How to size the chilled water pipe for AHU and FCU?

First Step -  We need to know the total cooling capacity of AHU or FCU. 

Let's say , Total Cooling Capacity of FCU is 22kW and Total Cooling Capacity of AHU is 250kW.

Second Step - We need to calculate the chilled water flow rate. 

Based on Sensible Heat Equation  : q = m c ΔT  

q  = total cooling capacity of AHU or FCU or Chiller  (kW)
c   = specific heat capacity of water  ( 4.19 kJ/kgK)
ΔT = Chilled water supply temp and chilled water return temp (K) . (Normally Ts- 6.7C and Tr- 12.2C)

m = Chilled water flow (kg/s) For Water (kg/s = l/s) Since density of water is 1000 kg/m3. 

For FCU - 22 kW , 

Chilled Water Flow Rate = q / ( c ΔT) = 22 / (4.19 x 5.5) 

So chilled water flow rate for FCU (22-kW) is 0.955 l/s (kg/s).

For Chilled Water Piping ,As per ASHRAE Guideline; Less than or equal of 50mm Pipe size ,

Water Velocity shall be less than 1.2 m/s . 

Normally we design 1 m/s for less than or equal 50mm pipe size . 

Based on Continuity Equation          Q = A V  

Q = Volumetric Flow Rate (m3/s)
A = Area ( m2 ) 
V = Average Water Velocity  (m/s)

A = Q / V = 0.955 x 10^-3 / 1 

A = 955 mm2 

r = 17.4 mm ( A = π r2 ) 

D = 34.8 mm  ,  Nominal Diameter is 32mm and Actual inside diameter of 32mm is 35mm. 

So that chilled water pipe size for FCU (22kW) is 32mm.

For AHU (250 kW) , 

q = mc ΔT 

m = Q = q/ c ΔT   = 250 / (4.19 x 5.5 ) = 10.85 l/s ( kg/s) 

Q = 10.85 l/s . 

For AHU Chilled Water Pipe size ,  As per ASHRAE Guideline ,

Water Pressure Drop Shall be not more than 400 Pa/meter run. 

Most of the project design are using 250 Pa/m  

Water Velocity shall be less than 2.4 m/s 


Based on Flow Rate - 10.85 l/s and not more than 250 Pa/m , Pipe size shall be 100mm. 

If we choose pipe size 100mm, Water Pressure drop shall be 175 Pa/m and

Water Velocity shall be 1.32 m/s . 

Thanks a lot. 

Zaw Moe Khine

RSED Engineering.