Contents. Air-side systems. Fundamentals of HVAC&R Part 2 Presented by: Ir Dr. Sam C. M. Hui

Transcription

1 Joint Comprehensive Certificate Course on HVAC&R System, 2012 Feb-Apr 2012 Joint Comprehensive Certificate Course on HVAC&R System, Fundamentals of HVAC&R Part 2 Presented by: Ir Dr. Sam C. M. Hui March 1, 2012 Fundamentals of HVAC&R Part 2 Dr. Sam C. M. Hui Department of Mechanical Engineering The University of Hong Kong cmhui@hku.hk Contents Air-side systems Air duct design Space air diffusion 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 33 o C, 28 o C Control Loop Can you describe each component? 33 o C 27 o C 25 o C Airside System 12 o C 12 o C Chilled Water System 10 o C 7 o C 7 o C 3 o C Refrigeration System 49 o C 38 o C 35 o C Heat rejection 29 o C 35 o C 29 o C 13 o C 13 o C

2 Air-side systems The fluid = AIR Fluid properties Air density = kg/m 3 Air specific volume = m 3 /kg Specific heat (C p ) = 1.0 kj/kg.k Fluid pressure Static pressure/head 1 standard atmospheric pressure = kpa ( 1 bar) Absolute pressure & gauge pressure What is the difference between laminar and turbulent flow? It is useful to remember some typical data of air. Velocity profile Air-side systems Duct pressure changes (c.f. atm pressure) Static pressure (SP), Pa Velocity pressure (VP), Pa = ρv 2 / 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) AXIAL FANS Propeller Tube-axial Centrifugal and axial fan components Tube-vane Can you suggest where are they being used?

3 Air-side systems Tubular centrifugal fan Centrifugal roof ventilator Fan performance Fan volume flow rate (m 3 /s or l/s), V f Fan total pressure Δp tf, fan velocity pressure p vf & fan static pressure Δp sf (Pa) Fan power & efficiency Fan power or air power (W) = Δp tf x V f Fan power input on the fan shaft (brake horsepower), P f Fan total efficiency: η t = Δp tf x V f / P f Combined index of aerodynamic, volumetric & mechanical efficiencies Air temp. increase through fan, ΔT f = Δp tf /(ρc pa η t ) Performance curves for controllablepitch vane-axial fans Typical fan performance curve Air-side systems Fan-duct systems Flow resistance R, pressure drop Δp and volume flow rate V o p Duct sections in series: Duct sections in parallel: 1 R R V R 1 2 R R s 1 2 R n R 1 R p 1 2 R n 1 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)

4 Air-side systems Inlet conditions System effect Δp ts 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 swirl Outlet duct Inlet Outlet (Source: Air Movement and Control Association (AMCA)) A typical AHU with unhoused plug/plenum return fan Simple air-handling unit (AHU) Air-side systems Main components of AHU Casing Fans Coils Filters Humidifiers (optional) Outdoor air intake, mixing & exhaust section Controls Water cooling coil

5 Air duct design HEPA and ULPA filters Low efficiency (panel-type) Activated carbon filter Medium efficiency (bag-type) 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 Transverse joint reinforcement 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 H f = friction head loss, or Δp f = 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/s 2 ) ρ = density of fluid (kg/m 3 ) g c = dimensional constant, for SI unit, g c = 1

6 Air duct design Circular equivalent Hydraulic diameter, D h = 4 A / P A = area (mm 2 ); P = perimeter (mm) Rectangular duct: Flat oval duct: Friction chart for round duct (Source: ASHRAE Handbook Fundamentals 2001) Air duct design Dynamic losses Result from flow disturbances caused by ductmounted 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 90 o round elbow (Source: ASHRAE Handbook Fundamentals 2001) Rectangular elbow, smooth radius, 2 splitter vanes Airflow through a rectangular converging or diverging wye Mitered elbow and its secondary flow (Source: ASHRAE Handbook Fundamentals 2001)

7 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 Air duct design Typical supply duct system with symmetric layout & looping 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

8 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 (m 3 /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 Space air diffusion Occupied zone Thermal Comfort Draft & effective draft temperature Draft: unwanted local cooling of human body caused by air movement & lower space air temp. Turbulence intensity, I tur = σ v / v m σ v = standard deviation of air velocity fluctuation (m/s) v m = mean air velocity (m/s) Effective draft temperature: combines effects of uneven space air temp. & air movement θ = T x T r a (v x v rm ) (Source: Rock, B. A. and Zhu, D., Designer s Guide to Ceiling-based Air Diffusion.)

