Objectives

1. Objectives • Lean about energy transport by air • Calculate Cooling and Heating loads • Solve 1-D conduction • Design whether condition • Use knowledge of heat transfer to calculate • Solar gains • Internal gains

2. Equations for sensible energy transport by air • Energy per unit of mass Δhsensible = cp×ΔT [Btu/lb] cp - specific heat for air (for air 0.24 Btu/lb°F) • Heat transfer (rate) Qs = m × cp×ΔT [Btu/h] m - mass flow rate [lb/min, lb/h], m = V ×r V – volume flow rate [ft3/min or CFM] r – airdensity (0.076lb/ft3) Qs = 1.1 × CFM ×ΔT (only for IP unit system)

3. Equations for latent energy transport by air • Energy per unit of mass Δhlatent = Δw×hfg[Btu/lbda] hfg - specific energy of water phase change (1000 Btu/lbw) • Heat transfer (rate) Ql = m ×Δw×hfg [Btu/h] Ql = 1000 × WaterFloowRate (only for IP units)

4. Total energy transport calculation using enthalpies from chat • Energy per unit of mass Δh=h1-h2[Btu/lbda] • Heat transfer (rate) Qtotal = m ×Δh[Btu/h] Qtotal = Qsensible + Qlatent

5. Why do we calculate heating and cooling loads? Heating and Cooling Loads • To estimate amount of energy used for heating and cooling by a building • To size heating and cooling equipment for a building • Because my supervisor request that

6. Introduction to Heat Transfer • Conduction • Components • Convection • Air flows (sensible and latent) • Radiation • Solar gains (cooling only) • Increased conduction (cooling only) • Phase change • Water vapor/steam • Internal gains (cooling only) • Sensible and latent

7. 1-D Conduction l k A 90 °F 70 °F U U-Value[W/(m2 °C)] U = k/l k conductivity [W/(m °C)] l length [m] Q heat transfer rate [W] ΔT temperature difference [°C] A surface area [m2] Q = UAΔT

8. Material k Values 1At 300 K Table 2-3Tao and Janis (k=λ) values in [Btu in/(h ft2 F)]

9. Wall assembly l1 l2 • R = l/k • Q = (A/Rtotal)ΔT • Add resistances in series • Add U-values in parallel k1 k2 90 °F 70 °F R1 R2 Tout Tmid Tin

10. Surface Air Film h - convection coefficient - surface conductance [W/m2, Btu/(h ft2)] • Direction/orientation • Air speed • Table 2-5 Tao and Janis Tout Tin Rsurface= 1/h Ri Ro R1 R2 Rtotal= ΣRi Tout Tin

11. What if more than one surface? l1 l2 k1, A1 k2, A2 Qtotal = Q1,2 + Q3 Q1,2 A2 = A1 U1,2 = 1/R 1,2=1/(R1+R2) k3, A3 Q1,2 = A1U1,2ΔT Q3 Q3 = A3U3ΔT l3

12. Relationship between temperature and heat loss U1A1 U2A2 U3(A3+A5) U4A4 U5A5 A2 A3 A1 A4 Tin Tout A5 A6 Qtotal= Σ(UiAi)·ΔT

13. Which of the following statements about a material is true? • A high U-value is a good insulator, and a high R-value is a good conductor. • A high U-value is a good conductor, and a high R-value is a good insulator. • A high U-value is a good insulator, and a high R-value is a good insulator. • A high U-value is a good conductor, and a high R-value is a good conductor.

14. Example • Consider a 1 ft × 1 ft × 1 ft box • Two of the sides are 2” thick extruded expanded polystyrene foam • The other four sides are 2” thick plywood • The inside of the box needs to be maintained at 120 °F • The air around the box is still and at 80 °F • How much heating do you need?

