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Class Objectives
• Stress the importance of HVAC
• Identify and compare factors that affect thermal comfort in buildings
• Calculate quantities from a psychrometric chart
HVAC Affects:
• Energy use (outdoor air quality)
• Peak demand
• Indoor air quality
• Occupant comfort and productivity
• Building cost
Primary energy vs. End use energy
• End use energy
amount of energy delivered to consumer
electric, heating, cooling
1kWh (3.6MJ) of source energy (fuel) ≠ 1kWh of electric energy
amount of energy delivered to consumer
energy sources used to produce this energy
Example: conversion and transmission losses in power plant
and delivery systems
• Primary energy
+
Energy efficiency vs. Thermal efficiency
Power plant100%Primary energy (gas)
~ 33%Electricenergy
Energy efficiency 33%
Electric
heater
Thermal efficiency 100%
End use ~ 33%For heating
Transport 100%Primary energy (gas)
~ 97% Gasenergy
Energy efficiency 73%
Furnace
heater
Thermal efficiency 75%
End use ~73%For heating
Heating the residential house
Page 17 Tao and Janis
LEED – Leadership in Energy and Environmental Design
• Green building rating system
• High-performance and sustainable buildings
• Voluntary (at this moment) national standard
Affect • Location environment – sustainable site
• Use of energy
• Use of water resources
• Use of building materials
• IAQ
• Design process
Thermal Comfort
• Combination of- Indoor environmental factors - Personal factors
• Human body - in thermal equilibrium with the environment
Thermal comfort equation - P.O.Fanger
Metabolic Heat - Work = Energy that body release
Energy balance for human body
Thermal comfort
PPD – Predicted Percentage of Dissatisfied Scale 5-100%
• Metabolism – health condition and activity
• Clothing level
• Air Temperature
• Mean Radiant Temperature
• Air Velocity
• Humidity
P.O. Fanger
Thermal comfort Factors that we control in buildings
• Air Temperature
• Humidity
• Air velocity• Surrounding Temperature
Overview
• Psychrometric quantities
• Heat loss and gain• Cooling, heating, and ventilation loads
• Cooling and heating equipment
• Air equipment and controls
Willis Haviland Carrier - Carrier
1911 "Rational Psychrometric Formulae”
Psychrometric Chart
• Need two quantities for a state point• Can get all other quantities from a state point
• Can do all calculations without a chart• Often require iteration• Many “digital” psychrometric charts available
• Can make your own
• Best source is ASHRAE Fundamentals (Chapter 6)
• Absolute Temperature (T) (K, R)• Dry-bulb temperature (t) [°F, °C]• Wet-bulb temperature (t*)• Dew-point temperature (td)
• Mean radiant temperature (tr)• Operative temperature (to)• Effective temperature (ET*)
Temperature
Which temperature do you expect to be higher?
A. Wet-bulb
B. Dry-bulb
Wet-bulb temperature (t*)
• Temperature measured by a psychrometer
• Lower than dry-bulb temperature• Evaporating moisture removes heat from
thermometer bulb• The higher the humidity
• Smaller difference between wet-bulb and dry-bulb temperature
Dew-Point Temperature, td
• Define temperature at which condensation happen• td is defined as temperature of that air at saturation
• i.e. RH = 100%
• Surfaces below the dew point temperature will have condensation
• Measured with a chilled-mirror apparatus
If you have a sample of air at itsdew-point temperature?
A. The water will condense out.
B. It will be pure water vapor.
C. Putting the sample in a sealed container and heating it will cause condensation.
D. Putting the sample in sealed container and cooling it will cause condensation.
Humidity
• Humidity ratio (W) [lb/lb, g/kg, grains]
[grains/lb = 1/7000 lb/lb]
• Relative humidity (RH, ) [%]• Saturation
Humidity Ratio, W
• Mass of water vapor/divided by mass of dry air
• Orthogonal to temperature• Not a function of temperature
• Most convenient form for calculations involving airflow
• Very hard to measure directly
Relative Humidity, RH or
• Ratio of partial pressure of water vapor to partial pressure of water vapor at same T and P at saturation
• Strong function of temperature• For constant humidity ratio
• Higher temperature, lower relative humidity
• Saturation
Relative Humidity
• Driving force for moisture transport• Human comfort• Moisture absorption/desorption
• Can be measured with• Resistive sensors• Capacitive sensors• Horse hair
Conclusions
• Define thermal comfort parameters
• Define all quantities on a psychrometric chart and use it to do calculations
Reading Assignment
Chapter 1 and
Chapter 2 (Section 2.1 & 2.2) Tao and Janis
Next class – Thursday
5 minutes quiz at the beginning of the class