Air Conditioning System

Air Conditioning System

Air Conditioning System Air conditioning for people is the control of temperature, humidity, air movement and air cleanliness, heat radiation sometimes [e.g. by chilled ceiling ], normally with mechanical means, to achieve human thermal comfort. . See http://fridge.arch.uwa.edu.au/topics/thermal/index_thermal.html for thermal comfort and related topics. Air conditioning systems can be categorized according to the means by which the controllable cooling is accomplished in the conditioned space. They are further segregated to accomplish specific purposes by special equipment arrangement. In selecting a suitable air conditioning system for a particular application, consideration should also ven to the following:- System constraints : Cooling load, Zoning requirements, Heating and ventilation - Architectural Constraints : Size and appearance of terminal devices, acceptable noise level, Space available to house equipment and its location relative to the conditioned space, acceptability of components obtruding into the conditioned space - Financial Constraints : Capital cost, Operating cost, Maintenance cost For case studies, see http://arch.hku.hk/teaching/case.htm#case There are four basic system categories: 1

Central chilled water air conditioning systems - All Air Systems Single zone 1.1 Reheat 1.2 Variable Air Volume 1.3 Dual Duct 1.4 Multizone 1.5

2

Central chilled water air conditioning systems - Air-and Water Systems Induction 2.1 Fan Coil 2.2 2.3 Two-pipe Three-pipe 2.4

3

Central chilled water air conditioning systems - All Water Systems, including cooling towers which can also be applied to systems 1, 2 above 3.1 Fan-coil units 3.2 Central chilled water air conditioning system with fan coils and other devices 3.3 Water cooling tower

4

Direct expansion Systems [i.e. direct expansion of refrigerant, without the chilled water cooling medium ] 4.1 Window air conditioners Unitary and Rooftop Air Conditioners 4.2 Split type and package air conditioning systems 4.3 4.4 Heat pumps

1. Central chilled water air conditioning systems - All Air Systems An all-air system provides complete sensible and latent cooling capacity in the cold air supplied by the system. Heating can be accomplished by the same air stream, either in the central system or at a particular zone. All-air systems can be classified into 2 categories:-Single duct systems -Dual duct systems System Advantages 1.

The central plant is located in unoccupied areas, hence facilitating operating and maintenance, noise control and choice of suitable equipment.

2.

No piping, electrical wiring and filters are located inside the conditioned space.

3.

Allows the use of the greatest numbers of potential cooling seasons house with outside air in place of mechanical refrigeration.

4.

Seasonal changeover is simple and readily adaptable to climatic control.

5.

Gives a wide choice of zonability, flexibility, and humidity control under all operating conditions.

6.

Heat recovery system may be readily incorporated.

7.

Allows good design flexibility for optimum air distribution, draft control, and local requirements.

8.

Well suited to applications requiring unusual exhaust makeup.

9.

Infringes least on perimeter floor space.

10.

Adapts to winter humidification.

System Disadvantages 1.

Requires additional duct clearance which can reduce the usable floor space.

2.

Air-balancing is difficult and requires great care.

3.

Accessibility to terminals demands close cooperation between architectural, mechanical and structural engineers.

Case studies See http://ug.arch.hku.hk/course/intgtech3/grad_hs/pt1/HVAC/index.html ; http://ug.arch.hku.hk/course/intgtech3/grad_hs/pt2/computer/air.html ;

