1. Heating and Air Conditioning I
Principles of Heating, Ventilating and Air Conditioning
R.H. Howell, H.J. Sauer, and W.J. Coad
ASHRAE, 2005
basic textbook/reference material
For ME 421
John P. Renie
Adjunct Professor – Spring 2009

2. Chapter 7 – Nonresidential Load Calculation
Principles.
Primary basis for design and selection of heating and air-conditioning systems and components
First costs
Comfort and productivity of occupants
Operation and energy conservation
This chapter discusses the common elements of load calculations and several methods of making load estimates – focuses on the ASHRAE Radiant Time Series (RTS) method
Cooling Loads – Conductive, convective and radiative
External – walls, roofs, windows, ceilings, etc.
Internal – people, lights, appliances, equipment
Infiltration – air leakage and moisture migration
System – ventilation, duct leakage, reheat, fans, pump power
Variables affecting cooling loads – interrelated and vary over 24 hour period – not always in phase – zone dependent

3. Chapter 7 – Nonresidential Load Calculation
Principles.
Heat flow rates – for air conditioning design
Space heat gain – rate heat enters into or is generated within a space at a given instant
Classified by mode in which it enters
Solar radiation through transparent surfaces
Heat conduction through walls and roofs
Heat conduction through interior partitions, ceilings and floors
Heat generated by occupants, lights or appliances
Energy due to ventilation and infiltration or outside air
Miscellaneous heat gains
Classified by whether it is sensible or latent
Sensible is directly added by conduction, convection, or radiation
Latent occurs when moisture is added to space (by occupants or equipment) – must be removed by condensation on cooling apparatus - coils

4. Chapter 7 – Nonresidential Load Calculation
Principles.
Heat flow rates – for air conditioning design
Space cooling load – rate at which heat must be removed from the space to maintain a constant space air temperature – this doesn’t necessarily equal the sum of space heat gains above at given time.
Radiant heat gains is not immediately converted into cooling load – first must be absorbed by the surfaces and objects in the space – then once they become warmer than air temperature, heat is transferred due to convection
This thermal storage effect is critically important in differentiating between instanteous heat gain for a given space and its cooling load for that moment.
Space heat extraction rate – the rate at which heat is removed from the conditioned space equals the space cooling load only to the degree that room air temperature is held constant.
Intermittent operation of cooling system and minor cyclic variation or swing in room temperature

5. Chapter 7 – Nonresidential Load Calculation
Principles.
Heat flow rates – for air conditioning design
Cooling coil load – rate at which energy is removed at the cooling coil that serves one or more conditioned spaces equals the sum of the instantaneous space cooling loads (or space heat extraction rate if is assumed that the space temperature does not vary) for all the spaces served by the coil, plus any external loads.
External loads include heat gain by the distribution system between individual spaces and the cooling equipment, the outdoor air heat and moisture introduced into the distribution system through the cooling equipment.
Cooling Load Estimation in Practice
Usually the cooling load is needed to be known before all parameters can be completely defined
Heat balance fundamentals
Engineering judgment
Space requirements, partitions, lighting, etc.

6. Chapter 7 – Nonresidential Load Calculation

7. Chapter 7 – Nonresidential Load Calculation
Principles.
Heat balance fundamentals
The calculation of cooling load for a space involves calculating a surface-by-surface conductive, convective, and radiative heat balance for each room and a convective heat balance for the room air.
Requires a laborious solution of energy balance equations involving the space air, surrounding walls and windows, infiltration and ventilation air, and internal energy sources.
Consider a case of a four wall, ceiling, floor with infilitration air and internal energy sources.
The energy exchange at each surface at a given time can be calculated from the following equation.

8. Chapter 7 – Nonresidential Load Calculation
Principles.
Heat balance fundamentals - continued

9. Chapter 7 – Nonresidential Load Calculation
Principles.
Heat balance fundamentals - continued

10. Chapter 7 – Nonresidential Load Calculation
Principles.
Conduction Transfer Function – solved simultaneously with (7-1)

11. Chapter 7 – Nonresidential Load Calculation
Principles.
Space Air Energy Balance – also simultaneously

12. Chapter 7 – Nonresidential Load Calculation
Principles.
Total Equivalent Temperature Difference Method (TETD)
Series of representative wall and roof assemblies used to calculated TETD values as a function of sol-air temperature and room temperature
See text for methodology
Transfer Function Method
Use of CTF followed by room transfer function (RTF)
Heat Balance Method (HB)
Exact solution – computer essential
Use of simplifying models, thus approximate
Well-mixed model
Uniform surface temperatures
Diffuse radiating surfaces
Uniform long wave (LW) and shortwave (SW) irradiation
Radiant Time Series Method (RTS)
New simplified method – rigorous but not iterative, transparent – for peak load calculation only

13. Chapter 7 – Nonresidential Load Calculation
Initial Design Considerations.
Building characterizations
Configuration
Outdoor design conditions
Indoor design conditions
Internal heat gains and operating schedules
Areas
Gross surface area
Fenestration area
Net surface area
Additional considerations

