Transcript Slide 1
Objectives
• Use psychrometric chart • Describe operation of technologies/techniques used to make ventilation more efficient • Model infiltration driving forces • Stack effect • Wind
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) • For comfort parameters use Chapter 8
Temperature
• Absolute Temperature (
T
) (K, R) • Dry-bulb temperature (
t
) [°F, °C] • Wet-bulb temperature (
t*
) • Dew-point temperature (
t d
)
Humidity
• Humidity ratio (
W
) [lb/lb, g/kg, grains] • 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,
) [%] • Saturation
What is enthalpy?
• Enthalpy is total energy in the air • Sensible plus latent • You can choose to track enthalpy, but then you don’t get any sense of sensible/latent split
Examples
• What is enthalpy of air in the classroom right now?
• Condensation on windows when taking a shower • How cold does it have to be outside for condensation to form on windows?
– Assumption is that windows are the same temperature as outside air – 80 °F, RH = 80%
What conditions should you use for calculations?
• Design • Outdoor – ASHRAE 1% and 99% values • Indoor – ASHRAE comfort zone • Energy use (i.e. operating) • Hourly data • http://www.ncdc.noaa.gov/oa/climate/climatedata.html#HOURLY • TMY data • http://rredc.nrel.gov/solar/old_data/nsrdb/tmy2/
ASHRAE Weather
• 2001 Fundamentals ch.27
Summary
• Calculate sensible and latent energy separately • Can combine into enthalpy • Ventilation energy consequences are linear with • Mass flow rate of air • Humidity ratio difference (latent) • Temperature difference (sensible)
Ventilation and Energy Efficiency
• Avoid losses from ventilation • Air-to-air heat exchanger • Eliminate needs for fans • Passive ventilation • Offset cooling/heating load • Economizer • Nighttime flush
Avoid losses from ventilation
• Need to supply some amount of air • Air-to-air heat exchanger • Adds efficiency multiplier to sensible (and sometimes latent) heat losses/gains due to ventilation
Heat recovery ventilation
• Several strategies • Counterflow or crossflow heat exchanger • Microporous membrane • Condensate removal if surface below dew point • Desiccant/polymer wheel • Issues • Energy exchange effectiveness (consider sign) • Carryover/leakage • Maintenance
Ref: ASHRAE HVAC Systems and Equipment (2000), ch. 44
Ref: ASHRAE HVAC Systems and Equipment (2000), ch. 44
Summary
• Energy recovery ventilation uses conditioned air to preheat/precool ventilation air • Some ERVs also exchange moisture • Typical effectiveness: • 50-90 % for sensible • 30-60% for latent
Passive Ventilation
• Provide driving force for ventilation • Designing buildings to take advantage of prevailing winds • Cupola – stack effect
What is a leak?
• Hole + driving force (pressure difference) • Flow can be either direction • Driving forces • Stack effect • Wind • HVAC system
Given a crack
Q
c d A
2
P
If
Re
1 ,
c d
constant of O(1)
Q
P
If
Re Q
1 ,
c d
P
P
Baker et al. (1987)
Building and Environment
Stack Effect
Stack Effect
• Consider a wall T in T out
PV=nRT
NL d
p/
d
z=-ρg
Major Steps in Stack Effect Derivation
• Use d
p/
d
z=-ρg
• Compare points on the inside and outside of wall • Assume constant inside and outside densities •
p NL
–
p in = -ρg
(
h NL
–
h in
)
, p NL
–
p out
• Rearrange to get
= -ρg
(
h NL
–
h out
) •
p out
–
p in = (ρ out
–
ρ in )g(∆h
) • Use ideal gas law to get:
p out –p in
ρ out ( T o u t
T in T in )g(
h)
Wind
• From Bernoulli Equation • Drag on a body at high Reynolds numbers
P
C P
U
2 2 • Get
C P
from measuremements or from
ASHRAE Fundamentals
Chapter 16
Unbalanced Leakage
Q s -Q r
Combining driving forces
• Get pressure difference caused by each effect for particular building • Use crack pressure flow relationship to determine flow through each leak • This is quasi-empirical model:
Q ws
Q w
2
Q s
2 Ref: Sherman (1992)
Indoor Air
Natural Ventilation / Cooling
• 13 th century Persia – Middle East • Passive ventilation and evaporative cooling • How much ventilation?
• How much cooling?
Calculations
• Pressure difference (assuming no wind)
∆P
≈ 0.04
∆T z
• Flow rate
Q
c d A
2
P
• Energy transfer
q = M∙h fg
(based on water flow rate)
q = MC∆T
(based on air flow rate)