Correct heat load estimation factors

Correct heat load estimation

You can use E20 excel sheet or HAP software to calculate heat load. Interface of E20 excel sheet is shown in figure below

Heat load estimate

Video mentioned below I showed proper way for heat load calculation for summer and monsoon conditions using E20 spreadsheet 

Design conditions

For most of the comfort system the recommended indoor design conditions like temperature and relative humidity are as follows

Dy bulb temperature (DBT) = 22.78˚C to 26.11˚C and Relative humidity (RH) = 50% for summer
Dry bulb temperature (DBT) = 22.11˚C to 22.22˚C and Relative humidity (RH) = 20-30% for winter

The cooling load of the building should be selected is based on 23˚C (74˚F) dry bulb temperature and 50% relative humidity indoor design conditions

Outdoor design conditions are determined from published data for the specific location, based on weather bureau or airport records

Heat load calculation formulas

Following formulas must be used while determining heat load for some particular space

Heat load through walls & roof is given by formula
Q-Wall = U * A * CLTD --------- (1)
where
 Q Wall = Load through the walls in Btu/hr
U = Thermal Transmittance for walls in Btu/ (h ft2.F)
A = area of walls in ft2
CLTD = Cooling Load Temperature Difference for walls in °F

 
Heat load though glass is given by formula
Q-Glass Conductive = U x A x CLTD --------- (2)
Q-Glass Solar = Ax SCx SCL --------- (3)
where
Q Conductive = Conductive load through the glass in Btu/hr
Q Solar = Solar transmission load through the glass in Btu/hr
U = Thermal Transmittance for glass in Btu/ (h ft2.F)
A = area of glass in ft2
CLTD = Cooling Load Temperature Difference for glass in °F
SC = Shading coefficient
SCL = Solar Cooling Load Factor

 
Heat load through partitions, floor & ceiling is given by formula
Q = U x A x (Ta - Ti) --------- (4)
where
A = area of partition, ceiling or floor in ft2
Ta = Temperature of adjacent space in °F that is outside temperature minus 5 °F
Ti = Inside design temperature of conditioned space in °F

Heat load due to People is given by formula
Q sensible due to people = N x (QS) x (CLF) --------- (5)
Q latent = N x (QL) --------- (6)
Where
N = number of people in space
QS, QL = Sensible and Latent heat gain from occupancy
CLF = Cooling Load factor by hour of occupancy of people in space

Lighting load is given by formula
Q = 3.41 x W x BM x (CLF) --------- (7)
Where
W = Watts input from electrical lighting plan or lighting load data
BM = lighting ballast multiplier normally it's 1.02 to1.08
CLF = Cooling Load factor by hour of occupancy in space

Equipment load is given by formula
Q = 3.41 x W x (CLF) --------- (8)
Where
W = Watts input from electrical lighting plan or lighting load data
CLF = Cooling Load factor by hour of occupancy in space

Infiltration Air load is given by formula
Q sensible = 1.08 x CFM x (To – Ti) --------- (9)
Q latent = 4840 x CFM x (Wo – Wi) --------- (10)
 
Where
CFM = Infiltration air flow rate
To & Ti = Outside/Inside design dry bulb temperature in °F
Wo & Wi = Outside/Inside design humidity ratio in lb water/lb dry air
ho & hi = Outside/Inside design air enthalpy in Btu per lb (dry air)

So you have to calculate space sensible heat, solar & transmission gain through wall & roof, transmission gain through glass, partition, ceiling & floor, infiltration & by pass air load, Internal heat gain load due to people, lighting & equipment's, room latent heat load due to infiltration, outside air & people. After calculating all the loads you have to calculate total sensible & total latent heat load and get effective sensible heat ratio (ESHR) by formula given below

ESHR = Total sensible heat load / (Sensible heat load + Latent head load) --------- (11)

Once you get the ESHR then you will calculate total required load in btu/hr
let's say if we get total heat load of 30000 btu/hr then tonnage will be given as

Tonnage = Total heat load / 12000 --------- (12)
Tonnage = 30000/12000
Tonnage = 2.5

After tonnage you have to calculate apparatus dew point (ADP) and in the end you can calculate dehumidified CFM by given formula

Dehumidified CFM = Total sensible heat / (1.08 x dehumidified rise) --------- (13)
Where

 
Dehumidified rise = Contact factor x (Room temperature - ADP) --------- (14)

Contact factor = (1- by pass factor) --------- (15)

              
where by pass factor is normally 0.12 which is a standard value So contact factor can be calculated by using equation 15
Contact factor = (1- 0.12) = 0.88

let's say we got 54 deg F apparatus dew point (ADP) based on 75 deg F inside room temperature, 50% relative humidity & effective sensible heat ratio is 0.92 and total sensible heat is 24000 Btu/hr after performing load calculation

So dehumidified rise is given by equation 14

Dehumidified rise = 0.88 x (75 - 54) = 18.48 deg F

then dehumidified CFM is given by equation 13

Dehumidified CFM = 24000 / (1.08 x 18.48)

Dehumidified air flow rate = 1203 CFM

Air conditioning equipment selection

After the load is evaluated, the equipment must be selected with capacity sufficient to offset this load. The air supplied to the space must be of the proper conditions to satisfy both the sensible and latent loads estimated. Psychrometrics analysis should be done to determine the criteria from which the air conditioning equipment is selected.