Design of HVAC Systems

8/3/2019 Design of HVAC Systems

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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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Documents

Design of HVAC Systems