HVAC

Design

Design principles of HVAC can be obtained from guidelines presented in ANSI / ASHRAE Standard 90.1, ASHRAE (2004b), ASHRAE (2005), MEWR (1996), works done by Bearg (1993); McDowell (2006); Mull, 1997; Ross, (2004) etc. and the scoring system is developed for the same. However, accessibility has been reported to be the main requirement for high maintainability.

AHU and FCU

Air handling unit (AHU) is a factory made encased assembly consisting of fan(s) and other equipments to perform one or more of the functions – circulating, cleaning, heating / cooling, dehumidifying and mixing of air but does not contain a source of heating or cooling. On the contrary, function of fan coil unit (FCU) is limited to only circulation, cooling and filtration. It doesn’t have any source of heating or cooling.

For larger buildings, AHU combined with VAV boxes are more energy efficient and flow rate can be changed by changing speed of supply fan motor and VAV terminals. Number of FCUs equivalent to AHU-VAV combination is very high and maintenance is expensive. Moreover a FCU receives lesser amount of water as its distance from chiller increases. To overcome this, ‘reverse-return’ header with additional piping or auto flow control valve or regular testing & balancing (TAB) to throttle the flow is required. Moreover FCUs usually have constant air volume (CAV) operation. Hence for a large building FCU is not economical and more suitable for smaller buildings.

Chiller

A chiller is essentially comprised of an evaporator, condenser, compressor and various control devices. Refrigerant carrying the heat of return air comes into contact with cold water or air from cooling tower and simultaneously undergoes various thermodynamic cycles and finally returns to AHU at a very low temperature. Efficiency of chiller is enhanced by increasing in evaporator temperature and reducing condenser temperature, but it should be in tandem with other components of HVAC system. It is important to produce exact amount of tonnage as the requirement varies from time to time.

Cooling tower

Water from chiller rejects heat to the atmosphere at cooling tower and goes back to chiller again to cool down the refrigerant. Apart from its cooling efficiency, and quiet operation, the other major concern about cooling tower is legionella bacteria. Cooling towers are factory assembled tested products. As its parts are always in contact with water, it is essential to prevent corrosion, scaling and biofouling.

Design of air distribution system

Mainly ducts, terminals (supply air diffuser and return air grille), variable air volume (VAV) box and controllers constitute air distribution system. Supply air moves through ducts; get distributed inside the air-conditioned space through the outlets. After circulating through the room, it goes back to the AHU as return air via ceiling plenum. Design is a conscious balance between energy efficiency (large ducts, filters and coils) and available space to route the ductwork through areas that often contain other services.

