Heating and Air Conditioning I

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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 5 Design Conditions

• This chapter covers the fundamental elements that
  accompany the load calculations for sizing
  heating and cooling systems. This includes the
  estimation of the outside air quantities and the
  evaluation of the overall heat transfer
  coefficient for building components.
• Basic components of heating and cooling loads
• Transmission
• Solar Radiation
• Lights
• Power
• Infiltration (uncontrolled)
• Fresh Air (controlled)
• People
• Appliances
• Material in and out
• First four terms include Q
• Last five includes Q and mwater

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Chapter 5 Design Conditions

• General Considerations
• Knowledge of thermal insulation and thermal
  behavior of materials
• Flowing fluids carry heat and moisture
• Heating and cooling include simultaneous transfer
  of heat and mass
• Humidification and dehumidification depends on
  ventilation and other sources of moisture gain
  and loss.
• Outdoor Air Load Components
• Outdoor air flows through building dilute and
  remove indoor air contaminants significant
  portion of total space load
• Natural infiltration in addition to intentional
  mechanical ventilation
• Ventilation (both natural and mechanical) versus
  infiltration
• Air leakage into and out (exfiltration) due to
  pressure difference
• Modern building usually over pressurized
• Modern building not desirable to have open
  windows
• Residence building reliant on infiltration and
  natural ventilation

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Chapter 5 Design Conditions

• Outdoor Air Load Components
• Small exhaust fans code (condensation?)
• ASHRAE Standard 62 guidance on ventilation and
  indoor air quality
• Ventilation Rate Procedure
• Control of six pollutants
• Outdoor air supply gt minimums (5 cfm per person
  and 0.06 cfm/ft2)
• Indoor Air Quality Procedure
• Nine pollutants indoor maintained below standard
  level
• Subjective (0.5 AC/hr) local codes
• Terminolgy
• Air handling unit (AHU) Figure 5-2
• Return air (ra)
• Exhaust air (ea)
• Recirculated air (ca)
• Outside air (oa) not fresh outside air
  fraction Xoa or percent
• Mixed air (ma)
• Supply air (sa)
• Bypass air (ba)

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Chapter 5 Design Conditions
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Chapter 5 Design Conditions

• Outdoor Air Load Components.
• Terminolgy
• Makeup air unit (MAU) all air is outside
• Outside air
• Large part of the total space conditioning
  (heating, cooling, humidification, and
  dehumidification) 20 to 40
• Sensible heating qs Q r cp Dt
• Latent heating qL Q r hfg DW
• Dehumidification in summer
• Humidification in winter
• Infiltration
• Air leakage through cracks and interstices around
  windows and doors, and through the floors and
  walls depends on construction, building
  condition
• Pressure difference between indoor and outdoor
  caused by wind or differences in density
  chimney or stack effect also differences in
  temperature

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Chapter 5 Design Conditions

• Outdoor Air Load Components.
• Estimation of the air infiltration
• Crack method measured leakage characteristics
  of the building components
• Air change method (simplicity)
• Assumption of the number of air changes per hour
  (ACH) that a space experiences
• Q ACH x VOL/60 where VOL is gross volume of
  space
• Table 5-1 Change Rates as Function of
  Airtightness
• Outdoor design temperature
• Tight/Medium/Loose
• Figure 5-3 Building Pressure due to Wind Effects
• Impossible to accurately predict infiltration
  from theory alone because of the many unknowns.
  However, semi-empirical expressions can be used
  to estimate infiltration rate.
• Leakage function infiltration cause by pressure
  difference
• Q CD A (2 Dp/r)n
• Where CD is discharge coefficient (geometry and
  Reynolds number)
• Dp Dps Dpw Dpp (stack, wind, building
  pressurization)

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Chapter 5 Design Conditions

• Outdoor Air Load Components

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Chapter 5 Design Conditions

• Outdoor Air Load Components

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Chapter 5 Design Conditions

• Outdoor Air Load Components.
• Estimation of the air infiltration
• Stack effect occurs when air densities are
  different on the inside and outside of the
  building density decreases with increasing
  temperature
• Winter lower outside temperature at higher
  pressure than inside
• Summer cooler air inside air infiltrates at
  top and flows downward
• Neutral axis
• Dps 0.52 pb h (1/To) (1/Ti) where pb is
  barometric pressure
• Wind velocity effect (velocity pressure)
• Dpw 0.5 Cp r Vw2
• Figure 5-5 gives average pressure coefficients
  for tall buildings
• Building pressurization depends on design and
  operation of the HVAC system.
• Positive pressurization causes less infiltration
  from stack and wind effects
• Looking at crack coefficient equation - Q K A
  (Dp)n
• With A is wall area and K is leakage coefficient
• Table 5-2 Curtain Wall Leakage Coefficients
  Figure 5-6
• Tight (K 0.22), Average (K 0.66), and Loose
  (K 1.30)

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Chapter 5 Design Conditions

• Outdoor Air Load Components

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Chapter 5 Design Conditions

• Outdoor Air Load Components

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Chapter 5 Design Conditions

• Outdoor Air Load Components

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Chapter 5 Design Conditions

• Outdoor Air Load Components

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Chapter 5 Design Conditions

• Outdoor Air Load Components.
• Estimation of the air infiltration
• Typical infiltrations values for North America
  residential housing varies by order of
  magnitude
• Figure 5-7 Histogram of Infiltration Values New
  Construction
• Figure 5-8 Histogram of Infiltration Values
  Low-Income Construction
• Commercial building envelops are thought to be
  nearly airtight.
• At 0.30 in water 0.1 to 0.6 ACH
• Regression equation for rate (I ACH) based on
  Dt and V
• I K1 K2 Dt K3 V
• Not appropriate for simulations
• Multi-cell modeling complexity
• Residential single zone effective leakage area
  at 0.016 inch water
• Q AL (CS Dt CW V2)0.5
• Cw based on shielding class (Table 5-4) wind
  coefficient Table 5-6
• Cs values based on number of stories (Table 5-5)
  stack coefficient
• Commercial doors different characteristics
  swinging door Figure 5-9
• Figure 5-10 through 5-14 different styles, with
  traffic, etc.

