Application Block Library Fan Control Optimization

Transcription

1 Application Block Library Fan Control Optimization

2 About This Document This document gives general description and guidelines for wide range fan operation optimisation. Optimisation of the fan operation can be a complex task especially in VAV based systems, which are susceptible to a wide range of rapid flow changes. If an AHU system operates with a constant set point and constant flow, then a traditional approach is usually sufficient since the only period of time when the pressure varies is during the start-up procedure. However, in the variable flow systems (e.g. VAV or CO2 control based systems) or in a pressure sensitive environment (clean rooms, hospital operating theatres, pharmaceutical factories), the pressure PID controller tends to be too sluggish to respond efficiently to pressure variations, therefore optimisation would be required., Distech Controls Inc All rights reserved. While all efforts have been made to verify the accuracy of information in this manual, Distech Controls is not responsible for damages or claims arising from the use of its use. Persons using this manual are assumed to be trained HVAC professionals and are responsible for using the correct wiring procedures, correct override methods for equipment control and maintaining safe working conditions in fail-safe environments. Distech Controls reserves the right to change, delete or add to the information in this manual at any time without notice. Distech Controls, the Distech Controls logo, Innovative Solutions for Greener Buildings, ECO-Vue, and Allure are trademarks of Distech Controls Inc. 2 / 10 Fan Control Optimization

3 Table of Contents 1. Test installation setup Original fans speed control setup Functional tests Unit start-up Response to VAV generated flow changes PI regulator optimisation Basic approach Fan speed to pressure characteristic analysis Identification fan response time constant and PI regulator tuning Frequency inverter ramp adjustment Fan speed to pressure control time constant estimation PID controller parameters recalculation PI regulator test Conclusions... 9 Fan Control Optimization 3 / 10

4 1. Test installation setup The field tests that were necessary to prepare this document were executed on a supply/return AHU, providing air supply to a mixed type ventilation system combining constant flow and VAV subsystems. Nominal air output of the test unit was 6000[ m3 h ]. Maximum demand of the VAV system was 4900[ m3 ], and minimum demand was 720[m3 ]. h h The system was balanced with a static discharge air pressure setpoint at 300[Pa]. Control is provided by independent discharge and return fan frequency inverters, with a PI regulator and pressure sensor for each. 1.1 Original fans speed control setup Original parametrisation of the PI controllers was done with the standard (and rather conservative) approach, to ensure stable operation. PBand was set to 500[Pa] and T i integration time was set to 180[s]. This, combined with a supply and return fan frequency inverter rise and fall ramp time set to 45[s], ensured stable operation and lack of fan speed pressure oscillations. 2 Functional tests The AHU system, as described before, was tested using basic functionality of the EC-gfxProgram to gather an online trend log with the Live Trend Log control block. The block was used with the sampling rate set to 2000[ms] and inputs connected as follows: Y1 Discharge Air Pressure [Pa] sensor Y2 Discharge Fan Speed Control [%] signal Y3 Return Air Pressure [Pa] sensor Y4 Return Fan Speed Control [%] signal For the purpose of this document, we will focus on the Discharge Air Pressure control, but please note that an identical procedure must be applied for the return air side, and system s operation as a whole must be checked. 2.1 Unit start-up At start-up, the AHU program checks outside air temperatures, and if necessary, preheating is activated. Dampers are then enabled, and when their opening is confirmed, fans are activated and pressure control is started. Figure 1 shows Discharge Air Pressure and Fan Speed Control at unit start up (VAVs were set to 75% of maximum flow). We can see that PI jumps slightly over 60%. This is defined by the proportional action at t 0, when the measured pressure is 0[Pa]: P(t 0 ) = 100% P Band E(t 0 ) = 100% 500[Pa] 300[Pa] = 60%, then the PI output is further increased by the integral action and reaches about 65%. Next, we can see a response from a pressure sensor as the fan is started Figure 1 - Discharge air pressure and fan speed control at start up. 4 / 10 Fan Control Optimization

