A dual-input pneumatic controller is used for reset control. The combined actions which determine the type of reset, or reset action, are similar to those discussed earlier under “The Basic Pneumatic Controller Equation.”
The sensor for the primary variable is connected into the port marked “sensor,” or a similar term, and the sensor for the secondary variable is connected into the port marked “reset.” The controller set-up includes a setting for % authority, which is the percentage change in the secondary variable required to give a given change in the primary vari-able setpoint. The % authority setting may vary from as low as 10% to as high as 200% of primary sensor span.
As an example, a dual-input controller is to control temperature of air leaving a heating coil and is to be reset from space temperature. The discharge temperature is designated the primary or reset variable and the room temperature is designated the secondary or resetting variable.
According to the performance parameters given in the HVAC sys-tem design, a reset schedule is prepared. The reset schedule establishes the coil discharge air and space temperatures at each end of the scale and the required controller action, that is DA or RA.
The lowest primary variable temperature may be called Condition A and the highest primary variable temperature called Condition B. If the primary variable increases as the secondary variable increases, the action is DA. If the primary variable decreases as the secondary variable increases, the action is RA.
The output pressures of a two-input system are calculated using the basic pneumatic controller equations for dual-input controllers as follows:
Pout= Psp ±T1± SP1
TR1 × PR ±T2± SP2
TR2 × PR (4-15)
Pout= Psp± T1± SP1
PB1× Span1 × PR ± T2± SP2
PB2× Span2× %A × PR (4-16)
Calculating % Authority (%A). The % authority for a reset control system is calculated as:
% Authority =PB1
PB2× 100 (4-17)
From the definition of PB of a controller and sensor as being TR divided by Span, we can restate the above equation as follows:
% Authority =TR1/Span1
TR2/Span2× 100 (4-18)
When the spans of the two sensors are equal, the span values can-cel and the % authority becomes the ratio of throttling ranges, as fol-lows:
% Authorityequal spans=TR1 TR2 × 100
The reset action which will occur depends on the actions of the two inputs which are used in the reset control. These combinations deter-mine the reset action:
a. Direct-acting/direct-acting = reverse reset b. Reverse-acting/reverse-acting = reverse reset c. Direct-acting/reverse-acting = direct reset d. Reverse-acting/direct-acting = direct reset A typical reset schedule would be:
Condition Coil Air Outside Air Sensor Span
A 75°F 65°F 50°F
B 95°F 10°F 100°F
An example of the method for determining the type of reset action is shown in the following reset schedule for control of heated water
supply temperature leaving a convertor using open loop reset from outdoor air temperature:
Hot Water Outdoor Air Output Condition Temperature Temperature Pressure
A 200°F -10°T 3 psig
B 100°F 70°F 13 psig
The primary, or reset, variable is the water temperature leaving the convertor and the secondary, or resetting, variable is the outdoor air temperature. The reset schedule shows that a decrease in primary variable setpoint will be required upon an increase in secondary vari-able value, which is reverse reset.
To determine which combination is to be used to get reverse re-set, the control action of the primary controlled device must be con-sidered first. If a normally open hot water or steam valve with a 3 to 13 psig spring supplying heat to the convertor is to be controlled, the final control action will require an increase in output pressure to close the valve on increase in hot water supply temperature, which is di-rect action.
Examination of the table of action combinations on page 109 shows that the resetting control must be direct-acting to get reverse reset with a direct-acting controller for the heating valve.
A review of the change in the heating valve position (which fol-lows a change in hot water temperature leaving the convertor) shows that the valve moves toward the closed position as the hot water sup-ply temperature increases.
The air pressure signal required to close the valve is 13 psig. As the reset schedule shows, when the supply water temperature reaches 200°F, the controller output will be 13 psig and the valve will be fully closed. Thus we see that an increase in temperature causes an in-crease in controller output pressure, which is direct action. The con-troller must therefore be selected for direct-acting/direct-acting to give reverse reset.
Selecting the throttling ranges. The next parameters to be consid-ered are the throttling ranges. A trial throttling range is assigned for the primary variable on the basis of experience factors. After the pri-mary variable throttling range is assigned, the throttling range of the second variable is calculated according to the following equation:
1. For reverse reset; using DA/DA Controller:
TR2= T2B± T2A OPB ± OPA
PR ± T1B± T1A TR1
(4-19)
2. For reverse reset; using RA/RA Controller:
TR2= T2B± T2A OPA ± OPB
PR ± T1B± T1A TR1
(4-20)
3. For direct reset; using DA/RA Controller:
TR2= T2B± T2A OPA ± OPB
PR ± T1A± T1B TR1
(4-21)
4. For direct reset; using RA/DA Controller:
TR2= T2B± T2A OPB ± OPA
PR ± T1B± T1A TR1
(4-22)
Where:
TR1 = Throttling range of first variable, °F.
