1. Cooling/Heating Load calculation- Sensible heat load (qS), Latent heat load (qL)

2. Thermal distribution systems- A/C type: Split-type, Package-unit, Chilled-water- Refrigeration system: Vapor-compression, Absorption- Control system: electronic thermostat, inverter control- Air system: flow rate of Supply Air, Return Air, Outside Air- Water system: water flow rate, pump

3. Fan-and-Duct systems- Layout of duct system, duct sizing, fitting- Fan sizing

4. Refrigeration equipments- Compressor, Condenser, Evaporator, Expansion devices- Refrigerants

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Topic6-1 Air Conditioning Systems Design

1. Heating air without adding moisture

2. Heating with humidification

3. Cooling with constant enthalpy adiabatic cooling

4. Cooling with constant-moisture content

5. Cooling with dehumidification

6. Mixing of air quantities

7. Apparatus dew point

8. Sensible heat factor

2

Changing the Condition of Air

Outside air control: min 10-20% OAhospital – 100% OA

Load calculation (qS, qL)Space load: qt = qS + qL

qL = hfg(Ws – Wi) = 3010(Ws – Wi)qS = cpQs(ts – ti) = 1.23Qs (ts – ti)Supply air: Qs = qS /1.23(ts – ti)

9.2 Thermal distribution systems

Supply Air (SA)

Return Air (RA)

Outside Air (OA)

Air at 10 – 20 C 1.22 kg/m3

cp 1.007 kJ/kgKhfg of vapor water 2467 kJ/kgcp = 1.23 kJ/m3Khfg = 3010 kJ/m3

Qs = supply air flow rate m3/s

AHU

CMH = m3/h, CFM = ft3/min, Btu/h

Cooling coil

Load-ratio, SHF = qS /(qS + qL)Space load : qt = qS + qL = Qs(hs – hi)

Thermal distribution systems

Supply Air (SA)

Return Air (RA)Outside Air (OA)

Mixed air

4

qS = Qscp(ts – ti)qS /(qS + qL) = cp(ts – ti)/(hs – hi) RA

OA

Coil load: qcoil = Qs(hc – hi)

ts – ti

5

Ex 5-8: qs = 65 kW, qL= 8 kWSpace; 24C, 50% RH (=Ref.)Outdoor; 35C, 25CWBVentilation; OA:RA = 1:4Air condition leaving coil?Cooling capacity of the coil?

Method: ti,i, qcoil = ma(hc – hi)Load-ratio SHF = qS /(qS + qL)SHF = 65/(65+8) = 0.89Ref. 24C, 50% RH

mO:mS = 1:4Energy balance: m=Qmhc =mShS + mOhO

Mass balance of water:mWc =mSWS + mOWO

S

O

C

i

Air condition leaving coil qS /(qS + qL) = cp(ts – ti)/(hc – hi) = 0.89Assume ti, find hi until get hi@ti

Mostly air leaving coil 92-98%RHQs = qS/cp(tS – ti) = qt/(hS – hi) Coil load: qcoil = Qs(hc – hi)

Sen

sib

le H

eat

Fact

or

SHF

Ref.

6

When is Reheat coil needed?Ex: SHF = qS /(qS + qL) = 0.7 clean room; 20C, 40% RHOutdoor; 35C, 25CWBVentilation; OA:RA = 1:4

Load-ratio SHF lineSHF = 0.7 and Ref. 24C, 50% RH

S

O

C

i

Sen

sib

le H

eat

Fact

or

SHF

Ref.

clean room; 20C, 40% RHSHF line not cross the saturation line !

Reheat needed!

7

7 Processes in Air Conditioning Systems

SA

RAF CC DH

HC H

6. Air circulation processSA - Supply Air RA - Return Air

7. Air ventilation processOA – Outside/Fresh AirMA – Mixed Air

OA

MA

1. CC for Cooling process2. DH for Dehumidifier process3. HC for Heating process4. H for Humidifier process5. F – Filter process

7 processes in a Year-round air conditioning system

1.Hotwater boiler

2.Hot water coil

3.Refrigerating machinery

4.Cooling coil6.Blower Fan

12.Filter5.Humidifier

11.Outside air intake

10.Return-air duct

8.Supply air diffuser, Qs

7.Supply air duct

9.Return-air grille

AHU

Mechanical room

Conditioned Space

Room load

Coil load

AHU - 5 processes1. CC for Cooling process2. DH for Dehumidifier process3. HC for Heating process4. H for Humidifier process5. F – Filter process

Air distribution system – 2 processes5. Air circulation process6. Air ventilation process

Air circulation process Air ventilation process

Piping- Water- Refrigerant

Selection of a suitable A/C system depends on:1. Capacity, performance and spatial requirements 2. Initial and running costs3. Required system reliability and flexibility 4. Maintainability5. Architectural constraints

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9.3 Selection Criteria for Air-Conditioning (A/C) Systems

tonR – a ton of refrigeration = 12000 Btu/h = 3517 W

Classification of A/C system: size based1. Central AC system2. Unitary AC system or Package system

Classification of A/C system: fluid media basedC1. All air systemsC2. All water systemsC3. Air-water systemsC4. Unitary refrigerant based systems

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Central Air-Conditioning Systems

1. Cooling capacity > 50 tons or TR (small system up to 250 tons)

2. Circulating air > 20,000 cfm (small system up to 100,000 cfm)

3. Primary components - in a central mechanical room4. Air sent to conditioned spaces – extensive ductwork5. A single large building or multiple zones within the building6. Carefully plan/specifications – a licensed mechanical engineer

Heart of a central-station unit - AHUSmall systems < 250 tons use of - factory-assembled- component-selected- zone AHU

1) Fan section2) Coil section - CC&DH,F,H

3) Pan/Drain section4) Control device - dampers Large AHU for a central station system

AHU supplied with- chilled water or- direct-expansion coil

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Multi-zone air distribution from a central AHUMulti-zone section technique

Mixing air supplied to each space, rang of 100% heating to 100% cooling from the same central station unit.

From AHU

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C1. All air systemsAir is processed in the A/C plant → conveyed to the conditioned space through insulated ducts using blowers and fans

C1.1 Single duct systems1.1.1 Constant volume, single zone systems1.1.2 Constant volume, multiple zone systems1.1.3 Variable volume systems

C1.2 Dual duct systems1.2.1 Dual duct, constant volume systems1.2.2 Dual duct, variable volume systems

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C1.1 Single duct, constant volume, single zone systems

Constant volumetric flow rate of supply air (SA)Regulating supply air temperature (T) and humidity ratio (H).Dampers controls amount of outside air (OA)

SA

RA

OAMA

EA

RCA

T – ThermostatH - Humidistat

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C1.1 Single duct, constant volume, single zone systems

Thermostat and Humidistat are indicative of zone conditionsSupply air conditions are controlled by- Coil control

- Face-and-bypass control

→ Simple control by Varying flow rate of cold and hot water→ Not possible to control humidity precisely→ i.e. lower cold water flow rate, higher humidity ratio

→ Constant flow rate of cold and hot water→ Air flow rate over coils are controlled by Bypass damper→ possible to control humidity precisely→ Occupies more space and more expensive

Applications:- Spaces with uniform loads, such as large open areas with small

external loads e.g. theatres, auditoria, departmental stores.- Spaces requiring precision control such as laboratories

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C1.2 Single duct, constant volume, multiple zone systems

For very large buildings, several zones, different cooling requirements - multiple zone system with terminal reheat coils

- zone thermostat controls amount of reheat, hence supply air temp. - reheat coil - electricity or hot water.