5 (please note that fans need 45s to ramp up from 0%-100%) and the corresponding PI regulator reacts by slightly declining the control signal from its previous position (at t=10:44). When the pressure response to the initial PI output levels out at about 100[Pa], the regulator gradually increases the fan control signal (this action is based entirely on its integral action). We can see that even though regulation seems to be very stable, this asymptotic climb takes about 12min to reach the 5% zone around the setpoint. 2.2 Response to VAV generated flow changes To test the pressure control response to VAV flow variations, we forced changes of the VAV setpoints from maximum to minimum, and then to 75%. These changes were executed with a time span reaching up to 7 min. As we can see from Figure 2, the PI regulator is not able to stabilise the pressure control in the 5% zone around the setpoint nor it is able to quickly return to this zone after a major VAV flow change. Figure 2 - Fan speed control and discharge air pressure response to VAV flow variations. This example clearly shows the default PI setting, which may work sufficiently well for a constant flow system, is not able to provide reliable control for a VAV based system. 3 PI regulator optimisation In order to improve system reaction to installation state changes, a PI parameter optimisation attempt was undertaken. 3.1 Basic approach In our basic approach, P Band and T i were readjusted to new values to verify if it would be possible to obtain satisfactory results with the basic PI setup. As a first approach, P Band was set to 250[Pa] and T i to 90[s]. The test was then performed to determine the performance of the newly parametrized regulation algorithm. The results in Figure 3 clearly show that this basic approach produced some improvement as stabilisation occurred in about 4-5 minutes, however some disturbing side effects were visible (increased level of oscillations in both control signal and pressure readings). This was a sign that the system may have been close to the stability margin. To verify if that was the case, an additional test was conducted. Figure 3 - Fan speed control and discharge air pressure response t0 VAV flow variations (P Band = 250[Pa] and T i = 90[s]). Fan Control Optimization 5 / 10

6 Figure 4 - Stability issues with P Band = 250[Pa] and T i = 90[s]. Pressure setpoints were changed at different VAV flow settings. The results of this additional test (Figure 4) show that a setpoint change at minimum airflow values caused the system to lose stability, proving that our suspicion that the system may have been close to the stability margin were fully justified. As a result of this test, the entire approach to the optimisation procedure had to be changed. 3.2 Fan speed to pressure characteristic analysis The fact that the same change of pressure setpoint at different loads led to dramatically different results (as seen in Figure 4) suggested that in might have been the nonlinearity of the fan speed to pressure characteristics that was responsible for the loss of stability.to verify this, we decided to take a deeper look at centrifugal fan characteristics. Figure 5 shows an example of Pressure to Volumetric Flow characteristic of a centrifugal fan. We can observe that the fan pressure relation to the volumetric flow (which is proportional to the rotational fan speed), is square rather than linear. Thus, if the system operates under these given conditions (pressure and flow) and any of those parameters changes, the PI regulator would not be able to sustain the same quality of control because it operates on the assumption of a linear relation between the control signal and controlled variable. The most obvious way of counteracting this problem is the linearization of the fan speed to pressure relationship. To do this, it is necessary to recalculate the PI s operation from pressure-based to square root of the pressurebased. This way we can obtain a linear relationship between the PI s control signal and a controlled variable. This involves a recalculation of the measured pressure and pressure setpoint to their square root values (Figure 6 mark 1 and 2) and recalculation of the P Band to its square root (Figure 6 mark 3). Figure 6 - PID readjusted for pressure square-root based operation. Figure 5 - Fan flow to pressure characteristic. 6 / 10 Fan Control Optimization

7 3.3 Identification fan response time constant and PI regulator tuning As we can see from Figure 1, actual pressure response to a change in the control signal of the frequency converter is relatively fast, thus it gives us room for optimisation Frequency inverter ramp adjustment The first step to improvement was the exact adjustment of the frequency inverter time ramp up and down time. This parameter is used to ensure that when the fan speed is being increased or decreased, the frequency inverter would not be subjected to the overcurrent which forces it to cut the power supply to the electric motor. The default values tend to be set in a very secure way, especially when the fan motors are not burdened by a heavy start-up (initial currents are not excessively high due to quadratic torque characteristics). This procedure should be done with all the dampers opened to nominal positions and all VAVs at maximum flow settings. The motors should then be stopped completely, and the ramp decreased (it can be safely decreased by 20-30% in each step) and the motor should then be restarted. After the motor is restarted, the current consumption must be observed to ensure the current limit is not exceeded (most of the frequency inverters would show a warning and limit power supply to the motor when that happens). If the current limit is not reached, this operation can be repeated, on the other hand, if the current limit is reached, we need to go back a notch to the previous settings and verify if the margin is sufficient to ensure the prevention of the motor current reaching its limitation at normal operation. In our case, the initial value of the ramp up/down time was 45s, and during the tests we had overcurrent warnings appearing at ramp up/down equal to 10s, so we tested and set 15s as a secure value. Please note that even with the security margin equal to 50% of the ramp up/down time, the final ramp value is only 1/3 of the initial one. NOTE. Some AHU manufacturers do not allow a user to change frequency converters up/down ramp times. If that is the case, this step must be omitted Fan speed to pressure control time constant estimation The next step was the estimation of fan speed to pressure control loop time constant. To do that we switched the AHU on, forced all the VAVs to 75% of maximum flow, let the pressure control stabilise, and locked discharge and return fan speed control outputs. Then we forced the discharge fan speed to 10% and waited until the response stabilised and forced output to 100%. NOTE. Pressure must be constantly observed. Over pressurization of the installation might lead to physical damage. If the pressure rises too high, the unit must be immediately stopped and the entire procedure restarted with a lower fan speed setting. As a result, we received a response pressure over time characteristic (Figure 7). This characteristic type is typical for a serial connection of multiple first order inertial blocks with delay. It can be modelled with sufficiently high accuracy by a first order inertial object with delay. Its time response can be modelled using main time constant T and delay time τ. To estimate those parameters, we applied a Figure 7 - Time constant estimation. graphical method (see Figure 7). It required that we draw a tangent at the highest inclination point, where we then found cut-off points of this tangent with maximum and minimum values of the characteristic. The time necessary for the tangent to reach the lower cut-off point to the higher cut-off point is our time constant. In our case, its value was T = 8[s]. The time Fan Control Optimization 7 / 10