http://arch.hku.hk/teaching/cases/shingmun/shingmun.html and http://arch.hku.hk/teaching/project/project502.html ; Distribution systems have a number of important components: 1. The Air Handling Unit is a cabinet that includes or houses the central furnace, air conditioner, or heat pump and the plenum and blower assembly that forces air through the ductwork. 2. The Supply Ductwork carries air from the air handler to the rooms in a house. Typically each room has at least one supply duct and larger rooms may have several. 3. The Return Ductwork carries air from the conditioned space back to the air handler. Most houses have only one or two main return ducts located in a central area. 4. Supply and Return Plenums are boxes made of duct board, metal, drywall or wood that distribute air to individual ducts or registers. 5. The Ductwork is a branching network of round or rectangular tubes generally constructed of sheet metal, fiberglass board, or a flexible plastic and wire composite material located within the walls, floors, and ceilings. The three most common types of duct material used in home construction are metal, fiberglass duct board, and flex-duct. 6. Flex-duct is installed between the register and plenum box, or plenum box and air handler, usually in a single, continuous piece. While flexduct has fewer seams, the inner lining and outer insulated covering can tear or be pinched closed. Also longer flex-duct runs can restrict the flow of air; proper design and installation is very important. 7. Both metal and fiberglass duct board are rigid and installed in pieces. Fiberglass duct board, like flex-duct, is made of an insulation material. Ducts are built of sections of the duct board. The seams in the duct board should be carefully sealed with mastic or high quality duct tape. 8. Rectangular metal duct, especially the kind used for plenums and larger trunk runs, is often insulated on the inside with fiberglass duct liner. If it is not insulated on the inside, metal ducts should be insulated on the outside using a fiberglass batt with an attached metal foil vapor retarder. The insulation should be at least two inches thick, and the vapor barrier installed on the outside of the insulation facing away from the duct. The seams in the insulation are usually stapled together around the duct and then taped. All of the seams should be sealed before insulation is installed. All return and supply ducts located outside the conditioned space, in attics, crawlspaces, or basements, for example, should be sealed and insulated. 9. Ductwork Joints join pieces of ductwork. 10. Elbows are manufactured pieces of duct used for turns. 11. Boots connect ductwork to registers. 12. Registers and Grilles are the coverings for duct openings into the conditioned space.

Fig 1b. shows the control of chilled cooling coil and fan

These are components will bring about:-Heat balance: The amount of heat extracted out of the air conditioned room (by the cooling system, exhaust air systems, building leakage, must be equal to the

amount of heat generated inside the room (by human being, electrical appliances, etc.) and transferred into the room (by conduction through the building envelope, radiation via the glass, hot air leakage into the room through gaps in windows, doors, fresh air introduced into the room, etc.) i.e. Total kW going into room = Total kW going out of the room.

-Air balance: The mass flow rate of the air going into the room = The mass flow rate of air going out of the room. Fresh air coming into the room : 2.5 l/s per person, non-smoking, 5 l/s per person for smoking accommodation, good indoor air quality (IAQ) is important. 1.1

Single Zone System The all-air single-zone air conditioning system is the basic central system which can supply a constant air volume or a variable air volume at low, medium or high pressure. Normally, the equipment is located outside the conditioned space but can also be installed within the conditioned are if conditions permit. Typical applications include:-Space with uniform loads -Small spaces requiring precision control -Multiple systems for large areas

Fig 1.1 1.2

Reheat System The reheat system is a modification of the single-zone system. It provides:-Zone or space control for areas of unequal loading. -Heating or cooling of perimeter areas with different exposures. -Close control for process or comfort applications. In the reheat system, heat is added as a secondary process to either preconditioned primary air or recirculated room air. The heating medium can be hot water, steam or electricity. Advantages : Closely controls space conditions Disadvantages : Expensive to operate

Fig 1.2 1.3

Variable Air Volume System The variable air volume system compensates for varying cooling loads by regulating the volume of cooling air supplied through a single duct. (a)

Simple Variable Air Volume (VAV) Simple VAV systems typically cools only and have no requirement for simultaneous heating and cooling in various zones.

(b)

Fig 1.3a Variable Air Volume – Reheat It integrates heating at or near the terminal units. It is applied to systems requiring full heating and cooling flexibility in interior and exterior zones. Heating is turned on when the air flow reaches a predetermined minimum.

Fig 1.3b

Advantages a) b)

Capital cost is lower since diversities of loads from lights, occupancy, solar and equipment of as much as 30% are permitted.

c)

Virtually self-balancing.

d) e) f)

1.4

When combined with a perimeter heating system, it offers inexpensive temperature control for multiple zoning and a high degree of simultaneous heating-cooling flexibility.

It is easy and inexpensive to subdivide into new zones and to handle increased loads with new tenancy or usage if load does not exceed the original design simultaneous peak. No zoning is required in central equipment. Lower operating cost because Fans run long hours at reduced volume (i) Refrigeration, heating and pumping matches diversity of loads (ii) Unoccupied areas may be fully cut-off (iii)

g)

Reduced noise level when the system is running at off-peak loads.

h)

Allows simultaneous heating and cooling without seasonal changeover.

