Roberts Gorden CRV-B-4 - Manual (Page 21)

SECTION 6: R ADIANT T UBE AND T AILPIPE
15
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 down-
stream 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 corre-
spond to areas of major heat loss is the key to provid-
ing uniform comfort levels. The use of adequate
tailpipe is the key to high combustion efficiency 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 radi-
ant 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 varied
according to the flow units in the system and the
designed radiant length. Longer lengths of tailpipe will
attain higher operating efficiencies and therefore con-
densation 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 con-
tained in ANSI Z83.20/CSA 2.34 (latest edition).
Actual installation variables (gas BTU content, air tem-
perature and operation cycle, etc.) may effect efficien-
cies (positively or negatively). Page 16, Figure 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.