This preview shows page 1. Sign up to view the full content.
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. airconditioner) (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
AirConditioner 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 airhandlers 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 FlowPressure 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 hourbyhour simulation
calculation tool in North America: “DOE2”
U.S. Department of Energy DOE2:
• software developed by US DOE, original version in 1970’s • “hourbyhour” simulation tool—simulates energy use on a
hourly basis for 1 year (8760 hrs) • “quasisteadystate” – treats conditions as approximately
constant over each 1 hr timest...
View
Full
Document
This note was uploaded on 10/04/2012 for the course ME 760 taught by Professor Davidmather during the Spring '12 term at Waterloo.
 Spring '12
 DavidMather

Click to edit the document details