121010 psychrometrics

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Fundamentals of HVAC - PSYCHROMETRICS

PSYCHROMETRICS

Psychrometric Chart

• Dry Bulb• Wet Bulb• State Point• Relative Humidity• Dewpoint

HVAC Load Calculations

Psychrometric Chart• Definitions• Psychrometrics - the study of the physical and thermodynamic properties of

the atmosphere and its water vapour• Absolute humidity - actual mass of water vapor present in the air water vapor

mixture. The absolute humidity may be expressed in pounds of watervapor (lb).

• Specific Humidity Ratio or Humidity Ratio - ratio between the actual mass of water vapor present in moist air - to the mass of the dry air.

• Humidity Ratio - normally expressed in pounds of water vapor per pound of dry air or in Grains of moisture per pound of dry air. There are approximately 7000 grains in a pound.

• Relative Humidity - The ratio of the amount of water vapor in the air at a specific temperature to the maximum amount that the air could hold at that temperature, expressed as a percentage.

• Dew Point temperature - temperature below which moisture will condense out of air. Air at a given Humidity Ratio has a constant Dew Point. If air is cooled below this point, moisture condenses out thus changing its humidity ratio.

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PSYCHROMETRICS

Psychrometric Chart



Sensible Heating and Cooling


Humidification


Dehumidification

PSYCHROMETRICS

Psychrometric Chart



Common Formulas• Sensible Btuh = Cfm * ΔT * 1.08

– Derive the 1.08 factor as follows:– 1 CFM x 60 = 60 CFH– 60 CFH x .075 lbs of air/cu ft = 4.5 lbs of air/hr– 4.5 x 0.24 Btu/lb -°F (specific heat of air) = 1.08 Btu/hr °F - CFM.

• Total heat = Cfm * ΔH * 4.5– Derive this one on your own?Mixed Air ºF = (Cfm RA * ºF) + (Cfm OA* ºF)

Cfm SA (Total Cfm)

Fundamentals of HVAC – PSYCHROMETRICS

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121010 psychrometrics

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