Main Highlights of this MEP Specification
- Air Handling Units
- Fans - General Requirements
- Centrifugal Fans
- Axial Flow Fans
- Propeller Fans
- Protectively Coated Fans and Special Enclosures
- Mechanical Roof Extract Units
- In-Line Centrifugal and Mixed Flow Fans
- Twin Fan Units
- Cooling Coils
- Air Heater Batteries
- Air Filters
- Fan Coil Units
- Packaged Air Conditioners
1. Air Handling Units (AHU) of Air Conditioning And Ventilation Installations
General Requirements for AHU
All individual components of packaged air handling units shall comply with the appropriate Sections of this Specification unless otherwise specified within the Schedules or on the drawings.
Construction of AHU
Each AHU assembly shall be of compact construction and shall consist of a rigid metal framework, with insulated double skin sheet metal panels. The metal shall be 1.2mm minimum thickness strengthened as necessary to prevent distortion and drumming. The insulation shall be 25mm thick and comply with the relevant clauses in the section covering insulation.
The panels shall be easily removable using quick release fasteners to obtain access for inspection and maintenance. All items shall be assembled by means of bolts, anti-vibration lockwashers and nuts or accepted quick release fastenings. All units shall be mounted on accepted type anti-vibration mountings.
Dampers, Filters and Fans Used in AHU
Where air mixing sections are required the damper controls shall be suitable for connection to a single damper motor; a motor mounting bracket shall be provided. The arrangement of dampers shall ensure adequate mixing. Dampers shall be of the opposed blade multi-leaf type.
Unless otherwise indicated in the Particular Specification, a flat panel pre-filter and bag type main filter shall be provided, each in accordance with Clause 12 of this Specification.
Both fresh air and recirculated air shall be filtered.
Fans shall be of either forward or backward bladed centrifugal type; unless otherwise indicated fan motors shall be mounted internally in the unit and there shall be means of adjusting the motor position. The fan and motor assembly shall be provided with anti-vibration mountings to isolate vibration from the casing.
Fans fitted to all air handling units shall be provided with external lubricators arranged to terminate adjacent to the access door to the fan section of the air handling unit.
Anti-Condensation Features for AHU
Where units incorporate humidifying plant and/or cooling coils the internal surfaces of the units liable to be affected by any free water produced shall be protected by anti-corrosion paint or a similar finish. Such units shall have adequate drain trays to collect water; the drain trays shall be extended, or other means of collection shall be provided, to ensure the removal of any water deposited or condensed in adjacent sections. Trays collecting cooled water shall be insulated to prevent condensation on the outer surface, and shall be provided with a trapped outlet to drain.
Thermal Insulation of AHU
Thermal insulation shall be provided between the inner and outer members of the double skin panels and in any hollow members of the framework. The insulation shall be of the closed-cell foamed plastic type, with a minimum thickness of 25mm, and a maximum conductivity of 0.036W/m oC.
Additional insulation shall be provided as required to prevent any risk of condensation on the external surfaces.
2. Fans - General Requirements in Air conditioning and Ventilation System
Fans shall be capable of giving the specified performance when tested in accordance with BS 848. Although estimated values of the resistance to airflow of items of equipment may be indicated this does not relieve the Contractor of the responsibility for providing fans capable of delivering the required air volume through the system.
The make and design of fans shall be accepted by the Engineer and evidence supporting claimed noise levels and fan efficiencies shall be submitted for acceptance prior to placing the order. Where fans are supplied with noise attenuators full details of the attenuators shall be given.
Belt driven fans shall be fitted with pulleys suitable for V-belts; pulleys of the taper lock type may be used for drives up to 30 kW output. Alternatively, and in any case above 30 kW output, pulleys shall be secured to the fan and the motor shafts by keys fitted into machined key-ways. Pulleys shall be keyed to the fan shaft in the overhung position. Keys shall be easily accessible so that they can be withdrawn or tightened and they shall be accurately fitted so that the gib head does not protrude beyond the end of the shaft.
