The various steps to get the same are as under:

• Step 1 (Outdoor Design Condition):  The outdoor design conditions are taken from the Climatological Table. Where data for particular station is not available, nearest available with almost similiar condition and lattitude may be referred. For Summer season it may be normal design condition or maximum design condition. The normal design conditions are the simultaneoulsly occuring dry-bulb and wet-bulb temperatures and moisture content which can be expected to exceed a few times a year for short periods. The maximum design conditions are recommended for laboratories or industrial applications where exceeding the room design conditions for even short periods can be detrimental to process. The maximum design dry-bulb and wet-bulb temperatures are simultaneous peaks (not individual peaks).

  In Department the outdoor conditions are determined in the following manner:

  • SUMMER CONDITION:

    1. Select three months in which afternoon temperature remains highest.
    2. Note highest DB temperature of selected three months.
    3. Note afternoon DB & WB temperatures of selected three months.
    4. Read the grains of moisture per pound of dry air with the help of pschrometric chart corresponding to above DB & WB.

    Then selected condition is arrived as below:

    • Design DB = Average of the highest DB temperature (°F).
    • Design grains of moisture = Max moisture grain in 3 months.
    • Design WB and RH can be read from pschrometric chart corresponding to above two conditions.

  • MONSOON CONDITION:

    1. The calculation for monsoon condition is same as above, but it is based on the datas of thre months when rainfall is mamximum.

  • WINTER CONDITION:

    1. Select three months in which forenoon temperature remains lowest.
    2. Note lowest DB temperature of selected three months.
    3. Note forenoon DB & WB temperatures of selected three months.
    4. Read the grains of moisture per pound of dry air with the help of pschrometric chart corresponding to above DB & WB.

    Then selected condition is arrived as below:

    • Design DB = Average of the lowest DB temperature (°F).
    • Design grains of moisture = Max moisture grain in 3 months.
    • Design WB and RH can be read from pschrometric chart corresponding to above two conditions.
• Step 2 (Inside Condition) : Note down the inside design conditions to be maintained.

• Step 3 : Work out fresh air requirement.

  cfm (required) = Volume × No. of air changes per hour / 60 (considering one air change for switch room; for other areas 4/3 air change may be taken).

• Step 4 : Mark the orientation of the building.
• Step 5 : Calculate the glass area, net masonary wall area for each direction.

• Step 6 : Calculate the partition wall area, ceiling, floor & roof area etc.

  Area of ceiling shall not be considered if the space above is air conditioned, similarly the area of the floor shall not be considered when the space below is air conditioned. Partition walls dividing the two air conditioned spaces shall not be considered.

• Step 7 : Find out the occupancy & heat load due to equipment & light./li>

• Step 8 : 

  1. Calculate solar heat gain through glass (radiation) using the following formula :

     Solar heat gain through glass = Area × Solar Heat Gain (from table 15 corresponding to 4:00 PM in required month; mostly May for Summer and July for Monsoon) × Heat Gain factor (from table 15 Corr and table 16).

     For ordinary glass Heat Gain Factor may be taken as 1 otherwise find it out from table 21-25 for wall; 27-28 for roof (only without storage gain is considered).

  2. Calculate solar/trans heat gain through walls and roofs using the following formula :

     Solar heat gain through wall and roof = Area x [equivalent temperature difference ( refer from table 19 for walls; table 20 for roof) + correction factor ( taken from table)]x transmission coefficient ( refer the relevant table)

  3. Transmission heat gain to conditioned space except through walls and roofs ( i.e. due to all glass, partition wall, ceiling & floor ):

     Here the same transmission coefficient or U-factor (table) is used for glass, walls, ceiling and floor. For temperature difference of partition walls and intermediate floors, assume the inside temperature as 50F from the actual using the following formula:

     Heat gain = Area (ft²) × U value × (outdoor temperature - Inside temperature - 5 °F)

  4. Infiltration and outside air heat gain is calculated as below :

     Infiltration Heat gain = 1.08 x cfm x temperature difference

     outside air heat gain = 1.08 × cfm x temperature difference × bypass factor.

     Bypass factor : [ 0.1 for DX system ] [ 0.2 for chilled water system ]

  5. Internal Sensible heat gain

     1. due to occupancy = number of people × 240 Btu/h.
     2. due to equipment = kW × 3410 Btu/h.
     3. due to light = kW × 3410 Btu/h.

     Room sensible heat total = room sensible heat subtotal + duct heat gain @ 12.5 % = A (say)

  6. Room Latent heat :

     1. Outside air latent heat = 0.68 × cfm × grain difference x bypass factor.
     2. Infiltration air heat gain = 0.68 × cfm × grain difference
     3. Latent heat gain due to people = number of people × 310

     Room latent heat (B) = room latent heat subtotal + 10 % heat gain through duct

  7. Outside Air Heat Gain :

     Sensible heat = 1.08 × cfm × temperature difference × (1 - B.F.)

     Latent heat = 0.68 × cfm × grains difference × (1 - B.F.)

     Outside air heat gain = O.A.S.H + O.A.L.H = C (Say)

     Where,

     O.A.S.H = Outside air sensible heat gain

     O.A.L.H = Outside air latent heat gain

     Total heat gain = A + B + C

     Heat gain through return duct @ 5% of (A + B + C) = say D

Grand total heat (G.T.H) = A + B + C + D

TR required = (G.T.H. / 12 000)

Now effective Sensible heat factor = (Room Sensible Total heat / room total heat) = A / (A + B)

Select A.D.P from either table or Psychometric chart

Where,

A.D.P = Apparatus dew point.

Dehumidified air rise = (1 - B.F.) x (Room temperature - A.D.P.)

Dehumidified air taking 20 % extra = (Room Sensible heat (A) × 1.2) / (1.08 × Dehumidified rise)

The various tables related with heat gain from various appliances/human beings, though not much useful for the table for calculation of heat gain in Telephone Exchanges, nevertheless is given below: