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
1kWh (3.6MJ) of source energy (fuel) ≠ 1kWh of electric energy
• End use energy
amount of energy delivered to consumer
electric, heating, cooling
• Primary 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
Energy efficiency vs. Thermal efficiency
Heating the residential house
Power plant
100%
Primary
energy (gas)
Energy efficiency
33%
Transport
100%
Primary
energy (gas)
Energy efficiency
73%
~ 33%
Electric
energy
~ 97%
Gas
energy
Electric
heater
Furnace
heater
Thermal
efficiency 100%
End use
~ 33%
For heating
Thermal
efficiency 75%
End use
~73%
For heating
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
Energy balance for human body
Metabolic Heat - Work = Energy that body release
Thermal comfort
•
•
•
•
•
•
Metabolism – health condition and activity
Clothing level
Air Temperature
Mean Radiant Temperature
Air Velocity
Humidity
P.O. Fanger
PPD – Predicted Percentage of Dissatisfied
Scale 5-100%
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)
Temperature
•
•
•
•
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*)
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 its
dew-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