15. The Moral of the Story • Calculate R-values for each series path • Convert them to U-values • Find the appropriate area for each U-value • Multiply U-valuei by Areai • Sum UAi • Calculate Q = Σ(UAi)ΔT

16. Heat transfer in the building Not only conduction and convection !

17. Infiltration • Air transport Sensible energy Previously defined • Q = m× cp × ΔT [BTU/hr, W] • ΔT= T indoor – T outdoor • or Q = 1.1BTU/(hr CFM °F)× V × ΔT [BTU/hr]

18. Latent Infiltration and Ventilation • Can either track enthalpy and temperature and separate latent and sensible later: • Q total= m× Δh [BTU/hr, W] • Q latent = Q total - Q sensible = m× Δh - m× cp × ΔT • Or, track humidity ratio: • Q latent = m× Δw ×hfg

19. Ventilation Example • Supply 500 CFM of outside air to our classroom • Outside 90 °F 61% RH • Inside 75 °F 40% RH • What is the latent load from ventilation? • Q latent = m×hfg× Δw • Q = ρ × V×hfg× Δw • Q = 0.076 lbair/ft3 × 500 ft3/min × 1076 BTU/lb × (0.01867 lbH2O/lbair - .00759 lbH2O/lbair) × 60 min/hr • Q = 26.3 kBTU/hr

20. What is the difference between ventilation and infiltration? • Ventilation refers to the total amount of air entering a space, and infiltration refers only to air that unintentionally enters. • Ventilation is intended air entry into a space. Infiltration is unintended air entry. • Infiltration is uncontrolled ventilation.

21. Where do you get information about amount of ventilation required? • ASHRAE Standard 62 • Table 2 • Hotly debated – many addenda and changes • Tao and Janis Table 2.9A

22. Ground Contact • Receives less attention: • 3-D conduction problem • Ground temperature is often much closer to indoor air temperature • Use F- value for slab floor [BTU/(hr °F ft)] • Note different units from U-value • Multiply by slab edge length • Add to ΣUA • Still need to include basement wall area • Tao and Janis Tables 2.10 and 2.11 More details in ASHRAE handbook -Chapter 29

23. Ground Contact • 3-D conduction problem • Ground temperature is often much closer to indoor air temperature • Use F- value for slab floor Multiply by slab edge length and Add to ΣUA

24. Summary of Heating Loads • Conduction and convection principles can be used to calculate heat loss for individual components • Convection principles used to account for infiltration and ventilation

25. Where do you get information about amount of ventilation required? • ASHRAE Standard 62 • Table 2 • Tao and Janis Table 2.9A

26. Weather Data • Table 2-2A (Tao and Janis) or • Chapter 28 of ASHRAE Fundamentals • For heating use the 99% design DB value • 99% of hours during the winter it will be warmer than this Design Temperature • Elevation, latitude, longitude

27. Weather Data • Forcoolinguse the 1% DB and coincident WB for load calculations • 1% of hours during the summer will be warmer than this Design Temperature • Use the 1% design WB for specification of equipment

28. Solar Gain • Affects conductive heat gains because outside surfaces get hot • Use Q = U·A·ΔT Replace ΔT with TETD – total equivalent temperature differential Q = U·A· TETD • Tables 2-12 – 2-14 in Tao and Janis Replace ΔT with CLTD (Tables 1 and 2 Chapter 29 of ASHRAE Fundamentals)

29. Solar Gain TETD depends on: • orientation, • time of day, • wall properties • surface color • thermal capacity

30. Glazing • Q = U·A·ΔT+A×SC×SHGF • Calculate conduction normally Q = U·A·ΔT • Use U-values from NFRC National Fenestration Rating Council • ALREADY INCLUDES AIRFILMS • http://cpd.nfrc.org/pubsearch/psMain.asp • Use the U-value for the actual window that you are going to use • Only use default values if absolutely necessary • Tao and Janis - no data • Tables 4 and 15, Chapter 31 ASHRAE Fundamentals

31. Shading Coefficient - SC • Ratio of how much sunlight passes through relative to a clean 1/8” thick piece of glass • Depends on • Window coatings • Actually a spectral property • Frame shading, dirt, etc. • Use the SHGC value from NFRC for a particular window SC=SHGC/0.87 • Lower it further for blinds, awnings, shading, dirt • http://cpd.nfrc.org/search/cpd/cpd_search_default.aspx?type=W

32. More about Windows • Spectral coatings (low-e) • Allows visible energy to pass, but limits infrared radiation • Particularly short wave • Tints • Polyester films • Gas fills • All improve (lower) the U-value

33. Low- coatings

34. Internal gains • What contributes to internal gains? • How much? • What about latent internal gains?

35. Internal gains • ASHRAE Fundamentals ch. 29 or handouts • Table 1 – people • Table 2 – lighting, Table 3 – motors • Table 5 – cooking appliances • Table 6 -10 Medical, laboratory, office • Tao and Janis - People only - Table 2.17

36. Readings: • Tao and Janis 2.4-2.8.10