Advantages:- Relatively small space - Excellent T and H control over a

wide range of zone loads - Proper ventilation and air quality

in each zone is maintained

Disadvantages:- High energy consumption for cooling,

air is cooled to a very low temp. and is then heated in the reheat coils

- Simultaneous cooling and heating is not possible

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C1.3 Single duct, Variable air volume (VAV) systems

Variable air volume is controlled by a zone damper which controlled by zone thermostatTemp. of supply air remains constant, whereas its flow rate varies depending upon the load

Advantages:Lower energy consumption by - no reheat - lower fan power input due to lower flow rate, for low load

Disadvantages:- Improper ventilation by controlled flow rate- difficult to control humidity precisely

* combining VAV systems with terminal reheat to maintain the air flow rate at proper ventilation

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C2.1 Dual duct, constant volume systemsSupply air fan splits the flow into two streams: CC and HC Cold and hot air streams are mixed in required proportions using a mixing box arrangementSupply air temperature varies depending upon load

Advantages:- proper ventilation - Simultaneous cooling and heating - responsive to load variations

Disadvantages:- Occupies more space - Not very energy efficient

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C2.2 Dual duct, variable volume systems

Similar to dual duct, constant volume systems → the mixing boxes reduce the air flow rate as the load on the zone drops

Outdoor air control in all air systemsMass balance: MA = SA = RA, OA = EA(unless building pressurization or de-pressurization)

Minimum amount of OA (10 - 20% of SA for

ventilation) when the outdoor is too warm (> 24C)

For energy conservation, 100% OA can be used when To =13 - 24C

RA

OAMA

EA

RCA

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C1. All air systems - ConclusionAdvantages:1. energy conservation by utilizing OA effectively. 2. high-quality controls ± 0.15C, ± 0.5%3. dual duct → simultaneous cooling and heating → Changeover from summer to winter is simple 4. provide good room air distribution and ventilation under all load conditions5. Building pressurization can be achieved easily 6. A/C plant located away from conditioned space → wide variety of air filters and avoid noise

Disadvantages:1. occupy more space 2. Retrofitting may not always be possible due to the space requirement 3. Balancing of air in large with variable air volume systems could be difficult

Applications: to building- require individual control of multiple zones, such as office buildings, classrooms, laboratories, hospitals, hotels, ships etc.- require very close control of the conditions in the conditioned space such as clean rooms, computer rooms, operation theatres, research facilities etc.

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C2. All water systemsWater transports energy between conditioned space and A/C plant. a. 2-pipe system

b. 4-pipe system

2-pipe system

- cooling only or heating only application- one-supply water, another-return water- flow control valve is controlled by zone thermostat.- a separate arrangement for required ventilation air - A pressure relief valve (PRV) - balanced flow rate. - two-supply water (hot, cold), two-return water pipelines- cold and hot water are mixed in a required proportion .

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C2. All water systems - 4-pipe system Heat transfer between the cold/hot water and conditioned space by1. Fan coil units - By controlling the cold water flow rate or air flow rate or both.

- unit ventilator separate arrangement of ventilation

2. Convectors – finned tube coil- natural convection, no fans- Heating applications

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C2. All water systems - ConclusionAdvantages:1. requires very less space, absence of large supply air fans → small plant size 2. Individual room control is possible, benefits of a large central system 3. small Thot water → possible to use solar or waste heat for winter heating 4. It can be used for new as well existing buildings (retrofitting). 5. Simultaneous cooling and heating is possible with 4-pipe systems

Disadvantages:1. Requires higher maintenance, particularly in the conditioned space 2. Draining of condensate water - messy, create health problems *eliminated by dehumidification – by a central ventilation system 3. Control of humidity is difficult using chilled water control valves.

Applications: - All water systems using fan coil units - buildings requiring

individual room control, such as hotels, apartment buildings and office buildings

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C3. Air-water systemsprimary air- air supplied to conditioned space from central plant- ducting system with fans for conveying air

secondary water - water supplied from central plant - water pipelines and pumps for conveying water

water pipelines and pumps

ducting system with fans

chilled water coil - only sensible load → condensation avoided

no need for separate ventilation systems

secondary water lines - 2-pipe, 3-pipe or 4-pipe type

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C3. Air-water systems- ConclusionAdvantages:1. Individual zone control is possible using room thermostats, 2. possible to provide simultaneous cooling and heating 3. Space requirement is reduced 4. Positive ventilation can be ensured under all conditions 5. coil operates dry → its life increases → odors, fungal avoided 6. heating coil and secondary air → avoiding supply of primary air during winter7. Service of indoor units is relatively simpler