Grading for design of AHU and FCU

Factors	Grading criteria		Grade
AHU room & housing
Location	In spite of precautions, AHU may generate some noise or vibration. Hence isolation by proper planning is desirable.
	In acoustically treated plant room located in service area.		5
	In acoustically treated plant room near occupancy space.		3
	Above ceiling.		1
Enclosure	AHU needs to handle both temperature and pressure. Hence a good leakage resistant and thermally insulated construction is mandatory. Access opening should not affect these desirable properties. Metal wall panels bolted into structural frame and strong enough to handle 1.5 times maximum anticipated operating pressure with less than 1% leakage of the design air volume. Fire proof insulation of min 2” thick fibreglass (NFPA 90A#) sandwiched between two layers of galvanised sheet metal. The insulation should also be able to attenuate noise. Door of same construction as the unit casing and of full height. Neoprene rubber gasket installed between the frames and around door. The gasket is fixed with high quality mastic.		Yes: 5 No: 1
Access	AHU door: Min width of 600mm or 2ft and held against air pressure. Access panel: individual sections (filter, motor, cooling coil, fan etc) are accessed by min (600 x 600) mm removable glass panels. The panels have double skins of reinforced / plexus glass fitted in a gasket frame and air space sealed in between. Clearance: min 800 mm for at least 3 sides of all components for easy inspection, services, repairs and replacement without disassembling. A haphazard or cluttered layout should be avoided. No equipment should be located at very high level or obstructed by structure or services.		Yes: 5 No: 1
Equipment mounting	Correct mounting techniques can reduce noise and vibration significantly. Concrete inertia pad with isolators is must for AHU at ground floor and highly recommended for AHUs at higher levels (PWD, 1997). Min 50mm horizontal clearance between isolators and concrete bases. Min 50mm vertical clearance between all isolated equipment and the building structure.		Yes: 5 No: 1
Isolator material	Steel spring isolators – strong and corrosion resistant.		5
	Elastomeric mounts.		3
	Resilient mounts.		1
Services	Fluorescent lamps fitted in non-corrosive and vapour tight luminaries. Each accessible service compartment should have separate lighted toggle switches placed on AHU exterior. Fitted with smoke detector and fire damper (SS CP 13). A tap installed within the AHU room for easy cleaning of cooling coil. All monitoring gauges to be installed at average height of 1.5m		Yes: 5 No: 1
	Air intake & distribution
Air intake location	At external or roof to pick outdoor air free of contaminant or odours. Hence min 5m away from any exhaust discharge/ kitchen, toilet, car park, cooling tower, rubbish dump or plant room (SS CP 13). Should have easy access for inspection and maintenance. Above a height of min 2.1m from ground for safety reasons.		Yes: 5 No: 1
Protection of intake	Protection from	Preventive feature	Yes: 5 No: 1
	Insect	½” mesh screen cover
	Rain	Wall louvers of extruded aluminium sloped at 45 ° to outdoor drain or fitted with a pipe to machine room floor drain.
	Intake velocity > 5m/s	Adjusted size of wall louvers
Fan & motor	Size should match cooling coil capacity. Else moisture carry over occurs. Fan belts should be completely covered by guards with access slots/holes. These slots allow use of contact type tachometer for testing rotating shafts of motors and fans at any position. Guards shall be readily removable for maintenance and adjustment.		Yes: 5 No: 1
	Condensate /drain pan
Material	Pan is in constant contact with water. Should be corrosion & leakage proof.
	Stainless Steel + minimum of 2” thick polystyrene insulation on outside.		5
	Galvanized Steel.		3
	Mild Steel.		1
Drainage	Condensate should be totally drained by: Drainage connection from bottom of casing. Pan sloped from all directions towards the drain. To maintain a proper slope, pan is fitted on concrete housekeeping pads of min height 4”. Drain pan extends to the entire length of cooling coils including headers and return bends and at least 43mm deep. Secondary condensate pan provided. For cooling coils two or three bank high, an intermediate drain pan with plastic drain tubes extending into the main drain pan should be installed on the air leaving side of each coil. A U-bend on pipes leaving from AHU and a break between condensate drain pipe and floor trap prevents carry over of water.		Yes: 5 No: 1
Drain off pipe detail	Min 2” uniform dia. to drip off condensate indirectly into floor drain. ‘Tees’ with cleanout plugs and unions for maintenance of turns, traps, etc. Piping is insulated with polystyrene material. Pipe is elevated from ground with bracket at max interval of 1m.		Yes: 5 No: 1
	Filters
Selection	Selected as per efficiency (arrestance, spot and fractional), minimum efficiency reporting value (MERV) and available space (ASHRAE Standard 52.2). With pleats both the efficiency and the space requirement increase.		Yes: 5 No: 1
	Disposable pre-filter + primary + secondary filter		5
	Disposable pre-filter + primary filter		3
	Primary filter only		1
	Various types of air filters
Layout	Located at suitable position. Min. 1m space in front of filter bank for cleaning, media removal and inspection.		Yes: 5 No: 1
Pressure gauge	Used to calculate pressure differential. for each filter bank. Located outside the casting for easy access and measurement.		Yes: 5 No: 1
	Cooling coil
Material	Coil and coil headers: copper Fins: aluminium. Support for coil and casing. stainless steel Note: Copper fins are used only in aggressive environment (e.g. marine).		Yes: 5 No: 1
Coil row	The layout should facilitate heat transfer and easy cleaning of scales which may blanket its thermal conductivity. No of rows per coil £ 8. For more rows more coils are recommended. Coils should be separated by min 30” at both the entering and exiting side of coil faces. This facilitates maintenance. Spacer between coils to limit face velocity of airflow within 2.5m/s.		Yes: 5 No: 1

Grading for design of FCU

Factor	Grading criteria	Grade
Location	Away from obstructing structures and services. Air intake location and its protection same as AHU.	Yes: 5 No: 1
Housing	Units are internally lined with min 25mm thick fibreglass and neoprene.	Yes: 5 No: 1
Access	Access panel (min 600×600 mm) located min 1m away on filter side. Access panels not blocked by structural steel, piping, or other obstruction. Clearance or easy access of all equipment for maintenance or replacement without disassembly. Piping should be installed next to access panel to facilitate maintenance.	Yes: 5 No: 1
Isolator	Units suspend from structure with rubber-in-shear vibration isolators i.e. rubber hangers (Schaffer, 1991).	Yes: 5 No: 1
Cooling coil	Copper tubes, with mechanically bonded aluminium fins. Spacer between coils to limit face velocity of airflow within 2.5m/s.	Yes: 5 No: 1
Drain pan material	Same as AHU.	Yes: 5 No: 1
Drainage	Same as AHU.	Yes: 5 No: 1
Drain off pipe	Min 25 mm uniform dia. to drip off condensate indirectly in floor drain. ‘Tee’s with cleanout plugs and unions for maintenance of turns, traps, etc. Piping is insulated. Pipe is elevated from ground with bracket at max interval of 1m.	Yes: 5 No: 1
Filter type	Disposable pleated.	5
	Disposable glass fibre.	4
	Washable foam.	2