Dual Duct System The dual-duct system employs two air ducts to supply cold air and warm air to a mixing terminal unit which proportions the cold and warm air in response to a thermostat located in the conditioned space. The system is well suited to provide temperature control for individual spaces or zones.

Fig 1.4 Advantages (in addition to those common to all air systems) 1.

Systems with terminal volume regulation are self-balancing.

2.

Zoning of central equipment is not required.

3.

Instant temperature response is achieved because of simultaneous availability of cold and warm air at each terminal unit.

4.

No seasonal changeover is necessary.

Disadvantages

1.5

1.

Initial cost is usually higher than other VAV systems.

2.

Does not operate as economically as other VAV systems.

Multi-zone System The multi-zone system applies to a relatively small number of zones served by a single, central air-handling unit. Different zone requirements are met by

mixing cold and warm air through zone dampers at the central air handler in response to zone thermostats.

Fig. 1.5 Advantages (in addition to those common to all-air systems) 1.

Easy to balance.

2.

Air transmission and distribution is simplified.

2.

Central chilled water A/C systems - Air-and-Water Systems An air-and-water system is one in which both air and water (cooled or heated in central plant room) are distributed to room terminals to perform cooling or heating function. The air side is comprised of central air conditioning equipment, a duct distribution system, and a room terminal. The supply air, called primary air, usually has a constant volume which is determined by: 1.

The ventilation requirement.

2.

The required sensible cooling capacity at maximum cooling load.

3.

The maximum sensible cooling capacity following changeover to the winter cycle when chilled water is no longer circulated to the room terminal.

The water side consists of a pump and piping to convey water to heat transfer surfaces within each conditioned space. The water is commonly cooled by the introduction of chilled water from the primary cooling system and is refereed to as the secondary water loop. Individual room temperature control is by regulation of either the water flow through it or the air flow over it. 2.1

Induction System The inducting system is designed for use in perimeter rooms of multi-storey, multi-room building that may have reversing sensible heat characteristics. It is especially adapted to handle the loads of skyscrapers with minimum space requirements for mechanical equipment. In the induction system, ducted primary air is fed into a small plenum chamber where its pressure is reduced by means of a suitable damper to the level required at the nozzles. The plenum is acoustically treated to attenuate part of the noise generated in the duct system and in the unit. The primary air is then delivered through nozzles as high velocity jets which induce secondary air from the room and over the secondary coil. Induction units are usually installed at a perimeter wall under a window. Some hotel rooms are provide with induction coils.

Fig. 2.1 The induction system employs air ducts to convey treated air with higher pressure levels and of the right adjustable quantities to various cooling/heating coil units. These coil units are built in with induction nozzles such that when high pressure air goes through them, air room the room is inducted across the fin surface of the water-circulated coils. This inducted air stream is either cooled or heated after passing through the coil, and then mixed with the air coming out

of the nozzle. The right quantity of high pressure air is adjusted automatically in response to a thermostat located in the conditioned space. The system is well suited to provide temperature control for individual spaces or zones. Advantages 1.

Individual room temperature control.

2.

Separate sources of heating and cooling for each space available as needed to satisfy a wide range of load variations.

3.

Low distribution system space required as a result of reducing the air supply by use of secondary water for cooling and high velocity air design.

4.

Reduced size of central air handling equipment.

5.

Dehumidification & filtration performed in a central plant room remote from conditioned space.

6.

Outdoor air supply is positive.

7.

Minimal maintenance required for individual induction units which have no moving parts, i.e. no fans

8.

Air duct dimensions are smaller than VAV systems or CAV systems

9.

Zoning of central equipment is not required.

10.

No fan comes together with the coil, making the conditioned space quiet.

Disadvantages

2.2

1.

Limited to perimeter space.

2.

The primary air supply is usually constant with no provision for shutoff.

3.

Not applicable to spaces with high exhaust requirement.

4.

Higher energy consumption due to increased power required by the primary pressure drop in the terminal units.

5.

Controls tend to be more complex than for all-air systems.

6.

A low chilled water temperature is needed to control space humidity adequately.

