Thermolec Application &Technical Notes - Manual (Page 26)

THREE EASY STEPS
STEP 3 - (continued)
ON - OFF Control Mode
Air temperature is controlled by switching on and off selected control stages of the heater or, on single
stage heaters, the entire heater. The ON / OFF mode is practical up to two control stages.
It is recommended for most COARSE control accuracy applications and is satisfactory up to a maximum
of 25 ˚F
D
T temperature rise per stage.
CONTROL STAGES
A control stage is a "portion of the total KW capacity of a heater, wired to respond separately to
temperature control". Contrary to a common misunderstanding, the number of control stages to be
specified for an ON / OFF control system does
not depend on total KW capacity nor on total amps
of a heater.
With ON - OFF control, the temperature rise per control stage (
D
T/stage) determines the inherent
temperature control accuracy of the duct heater. Therefore the correct number of control stages is a
function of the total temperature rise of the heater and of the desired control accuracy.
For example, a 200 KW heater, handling 120,000 CFM for a design temperature rise of 5 ˚F will give
FINE control accuracy with only one control stage. The temperature controller for this heater will be
a 1 stage ON - OFF thermostat (if, for any reason, it is not desirable to switch 200 KW in a single step,
several "power stages" can be used with automatic time delay).
On the other hand, with a 10 KW heater, handling 600 CFM for a design temperature rise of 50 ˚F, FINE
temperature control requires FULL SCR control.
POWER STAGES
A power stage is a part of a control stage, wired for delayed switching ON. The need for and the capacity
of power stages is determined, not by temperature control requirements, but by local codes
and conditions governing the increments of capacity that may be brought on line at any moment
without causing excessive voltage drop. When required, Thermolec will break down high amperage
control stages into several power stages.
MODULATING CONTROL MODE
This method is also referred to as
"proportional". The heater is electronically controlled to deliver
anywhere from zero to 100% of it’s capacity, precisely and smoothly matching the heat demand of the
system. This is achieved by means of an SCR controller connected to a proportioning thermostat which
may be either a duct type for a Fresh Air make-up or a room type for zone heating.
The word proportional refers to the portion of a time period in which a heating element is turned
ON and OFF (e.g. 10% ON and 90% OFF meaning 10% of the heating capacity).
According to the thermostat demand, the heater is pulsed in different proportion of ON time and OFF
time to match the heating demand. The longer the element is turned ON, the more heat it is generating
and vice versa. The heat produced during the ON period continues to dissipate during the OFF period,
thus creating an average temperature output matching the thermostat set-point.