Machined bolts, nuts and washers only shall be used for the assembly of fans; all bearing surfaces for the heads of bolts or washers shall be counter-faced. Holding-down bolts for fans and motors shall be square section under the head or be fitted with lugs to prevent them turning in the fan baseplate when the nuts are tightened. Any fan which is too large or too heavy for safe manhandling shall be provided with eye-bolts or other lifting facilities to enable mechanical lifting equipment to be used.
3. Centrifugal Fans in Air-conditioning and Ventilation System
Unless otherwise indicated centrifugal fans larger than 7.5 kW output shall be of the backward curved blade type having fan total efficiencies of not less than 78%. Below 7.5 kW fans may be either forward curved or backward curved type. Fan casings shall be constructed of mild steel plates with angle stiffeners and base angles to ensure freedom from drumming and shall be suitable for operation at the maximum static pressure of the system. Fan casings shall be constructed so that impellers can be easily withdrawn after installation. Outlets shall be flanged and inlets shall be flanged or spigotted as indicated except that for suction pressures greater than 1000 Pa inlets shall be flanged.
A drain and plug shall be fitted to the fan casing at its lowest point. For all kitchen extract ventilation fans an access door, for inspection and cleaning, shall be fitted to the scroll casing in an accessible position; it shall be the full width of the impeller.
Impellers shall be of mild steel (or aluminium where indicated) of riveted or welded construction, with spiders or hubs of robust design, and shall be capable of running continuously at ten per cent in excess of normal speed. Impellers shall be keyed to a substantial mild steel shaft and the impeller with shaft shall be statically and dynamically balanced and tested for over-speed before leaving the maker's works.
Shaft bearings of belt driven single inlet fans shall be truly aligned and rigidly mounted on a pedestal common to both bearings.
Double inlet, double width fans shall have a pedestal mounted bearing at each side of the fan.
Fan bearings shall be ring oiled sleeve bearings, or ball or roller type. Where silence is important the bearing pedestal shall not be attached to the fan casing, and ring oiled sleeve bearings shall be applied.
Unless otherwise indicated centrifugal fans shall be driven by electric motors through V-belt drives.
The maximum fan outlet velocity shall be 10 m/s.
4. Axial Flow Fans in Air-conditioning and Ventilation System
Axial flow fans shall be either the single stage type or the multi-stage contra-rotating type with each impeller mounted on an independent motor, and having an efficiency of not less than 60%. Casings shall be rigidly constructed of mild steel stiffened and braced to obviate drumming and vibration. Cast iron or fabricated steel feet shall be provided where necessary for bolting to the base of supports. Inlet and outlet ducts shall terminate in flanged rings for easy removal. The length of the duct casing shall be greater than the length of the fan(s) and motor(s) in order that the complete section may be removed without disturbing adjacent ductwork. Electrical connections to the motor(s) shall be through an external terminal box secured to the casing.
Impellers shall be of steel or aluminium; the blades shall be secured to the hub or the blades and the hub shall be formed in one piece.
The hub shall be keyed to a substantial mild steel shaft and the whole statically balanced. Blades shall be of aerofoil section. Shafts shall be carried in two bearings which may be ball, roller or sleeve type. Lubricators shall be extended to the outside of the casing.
Where axial flow fans are driven by a motor external to the fan casing the requirements for pulleys and V-belt drives and guards as specified elsewhere shall be met. Unless otherwise indicated a guard is not required for any part of a drive which is inside the fan casing. An access door of adequate size shall be provided.
Where axial flow fans of the bifurcated type are indicated the motors shall be out of the airstream. Motors may be placed between the two halves of the casing in the external air or may be placed within the fan casing provided that effective ventilation is given to the motor. Where hot gases or vapours are being handled the motor and the bearings shall be suitable for operation at the temperature they may experience.
Fans which do not connect to a suction duct shall be supplied with a bellmouth inlet.
5. Propeller Fans in Airconditioning and Ventilation System
Impellers shall be of steel or aluminium; the blades shall be fastened to the hub or the blades and hub shall be formed in one piece. The bearings may be ball, roller, or sleeve type. Propeller fans may be ring mounted or diaphragm mounted in a casing, as indicated. Where they are mounted in a casing the casing shall be longer than the length of the fan and motor.
