fundamentals of hvac&r part 2 by: sam c. m....

Joint Comprehensive Certificate Course on HVAC&R System, 2012

2012年度暖通空調及製冷系統綜合証書課程

Fundamentals of HVAC&R Part 2

Presented by:Ir Dr. Sam C. M. Hui

March 1, 2012

Joint Comprehensive Certificate Course on HVAC&R System, 2012Feb-Apr 2012

Dr. Sam C. M. HuiDepartment of Mechanical Engineering

The University of Hong KongE-mail: [email protected]

Fundamentals of HVAC&R Part 2

Contents

• Air-side systems

• Air duct design

• Space air diffusion

• Ventilation design

Air-side systems

• To better understand HVAC systems, they can be divided into five subsystems or loops:• Air-side• Chilled water• Refrigeration equipment• Heat rejection• Controls

13 oC 13 oC

12 oC

7 oC 3 oC38 oC

49 oC10 oC

35 oC

29 oC

33 oC, 28 oC

7 oC

12 oC

29 oC

35 oC

27 oC33 oC

25 oC

Control Loop

Hea

t rej

ectio

n

Ref

riger

atio

nSy

stem

Chi

lled

Wat

erSy

stem

Airs

ide

Syst

em

Can you describe each component?

Air-side systems

• The fluid = AIR• Fluid properties

• Air density = 1.204 kg/m3

• Air specific volume = 0.831 m3/kg• Specific heat (Cp) = 1.0 kJ/kg.K

• Fluid pressure• Static pressure/head• 1 standard atmospheric pressure = 101.325 kPa (≈1 bar)• Absolute pressure & gauge pressure

It is useful to remember some typical data of air. Velocity profile

What is the difference between laminar and

turbulent flow?

Air-side systems

• Duct pressure changes (c.f. atm pressure)• Static pressure (SP), Pa• Velocity pressure (VP), Pa = ρV2 / 2• Total pressure (TP), Pa = SP + VP

• Fan: a pumping device• Fan (total) pressure = pressure difference between

fan inlet and fan discharge• At fan suction/inlet, SP = negative (c.f.

atmospheric); at discharge, SP = positive

Air-side systems

• Fans• Fan types

• Centrifugal fans: radial, forward curved, air foil (backward curved), backward inclined, tubular, roof ventilator

• Axial fans: propeller, tube-axial, vane-axial• Arrangement

• Motor location, air discharge orientation, drive train type (direct drive or pulley drive)

• Centrifugal: single width single inlet (SWSI), double width double inlet (DWDI)

Centrifugal and axial fan components

Propeller Tube-axial

Tube-vane

AXIAL FANS

Can you suggest where are they

being used?

Tubular centrifugal fan Centrifugal roof ventilator

Air-side systems

• Fan performance• Fan volume flow rate (m3/s or l/s), Vf• Fan total pressure Δptf, fan velocity pressure pvf &

fan static pressure Δpsf (Pa)• Fan power & efficiency

• Fan power or air power (W) = Δptf x Vf• Fan power input on the fan shaft (brake horsepower), Pf• Fan total efficiency: ηt = Δptf x Vf / Pf

• Combined index of aerodynamic, volumetric & mechanical efficiencies

• Air temp. increase through fan, ΔTf = Δptf /(ρcpaηt)

Typical fan performance curve

Performance curves for

controllable-pitch vane-axial

fans

Air-side systems

• Fan-duct systems• Flow resistance R, pressure drop Δp and volume flow

rate V

• Duct sections in series:• Duct sections in parallel:

2VRp o

ns RRRR 21

np RRRR1111

21

Air-side systems

• Fan Laws• Speed (n)• Volume flow (V)• Total pressure loss (Δp )• Air density (ρ)• For air systems that are

geometrically & dynamically similar: (D = impeller diameter)

Air-side systems

• System effect Δpts• Its additional total pressure loss caused by uneven

or non-uniform velocity profile at the fan inlet, or at duct fittings after fan outlet

• Due to the actual inlet and outlet connections as compared with the total pressure loss of the fan test unit during laboratory ratings

Inlet Outlet

Inlet swirl

Outlet duct

Inlet conditions

(Source: Air Movement and Control Association (AMCA))

Simple air-handling unit (AHU)

A typical AHU with unhousedplug/plenum return fan

Air-side systems

• Main components of AHU• Casing• Fans• Coils• Filters• Humidifiers (optional)• Outdoor air intake, mixing & exhaust section• Controls

