CEMP-E
TI 810-11
30 November 1998
(a) As a method of scaling an outside air temperature signal to schedule water temperatures
for heating.
(b) As a method of space temperature control for single-zone air handling units.
(c) As a method of controlling terminal units that can be modulated.
(2) Figure 2-13 shows the kind of control that would be expected if the controller in figure 2-12
were configured for the proportional control mode. The controller modulates its output signal in proportion
to variations of the input signal. For example, the controller, operating through the IP, sends a 21 kPa (3
psig) air signal to the normally open preheat coil valve when the discharge temperature is 6 degrees C (43
degrees F), and it sends a 103 kPa (15 psig) signal when the discharge temperature is 8 degrees C (47
degrees F). The 21 kPa (3 psig) signal completely opens the valve to heating, and the 103 kPa (15 psig)
signal completely closes the valve to heating. The controller/IP combination has a proportional sensitivity of
37.2 kPa per degree C (3 psig per degree F), and throttles the valve over a range of 2 degrees C (4
degrees F). The setpoint of the controller is 7 degrees C (45 degrees F), but the temperature at which it is
controlling is somewhere between 6 degrees C (43 degrees F) and 8 degrees C (47 degrees F). It takes a
2 degree C (4 degree F) change in controller output signal for the valve to go from full heating to no
heating.
Figure 2-13. Proportional control mode.
(3) On a fall in outside air temperature, the heating load increases. The resultant drop in
temperature at the sensor causes the transmitter to signal the controller to lower its output to the IP
transducer. The IP transducer lowers the control air pressure to the normally open valve, allowing more
heating medium to pass through the coil, thereby returning the discharge temperature almost to the
temperature maintained before the fall in outside temperature. Increasing the controller's sensitivity would
shrink the full heating/no heating temperature range; decreasing its sensitivity would increase that range.
(4) The phenomenon that prevents the proportional control mode from achieving its control point
exactly at setpoint is called "offset due to load". This occurs when an equilibrium is reached between HVAC
system output and the load imposed on the HVAC system. When this equilibrium occurs, the discharge
temperature does not change. This means that the proportional mode controller does not change its output
and cause a change in the HVAC system output. Conversely, without the change in HVAC system output to
drive the temperature toward the controller setpoint, the controller output does not change. Therefore, at
these equilibrium points, the controls do not bring the system back to setpoint.
(5) Figure 2-13 shows the relationship between coil discharge temperature and controller/IP output
in response to changes in outside air temperature. Except between points A and B the controller/IP output
is proportional to the discharge air temperature. Between points A and B, the coil discharge air temperature
is the same as the outside air temperature. At points A and B, with a coil discharge temperature of 8
degrees C (47 degrees F), the output is 103 kPa (15 psig) and the valve is closed. At the fifth hour when
the discharge temperature is at the 7 degrees C (45 degrees F) setpoint the controller/IP output is 62 kPa
(9 psig) and the valve is half open. If the valve is closed at 8 degrees C (47 degrees F) outside air
temperature and is half open with a 11 degrees C (20 degrees F) pick-up at -4 degrees C (25 degrees F)
outside air temperature, then by extrapolation the valve is fully open at outside air temperatures of -16
degrees C (3 degrees F) and below. As soon as the air temperature leaving the coil, the incoming outside
air temperature, and the heating action of the coil reach an equilibrium, the valve remains at a given
position until the equilibrium is disturbed by changes in the outside air temperature.
(6) For a given load on the system, there is some optimal proportional sensitivity adjustment at
which the controller will be in stable control and close to the setpoint. In some HVAC control applications,
proportional control may function quite well with a high sensitivity adjustment. A high sensitivity adjustment
2-16
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