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Chapter 5 Design Conditions

• Outdoor Air Load Components

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Chapter 5 Design Conditions

• Outdoor Air Load Components

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Chapter 5 Design Conditions

• Outdoor Air Load Components

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Chapter 5 Design Conditions

• Outdoor Air Load Components

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Chapter 5 Design Conditions

• Outdoor Air Load Components.
• Ventilation Air
• Acceptable Indoor Air Quality (IAQ) debated for
  years
• Control of moisture, carbon dioxide, odors,
  tobacco smoke
• Additional pollutants not generated by occupants
  Table 5-7
• Ventilation rate procedure meet outside air
  quality standards Table 5-8
• Ventilation effectiveness ability to remove
  internally generated pollutants
• See Table 5-9 Minimum Ventilation Rates in
  Breathing Zone
• Indoor air quality procedure amount of outdoor
  air ay be reduced by recirculating air which
  offending contaminates have been removed or
  converted to less objectionable forms
• Figure 5-15
• Nomenclature (C contaminant concentration, Fr
  reduction factor, N contaminate generation
  rate, R recirculation rate)
• Filter placement
• Variable air volume (VAV) systems
• See Figure 5-16 for required outdoor air with
  recirculation and filtration.
• See Example 5-3 Chicago fast-food cafeteria

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Chapter 5 Design Conditions

• Outdoor Air Load Components

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Chapter 5 Design Conditions

• Outdoor Air Load Components

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Chapter 5 Design Conditions

• Outdoor Air Load Components

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Chapter 5 Design Conditions

• Outdoor Air Load Components

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Chapter 5 Design Conditions

• Outdoor Air Load Components

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Chapter 5 Design Conditions

• Outdoor Air Load Components

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Chapter 5 Design Conditions

• Outdoor Air Load Components

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Chapter 5 Design Conditions

• Outdoor Air Load Components

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Chapter 5 Design Conditions

• Outdoor Air Load Components

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Chapter 5 Design Conditions

• Outdoor Air Load Components

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Chapter 5 Design Conditions

• Outdoor Air Load Components - Example

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Chapter 5 Design Conditions

• Outdoor Air Load Components - Example

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Chapter 5 Design Conditions

• Heat Transfer Coefficients
• Modes of heat transfer (driven by DT)
• Conduction transfer of heat through molecular
  motion
• Convection enhancement of conduction through
  motion
• Radiation electromagnetic radiation between
  surfaces at DT
• U-Factor or R-value resistance to flow of
  energy
• Thermal resistance of building materials (Table
  5-11)
• Apparent thermal conductivity

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Chapter 5 Design Conditions

• Heat Transfer Coefficients
• Calculation of U requires
• Apparent thermal conductivity and thickness of
  material
• Thermal conductance of non-homogeneous material
• Surface conductance of surfaces
• Conductance of air spaces
• Surface Conductance
• Combined effects of radiation, convection, and
  conduction
• Surface properties (reflectivity, emissivity,
  roughness)
• Geometry, viewing angles,
• Flow behavior, etc.
• Other factors
• Workmanship
• Air-tightness
• Irregular areas, shading
• Shrinkage, settling, compression,etc.

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Chapter 5 Design Conditions

• R-Values - materials

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Chapter 5 Design Conditions

• Surface Conductances and Resistances

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Chapter 5 Design Conditions

• Emittance Values of Surfaces and Airspaces

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Chapter 5 Design Conditions

• Thermal Resistances of Plane Airspaces

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Chapter 5 Design Conditions

• Thermal Properties of Common Building Materials
  (pgs. 123-127)

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Chapter 5 Design Conditions

• Determining U-factors
• Total resistance (both conductances and air film)
  U 1/Rtot
• Parallel heat paths
• Add the U factors (on a percentage basis)
• Unequal areas (pitched roof and ceiling areas)
• Windows and doors - Tables 5-16 and 5-17

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Chapter 5 Design Conditions

• Determining U-factors

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Chapter 5 Design Conditions

• Determining U-factors fenestration products

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Chapter 5 Design Conditions

• Determining U-factors - Doors

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Chapter 5 Design Conditions

• The Overall Thermal Transmittance

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Chapter 5 Design Conditions

• Calculating Surface Temperatures
• Voltage divider network due to resistances
  driving DT
• Account for temperature effect on the resistance
  of material
• Look at example of structure wall

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Chapter 5 Design Conditions

• Calculating Surface Temperatures

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Chapter 5 Design Conditions

• Example 5-3 Determining Uoverall

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Chapter 5 Design Conditions

• Example 5-5 Determining Uoverall

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Chapter 5 Design Conditions

• Example 5-5 Determining Uoverall

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Chapter 5 Design Conditions

• Example 5-5 Determining Uoverall

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Chapter 5 Design Conditions

• Example 5-5 Determining Uoverall