Water cooling coil

Low efficiency (panel-type) Medium efficiency (bag-type)

HEPA and ULPA filters Activated carbon filter

Air duct design

• Types of air duct• Supply air duct• Return air duct• Outdoor air duct• Exhaust air

• Duct sections• Header or main duct (trunk)• Branch duct or runout

Rectangular duct Round duct w/ spiral seam

Flat oval duct Flexible duct(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)

Transverse joint reinforcement(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)

Air duct design

• Duct specification• Sheet gauge and thickness of duct material• Traverse joints & longitudinal seam

reinforcements• Duct hangers & their spacing• Tapes & adhesive closures• Fire spread and smoke developed• Site-fabricated or factory-fabricated

Air duct design

• Frictional losses: Darcey-Weisbach Equation• Hf = friction head loss, or Δpf = pressure loss

• f = friction factor (dimensionless)• L = length of duct or pipe (m)• D = diameter of duct or pipe (m)• v = mean air velocity in duct (m/s)• g = gravitational constant (m/s2)• ρ = density of fluid (kg/m3)• g c = dimensional constant, for SI unit, g c = 1

Friction chart for round duct(Source: ASHRAE Handbook Fundamentals 2001)

Air duct design

• Circular equivalent• Hydraulic diameter, Dh = 4 A / P

• A = area (mm2); P = perimeter (mm)• Rectangular duct:

• Flat oval duct:

Air duct design

• Dynamic losses• Result from flow disturbances caused by duct-

mounted equipment and fittings• Change airflow path’s direction and/or area• Flow separation & eddies/disturbances

• In dynamic similarity (same Reynolds number & geometrically similar duct fittings), dynamic loss is proportional to their velocity pressure Region of eddies and

turbulences in a round elbow 5-piece 90o round elbow

(Source: ASHRAE Handbook Fundamentals 2001)

Rectangular elbow, smooth radius, 2 splitter vanes

Mitered elbow and its secondary flow(Source: ASHRAE Handbook Fundamentals 2001)

Airflow through arectangular converging

or diverging wye

(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)

Abrupt enlargement Sudden contraction


Air duct design

• Duct layout• Symmetric layout is easier to balance

• Smaller main duct & shorter design path• For VAV systems, duct looping allows feed from

opposite direction• Optimise transporting capacity (balance points often

follow the sun’s position)• Result in smaller main duct

• Compare alternative layouts & reduce fittings• For exposed ducts, appearance & integration with

the structure is important

Typical supply duct system with symmetric layout & looping


Air duct design

• Duct sizing methods• Equal-friction method with maximum velocity

• Duct friction loss per unit length remains constant• Most widely used in normal HVAC applications

• Constant-velocity method• Often for exhaust ventilation system• Minimum velocity to carry dust is important• Limit velocity to reduce noise

Air duct design

• Duct sizing methods• Static regain method

• Normally used with a computer package for high velocity systems (e.g. in main duct)

• Size air duct so that ↑static pressure nearly offset the pressure loss of succeeding duct section along main duct

• T method• Optimising procedure by minimising life-cycle cost

• System condensing (into a single imaginary duct)• Fan selection (optimum system pressure loss)• System expansion (back to original duct system)

Concept of static regain method(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)

Air duct design

• Design information required• Client requirements• Required supply air condition• Type of system supplied• Ambient conditions• Duct material• Duct insulation• Duct system layout

Air duct design

• Key design inputs• Design volume flow rate (m3/s)• Limiting duct pressure loss (Pa/m)• Limiting flow velocity (m/s)

• Design outputs• Schematic of ductwork layout & associated plant• Schedule of duct sizes and lengths, and fittings

Space air diffusion

• Objective of space air diffusion• Evenly distribute conditioned & outdoor air to

provide healthy & comfortable indoor environment, or appropriate environment for process, at optimum cost

• Last process of air conditioning• Take place entirely within conditioned space• Directly affect the occupants, but it is difficult to

trace & quantify

Space air diffusion

• Important considerations:• Thermal comfort (temp., humidity, air velocity)

• Comfort conditions, local variations• Indoor air quality

• Airborne pollutants• Ventilation effects

• Noise control• Noise criteria, sound attentuation

• Occupied zone: 1.8 m from floor

Occupiedzone

(Source: Rock, B. A. and Zhu, D., 2002. Designer’s Guide to Ceiling-based Air Diffusion.)