Disadvantages:1. Complicated operation and control - controlling both primary air &secondary water

2. In general these systems are limited to perimeter zones 3. secondary water become dirty if the quality of filters is not good 4. const. primary air → shutting down supply air to unoccupied spaces not possible5. condensation may take place on the cooling coil of secondary water- high latent 6. Initial cost could be high

Applications: - exterior buildings with large sensible loads, humidity not

required, i.e. office buildings, hospitals, schools, hotels, apartments

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C4. Unitary refrigerant based systemsFactory assembled and tested as per standard specifications Package/single units of varying capacity and type- consist of refrigeration unit, fans, filters, controls - fraction to 100 TonR- Window air conditioners- Split air conditioners / Package unit / VRV or VRF- Ductable systems with air- or water-cooled condensing units- Fan-coil unit / Duct-type unit- Rooftop or Roof-mounted unit

A window type room air conditioner A package unit with remote condensing unit

26

C4. Unitary refrigerant based systems - ConclusionAdvantages:1. Individual room control is simple and inexpensive 2. individual air distribution with simple adjustment by the occupants 3. Performance of the system is guaranteed by the manufacturer 4. System installation is simple and takes very less time 5. Operation of the system is simple - no need for a trained operator 6. Initial cost is normally low compared to central systems 7. Retrofitting is easy as the required floor space is small Disadvantages:1. system is less flexible in terms of air flow rate, condenser and evaporator sizes 2. Power consumption per TR could be higher compared to central systems 3. Close control of space humidity is generally difficult 4. Noise level in the conditioned space could be higher 5. Limited ventilation capabilities 6. designed to meet the appliance standards, rather than the building standards.7. The space temperature may experience a a swing if on-off control is used 8. Equipment life is relatively short.

Applications: - individual rooms to large office buildings, classrooms, hotels,

shopping centers, nursing homes

27

Example- Central A/C system

• KKU-อาคารสารสนเทศ Chiller 250 tonR3 Oil-free magnetic bearing

• แฟรีพ่ลาซา่ Chiller 500 ton2 + 400 ton1

• BKK-สามย่าน Midtown Chiller 750 tonR6 special chiller controller and

pump, 0.65 kw/tons, oil-free ceramic bearing, new refrigerant-R1233zd

• Seagate โคราช Chiller 1,100 ton14

• KKU-ตกึ 50 ปี หอ้งเรยีน VRF FCU 1-4 ton43 (160 ton), SA 400 cfm/ton

CDU 22-50 ton4 (155 ton)

• KKU-ตกึ 50 ปี Chiller 150 tonR3 , 0.72 kw/tons, R134aหอ้งสมดุ+หอ้ง300ที่นั่ง AHU 7-42 Ton7 (180 ton), SA 420-530 cfm/tonหอจดหมายเหต ุหอ้งรบัรอง FCU 1.5-4 ton18 (60 ton), SA 400cfm/tonโถง ชัน้2-4 PCU 18-39 ton4 (100 ton), SA 400cfm/ton, SA 140 cfm/ton

• รพ.ศรนีครนทร์ Chiller 500 TonR5, ศนูยห์วัใจ Chiller 250 TonR3

Example- Unitary A/C system or Package system

• KKU-ตกึเพียรวิจิตร หอ้งสมัมนา ชัน้9 Package 20 ton2

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Selection of Air Conditioning Systems

1. Factors of comfort 2. Factors of economy

3. Factors of operation and maintenance characteristics

-Temperature-Moisture content-Air motion-Air quality-Noise

-Initial cost or First cost-Operating and Maintenance cost

-equipment value, interest, pay back- fuel, electric, water, maintenance,

repair, personel

Best air conditioning system:

-Ease of Maintenance-Controllable-Variable load-High Efficiency

-Basic Construction-Long life cycle-Ease to be repaired-Ease of installation