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Design of HVAC Systems
By
Priyantha Bandara
Senior Lecturer
Department of Manufacturing Technology
University of Vocational Technology
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Overview
Overall HVAC Design Process
Thermal Comfort
Sources of HVAC Loads?
Estimation of HVAC Loads
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Overall HVAC Design Process
Locate Forced AirUnits
Locate Grilles &Registers
Route Ducts
Sub-Zones(Trunks)
Static Pressure
Total Flow Rate
EquivalentLengths
Friction Rates
Roo
ma
irflow
isproportio
naltorooml
oad
Fric
tionrate&rooma
irflo
w
determineductsize
LocateThermostat
LocateCondenser
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Thermal Comfort
Human thermal comfort is defined byASHRAE as the state of mind that expresses
satisfaction with the surrounding environment(ASHRAE Standard 55).
Maintaining thermal comfort for occupants of
buildings or other enclosures is one of theimportant goals of HVAC design engineers.
http://en.wikipedia.org/wiki/ASHRAEhttp://en.wikipedia.org/wiki/HVAChttp://en.wikipedia.org/wiki/HVAChttp://en.wikipedia.org/wiki/ASHRAE -
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Thermal Comfort contd..
Thermal comfort is maintained when the heatgenerated by human metabolism is allowed to
dissipate, thus maintaining thermal equilibrium withthe surroundings. Any heat gain or loss beyond thisgenerates a sensation of discomfort.
Thermal comfort is very important to many work-
related factors. It can affect the distraction levels ofthe workers, and in turn affect their performance andproductivity of their work.
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Thermal Comfort Factors
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Sources of HVAC Loads
Human-occupancy loads
Weather-dependent loads Process, appliance and mechanical
equipment loads
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Human-occupancy loads
Human body generates heat energy withinitself and releases it by radiation, convection
and evaporation from the body surface(sensible) and by convection and evaporationin the respiratory tract (latent). The amount of
heat generated and released depends onsurrounding temperature and on the activitylevel of the person.
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Heat Gain from Activity Levels
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Weather Dependent Loads
Weather dependent loads are due to
Heat loss/gain due to indoor-outdoor
temperature difference Heat loss/gain due to solar-night time
radiation through openings and facades.
Heat-humidity loss/gain by ventilation Thermal resistance across the solid and
interface
Wind effects and infiltration
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Process, appliance andmechanical equipment loads
Lights Illuminants convert electricalenergy into light energy and sensible
heat. Lighting is either incandescent orfluorescent.
Electrical & Electronic Appliances
Motors
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Heat Gain from Equipment
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Sources of HVAC Loads
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Estimation of HVAC Loads
Provide information for equipment selection,system sizing and system design.
Provide data for evaluating the optimumpossibilities for load reduction.
Permit analysis of partial loads as required
for system design, operation and control.
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Estimation of HVAC Loads contd..
Transfer Function Method (TFM): This is the most complexof the methods proposed by ASHRAE and requires the useof a computer software or advanced spreadsheet.
Cooling Load Temperature Differential/Cooling Load
Factors (CLTD/CLF): This method is derived from the TFMmethod and uses tabulated data to simplify the calculationprocess. The method can be fairly easily transferred intosimple spreadsheet programs but has some limitations dueto the use of tabulated data.
Total Equivalent Temperature Differential/Time-Averaging(TETD/TA): This was the preferred method for manual orsimple spreadsheet calculation before the introduction of
the CLTD/CLF method.
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Estimation of HVAC Loads contd..
Fabric Heat Transfer
Ventilation Heat Transfer
Solar Irradiation
Equipment Loads
Occupancy Loads
Infiltration Loads
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Fabric Heat Transfer
This is caused by the transmission of heatthrough building elements such as walls, roof,
windows, floor etc. Governed by Fouriers Law of Heat Conduction.
Depends on the Overall Heat Transfer
Coefficient (U value) of the material.
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Fabric Heat Transfer contd..