Space air diffusion

• Draft & effective draft temperature• Draft: unwanted local cooling of human body

caused by air movement & lower space air temp.• Turbulence intensity, Itur = σv / vm

• σv = standard deviation of air velocity fluctuation (m/s)• vm = mean air velocity (m/s)

• Effective draft temperature: combines effects of uneven space air temp. & air movement

• θ = Tx – Tr – a (vx – vrm)

Thermal Comfort


Space air diffusion

• Air diffusion performance index (ADPI)• ADPI = (Nθ x 100) / N

• θ: effective draft temperature• Nθ: number of points measured in occupied zone in which -1.7 oC < θ <1.1 oC

• N : total number of points measured in occupied zone

• Higher the ADPI, higher % of occupants who feel comfortable

• ADPI is useful for cooling mode operation• For heating mode, temperature gradient ٪ 2 points may be a

better indicator of thermal comfort (< 2.8 oC typical)

Space air diffusion

• Air exchange rate• = Volume flow rate / interior volume• Unit: L/s or air change per hour (ACH)• May consider outside air, or supply air

• Time constant (τ)• Inverse of air exchange rate

• Air diffusion effectiveness• Perfectly mixing, perfectly displacing• Degree of effectiveness of air diffusion


Surface effect (or Coandă effect)

(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration) (Source: Rock, B. A. and Zhu, D., 2002. Designer’s Guide to Ceiling-based Air Diffusion.)


Primaryair

Secondaryair

Total air


(Source: Rock, B. A. and Zhu, D., 2002. Designer’s Guide to Ceiling-based Air Diffusion.) (Source: Rock, B. A. and Zhu, D., 2002. Designer’s Guide to Ceiling-based Air Diffusion.)


Space air diffusion

• Supply outlets• Grilles and registers• Ceiling diffusers• Slot diffusers• Nozzles

• Return & exhaust inlets• Light troffer diffuser & troffer-diffuser slot• Design issues: architectural setup, airflow pattern

needed, indoor requirements, load conditions

Supply grille and register


Ceiling diffusers

Square & rectangular

Removable inner-corePerforated ceiling diffuser


Slot diffusers(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)

Round nozzle Nozzle diffuser


Light troffer, slot diffuser and return slot combination(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)

Space air diffusion

• Five typical airflow patterns• Mixing flow (most common)• Displacement flow• Upward flow• Unidirectional flow• Projecting flow

• Principles of mixing flow• Supply air thoroughly mixed with ambient air• Occupied zone is dominated by induced recirculating flow• Creates relatively uniform air velocity, temperature,

humidity, and air quality

Mixing flow using high side outlets

Will the air jet enter the occupied zone?Will stagnant zone be formed?


Mixing flow using ceiling diffusers(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)

Stratified mixing flow in a large indoor stadium using supply nozzles

(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration) Hong Kong International Airport

Displacement flow characteristics(Source: http://www.price-hvac.com)

Upward flow underfloor air distribution system

Unidirectional flow for clean rooms(Source: Wang, S. K., 2001. Handbook of Air Conditioning and Refrigeration)

Hospital operating theatre (laminar flow with air curtains)(Source: http://www.price-hvac.com)

Industrial spot cooling system Desktop task air conditioning


Ventilation design

• What is Ventilation (通風)?• The process by which fresh air is introduced and

ventilated air is removed• Primary aim: to preserve the qualities of air

• May also be used to lower temperature & humidity

• Natural ventilation• By means of purpose-provided aperture (such as

openable windows, ventilators and shafts) and the natural forces of wind and temperature-difference pressures

Natural ventilation (e.g. cross ventilation 穿堂風)

(Source: Environmental Protection Department)

Ventilation design

• Mechanical (or forced) ventilation• By means of mechanical devices, such as fans• May be arranged to provide either supply, extract

or balanced ventilation for an occupied space

• Infiltration (air leakage into a building)• Uncontrolled random flow of air through

unintentional openings driven by wind, temperature-difference pressures and/or appliance-induced pressures across the building envelope

Mechanical ventilation (extract ventilation)


What will happen if the windows are closed?

