Service manual

SECTION 6: RADIANT TUBE AND TAILPIPE
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SECTION 6: RADIANT TUBE AND TAILPIPE
The main purpose of the tailpipe and the radiant tube
is to provide sufficient tube surface to transfer the
heat from the flue gases to the tube wall where it
radiates from the tube. Radiant tube is defined as
the tubing between burners firing in a radiant branch,
plus the radiant tubing immediately following the last
downstream burner. Tailpipe is defined as all tubing
between the radiant tube and the pump.
Most of the radiant heat supplied by each burner is
released from the radiant tube; the balance is
released by the tailpipe. The placement of radiant
tube to correspond to areas of major heat loss is the
key to providing uniform comfort levels. The use of
adequate tailpipe is the key to high combustion effi-
ciency and proper operation of the pump.
6.1 Radiant Tube Length
The considerations in the selection of the length of
radiant tube include the following:
6.1.1 Minimum Radiant Tube Length
Provides for the highest level of intensity per length
of radiant tube and good radiant heat uniformity
between burners. More tailpipe length is required to
maintain operating efficiency and pump capacity.
6.1.2 Maximum Radiant Tube Length
Provides the lowest level of intensity per length of
radiant tube, and consequently the largest span
between burners. The radiant intensity will be
reduced slightly for the last 5’-10’ (2-3 m) of radiant
tube before the next burner.
The length of radiant tube required varies according
to the burner input. Consideration has been given to
the use of a standard 10’ (3 m) length of tube or
lengths that can be cut from same without waste.
See Page 12, Table 1.
When positioning radiant tube to give the required
radiant distribution, it is important to consider:
• Clearances to combustible materials.
• Lighting equipment and other suspended objects.
6.2 Tailpipe
Tailpipe provides a low level of radiant intensity per
length. The length of tailpipe for systems can be var-
ied according to the flow units in the system and the
designed radiant length. Longer lengths of tailpipe
will attain higher operating efficiencies and therefore
condensation will occur.
Page 16, Figure 16 relates the effect on system ther-
mal efficiency of variations in radiant and tailpipe
lengths. The chart was created based on test data
obtained in accordance with methodology developed
by the National Bureau of Standards (NBSIR 80-
2110) and recommendations on flue loss calculation
contained in ANSI Z83.20/CSA 2.34 (latest edition).
Actual installation variables (gas BTU content, air
temperature and operation cycle, etc.) may effect
efficiencies (positively or negatively). Page 16, Fig-
ure 16 is presented as a guide to the designer for
information only.
NOTE: When accounting for the required tailpipe
lengths during the design process, it is important to
verify that the tailpipe for each branch is at least
equal to the specified minimum.