Three easy steps – Thermolec Application &Technical Notes User Manual
Page 25
DETERMINE METHOD OF CONTROL AND SELECT CONTROL
COMPONENTS
Temperature Control Modes and Staging
In selecting temperature controls the specification writer will generally wish to consider both control
accuracy and cost.
To provide an acceptably close match of heater output to the system's varying demand for heat it is
usually necessary to divide the total KW capacity into separately controlled increments or "control
stages". Each stage accounts for part of the total temperature rise
D
T through the heater.
This temperature rise per stage (
D
T / stage)
determines how accurately the temperature
in the system can be controlled. For selection
purposes Thermolec distinguishes FINE, MEDIUM,
and COARSE control, which we arbitrarily define:
When a heater is controlled by an SCR, it can be considered as having an infinite number of stages and
the accuracy is equivalent to FINE.
For a given accuracy and energy efficiency requirement, the number of control stages to be specified
depends on the total design temperature rise through the heater.
The following table shows the Thermolec recommendations for control.
Caution: It should be reminded that a coarse control puts an additional stress on the contactors since
they have to cycle more often.
THREE EASY STEPS
Temperature rise per
Temperature control
control stage, ˚F
accuracy
5 or less FINE (SCR)
6 to 20
MEDIUM
over 21
COARSE
Recommended Number of Control ON/OFF stages or SCR Proportional Control
Heating Load
D
T =
6°C
10°F
D
T =
11°C
20°F
D
T =
17°C
30°F
D
T =
22°C
40°F
D
T =
28°C
50°F
D
T >
28°C
50°F
Temperature
Control
Options
Coarse
Medium
Fine
1
stage
ON
/
OFF
SCR
--
SCR
SCR
SCR
SCR
SCR
SCR
SCR
SCR
SCR
1
stage
ON
/
OFF
2
stages
ON
/
OFF
2
stages
ON
/
OFF
2
stages
ON
/
OFF
STEP 3 -