14. Chapter 7 – Nonresidential Load Calculation
Heat Gain Calculation Concepts
Primary weather-related variable influencing a building’s cooling load is solar radiation
Heat gain through exterior walls and roofs
Sol-Air temperature – the temperature of the outdoor air that, in the absence of all radiation changes, gives the same rate of heat entry into the surface as would the combination of incident solar radiation, radiant energy exchange with the sky and other outdoor surroundings, and convective heat exchange with outdoor air.
Heat gain through exterior surfaces

15. Chapter 7 – Nonresidential Load Calculation
Initial Design Considerations.
Heat gain through exterior walls and roofs - continued

16. Chapter 7 – Nonresidential Load Calculation
Initial Design Considerations.
Heat gain through exterior walls and roofs – Sol-Air Temperatures

17. Chapter 7 – Nonresidential Load Calculation
Initial Design Considerations.
Sol-air temperatures – any other air temperature cycle can be determined from Table 7-1
Average Sol-Air Temperature

18. Chapter 7 – Nonresidential Load Calculation
Initial Design Considerations.
Hurly air temperature – Table 7-1 is based on a design temperature of 95 F and 21 range. For something different, take the percent of range and subtract it from the design temperature.
Say design temperature is 88 F and range is 19.9 (FW)
At 8:00 pm, 88 – (0.47)*19.9 = 78.6 F

19. Chapter 7 – Nonresidential Load Calculation
Initial Design Considerations.
Heat Gain Through Fenestration

20. Chapter 7 – Nonresidential Load Calculation
Initial Design Considerations.
Heat Gain Through Fenestration

21. Chapter 7 – Nonresidential Load Calculation
Initial Design Considerations.
Total instantaneous rate of heat gain … HB model

22. Chapter 7 – Nonresidential Load Calculation
Initial Design Considerations.
Fenestration heat gain

23. Chapter 7 – Nonresidential Load Calculation
Initial Design Considerations.
where

24. Chapter 7 – Nonresidential Load Calculation
Initial Design Considerations – Table 7-3 Solar Heat Gain

25. Chapter 7 – Nonresidential Load Calculation
Initial Design Considerations – Table 7-3 Solar Heat Gain

26. Chapter 7 – Nonresidential Load Calculation
Initial Design Considerations – Table 7-3 Solar Heat Gain

27. Chapter 7 – Nonresidential Load Calculation
Initial Design Considerations – Table 7-3 Solar Heat Gain

28. Chapter 7 – Nonresidential Load Calculation
Initial Design Considerations – Table 7-4 Glazing and Windows

29. Chapter 7 – Nonresidential Load Calculation
Initial Design Considerations – Tables
Table 7-5 – Solar Heat Gain Coefficients for Domed Horizontal Skylights
Table 7-6 – Solar Heat Gain Coefficients and U-Factors for Standard Hollow Glass Block Wall Panels
Table 7-7 – Unshaded Fractions (Fu) and exterior Solar Attenuation Coefficients (EAC) for Louvered Sun Screens
Table 7-8 – Interior Solar Attenuation Coefficients (IAC) for Single or Double Glazing Shaded by Interior Venetian Blinds or Roller Shades
Table 7-9 – Between Glass Solar Attenuation Coefficients (BAC) for Doubling Glazing with Between-Glass Shading
Table 7-10 – Properties of Representative Indoor Shading Devices Shown in Table 7-8 and 7-9
Table 7-11 – Interior Solar Attenuation Coefficients for Single and Insulating Glass with Draperies

30. Chapter 7 – Nonresidential Load Calculation
Initial Design Considerations
Effect of horizontal projection to provide for shading and considerable reduction in solar gain.
Applicable to south, southeast, and southwest exposures in late spring, summer, and early fall. East and west all year and south in winter the lengths would be to large.
Geometry

31. Chapter 7 – Nonresidential Load Calculation
Initial Design Considerations

32. Chapter 7 – Nonresidential Load Calculation
Initial Design Considerations – gain into a window

33. Chapter 7 – Nonresidential Load Calculation
Initial Design Considerations – gain into a window

34. Chapter 7 – Nonresidential Load Calculation
Initial Design Considerations – gain into a window

35. Chapter 7 – Nonresidential Load Calculation
Initial Design Considerations – gain into a window

36. Chapter 7 – Nonresidential Load Calculation
Solar Angles

37. Chapter 7 – Nonresidential Load Calculation
Solar Angles
Determine the Earth-Sun line at given time, data, position
The angles QV and QH are measure of this E-S line from the local vertical and a line normal to the vertical surface
Dropping a projection from the E-S line to the horizontal ground plane, forming a right angle
The angle b, the solar altitude, is the angle between the E-S line and this ground projection
The angle f, the solar azimuth, is the angle from the base leg of the E-S projection to the south direction.
The angle g is the angle between the base leg of the E-S projection and the perpendicular to the surface – wall solar azimuth
The angle y are the angle between south direction and the perpendicular to the surface
The profile angle W is determined from g and b – tabulated in Table 7-13

38. Chapter 7 – Nonresidential Load Calculation
Table 7-13 Solar Position and Profile Angles for 40 deg N Lat.