FME 706/FML 2007- 08Air Conditioning1 AIR CONDITIONING.

FME 706/FML 2007- 08 Air Conditioning 1

AIR CONDITIONING


SCOPE AND USE OF AIR CONDITIONING

Not restricted to cooling only but might include:o Control of temperature at all times by heating or coolingo Control of air humidity by humidification or dehumidificationo Control of air movement at a desirable velocityo Introduction of outdoor air as requiredo Control of air quality by removal of dirt particles and odorous gaseso Control of sound generated by the air conditioning equipment

Environmental control Used for two purposes:

o Comfort (people)o Process control (as required)


PSYCHROMETRICS

Study of air-water vapour (binary) mixtures Content of water vapour can change A/C processes may involve both sensible and latent heat transfer

SOME IMPORTANT PARAMETERS IN PSYCHROMETRICS

Dry Bulb Temperature (TDB) – sensed with a normal thermometer bulb/sensor Wet Bulb Temperature (TWB) – sensed by a thermometer whose bulb is wrapped with water

soaked wick in rapidly moving air Dew Point Temperature (TDP) – Temperature at which water vapour starts to condense at

constant pressure Humidity Ratio/Specific Humidity (W) – Mass of water vapour divided by the mass of dry air

(mv/ma kgv/kga) Relative Humidity ( or rh) – Ratio of actual water vapour pressure in the air to the water vapour

pressure at saturation at the mixture temperature va - volume of a mixture containing one kg of dry air (m3/kga) h – enthalpy contained in a mixture containing 1 kga (kJ/kga) va and h involve (1+W) kg of mixture


PSYCHROMETRICS (Cont’d 1)

PSYCHROMETRIC CHART


SOME IMPORTANT PSYCHROMETRIC PROCESSES

Thermodynamic Wet Bulb Temperature (Adiabatic Saturator)

Thermodynamic Saturator

Adiabatic wall

mw

Water

Air

Make-up water


SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 1)

Adiabatic Saturator H2O Process Sling Psychrometer

T

s

Pv2=Pvs2 Pv1

T2=Tw

f2

Handle

Dry bulb thermometer

Wet bulb thermometer



22221111 vvaawwvvaa hmhmhmhmhm

aaa mmm21

)WW(mm 12aw

)hh(Whh)hh(W wv2aawv1 2121

hw = hf2, hv1 = hg1, hv2 - hw = hfg2,

21

212

fg

fg2aa1 hh

hW)hh(W

2

2

2

22

622.0

s

s

PP

PW



IMPORTANT RELATIONSHIPS

Specific Humidity

Enthalpy

or

a

v

a

v

a

v

a

v

vv

av

a

a

v

v

a

v

P

P

P

P

P

P

M

M

RP

RP

VP

TR

TR

VP

m

mW 622.0

9645.28

01534.18~

vvaa hmhmmiH

va

va

a

hm

mh

m

mi hWhhWi va )1(

gopgopppgop WhtcWhtWccWtchtchvava

)()(



Datum: Dry saturated vapour at 0ºC, t in ºC For A/C purposes, cpa 1.005, cpv 1.87 kJ/(kg.K), W 0.01 kgv/kga, hgo = 2500.8 kJ/kg,

cp 1.024 kJ/(kga.K), and hence h 1.024t + 2500.8W kJ/kga

and W

; ; P = Pa + Pv ; and hence

Solution for W1 From Adiabatic Saturator

P

P=s

va

v

P

P622.0W

( ) ( )

0.622 0.622 0.622a v s

s s s

WP W P P W P P

P P P

21

2212

1

)(

fg

fgp

hh

hWttcW a



Cpa, t1, t2, hfg2, hg1, and hf2 from tables

Since at 2 air is saturated, 2 = 1 get W2 from

where Ps2 from tables at t2

Heating and Cooling at Constant W (Sensible)

2

2

s2

s22 PP

P622.0W

Q

(a) (b)

W



Cooling and Dehumidification

or

or

)())(()]()[()( 121212 1212ttcmttWccmhhWhhmhhmQ pappavvaaaa va

Q

(a) (b)

mw

A

21 vwv mmm )WW(mm 21aw

Qhmhmhm ww2a1a )WW(hm)hh(mQ 21wa21a



represents enthalpy carried away by the condensate ( 10ºC) which is negligible compared to the first term and hence

where and

Sensible Heat Factor (SHF) (Related to bypass factor)

Important in A/C calculations.

)W-W(hm 21wa

ls QQQ

)hh(m)tt(cm

QQ

2Aa21pa

2As

)hh(m

h)WW(mQQ

A1a

fg21aA1l 1

Q

QSHF s



Heating With Humidification

or

Equation of a straight line.

