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)