7.

Seasonal changeover is necessary.

8.

Initial cost is usually higher than fan coil systems.

Fan-Coil System The fan-coil system is similar to the inducting system, with the induction unit replaced by the fan-coil unit. The basic elements of the fan-coil units are a finned-tube coil and a fan section. The fan section recirculates air continuously from within the perimeter space through the coil which is supplied with either hot or chilled water. Auxiliary air may be delivered to the conditioned space for dehumidification and ventilation purposes.

Fig 2.2a

Fig 2.2b Advantages (in addition to those for induction units) 1.

System can be operated with the primary air turned off.

2.

The air velocity is fairly constant regardless of the primary air quantity.

3.

Primary air can either connect directly to fan-coil unit or supply the room separately.

Case studies See http://ug.arch.hku.hk/course/intgtech3/grad_hs/pt1/HVAC/index.html ; http://ug.arch.hku.hk/course/intgtech3/grad_hs/pt2/computer/air.html ; 2.3`

Two-pipe Systems In two-pipe systems for induction coil, fan-coil or radiant panel systems, the water distribution circuit consists of one supply and one return pipe. The secondary water is cold in summer and intermediate seasons and warm in winter. The primary air quantity is fixed and the primary air temperature is varied in reverse proportion to outside temperature to provide the necessary amount of heating during summer and intermediate seasons. During winter cycle operation, the primary air is preheated and supplied at about 10°C to provide a source of cooling.

Fig 2.3 Advantages 1.

Usually less expensive to install than four pipe systems.

Disadvantages

2.4 `

1.

Less capable of handling widely varying loads or providing widely varying choice of room temperature than four-pipe systems.

2.

Cumbersome to change over.

3.

More costly to operate than four-pipe systems.

Three-pipe Systems Three-pipe systems for induction coil, fan-coil and radiant panel systems have three pipes to each terminal unit, a cold water pipe, a warm water pipe and a common return. These systems are rarely used today because they consume excess energy.

Fig 2.4 2.5`

Four pipe Systems Four-pipe systems have a cold water supply, cold water return, warm water supply and warm water return. The terminal unit usually has two independent secondary water coils, one served by hot water, the other by cold water. The primary air is cold and remains at the same temperature year-round.

Fig 2.5a

Fig 2.5b Advantages (as compared with two-pipe systems) 1.

More flexible and adaptable to widely varying loads.

2.

Simpler to operate (No summer-winter changeover and primary air reheat schedule).

3.

Higher efficiency due to lower operating costs.

Disadvantages 1.

3.

Higher initial cost.

Central chilled water air conditioning systems - All-water Systems All-water systems are those with fan-coil, unit ventilator, or valance type room terminals with unconditioned ventilation air supplied by an opening through the wall or by infiltration. Cooling and dehumidification is provided by circulating chilled water through a finned coil in the unit. Heating is provided by supplying hot water through the same or a separate coil. System Advantages 1.

Flexible and readily adaptable to many building module requirements.

2.

Provides individual room control.

System Disadvantages

3.1

1.

No positive ventilation is provided unless wall openings are used.

2.

No humidification is provided.

3.

Seasonal change over is required.

4.

Maintenance and service work has to be done in the occupied areas.

Fan-coil units

A fan-coil unit basically consists of a finned tube coil, a filter and a fan section. The fan recirculates air continuously from the space through the coil, which contains either hot or chilled water.

Fig 3.1a

Fig 3.1b

Ceiling Building Services in 2/F Staff area, Dept. of Architecture, Knowles Building HKU

Fig 3.1c 3.2

Central chilled water air conditioning system with fan coils and other devices In this system, the following circuits do not mix with each other, and heat exchange is performed via various metal surfaces:-the chilled water circuit – nominally 12 deg .C entering water chiller, 7 deg. C leaving chiller, i.e. nominally 7 deg .C entering fan coil units [FCU] /air handling unit[AHU] /primary handling unit[PAU]- for treating fresh air, 12 deg. C leaving these devices – chilled water pumps move water through this circuit – CH. W. F- chilled water flow ; - CH. W. R- chilled water flow return. -refrigerant circuit – refrigerant compressors move the refrigerant through this circuit -cooling water circuit - nominally 35 deg .C entering water cooling tower , 30 deg. C leaving cooling tower, i.e. nominally 30 deg .C entering condenser of chiller assembly, 35 deg. C leaving condenser of chiller assembly – Condenser water pumps move condenser water through this circuit. See attached diagram and See http://www.ekingair.com/Screw.files/frame.htm