The casing shall be of steel, shall have flanged ends and shall incorporate an inspection door. A terminal box shall be mounted externally on the casing. The tip speed of fans shall not exceed 20 m/s.
Propeller fans shall not be installed to work against a resistance of more than 50 Pa.
6. Protectively Coated Fans and Special Enclosures
Protectively coated fans shall meet the appropriate requirements of the preceding clauses relating to fans generally and to particular types of fans; the form of protection shall be as indicated. Where a protective coating is required for use with corrosive gases the coating shall cover all parts of the complete fan, motor and casing assembly which will be in contact with the corrosive gases.
No fan shall be installed if the protective coating has been damaged in any way. Flame-proof enclosures shall comply with the requirements of BS 229 for the appropriate gas group.
7. Mechanical Roof Extract Units in Airconditioning and Ventilation System
The fans used in roof extract units shall meet the appropriate requirements of the preceding clauses relating to fans generally and to particular types of fans. Cowls and bases shall be of aluminium construction unless otherwise stated in the Particular Specification. Cases shall be formed so as to ensure a weatherproof fitting to the building structure.
Adequate access to motor terminals and lubricating points shall be provided by means of hinged cowls or otherwise as appropriate. Backdraught dampers and/or fire release dampers shall be provided where indicated.
8. In-Line Centrifugal and Mixed Flow Fans in Airconditioning and Ventilation System
In-Line centrifugal and mixed flow fans may be incorporated subject to the acceptance of the Engineer provided that full details of ratings, noise level, dimensions and construction are given in the tender. Fans of these types above 7.5 kW output will not normally be accepted.
9. Twin Fan Units in Airconditioning and Ventilation System
Duplicate fan assemblies shall comply with the appropriate sections of this Specification for the fans themselves and shall incorporate an automatic or manual changeover facility as indicated.
A non-return damper arrangement shall be fitted to prevent short circuiting.
10. Cooling Coils in Airconditioning and Ventilation System
All air cooling coil casings shall be of sheet steel not less than 1.2mm thick with angle framing at each end drilled ready to receive the counter flanges on the connecting ductwork and shall be galvanised after manufacture. Cooler batteries shall be constructed of copper tubes with aluminium fins, unless otherwise stated in the Particular Specification.
The secondary extended cooling surfaces shall be bonded to the primary cooling tubes. Headers for copper tube batteries shall be of copper or bronze. The bottom of a casing shall be made in the form of a watertight drip tray complete with drain connection.
The drain shall be fitted with a water seal to prevent the ingress and discharge of air to and from the system; the drain pipe shall be run to the nearest sump or gully.
A separate drain trough shall be provided for at least each 1.2 m height of coil. Water shall not be carried over from a cooler into the airstream; an eliminator section shall be fitted where necessary to prevent carry-over.
The resistance to air flow shall not exceed 125 Pa and the face velocity shall not exceed 2.5 m/s. Before leaving the maker's works coolers shall be tested to 1½ times the working pressure or to 7 bar whichever is the greater.
Eliminator plates shall be fitted if the face velocity exceeds 2.25 m/s.
Cooling Coils shall be supported so that their weight is not transmitted to ductwork and so that they can be removed without disturbing adjacent ductwork. Inspection doors shall be provided on both the upstream and downstream sides of the cooler. On any system where the air static pressure at the cooler exceeds 750 Pa airtight cover boxes shall be provided over the headers and bends; provisions shall be made for draining the cover boxes.
Coils with fins which have been badly damaged during transport or erection or which have become fouled or coated with builders materials will not be accepted.
Chilled Water Coils
Coils shall be arranged in a contra-flow pattern, with the flow of chilled water entering at the leaving air end and leaving at the entering air end. The flow and return connections and headers shall be so arranged to ensure an equal flow of water through all the tubes. Provision shall be made for expansion of the tubes, for effective venting of the cooler battery and connections and for draining of the cooler battery headers and tubes.
Connections up to and including 65mm bore shall be screwed or flanged; connections 80mm diameter and above shall be flanged. Isolating valves shall be provided on inlet and outlet connections, arranged so as to facilitate removal of the cooler.
