Custom Air Handling Units | Coil Selection Guidelines

I have included a collection of notes that we have pulled together over the years that guide us through typical coil selections for custom air handling units. 

Coils Basics:

  • Header Orientation –  Supply low leaving, return high entering
  • Coil Hand –  Face the entering air side of the coil to determine its hand connection. With air at your back which side (hand) are the headers on?
  • Plate Fin Type – Much more common, easier to clean
  • Spiral Fin Type – Less common.  Less pressure drop. Spiral Fin has advantage in steam heat coil as separate tubes can handle higher thermal expansion.
  • #Rows / Circuits = # of Passes

 

Coil Construction:

  • Tubes–  Copper is standard 
    • 5/8″ Tube O.D. is standard.  1/2″ Tube O.D. is more typical on DX
    • Tube thickness for water coils usually starts at  .020”, but we usually specify a .025” or .035”.   Belled ends allow for larger U-bends and less pressure drop.  We typically use them
  • Fins – Aluminum is standard  –  standard fin thickness .0075”, we sometimes use .0095” for hospitals/labs/high quality, .006 for lower cost

  • Casing– Galvanized is OK on Steam or HW Coils but Chilled Water (CW) coils are typically 16 gauge 304 stainless steel
    • typical casing is 1-1/2” leg but on most coils if spacing is tight can be 0.75” or 1”
  • Headers–  either Carbon Steel or Copper Headers are standard 
    • often use “non-ferrous headers” for cooling coils where there will a lot of moisture and condensation
    • MPT (Male Pipe Thread) is typical for coil connections
  • Coil Sizing – Fin Height
    • heating coils max FH of 60”  is max reasonable handling size in a shop
    • cooling coils – max FH for individual cooling coils with a drain pan expected to have significant condensation is 42” since condensation from upper part can cut airflow in bottom part
  • Coil Sizing – Fin Length
    • with no glycol, fin length can be whatever is needed up to 180”.  With glycol the WPD goes up significantly with higher FL so probably need to break big coils up into shorter length sections.
    • typical max coil width or length in an AHU is FL + 15” to outer dimensions of 2” wall AHU
  • Coatings – if a coil is going in unit within 3-5 miles of the ocean or in an area where corrosion might be a factor, need to include coating.  Electrofin is the best, good up to any number of rows, highly flexible so won’t chip easily. 

Selection Criteria:

  • Face Velocities
  Coil Type Allowable Velocity
  Hot Water (HW) 200 – 1200 FPM
  Steam 200 – 1200 FPM
  Chilled Water (CW) 200 – 550 FPM
  DX 200 – 550 FPM
    • Typical Hot Water Coil – 600 – 700 FPM
    • Typical Chilled Water Coil – 500 FPM
    • On a Cooling Coil – Air velocities above 500 fpm is the point where water droplets can leave the outer edge of the fin.  This is called moisture carry-over.   We design to avoid this.
  • Air Pressure Drop –  We typically design for less than an inch on all applications and look for a pressure drop near 1/2″.

  • Water Coil Pressure Drop –  25’ Water Pressure Drop (WPD) is about maximum – use bell ends to decrease pressure drop.
    • for HW coils, WPD up to 10ft is usually OK
    • for CHW coils, WPD up to 15-20ft is usually OK
  • Tube Velocity – stay below 8 ft/sec,  3 to 6 ft/sec is typical.  Number is higher with glycol
    • Optimize velocity through circuiting
  • Reynolds Flow Number –  under 2100 can cause laminar flow.  Can either change circuit (preferred) or add Turbulators (turbulators add pressure drop)
  • Max GPM – typically listed by connection sizes
  • Glycol –  Polypropylene (propylene glycol = PG) is more commonly used antifreeze
  • Fouling Factors –  Use 0 if you can.  Typical #’s are Ris (inside) .0005  Ros (outside) .001

Other Recommendations:

  • If space allows, making coil section larger to reduce velocity.  Moving to 300-400 FPM velocity adds a small % first cost but saves big in supply fan HP over life of unit. 
  • Glycol % has a drastic negative effect on transfer properties making GPM, coil and piping sizing go up. Always question unit design and operation if they request 40%
  • If coils are to be in a large VAV AHU application, velocity should not drop below 250 fpm or laminar air side will reduce effectiveness.  Face dampers may be needed to control air flow.

See manufacturer for further information on coils:  Aerofin

2 Comments

jim kahle

Does ASHRAE 90.1, 2010, or previous versions, provide limits on maximum air pressure drop through cooling and htg coils?
Thank you,

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