Estimation of Air Conditioning Work


In order to procure and install package type air conditioning system, it is essential to have the details of various items of work involved and their quantities. To have the above information in consolidated form, an estimate is prepared. The work is split into two sub-heads: sub head-I deals with the package unit and sub-head-II deals with associated items of work. The schedule of quantity and rates of various item is worked out.

Table of Contents

Sub-head-I (Air cooled packaged AC Units)

Air-cooled packaged AC units are available in various capacities and department has approved certain manufacturers for their particular. The number & capacity of the AC units is being selected keeping in view the heat load requirement of the area to be air-conditioned and standby provision to be made as per norms. However flexibility of the system and space constraints are also the factors that cannot be neglected during selection of the AC units.

Sub-head-II (Associated work)

  1. Fresh air package: 

    Fresh air package consists of fresh air fan, fine filter, damper for volume control and mounting arrangement etc. The capacity of fresh air fan is so selected that it is suitable for one air change per hour of the area to be air-conditioned and to overcome the static losses due to fine filters etc.

  2. GI sheet ducting / plenum: 

    Proper distribution of air, in the area to be conditioned is made through the supply air ducts and branches. Shortest and straight route of the duct is followed, to the latent is possible. The size of plenum, supply air duct are chosen keeping in mind the air velocity recommended passing through the route. The gauge of sheet out of which the duct piece is fabricated depends upon the longest side of the duct piece.

    750 mm and below - 24 gauge
    751 mm to 1500 mm - 22 gauge
    1501 mm to 2250 mm - 20 gauge
    2251 mm & above - 18 gauge

    Suitable supply and volume control dampers shall be provided in the branch ducts for balancing the air quantities.

  3. Supply air outlets: 

    1. Wall outlets: Supply air/outlets are provided on the walls where the distribution of air is to be made through the walls and run of duct is along the walls. It is kept in mind that the free area of supply air outlet should not be less than 60% of total area of outlet. The supply air outlets should be accompanied with the volume control dampers. The locations of supply air should be uniformly distributed so that these may not arise the hot pockets in the conductance space.

    2. Ceiling diffusers: In case the air is to be distributed from the top side, the ceiling diffuses are adopted and same points are kept in mind as that of wall outlets.

    3. Damper mounting on false floor: If the supply air ducts are laid beneath the false floor, the damper mountings along with the perforate sheets are provided on the false floor at the outlets of supply air.

  4. Return air inlets: 

    Suitable no. Of return air inlets of suitable size may be provide keeping in mind the symmetry of the installation inside the room and recommended passing through that outlet.

  5. Fire Damper: 

    Fire damper with feasible links and spring arrangement may be provided in supply air duct. The size of fire damper is normally the size of supply air duct so that if fully restrict the passage in case of closure.

  6. Fine Filters: 

    Fine filters as per specifications and standard may be chosen and fixed in the supply air passage preferably in the supply air plenum.

  7. MV switch board (Main): 

    MV switch board is got keeping in view the load on panel and provision for future extension and emergency supply. The ratings of incoming and outgoing is calculated on the basis of working current of the package units, fresh air system and other electrical loads on the panel. The size of Bus bar is normally chosen 1.5 times the maximum current of the system. Provision of coupler should also be made for the main supply and emergency supply for isolation of units on emergency supply. The rating of coupler shall be chosen depending upon the essential no. of units and their current load.

  8. Power wiring: 

    1.1 kV grade PVC insulated PVC sheathed armoured cables preferably are laid on the cable trays / racks / walls as per requirement. The size of the cable is chosen on the basis of current carrying capacity of cable and load requirement. The length of cable is calculated on the basis of linear measurements of cable route from supply end to the load point.

  9. Control panel / console & safety control interlocks wiring: 

    Operators room is equipped with the control panel console showing the operational status of the package units and giving the various failures/faults occurring time to time with re-settable hooter facility.

    Sequence of air-conditioning cycle / refrigeration cycle is maintained with the help of interlocking made and tripping arrangement in case of failure of sequence.

  10. Earthing: 

    The total system shall be properly earthed the panels etc may be got earthed with strip earthing and the power cables for package unit should accompany the earth wire for loop earthing. The quantity of earth strip and earth wire is total sum of linear measurement of run of cables.

  11. Thermal insulation: 

    1. Walls of AC unit room except the walls adjoining the AC space are insulated with the 25 mm resin bonded glass wool insulation.
    2. Glass windows & other openings in the wall may be insulated with 50 mm thick resin bonded insulation.
    3. Supply air ducts shall be insulated with 25 mm thick glass wool cladding overloaded with aluminum foil.

    The quantity may be calculated on the basis of wall area and supply air ducts area.


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