For Q = 0,

Q

(a) (b)

mw

)WW(mm 12aw

w12a12a h)WW(mQ)hh(m w12a12

12 h)WW(m

Q

WW

hh

w12

12 hWW

hh



hw = hgT1 – humidification at constant T1 (2’)

hw > hgT1 – heating with humidification (2’’)

hw < hgT1 - cooling with humidification (2)

Spray with liquid water at air wet bulb temperature – Twb remains constant. Basis of evaporative cooling

1

22

2



Adiabatic Mixing

; ;

Equation of a straight line (final state lies along this line)

(a) (b)

h1

h2

h3

W1

W2

W3

3a2a1a hmhmhm321

321 aaa mmm 3a2a1a WmWmWm

321

31

23

31

23

a

a

WW

WW

hh

hh

m

m

2

1

21

23

21

23

a

a

WW

WW

hh

hh

m

m

3

1

12

13

12

13

a

a

WW

WW

hh

hh

m

m

3

2



EXAMPLE OF A SIMPLE CENTRAL AIR-CONDITIONING SYSTEM

Space Q=Qs+Ql

Exhaust Outdoor air

Filters

Fan

Cooling & dehumid Coil

Heater

5,6,7

0

1 2

3

4



T4, Q, mao, 5, 6, 7, SHFroom and Qfan known.

Draw line from 5, 6, 7 to cross T4 (T5 – T4 10ºC)

Join 0 and 5 locate 1 – adiabatic mixing, i.e.

Hence

For known SHFcoil draw line 2-3, and hence 3-4 at constant W

Qcoil = ma1(h2 – h3), Qheater = ma3 (h4 – h3)

454 hh

Qma

1

6

60

10

a

a

m

m

WW

WW

1

12a

fan

m

Qhh


COMFORT AND HEALTH

Deep body temperature 36.9ºC If body can easily maintain an energy balance, then feeling of comfort results Body regulatory mechanisms:

Metabolism rate Increase of the rate of cutaneous blood circulation (capillary dilation) Sweating

Metabolism – depends on the level of activity 1 MET (metabolic rate) = 58.2 W/m2

Energy generated by an average sedentary MAN Area (man) 1.8 m2

1 MET 105 W Women 30% lower than men Latent and sensible

Comfort Conditions Depends on activity and clothing

1 clo 0.155 m2.K/W – heavy two piece suit with accessories 0.05 clo pair of shorts


COMFORT AND HEALTH (cont’d 1)

Examples of Cooling Load Due to Occupancy

Activity Example Male Adult Total Watts

Total Adjusted Watts

Sensible Watts

LatentWatts

Seated at rest Theatre, movie 115 100 60 40

Seated, very light work, writing

Offices, hotels, apartments

140 120 65 55

Standing, light work or walking slowly

Retail store, bank

235 185 90 95

Light Bench work Factory 255 230 100 130

Heavy work, heavy machine work, lifting

Factory 470 470 165 300

Heavy work, athletics

Gymnasium 585 525 185 340


COMFORT AND HEALTH (Cont’d 2)

ASHRAE Comfort Standard 55-81 (1981) (Sedentary)

W gv/kga

ºC 20 25 30

5

100

15

Summer Winter



Cooling T 24ºC Heating T 22ºC Humidity 40 – 50 % Velocity in occupied zone V 0.15 m/s For high activity – special charts (Fanger comfort Charts – ASHRAE HF)



OUTDOOR DESIGN CONDITIONS

Winter Summer

Station (Elevation)

Mean Annual Extrem

es

99%C

97.5%C

Design Dry Bulb C Outdoor Daily

Range C

Design Wet Bulb C

1% 2.5% 5% 1% 2.5% 5%

Nairobi (1820 m)

7 9 10 27 27 26 13 19 18 18

Addis (2363 m)

2 4 5 29 28 27 16 19 18 18

Lagos (3 m) 19 21 22 33 33 32 7 28 28 29

Dar es Salaam (14 m)

17 18 18 32 32 31 7 28 27 27



Mean of annual extremes: Average of the lowest temp. recorded each year over 25-30 years

99%: Temp. which has been equaled or exceeded 99% of the time during the three cold months (Ditto for 97.5%)

1%: Temp. equaled or exceeded or equaled 1% of the time during the time during the cooling months

Daily range: Difference between average maximum and minimum temp. for the warmest month – has an effect on the energy storage of structures.

