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