Moist Air & Thermal Comfort

Moist Air Properties of Moist Air If air and water vapour are present together in a confined space, a balanced condition will be reached where the air has become saturated with water vapour. If the temperature of the mixture is known, then the pressure of saturated vapour will be the pressure of steam at this temperature. mv P Pa Pv : ω T (o C ) Psat (kPa) ma 0 0.610 Moist Air & Thermal Comfort Dr. M. Zahurul Haq Professor Department of Mechanical Engineering Bangladesh University of Engineering & Technology (BUET) Dhaka-1000, Bangladesh 10 15 20 25 30 35 40 45 50 zahurul@me.buet.ac.bd http://teacher.buet.ac.bd/zahurul/ ME 415: Refrigeration & Building Mechanical Systems 1.224 1.704 2.337 3.166 4.246 5.628 7.384 9.593 12.35 ω c Dr. M. Zahurul Haq (BUET) Moist Air & Thermal Comfort ME 415 (2011) 1 / 19 mv ma P Pa Pv ω ma mv c Dr. M. Zahurul Haq (BUET) Moist Air Pv V /Rv T Pa V /Ra T Total (standard) pressure Partial pressure of dry air Partial pressure of water vapour Humidity ratio Mass of dry air Mass of water vapour 0.622 Pv Pa 0.622 Moist Air & Thermal Comfort pv P Pv ME 415 (2011) 2 / 19 ME 415 (2011) 4 / 19 Moist Air 25o C: For air at Pv 3.166 kPa Pa 97.159 kPa: saturated air. 3.166 ωs 0.622 97.159 0.02026 kg water/kg dry air. If mv 0.01013 kg water/kg dry air µ 0.5. ω saturation or degree of saturation, µ ω s The proportions of moisture can also be expressed as the ratio of the vapour pressures, and is then termed relative humidity: relative humidity, φ Pv Pv ,sat ω 0.01013 Pv 1.65 kPa φ 1.65/3.166 52 Dew-point temperature, Tdp is defined as the temperature at which condensation begins when the air is cooled at constant pressure. e957 e956 c Dr. M. Zahurul Haq (BUET) Moist Air & Thermal Comfort ME 415 (2011) 3 / 19 c Dr. M. Zahurul Haq (BUET) Moist Air & Thermal Comfort

Moist Air Moist Air Wet Bulb/Dry Bulb Temperatures ASHRAE-1989 Correlations φ Psat (Twb ) Pm Psat (Tdb ) Psat (Tdb ) saturated pressure corresponding to Tdb Psat (Twb ) saturated pressure corresponding to Twb Pm partial pressure of vapour due to depression of Twb below Tdb . Twb 273.2 Tdb Twb 1 Pm P 1514 873 e971 e970 If unsaturated air comes in contact with water at the same temperature, there will be change in vapour pressures. As a result, some of the water will evaporate. The resultant water temperature drop will provide a temperature difference, and a thermal balance will be reached where the flow of sensible heat from the air to the water provides the latent heat to evaporate a part of it. The equilibrium temperature is Wet-bulb Temperature. Moist Air & Thermal Comfort c Dr. M. Zahurul Haq (BUET) ME 415 (2011) 5 / 19 . Estimate relative humidity of air if Tdb 30o C & Twb 25o C. . Atmospheric air at 30o C and 50% RH is compressed isothermally to 1.0 MPa. Estimate the psychrometric conditions of the compressed air. c Dr. M. Zahurul Haq (BUET) Thermal Comfort Moist Air & Thermal Comfort ME 415 (2011) 6 / 19 ME 415 (2011) 8 / 19 Thermal Comfort Thermal Comfort Typical Climate Conditions Comfort zone according to ISO 7730 Comfort zone according to ASHRAE 55-1992 Thermal comfort is defined as that condition of mind which expresses satisfaction with the thermal environment. Factors affecting thermal comfort are: temperature, thermal radiation, humidity, air speed, personal activity and clothing. e959 c Dr. M. Zahurul Haq (BUET) c Dr. M. Zahurul Haq (BUET) Moist Air & Thermal Comfort ME 415 (2011) 7 / 19 Moist Air & Thermal Comfort

