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Enhancing Ventilation Efficiency through Psychrometrics and Energy Recovery Techniques

This informative resource delves into the critical relationship between psychrometrics and ventilation efficiency. It covers key principles such as the psychrometric chart, calculation of state points, and the driving forces of infiltration, including stack effect and wind influence. Methods for improving ventilation efficiency, such as air-to-air heat exchangers and passive ventilation design, are discussed. The material references ASHRAE fundamentals for comfort parameters and provides insights into energy recovery ventilation strategies, illustrating the importance of managing sensible and latent energy for optimal HVAC performance.

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Enhancing Ventilation Efficiency through Psychrometrics and Energy Recovery Techniques

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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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