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Cop useful heat transfer rate watts input power watts

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Unformatted text preview: n System heating input power usually electricity “Coefficient of Performance” For a refrigeration cycle (and some other situations), we typically use the terminology “Coefficient of Performance” (COP) instead of “efficiency”. COP = Useful Heat Transfer Rate [watts] Input Power [watts] dimensionless Numerical value can be (and often is) > 1 Example: A particular airconditioning unit requires 5 kW of electricity input to provide 4.5 tons (15.8 kW) of cooling. room cooling 15.8 kW A/C Unit reject heat ? kW 5 kW electric power COP = Useful Heat Transfer Rate Input Power outdoors Note: 1 ton of cooling ≈ 3.52 kW (heat flow rate) 15.8 kW = 3.2 = 5 kW In metric, COP is dimensionless: COP = Heat transfer rate (W) Electric input power (W) [dimensionless] In Inch#Pound units, heat transfer rates are usually stated in “btu/hr”. Equipment performance is often listed as an “Energy Efficiency Rating” (EER): EER = Heat transfer rate (Btu/hr) Electric input power (W) [Btu/hr/W] units of EER Note: 1 W = 3.412 btu/hr So… EER = COP × 3.412 Definition of COP depending on which heat flow is considered “useful” (i.e. purpose of the device): device cooling input power heating “Cooling COP” = “Heating COP” = (e.g. air-conditioner) (e.g. “heat pump”) COP = Useful Heat Transfer Rate (W) Input Power (W) Similar to: Input Power (W) = Useful Heat Transfer Rate (W) COP Input = Output Useful Heat Transfer Rate (W) “output” = Input Power (W) × COP “input” Note: The numerical value of the “output” watts can be greater than that of the “input” watts A particular home heating and cooling system: Furnace with cooling coil section above (indicated) AC “condenser unit” Household Air-Conditioner cooling heat absorbed from warm house air (through cooling coil) = heat rejected to hot outdoor air input electric power to compressor motor + condenser fan motor input = load measure of efficiency Note: Does not include power consumption of indoor fan motor Building Energy Performance – Spring 2012 - Topic 15 - Aspects of HVAC Fans HVAC air-handlers often utilize centrifugal fans to produce air movement (∆ ∙ ) Typical Centrifugal Fan Components “Backward Curved” Blades There are several type of blades (i.e. blade shapes), including “backward curved”. Airflow Recall: losses η fan mechanical power in Airflow Fan Δ × = Typical Backward Curved Fan Characteristics (at a particular shaft rotational speed) Pressure (Rise) Required Shaft Power Pressure (Rise), Fan Efficiency, Required Shaft Power Fan Efficiency Volumetric Flow Rate Typical Backward Curved Fan Characteristics (at a particular shaft rotational speed) Pressure (Rise) Required Shaft Power Pressure (Rise), Fan Efficiency, Required Shaft Power Volumetric Flow Rate Fan Efficiency reasonably constant in this region? Fan and System Flow-Pressure Curves Fan Curve Duct System Curve Pressure pressure drop approximately varies with Δ Volumetric Flow Rate “Fan Laws” Theoretical equations for predicting performance at differing conditions. (e.g. measure performance at one condition and predict performance at other condition by application of equations. Fan Laws for changes in shaft rotational speed - notation = shaft rotational speed (e.g. rpm or rad/s) = volumetric flowrate (e.g. cfm or m3/s) Δ = pressure rise developed (e.g. in. w.g. or Pa) ! "#$ = required shaft power input (e.g. hp or W) “inches of water gauge” Fan Laws for Changes in Shaft Rotational Speed…. Note: Based on assumption that is constant—so the equations are useful only within a certain range of operation… Volumetric flow is approximately proportional to shaft speed: = % % Pressure Rise Developed is approximately proportional to shaft speed squared: Δ = Δ% % Required Shaft Power is approximately proportional to shaft speed cubed: ! "#$ ! "#$ % = % varies with & & varies with Δ × ! "#$ = varies with assumed to be approximately constant Approximate Change in Fan Performance Curve with Shaft Speed % > > & % Pressure Δ & Volumetric Flow Rate Sample Fan Performance Curve at Several Shaft Speeds A reasonably simple means of achieving a permanent speed change is by changing diameter of pulleys on motor and/or fan shaft. Pulley Equation: = ()$)* diameter of pulley +()$)*,-.//01 +,-.//01 Airflow :345,;<==>? 2345 Fan Airflow Belt Drive + - Motor diameter of pulley 267879 :67879,;<==>? Duct System Curve Note similarity in impact of changing fan speed to “trimming” a pump impeller. Building Energy Performance – Spring 2012 - Topic 16 Aspects of Building Energy Simulation Building Energy Simulation Why simulate? - Tool to provide information e.g. estimate impact of varying some aspect of a building’s design or operation - Analysis tool to account for interactive effects within a building…primarily to calculate HVAC loads and energy use • Do I need to know if the HVAC system is running to calculate how much energy the lights use? • building envelope • lighting • HVAC A widely used hour-by-hour simulation calculation tool in North America: “DOE-2” U.S. Department of Energy DOE-2: • software developed by US DOE, original version in 1970’s • “hour-by-hour” simulation tool—simulates energy use on a hourly basis for 1 year (8760 hrs) • “quasi-steady-state” – treats conditions as approximately constant over each 1 hr time-st...
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This note was uploaded on 10/04/2012 for the course ME 760 taught by Professor Davidmather during the Spring '12 term at Waterloo.

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