Grading for design of chiller

Factor	Grading criteria					Grade
Capacity	Continuous running or frequent start-stop can shorten the lifespan of a chiller. Similarly for variable load requirement, running a chiller of higher capacity is wastage of energy. Hence a balanced design is essential (Law, 2003). Min 1 chiller that can handle the minimum operation loading at any time and does not go below 20% of its cooling capacity. E.g. combination of 2 small and 2 chillers. Routing in the event of replacement of any chiller is provided.					Yes: 5 No:1
Refrigerant	There are various parameters to be considered for refrigerant selection. Performance: power consumption, load on compressor (Hughes, 2003). Environmental impact: ozone depletion potential (ODP), global warming potential (GWP), thermal efficiency impact. Compatibility: should not react with tubing, seals, motors and wiring. Also not miscible with lubricants.
	Chlorofluorocarbon (CFC): banned by Montréal protocol, 1996.					1
	Hydro-chlorofluorocarbon (HCHC): will be phased out by 2020 (UNEP 2002)					3
	Other suitable hydro-fluorocarbon (HFC) refrigerants (e.g. R134A, R123).					5
Compressor	From variety of compressors, the selection is made depending on application size and requirement. Each type has its own pros and cons. For e.g. the chances of compressor drive shaft leakage is least in hermetic design, but its parts cannot be repaired at site.					Yes: 5 No:1
	Compressor		Displace	Design	Suitable for
	Reciprocating		Positive	Fully hermetic	Fractional– 10 T
				Semi hermetic	2-150 T
	Rotary screw/ vane		Positive	Semi/ fully hermetic	<100 T
	Rotary scroll		Positive	Hermetic	<100 T
	Centrifugal		Dynamic	Hermetic	80-several 1000T
	Various types of compressors
Condenser	For efficient heat transfer from hot refrigerant, the condenser pipes circulating cold water must remain free from rust, scale or dirt. Individually replaceable tubes made of integrated, finned, high efficiency copper tubing rolled into tube sheets and expanded into support sheets. Drain and vents with removable cover in water boxes facilitate tube cleaning. In-built automatic tube cleaning system is required.					Yes: 5 No:1
Purge units	Purge units improve refrigeration efficiency by removing non-condensable air and water from refrigerant. Else the condenser pressure is artificially high or moisture may react with refrigerant and form acid which can cause corrosion.					Yes: 5 No:1
	Advantages of automatic / thermal type (Ananthanarayanan, 2005): Lower operating temperature causes lower refrigerant loss. The pump doesn’t lie in the refrigerant path and no oil carry over to main system. Hence it handles much less moisture and rarely requires overhaul. Automatically monitors presence of air and purge pump works only when the pressure of non-condensable exceeds the predetermined value. More efficient moisture removal by condensation.					5
	Conventional					3
	Not provided					1
Mounting	Chiller has operating weight different from the installed weight. Hence special precaution should be taken to prevent vibration and noise during operation. Steel spring vibration isolators with neoprene acoustical friction pads between acoustical baseplate and support. But vertical limit stops are used to prevent spring extension when the load is removed (PWD, 1997). Inertia block of min 1.5 times of chiller weight. Ensure discharge line and the condenser shell to be acoustically insulated.					Yes: 5 No:1
Thermal insulation	Chilled water pipes should be prefabricated with insulation. Insulation material: closed-cell phenolic foam or pre-formed phenolic foam. Parts subjected to condensation (water shell, water boxes, suction connections to the compressor etc) require insulation as follows:					Yes: 5 No:1
	Servicing	Insulation type
	Regular	Removable type insulation box made of sheet metal covers filled with granulated cork.
	Not required	Min 25 mm thick first quality insulation with tough vapour proof skin.
Leak detector	Bypass for each socket (Sugarman, 2007).					Yes: 5 No:1
Plant room	Adequate space for maintenance – especially for cleaning of the condenser tubes and replacement of bigger components. Adequate acoustical insulation for both within and beyond the plant room.					Yes: 5 No:1