3.2

Water cooling tower A water cooling tower cools the water entering it from 35 deg. C to 30 deg. C nominally. The warmer water is sprayed inside the cooling tower admidst the stream of an upward air flow produced by the fan at the top of the tower. The air stream going out carries water particles. These water particles should not be taken into buildings, to avoid Legionnaire disease to occur. Condenser water pumps move condenser water through this circuit. Water in this circuit has to be treated. There is water loss to atmosphere in using cooling towers. For cooling towers, see http://www.engnet.com.tw/lc/A2.HTM , and

http://www.engnet.com.tw/lc/LRC-SAS.HTM

4.

Direct expansion Systems

4.1

Direct expansion Systems [i.e. direct expansion of refrigerant , without the chilled water cooling medium ] -Window Air Conditioners

[i.e. direct expansion of refrigerant, without the chilled water cooling medium ]

A window unit is an encased assembly designed primarily for mounting in a window, through a wall, or as a console. These units are designed for comfort cooling and to provide delivery of conditioned air to a room either without ducts or with very short ducts. They include a prime source of refrigeration, dehumidification, means for circulating and cleaning air, and may also include means for ventilating, and/or exhausting and heating.

Fig 4.1a

Fig 4.1b

Fig 4.1c

Fig 4.1d In a window air conditioner, the indoor unit and outdoor unit of the split system is put into one single unit. The refrigerant compressor now is part of the machine locating at the window area. Since this compressor gives out most noise, among other components, the window unit will make the room acoustically inferior to other air conditioning systems. Fresh air exchange for the room can be provided by :-(1) setting the “ventilator” switch of the window air conditioner to “open” position -(2) installing a ventilating extract fan in the room to extract room air to outside – caution- not to oversize the fan -(3) naturally leaking of air in and out of the room See http://www.aep.com/energyinfo/res_html/recrefr.htm and http://www.aep.com/energyinfo/res_html/recroom.htm 4.2

Direct expansion Systems [i.e. direct expansion of refrigerant , without the chilled water cooling medium ] -Unitary and Rooftop Air Conditioners

Fig 4.2 • These are commonly air-cooled units. •

The units are the floor – standing type designed for installation outdoors or on the roof.

•

A supply air duct and a return air duct are to be connected to the cooling unit.

• •

Application: For general air conditioning of stores, residences, schools, offices, etc. particularly suitable for single flat building with extensive floor areas. A remote controller should be installed on an easily accessible wall, incorporating a temperature selection switch & thermostat.

A case study of installing large unitary air conditioners for a church [ See photos and diagrams ] -church building built in 1945-1950, with no air conditioning -Air conditioning added to the church in 1990 with minimum interior change – only small portions of the windows inside the church hall were changed for fitting supply air grilles and return air louvers - the large unitary air conditioners were supported by s mild steel frame from the ground, detached from the church hall building, giving practically no structural loading problem and machinery vibration problem to the church hall -the fan power and the grilles were selected to prevent the use of distributing dir ducting inside the church hall. - 3 large unitary air conditioners were installed respectively to serve the front, middle and rear part of the hall, allowing partial operation of the hall with minimum use of air conditioning -fresh air mixes with return air before going through the air conditioner. Part of the cooled air coming out of the air conditioner will exhaust into the entrance hall before leaving the church building. 4.3

Direct expansion Systems [i.e. direct expansion of refrigerant , without the chilled water cooling medium ] - Split type and package air conditioning systems •

package air conditioning systems - Factory assembled (floor mounting) package, placed indoor, containing direct expansion coil, controls, fan and compressor, with the condenser remotely placed outdoor ; commonly used in Hong Kong for restaurants, café shops, factories, etc