TUAQf
Qf = Rate of fabric heat transfer (W)U = Overall heat transfer coefficient of the
element considered (W/m2K)
A = Cross sectional area of the element (m2)
T = Temperature difference across theelement (K)
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Elemental Heat Gains
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Ventilation Heat Transfer
3600
TNVC
QV
V
Qv = Rate of ventilation heat transfer (W)
Cv = Volumetric specific heat capacity of air(J/m3K) = 1300 J/m3K
N = Number of complete air changes per hour
V= Volume of the conditioned space (m3)
T = Temperature difference between inside
and outside air (K)
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Ventilation Heat Transfer contd..
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Gain from Solar Irradiation
Geographical latitude of the location
Orientation of the building
Season of the year
Local cloud conditions
Angles between Sun and the building surfaces
Material properties of building elements
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Heat Gain from Solar Irradiation
Solar Gain through transparent surfaces(windows) can be estimated as
AIQS
Qs = Rate of solar gain (W)
I = Radiation heat flux density (W/m2)
A = Surface area of the element (m2)
= Solar gain factor of the window glass
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Infiltration Load
iompooiompos TTcvTTcmQ ,
.
,
.
inf,
Rate of Sensible Heat Transfer (W)
Vo = Infiltration rate (m3/s)
o = Density of moist infiltrated air (kgm-3)
Cp,m = Specific heat of moist infiltrated air (J/kgK)
To = Outdoor dry bulb temperature (K)
Ti= Indoor dry bulb temperature (K)
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Infiltration Load contd..
iofgooiofgol WWhvWWhmQ ..
inf,
Rate of Latent Heat Transfer (W)
hfg = Latent heat of vapourization of water (J/kg)
Wo = Outdoor humidity ratio
Wi = Indoor humidity ratio
3600
.. VACHvo
Infiltration rate by air change method
ACH = No. of air changes per hour
V = Gross volume of the conditioned space (m3)
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Infiltration Load contd..
inf,inf,inf ls QQQ
Total heat load due to infiltration (Qinf)
Qs,inf = Rate of Sensible heat transfer (W)
Ql,inf = Rate of Latent heat transfer (W)
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HVAC Equipment Capacity
infQQQQQQQ oesvfT
QT = Total thermal load (W) = HVAC EquipmentCapacity (W or BTU/h)
Qf = Rate of fabric heat transfer (W)
Qv = Rate of ventilation heat transfer (W)
Qs= Rate ofSolar gain (W)
Qe= Equipment heat load (W)
Qo= Occupancy heat load (W)
Qinf = Infiltration heat load (W)
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HVAC Equipment Capacity
Based on the heat load calculations, theHVAC designer recommends the type ofthe HVAC system suitable for theapplication and the total size of thesystem. This helps in avoiding installingan over-sized system that can lead tohigh initial and running costs and also
avoids under-sized system that couldlead to under-cooling of the building.
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Dimensions and UnitsUsed in HVAC Applications
Dimension SI Unit IP UnitAcceleration m/s2 ft/sec3Area m2 ft2Density kg/m3 lbm/ft3Energy Nm, Joule (J) Btu, ft-lbForce (kgm)/s2, Newton (N) pound (lbf)Length m, meter (m) foot (ft)Mass kg, kilogram (kg) pound mass (lbm)Power J/s, Watt (W) Btu/hPressure N/m2, Pascal (P) psiSpecific Heat J/(kgC) Btu/(lbmF)Time second (s) second (sec)Absolute Temperature degree Kelvin (K) degree Rankine (R)
Temperature degree Celsius (C) degree Fahrenheit (F)Thermal Conductivity W/(mC) Btu/(hftF)Thermal Flux Density W/m2 Btu/(hft2)Velocity m/s ft/sec, ft/min, fpmVolume m3 ft3Volume Flow Rate m3/s ft3/sec, ft3/min, cfm
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Design Criteria for Ventilation
Systems Duct design in accordance with the HVAC Duct
Work Specifications, published by Heating &Ventilation Contractors Association or inaccordance with the ASHRAE Handbook.
Sections of the duct may be rectangular orcircular conforming to the preliminary drawings.
Duct assembly shall be air tight as to allowleakage of not more than 1% of the total flowrate.
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Ventilation Duct Systems
Type ofDuctwork
MaximumStatic Air
Pressure(kPa)
Maximumallowable
Air Velocity(ms-1)
Low Pressure 0.5 13
High Pressure 0.75 25.0 10
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