Grading for design of cooling tower

Factor	Grading criteria			Grade
Design	Simple and practical design without dead-legs, loops, bends or redundant pipe-work to prevent dirt accumulation and bacteria growth. Easy access to all parts of the system for inspection sampling, cleaning and disinfection is must. The space constraints in design should be considered.			Yes: 5 No:1
Type	Closed or indirect: Chiller water moves in a closed coil and rejects heat to the external water circuit which gets cooled by air. Hence the chiller water is not contaminated.			5
	Open or direct: Chiller water comes in direct contact of air and atmospheric pollutants.			3
Material	Easy cleanability, resistance to corrosion and chemicals are the basic criteria. All material should be opaque to sunlight as it promotes legionella growth.			Yes: 5 No:1
	Type	Element	Material
	Structural / heavy	Supports, cylinder, water basins, casing & fan deck	Heavy gauge hot deep galvanised steel
	Non-structural lightweight	Casing	FRP, rigid PVC
		Tower fill	High heat exchanging rigid PVC
		Drift eliminator	PVC film type
		Louver	Rigid PVC, stainless steel
Location	Min 5m from any source of exhaust air and other sources of warm air (e.g. kitchen, toilet) which may supply nutrients for bacterial growth. Away from fresh air intakes or the downwind of air intakes for the building or any occupied place. Louvers or other screening devices must not interfere with tower air flow, such as enclosure that restricts the air flow into or away from the tower and cause a recirculation of discharge into inlet.			Yes: 5 No:1
Access	Sufficient space for maintenance on all sides for each tower. Access doors located in each end wall casing for entry into the cold water basin and fan plenum area. Doors should be operable from both sides. Safe access via ladders to fan decks and hot basins, internal access platforms, stairs and catwalks over and around roof mounted piping.			Yes: 5 No:1
Vibration & noise reduction	Adequate size and location of the concrete bases fitted with steel spring isolator. Use of low noise multi-blade axial flow type fan.			Yes: 5 No:1
Legionella prevention	Isolation valves between cells or the supply and return piping, so that individual towers can be shut down for maintenance (MEWR, 2001). Easy dismantling, removal or replacement of infill & drift eliminators. Min. possible drift / spray along with an opaque enclosure of the tower pond.			Yes: 5 No:1

Grading for design of air distribution system

Factor

Grading criteria

Grade

Duct material

Comply with SMACNA standards (1995) for acceptable materials, material thicknesses and duct construction methods.
Sheet metal should be free from pitting, seam marks, roller marks, stains, discolorations and other imperfections.
Fire resistant material and construction (NFPA 90A and 90B).
Considerations for sealing materials and acceptable leakage rates.
Prefabricated and tested in factory with neatly finished joints and bends.

Yes:5

No:1

Insulation

To prevent heat gain and condensation, insulation used are (Gill,2000):

External: double sided aluminium foil with fibre glass or mineral wool
Internal: rigid rock wool insulation.

Yes:5

No:1

Return air path

Ducted for easy maintenance controlled mould growth.

Conventional – non ducted through ceiling plenum.

Air velocity

To meet the requirement of noise criteria (NC) of a space, ducts are designed to produce minimum possible noise and turbulence. For this purpose, the sizing is balanced with the space constraints.

Low:5

Mid:3

High:1

Access

Cover for access panels easy to open.
Access panels should not affect the integrity of the thermal, acoustic or fire insulation of ducts.
Unobstructed but safely protected access panels at suitable location and spacing. To be provided for each dimensional change, directional change, 7.5m of length, top or bottom of riser (BS EN 12097).

Adequate size of access opening (BS EN 12097)

Yes:5

No:1

Wet exhaust

Wet exhaust ducts that tend to carry moisture should be pitched downwards towards the source of moisture, with drainage facilities.

Yes:5

No:1

Location of terminals

Min 1m distance between supply air diffuser and return air grille.
Away from entrances or other sources of warm air.
Position not blocked by structures or services.
Easy access: e.g. not on high ceilings.
Design schemes suitable to type of ventilation – mixing or displacement.
Min 1 supply diffuser and 1 return grille for each enclosed area.

Yes:5

No:1

Terminal type

Uniform distribution and ease of cleaning are main focus (Int-Hout, 2004).

Louvered / round diffuser: integral conical or pyramidal transformation allows supply air to expand.

Linear slot diffuser: fairly uniform distribution of supply air.

Egg crate grille: partially obstructed unidirectional flow.

Various types of diffusers (Kavanaugh, 2006)

VAV box accessi-bility

Location of box and access panel not blocked by structures and services.
Access panels fitted on the sensors side.
Min size of access panel is 600 x 600 mm.
Adequate clearance for all services, repairs and component replacement without dismantling.

Yes:5

No:1

Damper selection

Opposed blade type with electrical control. It ensures uniform flow.

Direct blade type with electrical control

Damper blade arrangement: opposed and direct

Controller location

Location should facilitate accurate sensitivity and calibration.

Min one thermostat for each air-conditioned zone.
Should be away from the elements that can affect its sensitivity. E.g. supply air diffusers, entrance or any other sources of warm or cool air

Yes:5

No:1