Heater Coil Design

Basic Specifications

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Wattage
In the design of a custom open coil heating element, several factors need to be considered when selecting the optimum coil(s) for an application. First, the watts, volts, and resistance must be determined for each coil in the heater.  Typically, the line voltage is specified by the customer and/or end-user. The total wattage is calculated using the amount of airflow (SCFM) and the temperature rise (&#;T) desired.  (W=SCFM&#;&#;T /3 ) The wattage of each coil is the total wattage divided by the number of coils in the circuit.  Most heaters use identical coils throughout the circuit to ensure that the load is balanced. 

Voltage
The voltage across each coil is determined by the circuit used.  A single-phase heater will &#;see&#; full line voltage unless some of the coils are in series.  A three-phase heater is the same if it is wired as a delta circuit.  A wye (star) circuit will &#;see&#; only 57.7% of the full line voltage on each leg.  If there are two coils in series on the legs, the voltage each &#;sees&#; will be one half of the voltage on the leg.

Resistance
The resistance is calculated using Ohm&#;s Law (R = V2/W).  The result is Rhot. But Rhot is the resistance of the wire when it&#;s hot. Resistance increases with temperature, so compensation must be made to get Rcold.  Rcold is the resistance when at room temperature. The various alloys have different temperature factors (T.F.) from hot to cold. We generally use a T.F. of either -3% or -5% to figure Rcold from Rhot depending on the alloy.  For example, a W 120V heater would have a Rhot of 14.4&#;.  (120V2/W=14.4&#;)  Using -5% T.F. will result in Rcold of 13.68&#;.  (14.4&#; x .95 = 13.68&#;)

Detailed Specifications and Coil Geometry

Alloy
The next thing to be determined is which alloy to use for the resistance wire.  Each alloy has different properties that need to be considered for each application.  Some alloys are stronger at high temperatures, some resist oxidation better at high temperatures,  and some have non-magnetic properties. 

Dimensions
The coil designer must now fit the coil to the heater frame he will use.  This allows him to calculate how long each coil must be, and this is called the Stretch Length (S.L.).  A coil usually exits the coiling machine with the coil completely &#;closed&#;, with each wrap of the coiled wire against one another.  When the coil is stretched it will open up so that the wraps are not touching.  The amount it opens is referred to as the Stretch Ratio (S.R. = Stretch Length/Closed Length).  Stretch Ratio is the geometric pitch from one coil wrap to the adjacent one, expressed in units of the wire diameter.  A closed coil (not stretched) has an S.R. of 1.00.  An S.R. of 2.00 has a Space Between Turns (S.B.T.) equal to one diameter of the wire.  A properly designed coil will have its stretch ratio between the minimum and maximum values necessary for that type of heater, usually between 2.5 and 4.0.  This allows the air to circulate around the wire for better heat transfer.

The I.D. of the &#;donut&#; bushings used to support the coil, in a duct heater, for instance, determines the maximum O.D. for the coil.  The O.D. is controlled by specifying a suitable arbor on which to wind the coil.

Wire Gauge
The wire gauge selected has even more properties to consider, such as &#;the finer the wire, the higher the resistance&#;, measured in &#;/ft.  Using a finer wire increases the watt density (W/In2) for a given O.D. The arbor to wire ratio (A/W) will get larger with a finer wire.  If the A/W ratio is too large, the coil will be too flimsy and will sag.  If it&#;s too small the coil will be too stiff to stretch, and will also be more difficult to wind onto the heater frame.

Contact us to discuss your requirements of custom how to make a heat coil supplier. Our experienced sales team can help you identify the options that best suit your needs.

  We have produced thousands of open coil heater designs used in a wide range of applications and industries around the world. Contact us about your next project and see what Tutco-Farnam can manufacture for you.

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 .&#;, we sometimes use .&#; 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 TypeAllowable Velocity Hot Water (HW)200 &#; FPM Steam200 &#; FPM Chilled Water (CW)200 &#; 550 FPM DX200 &#; 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 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) .  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

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