Fundamentals of Heating. Ventilating, and Air-Conditioning

Air spaces and windows

• Glazing Air spaces Air conduction resistance R = 8/?i Where R = air conduction resistance 8 = air layer thickness X = thermal conductivity ( 0.023 W/m K for air) “Ideal case” for 0.02 m thick of air layer R = 8fk = 0.02/0.023 = 0.87 m2 K/W Because of convection and radiation effect, […]

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Overall heat transmission

It is necessary to consider the heat transfer from inside air to outside air. The heat transfer processes include conduction, convection, and radiation simultaneously. T„ Ri «2 1. O’ 1 X < Qi Q2 Q < R V 1Til T. X 11+1 X-   X.   Ti Rtv Rri X Q Ro = T0 — […]

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Combined convection and radiation

Rc=l/hc Tw W   Tw _o   Tair O_   Rr=l/hr R R Overall film resistance R R„ +R, T — T C r _ air_______ w R

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Radiation

Stefan-Boltzmann’s Law: q = oe1e2(T14-T24) Where q = heat flux due to radiation (W/m ) A = Stefan-Boltzmann constant (= 5.673xl0"8 W/m2 K4)or (= 0.1713×10"* Btu/h. ftz R) e = surface emittance (-) T = surface temperature (K) N R> R =l/hr The equation can be further written as: Q = oe1e2(T13+T12T2 + TjT2 + […]

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Convection

Newton’s Law: Q = hc (Tair — Tw) O Where q = heat flux due to convection ( W/m ) Hc = convective heat transfer coefficient (W/m K) Tair = bulk fluid (air) temperature (°C) Tw = surface temperature (°C) Newton’s Law can be written as T — T „ 1 With R = — […]

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Basic Heat Transfer Modes

Fourier’s Law: O Where q = heat flux (W/m ) K = thermal conductivity (W/m K) (material property, Table 5-1 of the text) dT/dx = temperature gradient (K/m) • Heat transfer in a single-layer wall In most practical uses, k is approximated as constant. For steady-state heat transfer, q is constant. Then the equation can […]

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Summary of Different Air-Conditioning Systems

System Advantages Disadvantages All Air • Central equipment location • No piping in occupied area • Use of outside air (free cooling) • Easy seasonal change • Heat recovery possible • Closest operating conditions • Duct clearance • Large ducts — space • Air balancing difficulties Air Water • Individual room control • Separate secondary […]

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Heat Recovery

Heat recovery is utilization of "waste” energy streams. Sources for heat recovery’ are: • Relief / exhaust air • Combustion gases • Coolant stream The recovered heat is used to: O Air-to-Air Heat Recovery System • Air-to-air HX (heat exchanger) • Heat wheel • Heat pipe • Heat pump _J 5 OUTSIDE AIR EXHAUST AIR […]

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Heat pumps

Air-to-air heat pumps Water-to-air heat pumps (water serves as heat source) 100° FIGURE 14.8a A simple through the wall air conditioner unit essentially consists of a compressive refrigeration machine. REVERSING VALVE OUTDOORS 40° F «t— FIGURE 14.8b In a "heat pump" the reversing valve allows the refrigerant to flow in either direction. In the winter […]

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Incremental units

Examples are motel units and larger single zone units. They are full heating, cooling and air handling systems with heating coils, cooling coils, refrigerator, and fans, etc. PACKAGED UNIT   FIGURE 14.10b Packaged units can contain both heating and cooling equipment.   FIGURE 14. lOd Packaged unit designed toi crawl space construction.   FIGURE 14. […]

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