Ventilation Mainly to control odour – recommended standards for different spaces (minimum 2.5 l/s) Filtration, washing, scrubbing, adsorption, odour masking and counteraction The smaller the particle, the more difficult to remove Fibrous media (viscous impingement and straining), electronic air cleaners



Cooling T 24ºC Heating T 22ºC Humidity 40 – 50 % Velocity in occupied zone V 0.15 m/s For high activity – special charts

Ventilation Mainly to control odour – recommended standards for different spaces (minimum 2.5 l/s) Filtration, washing, scrubbing, adsorption, odour masking and counteraction The smaller the particle, the more difficult to remove Fibrous media (viscous impingement and straining), electronic air cleaners


HEAT TRANSMISSION IN BUILDINGS AND COOLING LOAD

Cooling Load Temp. and humidity to be maintained at a comfortable level Heat must be extracted – cooling load Basis of equipment selection (cooling and dehumidification coil, heater, ducts, fans, piping, fans, pumps,

etc.)

Heat Gain

Radiation Heat storage in furnishings and structure

Convection, infiltration

Convection (delayed in time)

Cooling load


HEAT TRANSMISSION IN BUILDINGS AND COOLING LOAD (Cont’d 1)

Heat gain: Rate at which heat is being received in the space at any time (solar radiation, lighting, conduction, convection, people, equipment, infiltration, etc.)

Storage effect: Heat does not immediately go into heating the room air. Radiant component first absorbed by room materials before being absorbed by room air.

Cooling load: Rate at which heat must be removed to maintain room design conditions (temperature and humidity)

Cooling load

Morning Afternoon Evening

Heat Gain and Cooling Load

Instantaneous heat gain

Removal of stored heat

Heat being stored

Removal of stored heat


Heat Gain/Cooling Load Components Conduction through exterior walls, roof and fenestration (glazing/any light transmitting element) Conduction through interior partitions, ceiling and floor Solar radiation (short wave) through fenestration Lighting and equipment Occupancy Infiltration (Fans, duct heat gain, duct leakage)

HEAT TRANSMISSION IN BUILDINGS AND COOLING LOAD (Cont’d 2)


ROOM AIR DISTRIBUTION

Good air distribution is necessary for comfort Effective draft temp. difference from design condition between -1.7ºC and 1.1ºC within occupied zone

(approx. < 1.75 m) Air velocities 0.13 – 0.25 m/s (below or above cause discomfort)

AIR FLOW PATTERNS

The Horizontal Isothermal Jet


ROOM AIR DISTRIBUTION (cont’d)

I II III IV o

x

V

VCL

oAx


ROOM AIR DISTRIBUTION (cont’d 1)

Zone I – Constant centerline velocity Zone II – Transition zone Zone III – Most important and the longest fully developed flow) - Zone IV – Fast velocity decay – regarded as still air – very short

Throw – Distance to a specified velocity, e.g. 0.25 m/s

Important Characteristics Surface effects increase the throw and decrease the drop (c.f. free jet)

Jet parallel to a wall or ceiling tends to hug the surface (reduced entrainment –”ceiling effect” Obstructions e.g. beams, columns etc.

Cold jet – drop Warm jet - rise

x

AK

V

V o

o

x



High sidewall diffuser – good for cooling

Cooling Heating



Ceiling Diffuser Excellent for cooling Large diffusion surface area Handles large quantities of air

Beam



Slot Diffusers Long strip-shaped with one or more narrow openings

Plenum Ceilings Hung ceilings with slots or perforations for air supply (specialized suppliers/installation)

SELECTION CRITERIA FOR DIFFUSERS Capacity – Volumetric flow rate Throw – Axial distance (isothermal) jet travels till the maximum velocity is reduced to a specified

level, e.g. 0.75, 0.5, 0.25 m/s Noise Criterion (NC)

Tabulated Standards for different spaces, ducts, applications, fittings

Pressure - Ps and Pv or Po



Room Characteristic Length (L)

Ceiling Diffuser

L

High Sidewall Diffuser

L



Air Diffusion Performance Index (ADPI) Effective Draft Temperature (EDT)

q = (tx – tc) – a(vx – b)

tx - local temp., ºC

Tc – room average temp., ºC

vx – local velocity, m/s

a = 8, b = 0.15 Comfort Conditions: - 1.7 1.1˚C; vx < 0.35 m/s

ADPI – percentage of locations in occupied space of room which meet this criterion



Example

Terminal Device

Room Load (W/m2)

