How do you select and design a custom coil replacement?

Here is a step-by-step process for replacing a custom coil and A) Making sure it fits and B) Making sure it works.

Step #1: 

Gather the physical coil dimensions (more info will be needed at coil verification measure)

• Tube size (typical: 3/8″, 1/2″, 5/8″, or 1-1/8″)
• Number of Rows (1 or 2 rows typical for heating, 3-8 rows typical for cooling)
• Fins per inch (typical: 6-14)
• Fin height (typical max height is 60”. More than 60” coils are typically stacked)
• Fin length (typical max length is 240″)
• Coil depth (not critical for estimating, but typical is 3″-12″) 
• Circuiting and connections (single or dual circuit, intertwined or face split, left hand or right hand, same end or opposite end connections- not critical for estimating price)
• Flange sizes (Typical: 0″ – 2″ – not critical for estimating)
• Header and connection sizes (not critical for estimating)
• Pitched casing if steam coil (not critical for estimating)
  
  

Step #2

Gather all the design information:

• Chilled Water Coil:

  • cfm
  • Capacity required
  • Entering water temperature
  • Leaving water temperature
  • GPM 
  • Entering air temperature
  • Leaving air temperature 
  • Glycol type and percentage (none, propylene, ethylene, percentage 0-50%)
  • Application 
  • How is system controlled?
    
    

• Hot Water Coil:

  • cfm
  • Capacity required
  • Entering water temperature
  • Leaving water temperature
  • GPM
  • Entering air temperature
  • Leaving air temperature
  • Glycol type and percentage (none, propylene, ethylene, percentage 0-50%)
  • Application 
  • How is system controlled?
    
    

• DX Coil:

  • cfm
  • Refrigerant (R410-a, R-22)
  • Capacity required
  • Entering air temperature
  • Leaving air temperature
  • Condensing unit information (capacity, model number, staging, circuiting)
  • Application 
  • How is system controlled?
  • Accessories required? Hot gas bypass, TXVs, accumulators, filter driers, sight glasses, solenoid valves 
    
    

• Steam Coil:

  • cfm
  • Capacity Required
  • Entering air temperature
  • Leaving air temperature
  • Steam pressure 
  • Coil type
    • Steam distributing (no u-bends, designed for better freeze protection).  
    • Standard steam (looks like hot water coil). Not to be used for makeup air or for applications with an entering air temperature below 40 degF.
  • Application 
  • How is system controlled?
    
    

Step 3:

Verify performance

Chilled Water/Hot Water/Steam

For chilled water, hot water or steam coils, simply run a selection in the coil software program.

Direct Expansion (DX)

DX is a little trickier because the system performance depends on the paired condensing unit’s performance. 

DX Step 1: Run DX evaporator coil performance at 3 different saturated suction temperatures (SST) – example: 40, 45, 50

DX Step 2: Run condensing unit performance at the same 3 SSTs (40, 45, 50).

DX Step 3: Plot these two lines on a graph. The point where the two lines intersect is the System Design Capacity and SST. 

For a standard comfort cooling application you want to be between 45-50 deg SST. 

DX Step 4: Run the evaporator coil selection with the SST you found in Step 3. Verify that this new selection will work (check capacity, LAT, etc). If this doesn’t work, go back to Step 3 and add more coil surface area and/or change the condensing unit size and follow the steps again. 

If you need help replacing a custom coil let us know!

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Brad Telker

Vice President of Sales, Applied Systems Group at cfm Distributors, Inc.

Brad was born and raised in St Louis Missouri, where he spent the first 18 years of his life. At the ripe young age of 18, he embarked onto his next big adventure, traveling to the University of Missouri Columbia for a bachelor’s degree in Mechanical Engineering. Brad joined the cfm team in 2006, and now as the Vice President of Commercial Sales, he focuses on business development, as well as helping contractors and engineers find creative and unique solutions to any size project. When he’s not at work, Brad enjoys photography, running and spending time with his family.

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