Fig 4.3a

Fig 4.3b •

split air conditioning systems - Factory assembled (ceiling mounting) indoor unit of fan and direct expansion coil, controls, with the condensing unit [i.e. compressor and condensing coil ] remotely placed outdoor ; commonly used in Hong Kong for café shops, small offices, some domestic units, etc Case study – see http://arch.hku.hk/teaching/project/project302.html 4.3.1. The basic concepts of a split air conditioning system [ Small system] a. A split air conditioning system consists of an indoor unit and an outdoor unit connected together by refrigerant pipes. The refrigerant circulates between these 2 units [i.e. 2 parts of the system] to take heat from indoor to outdoor, by firstly having heat of the room air absorbed into the refrigerant via an airrefrigerant heat exchanger which is the indoor unit, then conveying the heat to the outdoor unit for disposal. See http://www.aep.com/energyinfo/res_html/reccent.htm b. The indoor unit comprises a finned coil and a fan which is driven by an electric motor. Refrigerant is circulated inside the finned coil to the outside unit and then back to the indoor unit. The fan pulls or pushes air around the outer surfaces of the coil inside the indoor unit, taking warm air from the room and injecting cooled air into the room in summer. The refrigerant has no direct contact with air. So the heat of the room air is transferred into the refrigerant in the indoor unit. Inside the coil, refrigerant evaporates, and the indoor unit is therefore commonly called an evaporator by the engineers. The indoor unit is wallmount or ceiling mount unit. See http://www.daikin.be/home.nsf/fFRM?ReadForm c. The outdoor unit The refrigerant then takes the heat from the indoor unit to the outdoor unit, which is commonly called a condensing unit. [ i.e. a unit for refrigerant to condense] In an air-cooled outdoor unit, heat exchange occurs in the same way as the indoor unit. However, the outdoor unit contains a refrigerant compressor, in addition to having a finned coil and motor-driven fan. The refrigerant does not have direct contact with air. Refrigerant going through this outdoor coil is losing its energy across the metal surface of the coil to the atmosphere, as outside air is drawn pass the surface of the finned coil by the fan. By passing through this finned coil, the outside air is heated up, by normally about 5 deg. rise in temperature. The outside air passing through the outdoor unit is an open circuit. That is, air path is not recirculated. See http://www.daikin.be/home.nsf/fFRM?ReadForm The refrigerant compressor, which usually is installed inside the outdoor unit, is pumping the refrigerant through the indoor unit and the outdoor unit. [ In the split system therefore the compressor – generating noise when pumping refrigerant- is located outdoor, inside the outdoor unit] The refrigerant takes up energy as it goes through the indoor unit, and rejects energy to the outside atmosphere as it goes through the outdoor unit. Energy rejected is the sum of the energy taken indoor plus the energy consumed by the compressor in pumping the refrigerant through the refrigerant circuit. This refrigerant circuit is a closed circuit, and if pipe joints are well installed , no leakage of refrigerant should occur. d. Air circuits for the indoor environment. The air passing through the indoor unit is cooled, say to 15 deg. C, before recirculated back to the room. A large part of air heated up in the room, say to 25 deg. [ Note : Design room temperature is 23 deg C in general for human comfort ] then goes back to the indoor unit for cooling. A small part of room air is extracted to outside by an exhaust fan, with an amount of fresh outside air coming in to replenish this

