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How to size a pump and piping system for the hydronic system in HVAC.

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188 views asked Mar 28, 2015 by Amol

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The first step is to layout the piping system including connection to the all AC units, then from the units’ specification sheet (cutsheet from the manufacturer) find the required flow (gpm) for each unit and write on the drawing next to the each appropriate unit. Start from the end unit in piping system and add up the units’ flow (gpm) on each section of piping (between the units) and write down the amount of the flow on each piping section, continue all the way to the start point of the piping system.  By using the pipe sizing chart for hydronic system from ASHRAE handbook at line 4 psi/100 ft size the supply pipe for each section (the water velocity in each pipe shall not exceed 4 to 5 fps (foot per second)). The return pipe would be the same size of the supply pipe.

For sizing the pump:  Measure the longest run (from pump location to the last and farthest AC unit), assume longest length as “L” foot:

  • Multiply the L by 2 for supply & return pipe total length  
  • Multiply the total length of supply & return pipe by 1.30 (considering 30% of total length of the supply & return as an equalint length for the fittings such as elbows, tees, etc)
  • Therefore the developed length will be:

DL= L x 2 x 1.3 = 2.6L

  • Multiply the DL by 4 psi/100 ft to find the total piping system pressure drop (pressure loss). Assume that as Lp
  • Find the pressure drop in water coil of the farthest AC unit from the cutsheet (manufacturer spec), assume that as Cp .
  • Find the pressure drop in balancing valve and control valve for the farthest AC unit from the manufacturer cutsheet, assume them as BVp and CVp .
  • The pump head is equal to:

(Lp + Cp +   BVp + CVp) = Lt     

  • The pump head by considering 10% safety factor would be:

                          Pump head = 1.10 Lt

  • The pump flow is the total gpm of the all AC units in system.

                          Pump flow = Total AC units flow

answered Apr 3, 2015 by Joe Mahini

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