Lecture-1Systems - 1 Energy Efficient Heating Energy Efficient Heating 2 Waste of heat in buildings results from • Poorly designed and insulated

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Unformatted text preview: 11/24/2009 1 Energy Efficient Heating Energy Efficient Heating 11/24/2009 2 Waste of heat in buildings results from: • Poorly designed and insulated heating systems x The room might have a high ceiling, like a church, allowing all the heated air to concentrate at the top not warming the occupants at floor level x The pipes might not be well insulated, allowing heating in undesired areas heating in undesired areas 11/24/2009 3 • Poorly designed and insulated building envelopes x If poorly insulated, heat escapes by conduction to the outside x If the envelope is not well sealed heated air will travel through cracks around doors and window frames • Poorly commissioned and maintained boiler plant x Heat meant to be warming water might be lost up the flue with combustion gases Poor control • Poor controls x If the system is old, its controls will be inadequate • Poor operating practices 11/24/2009 4 For best heating system in buildings For best heating system in buildings: • The building fabric and the mechanical heating system must be of equal importance We have to select the correct heating system for • We have to select the correct heating system for a particular application • We need to have an adequate control system • Building operatives should understand how to use a heating s stem correctl other ise it is use a heating system correctly, otherwise it is useless 11/24/2009 5 Environmental parameters that influence thermal comfort: • Air Temperature • Mean Radiant Temperature • Relative Humidity Relative Humidity • Air Velocity These parameters influence heat transfer by convection, radiation, and evaporation. 11/24/2009 6 Convective Heat Transfe Convective Heat Transfer • Influenced by air temperature and air velocity Takes place continually • Takes place continually • It is imperceptible in most buildings because of low air speeds (below 0.1 m/s) • To be perceptible, speeds must exceed 0.2 m/s 11/24/2009 7 Radiant Heat Transfer • Influenced by mean radiant temperature (t r ) of the surfaces within rooms • Occurs between the skin/clothes and surfaces seen by the human body • For cuboid shaped rooms, t r (at center of room): n n r t a t a t a t a t + + + + = ... 3 3 2 2 1 1 n a a a a + + + + ... 3 2 1 t r : mean radiant temperature ( ⁰ C) a 1 , a 2 , … : room component surface areas (m 2 ) t 1 , t 2 , … : room component surface temperatures (m 2 ) 11/24/2009 8 Evaporative Heat Los Evaporative Heat Loss • Influenced by relative humidity of air and air velocity Evaporative heat losses are low in humid areas • Evaporative heat losses are low in humid areas since the precipitation produced is hard to evaporate in such environments • Evaporative cooling important at air temperature above 25 C temperature above 25 ⁰ C • Above 29 ⁰ C, almost all heat losses by evaporation 11/24/2009 9 Dry Resultant (Operative) Temperature Dry Resultant (Operative) Temperature t res : dry resultant temperature ( ⁰...
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This note was uploaded on 02/07/2011 for the course MECH 673 taught by Professor Drnesreenghaddar during the Fall '09 term at American University of Beirut.

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Lecture-1Systems - 1 Energy Efficient Heating Energy Efficient Heating 2 Waste of heat in buildings results from • Poorly designed and insulated

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