amount exhausted. Now this make up air can be supplied by connecting a small air duct from an external opening to the indoor unit. See diagram attached e. Single splits and multiple splits -single split – one indoor unit is connected to one outdoor unit by insulated copper refrigerant pipes -multiple splits– several indoor units are connected to one outdoor unit by insulated copper refrigerant pipes See http://www.daikin.be/home.nsf/fFRM?ReadForm See also http://www.ambthair.com/multisplit.html for a design guide f. Energy saving options If heat rejection in the outdoor unit is taken care by cooling water , there would be a saving of 30% of energy. In urban areas,. cooling water can be provided by fresh water cooling towers. The water cooling tower can be placed at the top of a building, with a pump drawing water from it to circulate the condensing water to the outdoor units of the split system. After taking up heat from the outdoor unit, with an increase of unusually 5 deg. C, condensing water is circulated back to the cooling tower for cooling again. Of course the finned coil f the outdoor unit has to be replaced by a water –cooled condenser. See “water cooling tower” below. Another efficient option is to have a few additional valves and controls added to the basic split system to make it to serve as a heat pump system in winter. That is, the refrigerant will do a reverse job in taking heat from outside , and rejecting it to the indoor environment. Thus the refrigerant goes through a reversed cycle by taking heat from outside and rejecting heat to the room. One unit of energy for pumping the refrigerant will cause 4 units of energy to be taken from outside, i.e. ,totally 5 units of energy, into the room. For heat pumps, see http://www.spec-net.com.au/hitachi/reverse1.htm, http://www.nrc.ca/irc/cbd/cbd195e.html , http://www.heatpumpcentre.org/tutorial , http://www.iaheatpump.org/winter.html , http://www.heatpumpcentre.org/tutorial/buildings.htm , g. A variant of split air conditioning system - A packaged system If the refrigerant compressor of the outdoor unit of the split air conditioning system is installed together with the indoor unit, it is called a packaged system. The compressor now is put indoor, making the machine less quite than the split system. However this will allow a larger cooling capacity for the indoor unit, which then will be floor-mount usually. A packaged system is needed if the outdoor unit, now called a condenser, is put on the roof top, with the indoor unit a few floors below. h. Direct expansion air conditioning equipments consist of factory-matched refrigeration cycle components for inclusion is air-conditioning systems which are field designed to meet the needs of the user. The following list of variations is indicative of the vast number of types of unitary air conditioners presently available. 1. 2. 3. 4. 5. 6.

Arrangement: single or split. Heat rejection: air-cooled, evaporative condenser, water-cooled. Unit exterior: decorative for in-space applications, functional for equipment room and ducts, weatherproofed for outdoors. Placement: floor standing, wall-mounted, ceiling suspended, roof-mounted. Indoor air: vertical upflow, counterflow, horizontal, 90° and 180° degree turns, with fan, or for use with forced air furnace. Locations: Indoor - Exposed with plenums or furred in ductwork; concealed in closets, attic, crawl spaces, basements, garages or equipment room. Wall - Built-in, window, transom. Outdoor -Rooftop, wall-mounted or on ground.

4.4

Heat Pumps The term ‘heat pump’, as applied to a year-round air conditioning system, commonly denotes a system in which refrigeration equipment is used in such a manner that heat is taken from a heat source and given up to the conditioned space when heating service is wanted, and is removed from the space and discharged to a heat sink when cooling and dehumidification are desired. Heat pumps for air conditioning service may be classified according to a)

type of heat source and sink.

b)

Heating and cooling distribution fluid.

c)

Type of thermodynamic cycle.

d)

Type of building structure.

e)

Size and configuration.

4.3.1 Air-to-Air Heat Pumps The air-to-air heat pump is the most common type of heat pumps. It is particularly suitable for factory-built unitary heat pumps, and has been widely used for residential and commercial application. Air is used as the heat source and heat sink. Extended surface, forced convection heat transfer coils are normally employed to transfer the heat between the air and the refrigerant. When selecting or designing an air-source heat pup, two factors in particular must be taken into consideration: 1) 2)

the variation in temperature experienced in a given locality. the formation of frost.

4.3.2 Water-source Heat Pumps The water-source heat pump uses water and air as the heat source or heat sink depending on the mode of operation. When cooling, water is used as the heat sink, and the heat pump operates as a water-cooled air conditioner. When heating, water is used as the heat source and the equipment operates as a water chiller. The water-source heat pump is suitable for many types of multi-room buildings, including office buildings, hotels, schools, apartment buildings, manufacturing facilities and hospitals. Advantages 1.

Affords opportunity for energy conservation by recovering heat from interior zones and/or waste heat and by storing excess heat from daytime cooling for night time heating.

2.

No wall openings required.

3.

Longer expected life than air-cooled heat pumps.

4.

Lower noise level because condenser fans are eliminated.

5.

Energy for the heat pumps can be metered directly to each tenant.

6.

Total life cycle cost frequently compares favourably to central systems when considering relative installed cost, operating costs, and system life.

Disadvantages 1.

Space required for boiler, heat exchanger, pumps and heat rejector.

2.

Higher initial cost than for most other multiple-packaged unit systems.

3.

Reduced air flow can cause the heat pump to cycle cutout. Good filter maintenance is imperative.