T0.25/L for max ADPI

Max ADPI For ADPI greater than

Range of T0.25/L

Circular 250 1.8 76 70 0.7 – 1.3

Ceiling 190 1.8 83 80 0.7 – 1.2

Diffuser 120 1.6 88 80 0.5 -1.5

65 1.5 93 90 0.7 – 1.3


BUILDINGS AIR DISTRIBUTION

FAN Supply the required air to all conditioned space Must provide the required pressure drop to cater for ducts, diffusers, filters, etc. Types:

Axial : a) Vane axial - centerline of duct

- guide vanes before and after wheel (rotor) to control rotation of stream

- high speed (noisy)

b) Tube axial - no guide vanes

c) Propeller - low pressure applications

- high mass flow rates Centrifugal:

a) Forward curved (blades)

b) Radial

c) Backward curved (airfoil)

Most used in A/C – can move large or small quantities of air over wide ranges of pressure


BUILDINGS AIR DISTRIBUTION (Cont’d 1)

Fan Selection Fan characteristics

Capacity and total pressure Efficiency Reliability Size Weight Speed Noise Cost

Duct Design Layout (supply and return) – related to supply diffusers and return grilles, location of machine

room, and other structural and architectural considerations. Selection of size is a compromise between capital and running costs.


HVAC SYSTEMS, EQUIPMENT & CONTROL

HVAC systems may conveniently be divided into two broad categories: Equipment and systems which provide heating and cooling Systems which provide ventilation (air distribution and diffusion)

It is important to understand the (initial) design of the installation, modifications, operation/performance, utilization hours of operation and even maintenance record (for energy management purposes)

HVAC SYSTEMS Related to system organization Energy consumed depends on source of heating/cooling, air distribution, and whether working

fluid is simultaneously cooled or heated.

ALL AIR SYSTEMS Most common Moderate room air by providing conditioned air from a central source via ducts Control by altering the amount of air supplied or its temperature Provide best control of fresh outdoor air (quality) and humidity control


HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 1)

Can be used to provide outside air for cooling interior spaces while providing heating for perimeter zones

Drawback – energy consumed in distribution

Components of All Air Systems Air Handling Unit (AHU) – fan, (heating and cooling) coils, filters, humidifier (Supply and return) ducts circulate conditioned air. Sometimes plenum above suspended ceiling

used as part of return path Included in duct system is supplier of outdoor air and another for exhausting some of the return

air



Single Zone Air Conditioning System Layout

Outdoor air

Filters

Preheat coil (opt)

Cooling & dehumid coil

Heating or reheat coil (opt)

Supply fan

Supply air

Room(s)

Return air

Return fan

Exhaust air



Can be used for all year round control Can use 100% outdoor air – during intermediate cooling seasons – refrigeration

equipment not used

Control of proportion of outdoor air

Exhaust

Min

Max Outdoor air

Return

Mixed supply to AHU



Pre-heat coil – in cold climates to prevent cooling coils from freezing Face bypass – provides another method of controlling humidity – but not as good control

as reheat coil

Single zone systems suitable for large open spaces with uniform load, e.g. stores, factories, arenas, auditoriums, exhibition halls, etc

Bypass damper

Face damper

Cooling and dehumid coil



Variable Air Volume (VAV) Systems Same as single zone but individual thermostats control the amount of air supplied to

room

VAV



High degree of local temperature control Moderate additional capital cost AHU pressure increases (additional P for VAV) AHU needs regulation to balance varying duct P requirements (fan inlet and outlet dampers) Fan would operate off the optimum position – need variable speed drive Supplementary heating may be necessary (minimum air to space must be supplied) Single duct VAV systems most versatile and most widely used for large buildings (except where

high degree of humidity control is required or high air exchange)



Reheat Systems

o

Zone 1

Zone 2

Zone 3

AHU

Reheat coils

Return

m c s1

o

m

z1

s1

c



Provides individual zone control of temp. and humidity Wasteful – all air has to be cooled and then heated – double use (waste) of energy (cooling and

then reheating) Constant Air Volume (CAV) and VAV Reheat systems inefficient – highest level for all systems

(CAV reheat systems most inefficient. VAV reheat inactive except when air modulation cannot meet minimum temp. requirements)

CAV and VAV systems with reheat can provide extremely tight control conditions (with humidity control) e.g. museums, printing plants, textile mills and other industrial process settings)



Multizone Systems

Cooling

Heating



A variation of the single duct CAV reheat system (NOT any system with thermostatically controlled zones – misconception)

Most common systems produce two streams at ~ 38C and ~ 13C Streams blended with dampers to adjust room supply air temp.