Refrigerant Direct Expansion Air Cooling Coils
Coils shall be provided with refrigerant distributors, and connections to the tubes shall be designed to ensure an equal flow of refrigerant to each tube. Return (or suction) connections shall be arranged so as to ensure complete drainage of any oil in the cooler. Coolers shall be dehydrated and sealed after manufacture.
The thermostatic expansion valve which shall incorporate an external equaliser line shall maintain the design degree of superheat at the evaporator outlet; the remote sensing bulb shall be securely fixed to the evaporator outlet piping in a position where the degree of superheat can be correctly sensed.
All refrigerant spaces shall be tested at the maker's works to 1400 kPa for use with refrigerant 12 and to 2100 kPa for use with refrigerant 22.
11. Air Heater Batteries in Airconditioning and Ventilation System
Casings shall be of galvanised sheet steel no less than 1.2mm thick with angle framing at each end drilled ready to receive the counter flanges on the connecting ductwork. Heaters shall be supported so that their weight is not transmitted to ductwork and so that they can be removed without disturbing adjacent ductwork. Inspection doors shall be provided on both the upstream and the downstream sides of the heater.
The casing shall be airtight to meet the air pressure requirements of the system with appropriate arrangements to accommodate expansion where applicable.
Electric air heaters shall consist of a number of heating elements of the enclosed non corrodable type mounted in a steel casing. The elements shall be so installed that they can be removed for cleaning or renewal without dismantling ductwork. The surface temperature of the elements shall not exceed 150oC. A high temperature limit device with hand reset button shall be incorporated.
Each element shall be complete with a second set of terminals to operate a failure warning light.
The control of electric air heaters shall be interlocked with an air flow control of the vane or pressure type which shall isolate the heating elements from the electricity supply in the event of failure of air flow.
The heater shall be arranged with elements of the same number, or a multiple of the number, of steps as the control required. All heaters and heater sections of more than 3 kW loading shall be balanced over three phases and the complete heater bank shall be arranged for balanced operation on a 3- phase 4-wire system and be stepped by he controller in 3kW stages, unless otherwise indicated in the Schedules.
Below 3 kW the stepped loadings shall be in 1 kW increments. The connections from each element shall be taken to a readily accessible terminal box arranged for conduit entry. Each heater section shall be separately fused and the neutral point of all 3-phase star-connection sections shall be brought out to a link in the terminal box. Near any hot areas the wiring insulation shall be of appropriate quality.
The total resistance of the heater to air flow shall not exceed 25 Pa and the velocity through the free area shall not exceed 6 m/s.
The installation of electric heater batteries shall comply in all respects with the requirements of the local Fire Authority.
12. Air Filters in Airconditioning and Ventilation System
Filters shall operate to at least the efficiencies specified in this Section. They shall be complete with holding frames sufficiently robust to ensure that no distortion occurs in operation. All filters shall be installed with edge seals which shall prevent air bypassing the filters. The seals shall, remain effective even though the cells are periodically removed and refitted.
Filters shall be arranged so that there is easy access for cleaning and/or removal. A differential pressure gauge of the dial or inclined manometer type shall be provided for each filter bank and shall be fixed in such a position outside the duct system that it is accessible and easily read. The gauge shall be marked to show maximum pressure differential indicating a filter change requirement. Air velocity through filters shall be such that the clean resistance (or operating resistance of automatic filters) as indicated is not exceeded and that filter fabric or oil is not carried over into the system.
Filter assemblies shall be of a standard type and dimensions and shall be such that replacement filter medium is readily available from a number of manufacturers.
Where a flameproof filter medium is specified the material shall comply with the following requirements when tested in accordance with the relevant standard.
- Duration of flaming: No test specimen shall continue to flame for more than 8 seconds after the igniting flame has been removed.
- Extent of after-glow: After-glow shall not spread beyond the area of material damaged by flaming.
- Length of material which chars or melts: The average length of material which chars or melts on the six specimens shall not exceed 85mm and the maximum length of the charred or melted material in any one specimen shall not exceed 115mm.
Where washable type flameproof filters are offered or specified the filter medium shall comply with the requirements of clause (2) above both before and after the washing treatment prescribed in BS 3120 Appendix A.
