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Learn About Water/Steam Coils: Part 2

Part 2 –Standard Hot Or Chilled Water And Steam Coils          

Unlike booster coils, standard water coils are usually the primary source of heating or cooling for a space.  Unlike booster coils, they come in any size required, and are well constructed.  They will have headers on one end for same end-connected coils.  Fin spacing can vary according to need, as can the number of rows.  Figure 1 is a representation of a typical standard water coil.

Figure 1.

Usually, all the specifications needed to quote a standard water coil are available for a new installation.  The design engineer has calculated everything one needs to know to select the correct coil.  Sometimes, some of the specifications will be missing.  Just as in sizing booster coils, if some specifications are given, others can be calculated.  Because fin spacing and number of rows can vary, occasionally more than one coil will meet the given specifications.  In those cases, naturally the least expensive coil will be selected.

While the engineer’s specifications will probably include BTUH, G.P.M., DT for water, DP for airflow, etc., seldom will the performance specifications include the critical physical specifications for the connections.  These specifications are usually left to the installing contractor.  These specs determine if the coil is “same end” or “opposite end” connected and if it’s “right hand” or “left hand”.  Most water coils will be same end connected.  The supply and return connections will be on the same end.  Opposite end connections are unusual, but available.

On a same end coil, to determine which end the connections should be on, aim the coil face towards the inlet airside.  Then specify right or left hand connections, see Figure 2. 

Figure 2.

Specifying right or left hand is important, since coils should not be flip-flopped.  Coils should be supplied at the bottom and returned at the top.  The supply connection should always be on the leaving airside.  Improper piping (non-counter flow) can cause a loss of up to 10% of heat transfer.

Because of all the variables involved in coil construction, such as fin spacing, number of rows, circuitry, etc, it is impossible for you to select a coil to meet the performance and physical specifications without an extensive computer program.  Coil manufacturers, such as Witt, have developed coil selection programs that have greatly reduced coil selection time. 

Replacing an existing standard water coil is actually more difficult than supplying a “new” coil.  Physical size will be the most important specification as far as the contractor is concerned.  For replacement coils, seldom will there be performance specifications available.  As long as the physical specifications of a replacement coil match the old coil to be replaced, performance will be about the same.  This requires care be taken to measure and describe the coil to be replaced.  Be wary of coils with cast brass or cast iron headers!  Those types of coils are probably in unitary equipment.  Coils for unitary equipment must fit inside the unit.  If the replacement coil doesn’t fit within the walls, it will be useless. 

Note:  Be absolutely sure of the replacement coil’s dimensions before ordering.

Witt will supply the dimensioned drawing.  See Figure 3 for an example.

Figure 3.

In order to get a good drawing and eventually the correct replacement coil, the following information must be known:

Airflow describes the direction in which the air is flowing through the coil.  Visualize the coil in a room.  If the air is running through the coil from wall to wall, it is horizontal airflow.  If the air is flowing from floor to ceiling or ceiling to floor, it is vertical airflow.  Once it has been established that the coil is horizontal or vertical airflow, determine right or left hand as before (see Figure 2).

Figuring rows seems to be the most confusing aspect when trying to replace an existing coil.  “Rows” and number of tubes are often mixed up.  Rows are always counted in the direction of airflow, regardless of how the coil is mounted.  You count rows by viewing an end of the coil.  Determining the number of rows can be further complicated by staggered or uneven return bends. 

Tech Tip:       The best method to determine the rows on a coil is to take a straight edge and start at either the left or right side of an end and go from one row to another without the return bends confusing what you actually see (see Figure 4).   

These are only some of the patterns of return bends.  Return bends can be staggered many ways in order to keep a many-rowed coil relatively “thin”.  No matter how the “stagger” by using a straight edge laid along the edges of the return bends the rows can be easily ascertained.

Figure 4.

Count the number of tubes in each row.  See Figure 4 again.  Counting tubes is the reverse of counting rows.  It is the number of tubes in each row the straight edge was laid along.  While not as important as rows, this count will assist in determining circuitry.

Using a ruler, count the number of fins per inch.  Normal fin counts will be from 4 to 14 F.P.I.  Establish the connection sizes and type.  M.P.T. connections are most common, but F.P.T. are readily available.

Measure fin height and fin length.  Fin height is always measured in the direction of the fin, or put another way, perpendicular to the tubes (see Figure 3, “FH”).  Fin length is always measured in the same direction of the tubes (see Figure 3, “FL”). Casing height is always measured in the same direction as fin height, and casing length the same direction as fin length.  The coil manufacturer never supplies pieces of sheet metal that baffle or cover headers to prevent air bypass around headers. The end-user must supply them.  Casing depth is always measured in the direction of airflow.

Overall length includes everything except the connection stubs, casing length, return bends, and headers.  Remember on opposite end-connected coils, headers at both ends, this dimension is from the outside of one header to the outside of the header on the other end.  Study Figure 3, which shows examples of all these dimensions.

Steam Coils

Standard steam coils should only be used where the air temperature will always be above freezing.  While the coil manufacturer may leak test a coil to as high as 350 psig, standard steam coils should not be used with steam pressure over 100 psig due to expansion and contraction of the tubes.  Standard steam coils are always opposite end connected, supply coming in the top and condensate out the bottom.  Same end connected standard steam coils are never used, since steam should never be passed around return bends, which are required to make same end connections.

Steam distributing coils, sometimes called “non-freeze” or “dual-tube” coils, are used where below freezing air temperatures will be encountered.  There are no return bends, only plugged tubes at the back of the coil.  “Non-Freeze” is a bit of a misnomer.  Any coil can be eventually frozen under adverse conditions.  Dual-tube steam distributing coils are less prone to quick freeze-up because the tube in a tube construction distributes hot steam down the entire length of the tubes, thus warming the condensate that has been pushed into the tubes.  Dual tube coils are usually same end connected (no return bends are used) and have the option of two supply connections, one at each end to properly distribute steam evenly over the entire face.  This two-supply connection option should always be used on coils with a fin face length of 72 inches or more.  These coils are also limited to 100-psig-steam pressure.

With the addition of ascertaining the operating steam pressure, all the parameters for sizing a steam coil, new or replacement, are the same as for a standard water coil.

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