Psychrometry • Atmospheric air makes up t he envi ronment in almost every type of air conditioning system. • Henc e a thorough underst andi ng of the proper ties of atmospher ic air and the ability to analyze various processes involving air is fundamental to air conditioning design. • The moist air is a mixture of dry air (mixture of gases) and water vapour. • While the composition of dry air is constant , 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). • At a gi ven temperature and pr essure the dry air can onl y hold a certain maximum amount of moisture. When the moisture con tent is maximum, then the air is known as saturated air.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Psychrometry• Atmospheric air makes up the environment in almost every type ofair conditioning system.
• Hence a thorough understanding of the properties of atmospheric air
and the ability to analyze various processes involving air isfundamental to air conditioning design.
• The moist air is a mixture of dry air (mixture of gases) and watervapour.
• While the composition of dry air is constant, the amount of watervapour present in the air may vary from zero to a maximumdepending upon the temperature and pressure of the mixture (dry air+ water vapour).
• At a given temperature and pressure the dry air can only hold acertain maximum amount of moisture. When the moisture content ismaximum, then the air is known as saturated air.
• The humidity ratio (or specific humidity) W is the mass of watervapour associated with each kilogram of dry air
W = 0.624 (pv/pa)
• Dew-point temperature (DPT):
• If unsaturated moist air is cooled at constant pressure, then thetemperature at which the moisture in the air begins to condense isknown as dew-point temperature (DPT) of air.
• Degree of saturation µ:• The degree of saturation is the ratio of the humidity ratio W to thehumidity ratio of a saturated mixture Ws at the same temperatureand pressure.
• On a particular day the weather forecast states that the dry bulb
temperature is 37oC, while the relative humidity is 50% and thebarometric pressure is 101.325 kPa. Find the humidity ratio, dewpoint temperature and enthalpy of moist air on this day.
• Moist air at 1 atm. (760 mm of hg) pressure has a dry bulbtemperature of 32oC and a wet bulb temperature of 26oC. Calculatea) the partial pressure of water vapour, b) humidity ratio, c) relativehumidity, d) dew point temperature, e) density of dry air in themixture, f) density of water vapour in the mixture and g) enthalpy of
moist air using perfect gas law model and psychrometric equations.
• Using modified Apjohn equation and the values of DBT, WBT and
barometric pressure, the vapour pressure is found to be:• pv = 2.956 kPa
• The humidity ratio W is given by:• W = 0.622 x 2.956/(101.325-2.956) = 0.0187 kgw/kgda
• Relative humidity RH is given by:• RH = (pv/ps) x 100 = (pv/saturation pressure at 32oC) x 100• From steam tables, the saturation pressure of water at 32oC is
4.7552 kPa, hence,• RH = (2.956/4.7552) x 100 = 62.16%
• Dew point temperature is the saturation temperature of steam at2.956 kPa.
• Hence using steam tables we find that:• DPT = Tsat(2.956 kPa) = 23.8oC
• the ‘saturation line ’: this is the line of 100% relative humidity, andrepresents the locus of thermodynamic states that can hold themaximum amount of water in purely vapour form.
• When moist air is cooled belowits dew-point by bringing it incontact with a cold surface asshown some of the water vapourin the air condenses and leaves
the air stream as liquid, as aresult both the temperature andhumidity ratio of air decreases.
• This is the process air undergoes
in a typical air conditioningsystem.
• By applying mass balance andenergy balance equation
• By separating the total heat transfer rate from the cooling coil intosensible and latent heat transfer rates, a useful parameter calledSensible Heat Factor (SHF) is defined.
• SHF is defined as the ratio of sensible to total heat transfer rate.
• From the above equation, one can deduce that a SHF of 1.0corresponds to no latent heat transfer and a SHF of 0 corresponds
to no sensible heat transfer.• A SHF of 0.75 to 0.80 is quite common in air conditioning systems in
a normal dry-climate.
• A lower value of SHF, say 0.6, implies a high latent heat load such
• 0.2 kg/s of moist air at 45oC (DBT) and 10% RH is mixedwith 0.3 kg/s of moist air at 25oC and a humidity ratio of0.018 kgw/kgda in an adiabatic mixing chamber. Aftermixing, the mixed air is heated to a final temperature of40oC using a heater. Find the temperature and relative
humidity of air after mixing. Find the heat transfer rate inthe heater and relative humidity of air at the exit of heater.Assume the barometric pressure to be 1 atm.
• The required inside conditions for cold storage andindustrial air conditioning applications vary widelydepending on the specific requirement.
• However, the required inside conditions for comfort airconditioning systems remain practically sameirrespective of the size, type, location, use of the airconditioning building etc., as this is related to the thermal
Thermal comfort• Thermal comfort is defined as “that condition of mind which
expresses satisfaction with the thermal environment”.
• Factors affecting human comfort
• Effective temperature: the degree of warmth or cold felt by a
human body mainly depends upon dry bulb temperature, relativehumidity and air velocity.
• In order to evaluate the combined effect of these factors effectivetemperature is used. It is the index which correlates the combinedeffect of the above factors. It is represented by the ‘comfort chart’
• Physiological factorssuch as age, activity, sex and health.
These factors influence the metabolic rate. It is observed that ofthese factors, the most important is activity.