Thermal Comfort Thermal Comfort Bangladesh Outdoor Design Condition Ventilation: Outdoor Air Requirements To meet metabolic requirements of occupants. e993 To dilute the indoor air contaminants, odours and pollutants to maintain an acceptable air quality. Summer design is based on 4 summer month from June to September (Total (30 31 31 30)* 24 2928 hrs.) 2.5% means that temperature exceeds the value (33o C) only 2.5% of all the hrs in 4 months 73.2 hr. Winter: December to January (Total 2160 hrs). To support any combustion process or replace the amount of exhaust air required in laboratories, manufacturing processes or rest rooms. To provide make-up of amount of ex-filtrated air required when positive pressure is to be maintained at the conditioned space. 97.5% means temperature falls below the value for 54 hrs. For ordinary buildings, it is customary to design based on level of 2.5% in summer and 97.5% in winter. To add to or remove heat/moisture from the space. To,max 33o C, DR 11o C To,av To,max DR/2 27.5o C. c Dr. M. Zahurul Haq (BUET) Moist Air & Thermal Comfort ME 415 (2011) 9 / 19 c Dr. M. Zahurul Haq (BUET) Thermal Comfort Energy release per unit area of skin surface as a result of oxidative process in the living cell is the metabolic rate. It depends on the intensity of physical activities. Metabolic rate (W/m2 ) Mechanical work performed Convective radiative, or sensible heat loss from skin Evaporative heat loss from skin surface Evaporative heat loss from respiration 1 met 58.24 W/m2 Mechanical work, W µM , µ 0.2 to 0.24 Activity Sleeping Office work, reading or writing or seated Domestic work, cooking Walking, 2 mph Walking, 4 mph Heavy machine work In an air conditioned space, a steady-state thermal equilibrium is usually maintained between the human body and the indoor environment. c Dr. M. Zahurul Haq (BUET) Moist Air & Thermal Comfort 10 / 19 Metabolic Rate, M M W C R Esk Eres ME 415 (2011) Thermal Comfort Steady State Thermal Equilibrium M W C R Esk Eres Moist Air & Thermal Comfort ME 415 (2011) 11 / 19 c Dr. M. Zahurul Haq (BUET) met 0.7 1.0 1.6 - 2.0 2.0 3.7 4.0 Moist Air & Thermal Comfort ME 415 (2011) 12 / 19

Thermal Comfort Thermal Comfort Seven Factors Influencing Thermal Comfort 1 Activity level 2 Clothing 3 Expectation 4 Air temperature 5 Radiant temperature 6 Humidity 7 Air speed ASHRAE Comfort Zone e974 c Dr. M. Zahurul Haq (BUET) e264 Moist Air & Thermal Comfort ME 415 (2011) 13 / 19 c Dr. M. Zahurul Haq (BUET) Thermal Comfort 14 / 19 Outdoor Air Requirements to meet metabolic requirements of occupants As per ASHRAE 55-1992 & 90.1-1999 (o C) c Dr. M. Zahurul Haq (BUET) ME 415 (2011) Thermal Comfort Indoor Design Conditions Summer Winter 1. Commercial & public bld. 2. Health care bld. Moist Air & Thermal Comfort T 23.5 to 26.0 φ (%) 30 to 65 20.5 to 23.5 20.5 to 23.5 20 to 65 30 to 65 Moist Air & Thermal Comfort ME 415 (2011) to dilute the indoor air contraminants, odors and pollutants to maintain an acceptable air quality. to support any combustion process or replace the amount of exhaust air required in laboratories, manufacturing processes or rest rooms. to provide makeup of amount of exfiltrated air required when positive pressure is to be maintained at the conditioned space. 15 / 19 c Dr. M. Zahurul Haq (BUET) Moist Air & Thermal Comfort ME 415 (2011) 16 / 19

Thermal Comfort Thermal Comfort Outdoor Air as per ASHRAE 62-1992 Application Dining/conference room, office spaces, lounges Retail stores, transport waiting rooms, class rooms Hospitial patient rooms, residences Smoking lounges Common Air Contaminants L/s per person 10 7 15 30 e265 c Dr. M. Zahurul Haq (BUET) Moist Air & Thermal Comfort ME 415 (2011) 17 / 19 ME 415 (2011) 19 / 19 Thermal Comfort Filters for Air Cleaning Devices 1 Viscous impingment type 2 Dry screen type 3 Electrostatic precipitators 4 Air washers c Dr. M. Zahurul Haq (BUET) Moist Air & Thermal Comfort c Dr. M. Zahurul Haq (BUET) Moist Air & Thermal Comfort ME 415 (2011) 18 / 19

Thermal Comfort Seven Factors Influencing Thermal Comfort 1 Activity level 2 Clothing 3 Expectation 4 Air temperature 5 Radiant temperature 6 Humidity 7 Air speed e974 c Dr. M. Zahurul Haq (BUET) Moist Air & Thermal Comfort ME 415 (2011) 13 / 19 Thermal Comfort ASHRAE Comfort Zone e264 c Dr. M. Zahurul Haq (BUET) Moist Air & Thermal Comfort ME .