Thermolec Application &Technical Notes - Manual (Page 53)

FACTOR OPEN COIL TUBULAR FINNED TUBULAR
Selection guide for heating elements
Following are a few factors commonly used to determine the choice of construction.
ENGINEERING AND APPLICATION GUIDE
COIL
TEMPERATURES
AIRFLOW
UNIFORMITY
PRESSURE
DROPS
ELECTRICAL
CLEARANCES
AIR QUALITY
OUTLET AIR
TEMPERATURE
MECHANICAL
STABILITY
CONTROLLABILITY
SAFETY
COST
WEIGHT
Resistance coil, exposed
directly to airstream, runs
cooler than coils imbedded
in sheathed elements.
Airflow must be uniformly
distributed to prevent
hotspots. Pressure plates
can help even out airflow.
Lowest pressure drop
due to large percentage
of open space.
Large clearances
between live parts and
ground enable open coil
heaters to withstand
severe applications.
Use only with clean air
free of conductive
particles or water spray.
To a certain extent humid
air is acceptable.
1200ºF. maximum
Open coil heaters are-most
susceptible to damage
due to physical abuse.
Open coil heaters respond
quickly to step control
because of low thermal
inertia. This can produce
temperature fluctuations
if control system does not
compensate.
Since element is electrically
live, it is advisable and
safer to order protective
screen at all times, if
element may be touched
by conductive material or
accidentally by personnel.
For most applications,
open coil heaters are more
economical because
manufacturing operations
are simpler.
The lightest of all 3 types.
Tubes run hotter than open coil or
finned tubular. Temperatures
are kept within safe limits by
reducing watt densities.
Less susceptible to hotspots
than open coil, but more
susceptible than finned tubular.
Highest pressure drop
because of high percentage of
space occupied by tubes.
Clearances between live parts
and sheath are small, but filled
with compacted insulation.
Can be used with virtually
any of atmospheric conditions.
1200ºF. maximum
Tubular heaters are least
susceptible to damage due to
physical abuse.
Thermal inertia greater than
open coil, but less than finned
tubular.
Because the coil is enclosed in a
grounded metal sheath, electrical
shock hazard due to accidental
contact does not exist.
Generally most expensive of the
three because of conservative,
high temperature design.
Heavier than open coil.
Additional support required for
horizontally mounted units,
especially for extra long heaters.
Finned tubes run hotter than open
coil, but cooler than tubular due to
heat transfer effect of fins.
Finned tubulars heaters are most
tolerant of nonuniform airflow.
Hotspots tend to be dissipated
by sheath and fins.
Lower pressure drop than tubular,
but higher than open coil.
Clearances between live parts
and sheath are small, but filled
with compacted insulation.
Can be used with air containing
water droplets or conductive
particles unless particles are likely
to build up between fins.
600ºF. maximum
Finned tubular heaters can
withstand more physical abuse
than open coil.
Higher thermal inertia makes finned
tubular heaters slower to respond,
but can produce more uniform
temperatures if control system is
properly designed.
Because the coil is enclosed in a
grounded metal sheath, electrical
shock hazard due to accidental
contact does not exist.
Generally more expensive than open
coil, but less expensive than tubular
because watt densities are higher.
Heavier than open coil.
Additional support required for
horizontally mounted units,
especially for extra long heaters.
THERMOLEC