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**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

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**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

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**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

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**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

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**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.

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**Chapter 7 – Nonresidential Load Calculation**

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**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.

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**Chapter 7 – Nonresidential Load Calculation**

Principles. Heat balance fundamentals - continued

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**Chapter 7 – Nonresidential Load Calculation**

Principles. Heat balance fundamentals - continued

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**Chapter 7 – Nonresidential Load Calculation**

Principles. Conduction Transfer Function – solved simultaneously with (7-1)

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**Chapter 7 – Nonresidential Load Calculation**

Principles. Space Air Energy Balance – also simultaneously

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**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

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**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

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**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

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**Chapter 7 – Nonresidential Load Calculation**

Initial Design Considerations. Heat gain through exterior walls and roofs - continued

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**Chapter 7 – Nonresidential Load Calculation**

Initial Design Considerations. Heat gain through exterior walls and roofs – Sol-Air Temperatures

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**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

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**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

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**Chapter 7 – Nonresidential Load Calculation**

Initial Design Considerations. Heat Gain Through Fenestration

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**Chapter 7 – Nonresidential Load Calculation**

Initial Design Considerations. Heat Gain Through Fenestration

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**Chapter 7 – Nonresidential Load Calculation**

Initial Design Considerations. Total instantaneous rate of heat gain … HB model

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**Chapter 7 – Nonresidential Load Calculation**

Initial Design Considerations. Fenestration heat gain

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**Chapter 7 – Nonresidential Load Calculation**

Initial Design Considerations. where

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**Chapter 7 – Nonresidential Load Calculation**

Initial Design Considerations – Table 7-3 Solar Heat Gain

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**Chapter 7 – Nonresidential Load Calculation**

Initial Design Considerations – Table 7-3 Solar Heat Gain

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**Chapter 7 – Nonresidential Load Calculation**

Initial Design Considerations – Table 7-3 Solar Heat Gain

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**Chapter 7 – Nonresidential Load Calculation**

Initial Design Considerations – Table 7-3 Solar Heat Gain

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**Chapter 7 – Nonresidential Load Calculation**

Initial Design Considerations – Table 7-4 Glazing and Windows

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**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

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**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

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**Chapter 7 – Nonresidential Load Calculation**

Initial Design Considerations

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**Chapter 7 – Nonresidential Load Calculation**

Initial Design Considerations – gain into a window

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**Chapter 7 – Nonresidential Load Calculation**

Initial Design Considerations – gain into a window

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**Chapter 7 – Nonresidential Load Calculation**

Initial Design Considerations – gain into a window

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**Chapter 7 – Nonresidential Load Calculation**

Initial Design Considerations – gain into a window

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**Chapter 7 – Nonresidential Load Calculation**

Solar Angles

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**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

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**Chapter 7 – Nonresidential Load Calculation**

Table 7-13 Solar Position and Profile Angles for 40 deg N Lat.

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