Psychrometry HVAC CNST 305 Environmental Systems 1 Dr. Berryman 5cIndoor Air Quality, Psychometrics; Dry and Wet Bulb; Enthalpy; Condensation; Dew Point.

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Presentation transcript:

Psychrometry HVAC CNST 305 Environmental Systems 1 Dr. Berryman 5cIndoor Air Quality, Psychometrics; Dry and Wet Bulb; Enthalpy; Condensation; Dew Point Temp. Chapter 2.1 – 2.3 (Toa) 2.11 Mathematics and Science - Physics (1hr); 4.11 Analysis and Design of Construction Systems - Mechanical (2 hrs)

Psychrometry Psychrometry is the science dealing with the physical laws of air – water mixtures. When designing an air conditioning system, the temperature and moisture content of the air to be conditioned, and the same properties of the air needed to produce the desired air conditioning effect, must be known. Once these properties are known, the air conditioning task can be determined.

Psychrometric Chart

Properties of Air Dry-bulb temperature Wet-bulb temperature Dew-point temperature Relative humidity Humidity ratio

Thermometers Dry Bulb Wet Bulb Measured in degrees Fahrenheit ( o F)

Dew Point Condensation occurs at the Dew Point Temperature ( o F)

Saturation Fog DP, WB, DB same

Relative Humidity Amount of moisture that a given amount of air is holding = Amount of moisture that a given amount of air can hold 50% RH 100% RH - Saturated (percentage)

Humidity Ratio Actual weight of water in an air – water vapor mixture Pounds of moisture per pound of dry air 7000 grains of water in a pound at sea level one pound of 70°F air occupies approximately 13.5 cubic feet, and one grain of water in that air weighs about two-thousandths (0.002) of an ounce

wet bulb dew point 30 Dry-Bulb Temperature (°F) Humidity Ratio (grains/lb of dry air) humidity ratio dry bulb relative humidity Air Conditions on the Psychrometric Chart

Specific Volume specific volume lines (cubic feet / pound of dry air) specific volume lines (cubic feet / pound of dry air) 30 Dry-Bulb Temperature (°F) Humidity Ratio (grains/lb of dry air)

Enthalpy The total heat energy in one pound of air (Btu/lb) at its present condition Enthalpy (h) = Sensible Heat + Latent Heat

EXAMPLE Using the Chart Summer Design Conditions 95°F DB (dry bulb) 78°F WB (wet bulb) Find: Relative Humidity Humidity Ratio Dew Point

Answer 95°F 78°F 50% 40% 72°F

Condensation EXAMPLE Beach Conditions 92 o F 60% RH Beverage 45 o F

Condensation (cont.) EXAMPLE Winter: Glass surface temperature = 40 o F Inside Design: 77 o F 30% RH

Comfort Zone dry-bulb temperature humidity ratio wet-bulb temperature 80°F [26.7°C] 70°F [21.2°C] 60 % RH 30 % RH A

Air Conditioning Clinic TRG-TRC001-EN © American Standard Inc Effect of Adding Sensible Heat

Air Conditioning Clinic TRG-TRC001-EN © American Standard Inc Effect of Removing Sensible Heat

Air Conditioning Clinic TRG-TRC001-EN © American Standard Inc Effect of Adding Moisture

Air Conditioning Clinic TRG-TRC001-EN © American Standard Inc Effect of Removing Moisture

Air Conditioning Clinic TRG-TRC001-EN © American Standard Inc Removing Sensible Heat and Moisture

A A Air Conditioning Clinic TRG-TRC001-EN © American Standard Inc Removing Sensible Heat and Moisture

A A B B C C D D sensible load latent load Air Conditioning Clinic TRG-TRC001-EN © American Standard Inc Sensible and Latent Coil Loads

Next Time Calculating Heating Loads Section 2.5 Toa Heat Transmission