1. Outdoor air2. Supply air3. Indoor air4. Transferred air5. Extract air6. Recirculation air7. Exhaust air8. Secondary air9. Leakage10. Infiltration11. Exfiltration12. Mixed air

Analysis of air flow

Ventilation design

• Ventilation for supporting life• Maintain sufficient oxygen in the air• Prevent high concentration of carbon dioxide• Remove odour, moisture & pollutants

• Poor ventilation and indoor air quality• Impact on human health & productivity

• CO2 as an index of air quality• < 1,000 ppm, corresponds to fresh air 7 l/s/person• < 800 ppm, corresponds to fresh air 10 l/s/person

Ventilation to remove pollutants and moisture


* Remember, source control is usually more effective than ventilation

Ventilation system design should avoid intake of vehicle exhaust


* Also ensure outdoor air intake is of adequate quality

Ventilation design

• Purposes of ventilation• Maintain human comfort and health• Provide sufficient air/oxygen for human/livestock• Provide sufficient air/oxygen for processes• Remove products of respiration and bodily odour• Remove contaminants or harmful chemicals• Remove heat generated indoor• Create air movement (feeling of

freshness/comfort)

Ventilation design

• Ventilation calculations• For general mechanical ventilation:

• Ventilation Rate (m3/h) = Air Change Rate (/h) x Room Volume (m3)

• Ventilation Rate (m3/s) = Ventilation Rate (m3/h) / 3600

• For calculating fresh air ventilation rates• Fresh air rate (m3/s) = Fresh air rate per person (l/s/p) x

number of occupants

• Ventilation effectiveness• Depend on ventilation strategy, air distribution

method, room load & air filtrationChanges in the minimum ventilation rates in the USA

Current: ASHRAE 62.1-2010 (10 L/s/person)

(Source: ASHRAE)

Ventilation design

• Determine the required ventilation rate (Q):• (a) Maximum allowable concentration of

contaminants (Ci)• Ci = Co + F / Q

• (b) Heat generation inside the space (H)• Q = H / [cp x ρ x (Ti – To)]

• (c) Air change rates (ACH)• Q = V x ACH / 3.6

Ventilation design

• Mechanical ventilation• Movement of air through a building using fan

power• Ability to control the air flows

• Two types:• Unbalanced systems

• Air is either supplied or extracted• Balanced systems

• Air is supplied and extracted simultaneously

Ventilation design

• Design principles:1. Exhaust close to pollutant

generation2. Effective local extracts3. Supply to the breathing zone4. Supply air to clean areas5. More extract from “dirty”

areas6. Transfer air from “clean” to

“dirty” areas

Ventilation design

• Extract ventilation, e.g.• Commercial kitchens• Toilets and bathrooms• Underground carparks• Factories or industrial buildings• Localised industrial extraction

• Supply ventilation• Can be used to ensure adequate supply of outside

air, e.g. in boiler house ventilation

Kitchen hood

Makeup air

Dinning area

Supply airReturn air

Makeup air unitHVAC unit Kitchen exhaust

Kitchen

Example of kitchen ventilation system

Ventilation design

• Industrial ventilation• An important method for reducing employee

exposures to airborne contaminants• Dilution systems:

• Reduce the concentrations of contaminants released in a work room by mixing with air flowing through the room

• Local exhaust ventilation (LEV):• Capture or contain contaminates at their source before

they escape into the workplace environment

Ventilation design

• Supply and extract (balanced) systems• Central air handling unit (AHU) with separate

supply and extract fans• A heat recovery device can also be incorporated

• Energy implications & efficient ventilation• Heat recovery• Demand controlled ventilation (DCV)• User control ventilation• Ventilation system balancing

Ventilation design

• Hybrid ventilation (or mixed mode ventilation)• = Natural ventilation + Mechanical ventilation• Use them at different time of the day or season of

the year• Usually have a control system to switch between

natural and mechanical modes• Combine the advantages of both to satisfy the

actual ventilation needs and minimise energy consumption

Hybrid ventilation concepts

Low pressure mechanical ventilation

Demand-controlled mechanical ventilation

Constant air flow mechanical ventilation

Mechanical ventilationNatural ventilation

Air infiltration through cracks

Openable windows, supply and exhaust

grilles

Self-regulating supply and exhaust grilles

Demand-controlled natural ventilation

Is it feasible to use Natural Ventilation?

If situation prevents this,is it feasible to use

Mechanical Ventilation?


Hybrid/Mixed Mode Ventilation?


Cooling and Heating (without humidity control)?


Full Air Conditioning(with humidity control)?

Increasing:• energyconsumption• capital cost• running costs• maintenance• complexity

Ventilation design hierarchy

fundamentals of hvac&r part 2 by: sam c. m....

Documents