Dual Duct Systems Air not blended in the fan room Usually uses high velocity ducts (reduces size and cost of ducts but increased fan energy) with

mixing boxes Limited to buildings with strict temp. and humidity control requirements Dual duct with VAV has efficient control (c.f. CAV) but requires a lot more distribution energy



Heating

Cooling

Mixing box To zone

Hot duct

Cold duct

Return

Filters

To zone



ALL WATER (HYDRONIC) SYSTEMS Distribute hot or cold water from central plant Terminal units heat or cool room air Ventilation brought in through external wall directly to room or via terminal unit Lower capital cost and requires less space than all air system – H2O has higher density and

specific heat Useful when space is limited e.g. existing building not originally conditioned Disadvantages

Many units – maintenance Control of ventilation air quantities not precise Humidity control limited

Popular for low cost central systems in multi-room high-rise applications Water heated to 60 - 120C or chilled to 4 - 10C and piped to devices – finned heaters or coolers Steam also used

Latent heat 50 times more effective as water (T ~ 20C) But higher volume (~ 1600 times) Per m3 water requires less piping space



PIPING CONFIGURATIONS

Single Pipe Series System Least piping Maintenance of any unit necessitates shutdown of entire system Individual unit control not possible T diminishes with distance

Chiller/Heater

Terminal unit

Pump



One Pipe Main Offers individual control Special diverting tee – directs some of the water to the tee

Chiller/ Heater

Terminal unit

Pump

Diverting tee



Two Pipe - Direct Return Facilitates individual control

Central unit

Terminal units

Pump



Two Pipe – Reverse Return Balanced – provides nearly equal flow path

Central

unit

Terminal

units

Pump



Three Pipe System Separate heating and cooling supply pipes but common return with appropriate 3 way valves Possible to heat some rooms while cooling others Return can be direct or reverse

Hot Cold

Terminal units



Four Pipe System Two separate pipe systems – one for cooling and one for heating

Hot Cold Terminal units



HYDRONIC TERMINAL DEVICES

Radiators Hollow cast iron sections through which hot water flows – free convection

Convectors Heaters – free convection

Unit Heaters

etc



Fan Coil Units Small air handling unit No outside air provision (usually) Hot or cold water supply Can be placed anywhere – cooling near ceiling, heating near floor If with outdoor air, known as unit ventilators

Coil

Filter



AIR-WATER SYSTEMS Water and conditioned air from central system to individual terminal units Utilize best features of all air and all water systems Water carries most of the energy Usually distributed air only enough for ventilation – usually by high velocity ducts Supplied air distributed via fan coil units, or directly to rooms Most systems use induction units Central air – known as primary air. As it flows through unit at high velocity it inducts room air

(secondary air) – no fan required – minimizes maintenance Induction units popular with high rises Initial cost relatively high Primary air as low as 25% of all air system – not adequate for outside air cooling even for mild

climates – hence chilled water supplied to unit coils



High velocity air jets

Mixed air

Lint screen filter

Coil

Induction Unit

Primary air

Secondary air



UNITARY SYSTEMS Refrigeration and air conditioning packaged together, i.e. refrig. equipment, fan, fan coils, filters,

dampers and control Usually in or close to air conditioned space Can be all air, all water or air – water. Generally all air and largely inclined to the more simple

such as single zone with or without reheat, or multizone. Categorized as: Room units Unitary conditioners Roof units

Room Units Dampers adjustable to allow outdoor air through cooling coil Low cost and simplicity Ideal for existing building – electrical power upgrading may be necessary No flexibility to handle high latent heat or changing sensible heat ratio – no good humidity control High sound levels Air cleaning quality marginal (only large particles)



Maintenance for large number of units Energy wasteful Up to (approx) 3 tons (~ 10 kW)

Room air Filter

Evap. coil

Evap. fan

Cooled air

Motor

Cond. fan

Condenser

Compressor

Outdoor air Cond. Discharge air



UNITARY A/C UNITS

In or near space Heating sometimes included Available in vertical or horizontal package Limited ductwork can be connected if air distribution is desired Popular in small commercial application s Normally only condenser not packaged Split system

Condenser and compressor one package and cooling coil (with fan) inside (popular for residential heat pump)

Same advantages and disadvantages as room units Large units have multiple compressors Available up to ~ 50 tons (175 kW)



ROOFTOP UNITS (DIRECT EXPANSION – DX)

Outdoor installation All components packaged together or compressor and condenser may be remote Heating may be incorporated May be used with ductwork Do not use building space Relatively low cost Available with multizone arrangement Humidity control limited Popular in low cost one floor buildings (e.g. supermarkets and suburban commercial buildings)

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FME 706/FML 2007- 08Air Conditioning1 AIR CONDITIONING.

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