9 Space air diffusion Air diffusion performance index (ADPI) ADPI = (N θ x 100) / N θ: effective draft temperature N θ : number of points measured in occupied zone in which -1.7 o C < θ <1.1 o C 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 o C 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 (Source: Rock, B. A. and Zhu, D., Designer s Guide to Ceiling-based Air Diffusion.) Surface effect (or Coandă effect) (Source: Rock, B. A. and Zhu, D., Designer s Guide to Ceiling-based Air Diffusion.)

10 Total air Primary air Secondary air (Source: Rock, B. A. and Zhu, D., Designer s Guide to Ceiling-based Air Diffusion.) (Source: Rock, B. A. and Zhu, D., Designer s Guide to Ceiling-based Air Diffusion.) (Source: Rock, B. A. and Zhu, D., Designer s Guide to Ceiling-based Air Diffusion.) (Source: Rock, B. A. and Zhu, D., 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 (Source: Rock, B. A. and Zhu, D., Designer s Guide to Ceiling-based Air Diffusion.)

11 Square & rectangular Supply grille and register Perforated ceiling diffuser Removable inner-core Ceiling diffusers Round nozzle Nozzle diffuser Slot diffusers Space air diffusion Light troffer, slot diffuser and return slot combination 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

12 Will the air jet enter the occupied zone? Will stagnant zone be formed? Mixing flow using high side outlets Mixing flow using ceiling diffusers Stratified mixing flow in a large indoor stadium using supply nozzles Hong Kong International Airport Displacement flow characteristics (Source: Upward flow underfloor air distribution system

13 Unidirectional flow for clean rooms Hospital operating theatre (laminar flow with air curtains) (Source: Industrial spot cooling system Desktop task air conditioning What is Ventilation ( )? 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 ) 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 applianceinduced pressures across the building envelope (Source: Environmental Protection Department)

14 Mechanical ventilation (extract ventilation) Analysis of air flow 1. Outdoor air 2. Supply air 3. Indoor air 4. Transferred air 5. Extract air 6. Recirculation air 7. Exhaust air 8. Secondary air 9. Leakage 10. Infiltration 11. Exfiltration 12. Mixed air (Source: Environmental Protection Department) What will happen if the windows are closed? Ventilation to remove pollutants and moisture 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 CO 2 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 * Remember, source control is usually more effective than ventilation (Source: Environmental Protection Department) Ventilation system design should avoid intake of vehicle exhaust * Also ensure outdoor air intake is of adequate quality 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) (Source: Environmental Protection Department)

15 Ventilation calculations For general mechanical ventilation: Ventilation Rate (m 3 /h) = Air Change Rate (/h) x Room Volume (m 3 ) Ventilation Rate (m 3 /s) = Ventilation Rate (m 3 /h) / 3600 For calculating fresh air ventilation rates Fresh air rate (m 3 /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 filtration (Source: ASHRAE) Current: ASHRAE (10 L/s/person) Changes in the minimum ventilation rates in the USA Determine the required ventilation rate (Q): (a) Maximum allowable concentration of contaminants (C i ) C i = C o + F / Q (b) Heat generation inside the space (H) Q = H / [c p x ρ x (T i T o )] (c) Air change rates (ACH) Q = V x ACH / 3.6 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 Design principles: 1. Exhaust close to pollutant generation 2. Effective local extracts 3. Supply to the breathing zone 4. Supply air to clean areas 5. More extract from dirty areas 6. Transfer air from clean to dirty areas 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

16 HVAC unit Example of kitchen ventilation system Makeup air unit Kitchen exhaust Return air Supply air Dinning area Makeup air Kitchen Kitchen hood 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 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 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 Natural ventilation Air infiltration through cracks Openable windows, supply and exhaust grilles Self-regulating supply and exhaust grilles Demand-controlled natural ventilation Hybrid ventilation concepts Mechanical ventilation Constant air flow mechanical ventilation Demand-controlled mechanical ventilation Low pressure mechanical ventilation Is it feasible to use Natural Ventilation? If situation prevents this, is it feasible to use Mechanical Ventilation? If situation prevents this, is it feasible to use Hybrid/Mixed Mode Ventilation? If situation prevents this, is it feasible to use Cooling and Heating (without humidity control)? If situation prevents this, is it feasible to use Full Air Conditioning (with humidity control)? hierarchy Increasing: energy consumption capital cost running costs maintenance complexity