Three easy steps – Thermolec Application &Technical Notes User Manual
Page 12
THREE EASY STEPS
STEP 2 -
DETERMINE DUCT DIMENSIONS, AIR REQUIREMENT AND MECHANICAL OPTIONS
Air Flow requirements and Minimum Velocity
When there is a choice in heater face dimensions the specification writer has several good reasons for
favoring the smaller rather than the larger heater face area: The smaller sized, higher velocity electric coil
will perform better, weigh less and cost less per KW than an otherwise identical larger coil.
The main selection criteria are KW per square foot of face area (or duct size) and Design Velocity.
Kilowatts per square foot Table
Design Velocity Table
Unlike hot water or steam coils, electric coils will generate 100% of the heating capacity (i.e. the heat
output is constant as long as the heater is energized) regardless of the air flow. A drop in air flow through
an electric coil below the minimum required FPM (feet per minute) will increase both the coil temperature
and the exhausted air, which may result in high limit cut-outs tripping.
Another frequent cause of unnecessary cut-out tripping is the uneven distribution of air flow over the coil
surface, resulting in "hot spots". In order to achieve trouble-free performance, provide adequate air flow
as per "air flow requirement" chart (on page 14) and ensure even air flow distribution by following sound
industry practice in design and installation of ductwork and equipment.
The Thermolec CSA NRTL/C listing is limited to 22.5 KW per square foot of duct area for open coil
heaters and 13 KW per square foot of duct area for tubular element heaters.
Air flow requirements for ON-OFF Control
The minimum air flow required through a duct heater depends on the KW per square foot of face area for
the highest capacity ON-OFF stage. In general
400 FPM is adequate in most applications.
Air flow requirement for Full SCR control
An SCR controlled heater may be considered as having an "infinite" number of control stages, and
consequently the air flow requirements of the chart (on page 14) for minimum air flow does not apply.
As a general rule, heaters equipped with FULL SCR will require an air velocity of at least
300 FPM. For
lower design velocities please consult factory.
Exclusive Feature:
The special
Thermo-V unit for VAV applications allows the heater to perform down to 50 FPM.
Please see pages