Where the filter medium is required to be flameproof the casing shall be not less than 1.6mm thick for at least 1.8 m upstream and 1.8 m downstream of the filter; the immediate frames or cases of the filter elements shall be of material complying with BS 746 Part 1, Clause 7, Class 1.
Dry Replacement Media Type Filters
The filter shall be of the flat panel type contained in a galvanised steel front or side withdrawal frame. Each cell shall comprise a pad of glass fibre or synthetic fibre filter media, 50mm thick, retained within a rigid frame of durable cardboard. The filter shall have an efficiency of not less than 92% based on the tests specified in BS 2831 with test Dust No. 2, a maximum face velocity of 2 m/s and an initial pressure drop not exceeding 70 Pa.
Sufficient spares cells shall be provided to replace all of the filter bank.
The filter medium shall be processed washable open cell foamed plastic or plastic bonded synthetic fibre and shall have a minimum thickness of 15mm. The element shall be supported on plastic coated steel wire formers with a metal frame. The face velocity shall not exceed 2.25 m/s. Filters of this type shall have an efficiency of not less than 90% based on the tests specified in BS 2831 with test dust No. 2.
The filter shall comprise one or more 600 x 600 filter bag modules fitted into a purpose made galvanised steel side or front withdrawal frame.
Each module shall comprise a minimum of four separate bags banded or clipped into a galvanised steel header.
The bags shall have a minimum length of 600mm. and shall be of the fine medium to give an efficiency of not less than 99.6% based on the tests specified in BS 2831 with test dust No. 2. The medium shall be of multi-layer type with high mechanical strength. The maximum initial resistance shall be 100 Pa.
One complete set of spare bags shall be supplied.
Automatic Fabric Roll Type Filters
The filter shall comprise the complete assembly of filter frame, motor and drive and filter medium. The filter shall operate automatically, with provision for manual starting and stopping of filter movements; unless otherwise indicated automatic filter movement shall be controlled by the resistance across the filter. Provision shall be made for visual warning that the end of the clean filter medium is approaching. The driving motor shall be automatically switched off when the end of the filter medium is reached.
Unless otherwise indicated the resistance of the filter shall not exceed 125 Pa under normal operating conditions; the air velocity through the filter medium shall not exceed 2.5 m/s. Filters of this type shall have an efficiency of 95% based on the tests specified in BS 2831 with test dust No. 2.
Where the motor and/or gearbox is mounted in the air stream the electrical insulation and/or lubricants shall be suitable for the temperature range experienced.
A spare roll of filter medium shall be provided for every installed roll.
Grease eliminators shall be of the impingement type comprising fluoropolymer coated adjustable vertical baffles contained within a stainless steel casing. The bottom of the casing shall comprise a grease collecting trough, from which a drain shall be provided into a removable stainless steel grease collector.
The baffles shall be supplied in modules with a maximum size of 500mm x 500mm, each readily removable for cleaning. The use of mesh type grease eliminators will not be permitted.
High Efficiency Filters
High efficiency filters shall consist of asbestos cellulose or glass fibre pleated paper media in a rigidly constructed case with a completely positive seal.
The efficiency of the filter shall be equal to or better than that indicated and in any case shall be not more than 0.5% penetration based on the tests specified in BS 2831 with methylene blue or the sodium flame test specified in BS 3928.
Where fire protection is required or the air temperature may exceed 200oC the filtering medium shall be glass fibre paper, and the casing, spacers and seals shall all be of fire-resistant material.
The air velocity at the face of the filter shall not exceed 1.25 m/s. Filter changing arrangements must be such that dust is not released into the room during the changing operation. Two, or 10% whichever is the greater spares shall be provided.
Activated Carbon Type Filters
The cell casing shall be of steel protected against corrosion. The internal arrangement shall include a corrosion-proof framework of supports to ensure an equal disposition of individual panels across the cell, through sealing between panels and mechanical protection to the front and rear of the panels.
The carbon shall be of uniform bed thickness in the panels and shall be sufficiently densely packed to ensure that no settling down or gaps occur in use. For each rated 0.5m3/s capacity, 20 kg of carbon shall be provided. The resistance to air flow shall not exceed 125 N/m2.
