http://www.scribd.com/doc/111569258/Load-Calculation-Spreadsheets-Quick-Answers-Without-Relying-on-Rules-of-Thumb?secret_password=1lkzrbj8jnpsqznoyf54

PRECOOLED AHU CALCULATION

Just start with precooled AHU Calculation!!

When you want to size the precooled AHU,

you need to know fresh air volume and temperature set point.

Once you know this two info, you can size the AHU.

Just start calculate for 3000 l/s fresh air volume and set  point temp is 16 C.

We all know that, there are two kinds of heat for fresh air,Sensible and Latent Heat.

Cooling process for Sensible and De-humidification for  removing of moisture.

For Sensible Heat Load,(Cooling Process).

Q = m c ΔT 

Q = V ℓ c ΔT

Q = Cooling Capacity (kJ/s) (kW)

V = Air Volume (m3/s)

ℓ = Density of air (kg/m3) (1.204 kg/m3 for standard air )

c = Specific Heat Capacity of air (kJ/kgK) (1.004 kJ/kgK for standard air)

ΔT = Temperature differential.

m= mass of air (kg/s)

V= 3000 l/s = 3 m3/s. (1000 l/s = 1 m3/s)
ℓ = 1.204 kg/m3 for standard air @Sea level.

c = 1.004 kJ/kgK for standard air.
ΔT = Ambient temperature - Set  point temp. 

ΔT =(33 - 16 ) (Singapore Summer Ambient Temp - Set point temp)

Q  = 3 x 1.204 x 1.004 x ( 33-16)

Q = (m3/s) x ( kg/m3) x (kJ/kgK) x (K)

Q = 61.65 kW.

For Latent Heat Load, (De-humidification Process/Removing of Moisture )

Q = m hfg Δg

Q = V ℓ hfg Δg

Q = Cooling Capacity (kJ/s) (kW)

V = Air Volume (m3/s)

ℓ = Density of air (kg/m3) (1.204 kg/m3 for standard air )

hfg = heat of vaporization of water (kJ/kg) (2454 kJ/kg for standard air )
hfg= 2500 kJ/kg (Estimated for 16C)

Δg = Humidity Ratio (Moisture Content of dry air) (kg/kg)

m= mass of air (kg/s)

Q = V ℓ hfg Δg

Q = (m3/s) x ( kg/m3) x (kJ/kg) x (kg/kg)

Q = 3 x 1.204 x 2500 x  Δg

So that we need to see Psychrometric Chart for Humidity Ratio.
First Point is Ambient Temperature. (Singapore Summer Temp - (33 DB / 26.1 WB )
From Psychrometric Chart, g1 is 0.0186 kg/kg (18.6 g/kg) Based on 33DB/26.1 WB.
For Second Point, Supply Air Temp and Coil ADP or Coil Curve.
Coil ADP is based on Coil bypass Factor.
Normal Standard coil bypass factor is 0.1.
If bypass factor is 0.1 , Coil ADP is {16 - (33x0.1) }/(1 - 0.1) = 14.11 C.
Based on Supply Air temp 16C and Coil ADP (14.11 C),
From Psychrometric Chart , g2 is 0.0108 kg/kg (10.8 g/kg).
So that Δg = g1-g2

Δg = 0.0186 - 0.0108 = 0.0078 kg/kg.

Q = 3 x 1.204 x 2500 x  Δg

Q = 3 x 1.204 x 2500 x  0.0078

Q = 70.434 kW

Total Cooling Capacity for AHU is Sensible Load + Latent Load.

Total Load = 61.65 + 70.434 = 132.084 kW

NEXT OPTION CALCULATION WITH ENERGY METHOD

Q = m Δh
Q = V  ℓ  Δh

Q = Cooling Capacity (kJ/s) (kW)

V = Air Volume (m3/s)

ℓ = Density of air (kg/m3) (1.204 kg/m3 for standard air )

Δh = Enthalpy Differential of entering and leaving air to the coil (kJ/kg)

m= mass of air (kg/s)

Q = V  ℓ  Δh

V = 3 m3/s
ℓ = 1.204 m3/kg
Δh = Enthalpy Differential of entering and leaving air to the coil (kJ/kg)
Δh = Enthalpy of entering air (Off coil air) - Enthalpy of leaving air (On coil air)
From Psychrometric Chart,
First Point, h1 , Enthalpy of Entering Air is based on Ambient Air temp
(33 C DB/ 26.1 C WB), h1 = 81 kJ/kg.
Second Point, h2 , Enthalpy of Leaving air is
Based on Supply Air Temp  16C ,Coil ADP & Entering Point.

h2= 44 kJ/kg.
So that, Δh = h1-h2 = 81-44 
            Δh = 37 kJ/kg.

Q = V  ℓ Δh 

Q = 3 x 1.204 x 37

Q = 133.644 kW.