DetailCooling & Dehumidification in Detail

Applying the SFEE

Mass flow balance equation forthe water & steam

WaterAirAir mmm

31

13 hhmQ AirIn practice, water mass flow is very small compared to air mass flow. So a simplerequation could be used without undue error.

2)( 13 fWaterAir hmhhmQ



References

Psychrometric Chart Download–

http://www.ent.ohiou.edu/~thermo/me328/quiz.info/Nautica_SI.pdf



References



References

Overview of Psychrometrics: – http://www.handsdownsoftware.com/Overview_of_Psychrometrics.pdf



References



References

Coil Load Calculations: – http://courses.washington.edu/me425/425%20Psychrometric%20Review%202007.pd

f



References



References

Glossary: – http://air.1800anytyme.com/info/air_conditioning_glossary.php



References



References

Standard: British Standards Institution QC100.B862 BS EN ISO 2006– Ship and Marine Technology - Air-conditioning &

Ventilation of Accommodation Spaces - Design Conditions & Basis of Calculations



References



References

Thermodynamic Expert System– http://energy.sdsu.edu/testcenter/testhome/index.html

TEST - a visual platform to analyze thermo-fluids problems, verify hand calculations, pursue what-if scenarios, visualize thermal systems, and engage in a life-long journey to master thermodynamics.

Developed by Dr. S. Bhattacharjee, San Diego State University.



References



References

ABS Guide for Crew Habitability on Off-shore Installations, 2002

– "habitability" is defined as the acceptability of conditions of an offshore installation in terms of vibration, noise, indoor climate and lighting, as well as physical and spatial characteristics, according to prevailing research and standards for human efficiency and comfort



References



References

Simulation– Hui, S. C. M. and K. P. Cheung, 1998. Application of building energy

simulation to air-conditioning design, In Proc. of the Mainland-Hong Kong HVAC Seminar '98, 23-25 March 1998, Beijing, pp. 12-20.

http://www.arch.hku.hk/~cmhui/hvac98b.pdf



References



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Psychrometrics

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