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Objectives

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

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Objectives

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  1. Objectives • Use psychrometric chart • Describe operation of technologies/techniques used to make ventilation more efficient • Model infiltration driving forces • Stack effect • Wind

  2. 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

  3. Temperature • Absolute Temperature (T) (K, R) • Dry-bulb temperature (t) [°F, °C] • Wet-bulb temperature (t*) • Dew-point temperature (td)

  4. 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

  5. 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

  6. 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%

  7. 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/

  8. ASHRAE Weather • 2001 Fundamentals ch.27

  9. 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)

  10. 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

  11. 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

  12. 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

  13. Ref: ASHRAE HVAC Systems and Equipment (2000), ch. 44

  14. Ref: ASHRAE HVAC Systems and Equipment (2000), ch. 44

  15. 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

  16. Passive Ventilation • Provide driving force for ventilation • Designing buildings to take advantage of prevailing winds • Cupola – stack effect

  17. What is a leak? • Hole + driving force (pressure difference) • Flow can be either direction • Driving forces • Stack effect • Wind • HVAC system

  18. Given a crack Baker et al. (1987) Building and Environment

  19. Stack Effect

  20. Stack Effect • Consider a wall Tin Tout PV=nRT dp/dz=-ρg NL

  21. Major Steps in Stack Effect Derivation • Use dp/dz=-ρg • Compare points on the inside and outside of wall • Assume constant inside and outside densities • pNL–pin= -ρg(hNL–hin), pNL–pout= -ρg(hNL–hout) • Rearrange to get • pout–pin= (ρout–ρin)g(∆h) • Use ideal gas law to get:

  22. Wind • From Bernoulli Equation • Drag on a body at high Reynolds numbers • Get CP from measuremements or from ASHRAE Fundamentals Chapter 16

  23. Unbalanced Leakage Qs-Qr

  24. 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: Ref: Sherman (1992) Indoor Air

  25. Natural Ventilation / Cooling • 13th century Persia – Middle East • Passive ventilation and evaporative cooling • How much ventilation? • How much cooling?

  26. Calculations • Pressure difference (assuming no wind) ∆P ≈ 0.04 ∆T z • Flow rate • Energy transfer q = M∙hfg (based on water flow rate) q = MC∆T (based on air flow rate)

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