Heating and Air Conditioning I Principles of Heating, Ventilating and Air Conditioning R.H. Howell,...

Heating and Air Conditioning IPrinciples of Heating, Ventilating and Air

ConditioningR.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

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


• 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


• 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


• 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.

Chapter 7 – Nonresidential Load Calculation


• 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.


• Heat balance fundamentals - continued


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


• Space Air Energy Balance – also simultaneously


• 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

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

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


• Heat gain through exterior walls and roofs - continued


• Heat gain through exterior walls and roofs – Sol-Air Temperatures


• Sol-air temperatures – any other air temperature cycle can be determined from Table 7-1

• Average Sol-Air Temperature


• 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


• Heat Gain Through Fenestration


• Total instantaneous rate of heat gain … HB model


• Fenestration heat gain


• where

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

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

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

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

Chapter 7 – Nonresidential Load Calculation• Initial Design Considerations

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

Chapter 7 – Nonresidential Load Calculation• Solar Angles

Chapter 7 – Nonresidential Load Calculation• Solar Angles

• Determine the Earth-Sun line at given time, data, position

• Theangles V and H 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 the solaraltitude, is the angle between the E-S line and this ground projection

• The angle , the solar azimuth, is the angle from the base leg of the E-S projection to the south direction.

• The angle is the angle between the base leg of the E-S projection and the perpendicular to the surface – wall solar azimuth

• The angle are the angle between south direction and the perpendicular to the surface

• The profile angle is determined from and– tabulated in Table 7-13

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

Heating and Air Conditioning I Principles of Heating, Ventilating and Air Conditioning R.H. Howell,...

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