T1A = Temperature of first variable at condition A, °F.
T1B = Temperature of first variable at condition B, °F.
T2A = Temperature of second variable at condition A, °F.
T2B = Temperature of second variable at condition B, °F.
OPA = Output pressure of the controller at condition A.
OPB = Output pressure of the controller at condition B.
The pressure range 10 psig, used in all the above equations, indi-cates the pressure change between 3 to 13 psig. When using the equa-tions for other pressure ranges, substitute the correct pressure range for 10 psig.
Calculating % Authority
The throttling range for the second variable cannot be set up on the controller, so the % authority is set instead. Knowing the span of the sensors used and the throttling range of the primary and secondary variables, the % authority can be calculated. The next step is to calculate the percent authority.
Programming
The last step is to enter the set-up parameters. For dual-input pneumatic controllers, the setpoint for the first variable is selected and set directly on the controller. The setpoint of the second variable must be calculated by use of one of the following equations and the controller set-up for the two setpoints.
1. For reverse reset; using DA/DA Controller:
SP2= T2A+ TR2
Pmp± OPA
PR + T1A± SP1
TR1 (4-23)
2. For reverse reset; using RA/RA Controller:
SP2= T2A+ TR2
OPA ± Pmp
PR + T1A± SP1
TR1 (4-24)
3. For direct reset; using DA/RA Controller:
SP2= T2A+ TR2
OPA ± Pmp
PR + T1A± SP1
TR1 (4-25)
4. For direct reset; using RA/DA Controller:
SP2= T2A+ TR2
Pmp± OPA
PR + T1A± SP1
TR1 (4-26)
Where:
SP1 = Setpoint of primary variable.
SP2 = Setpoint of secondary variable.
Pmp = Pressure at midpoint, 8 psig for 3 to 13 psig system or 9 psig for 3 to 15 psig system.
Examples for Set-up of Dual-input Pneumatic Controllers
The following examples illustrate the steps in the setup and cali-bration of a dual-input pneumatic controller.
Assume a dual-input pneumatic controller with a 3 to 13 psig range. Determine all the factors required for setting and calibrating the controller for a sequence to reset the temperature of heated water leav-ing a convertor with reset from outdoor temperature in the followleav-ing reset schedule:
Hot Water Outdoor Air Output Condition Temperature Temperature Pressure
A 200°F -10°F 3 psig
B 100°F 70°F 13 psig
Step 1
Determine the controller and reset action. Because the reset sched-ule requires the primary variable to be set downward as the secondary variable increases, the required reset action is reverse reset. If a normally open heating source valve is to be positioned, the output pressure must increase as the primary variable increases, which requires direct action on the primary controller. With one direct acting controller required, it is necessary to make the secondary controller direct acting also to give reverse reset. From the tables above we found that DA/DA = reverse reset.
Step 2
Determine throttling ranges: Make hot water temperature the pri-mary variable with sensor 1. Assign a trial throttling range of 6°F to this input. With that throttling range and other parameters, use Equation 4-19 to calculate a throttling range for the secondary variable:
TR2= 70°F ± ± 10°F
Step 3
Select sensor ranges. Review of the HVAC system performance parameters shows that a hot water temperature sensor with a range of 200°F on a span of 40°F to 240°F and an outdoor air temperature sensor with a range of 100°F on a span of 25°F to 125°F will cover the tempera-ture ranges to be expected. With those sensors, the proportional bands are:
PB1= 6°F200°F × 100 = 3%
PB2= 4.5°F
100°F × 100 = 4.5%
% Authority = 3.0%
4.5%× 100 = 67%
Step 4
Select the secondary setpoint. Assume a primary setpoint of SP1 = 100°F. Calculate the value of SP2.
SP2=±10 +4.5% 8 ± 3
10 + 200 ± 100 °F
6°F = 67°F Step 5
Prepare calibration instructions for reset controller using values calculated above as follows:
1. Set the controller to DA/DA action.
2. Set PB to 3%.
3. Set % authority to 67%.
4. Set input to port 1 at 100°F or 6.6 psig (40° to 240°F).
5. Set input to port 2 at 67.6°F or 10.4 psig (–25° to 125°F).
6. Adjust the controller calibration point until the output is 8 psig.
7. Calibrate each sensor or compensate controller so that the inputs to the ports are correct for the measured temperatures.
8. Set the setpoint indicator to 100°F.
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