13. Fan Coil Units (FCU) in Airconditioning and Ventilation System
Fans, filters, cooling coils, heating coils, motors and thermal and acoustic insulation shall comply with the appropriate Sections of this Specification, with the following exceptions or alternatives:
1. Fans shall be of the forward curved centrifugal type and may be of aluminium, reinforced glass fibre or rigid plastic material.
2. Air Filters may be of washable synthetic fibre or cellular plastic material with a minimum thickness of 15mm and shall have a minimum efficiency of 90% when tested in accordance with BS 2831 using test dust No. 2.
3. Motors shall be quiet running and shall have sleeve bearings factory lubricated for life. Motor windings and electrical components shall be impregnated or protected to avoid trouble from condensation.
Casings of FCU
Casings shall be of sheet steel not thinner than 1.2mm and shall be protected against corrosion and finished inside and outside with stoved primer.
Casings of exposed units shall provide sufficient internal space to accommodate valves, controls etc. and shall incorporate return air grille and directional supply grille. The front panel shall be readily removable for access.
Concealed units shall be complete with supply air duct spigot and return air plenum. Valves and controls shall be external to the casing.
Cooling and heating coils shall be at least two-row and shall include an air cock. All units shall include an easily removable and replaceable air filter capable of treating the total air volume. Drain pans shall be either of sheet steel protected against corrosion or of plastic or reinforced glass fibre material and shall be arranged, and insulated where necessary, so as to ensure that no condensation is formed on the underside. Drain pans shall be large enough to collect all condensate from the coil, return bends and pipework connections; they shall be removable and shall have a slight fall to the drain connection.
Arrangement of Units
The arrangement of units, i.e. whether they be wall, floor or ceiling mounted, the position of inlet and outlet grilles if any, the need for sheet metal casing etc., shall be as indicated elsewhere.
Where units are concealed, the Contractor shall ensure that adequate access is provided for the removal of a complete unit.
Controls and Grilles
Fan coil units shall have a control giving three running speeds and an 'OFF' position. The specified duty shall be achieved using medium or low speed. Outlet grilles shall be adjustable to enable the direction of airflow to be reset. On floor mounted units, supply grilles shall be on the top of the unit.
Casings shall be lined with material to act as both thermal and acoustic insulation (Sections 10 and 11). Fan and motor assemblies shall be complete with anti-vibration mountings. Noise data shall be provided and shall be based on tests in accordance with the HVRA Test Code, or similar tests; full details of the testing technique and procedure shall be given. The data given shall include an octave band analysis of the sound power level of the unit.
14. Packaged Air Conditioners in Airconditioning and Ventilation System
Each unit shall comprise the following components, each of which shall comply with the appropriate sections of this Specification, except as otherwise indicated:
This shall consist of a heavy gauge steel chassis with sheet metal casing, minimum thickness 1.2mm and the whole shall be protected against corrosion and finished inside and outside with stoved primer. Casings shall include removable panels to provide ready access to all component parts.
Casings shall be lined with material to act as both thermal and acoustic insulation. Fan and motor assembles shall be complete with anti-vibration mountings. Noise data shall be provided and shall be based on tests in accordance with the HVRA Test Code, or similar tests; full details of the testing technique and procedure shall be given. The data shall include an octave band analysis of the sound power level of the unit.
Evaporator fans shall be of the belt driven forward curved centrifugal type and be of aluminium or galvanised steel construction. Split pulleys shall be provided to allow the fan speed to be adjusted.
Condenser fans shall be of the direct driven propeller type, with weatherproof motors.
Air filters shall be 50mm thick synthetic fibre or glass fibre material and shall have a minimum efficiency of 92% when tested in accordance with BS 2831 using test Dust No. 2.
Motors shall be quiet running and shall have sleeve bearings factory lubricated for life. Motor windings and electrical components shall be impregnated or protected to avoid trouble from condensation.
Compressors shall be hermetic or semi-hermetic type complete with resilient mountings.
Unless otherwise indicated condenser/evaporator coils shall be formed of copper primary surface tubes with aluminium secondary surfaces.
Drain trays and condensate disposal shall be as described in the relevant sections of this Specification.
Last Modified: 03-Nov-2018