8 delay was estimated as time between the point when output was forced to 100% to the lower cut-off point of the tangent. In our case τ = 6[s]. Please note that the small distortion of the characteristic, which can be observed on Figure 7 was caused by the limited resolution of our sampling (we used 1000ms sampling), and has little impact on final results PID controller parameters recalculation The estimated time constant and delay time values were used to calculate the P Band and T i parameter of our controller. Graphical representation of the parameters used in the equations (1) and (2) can be found in Figure 7. We used the PID tuning method designed for a static control object with a delay, aiming for a 0% overshoot: τ k P Band = 330[Pa] P 0.6 T Band 18 (1) T i = 0.8 τ T 9[s] (2) These values were used as a starting point for the final tuning of a PID controller. 3.4 PI regulator test The P Band and T i settings obtained as a result of our optimisation and test procedure were introduced into the PI regulator controlling the fan speed. A series of test were then executed. VAVs were forced to minimum flow, then maximum flow and then 75% of maximum flow to observe how quickly the system was able to stabilise pressure after a rapid change of VAV load. Next, for each of the VAV flow settings, the pressure setpoint was changed to 240[Pa] and back to 300[Pa] for an additional stability check (Figure 8). To fully estimate the effects of the conducted optimisation, a comparison with Figure 2 is necessary. When the VAVs load changes, the optimised PID barely steps out of the 5% comfort zone around the setpoint. Figure 8 - Final PID test. Transition to Minimum Flow: it reaches around 325[Pa] for the transition to minimum flow and stabilises at 300[Pa] after about 2min (it is a similar time to the actual run time of the VAV actuators), as opposed to 380[Pa] and lack of stabilisation at 300[Pa] even after 7 min, for the original setup. Transition to Maximum Flow: it drops to around to 260[Pa] for the transition to maximum flow, which is similar to the original PID settings, but then returns to the 5% zone in about 30s, which previously took over 5min. Also, it stabilises at 300[Pa] in about 1.5min as opposed to the lack of stabilisation in more than 6 min previously. Transition to 75% Maximum Flow: it reaches around 320[Pa] for the transition to 75% of flow and stabilises at 300[Pa] after about 2min, as opposed to 360[Pa] for the original setup and a lack of stabilisation at 300[Pa] even after 5min, when the previous test was concluded. Another test was performed to change the setpoint from 300[Pa] to 240[Pa] and back to 300[Pa] for all the flow settings. For the maximum flow and 75% of maximum flow, PID reached and stabilised at the setpoint in about 20s without any oscillations. For minimum flow, a very small overshoot and 2 to 3 oscillations could be seen, but pressure still 8 / 10 Fan Control Optimization

9 stabilised in about 20s. This performance was even more remarkable when compared to Figure 4, which showed a loss of stability with the previous PID setup, even despite an overall more sluggish response. From the executed tests, we can draw a conclusion that the results from the PID parametrisation done in gives such good results, that there is no need for further adjustment. 4 Conclusions The fan control optimisation process guaranties a much better and more efficient response to varying operating conditions in an AHU system compared to the traditional approach. This is especially important when an AHU is used in conjunction with a VAV distribution system or in a pressure sensitive environment (clean rooms, hospital operating theatres, pharmaceutical factories, etc.). Application of the optimisation procedure can be brought down to 4 steps. Three of them are considered mandatory, and one as recommended. 1. EC-gfxProgram code modification (mandatory) see Individual adjustment of fan frequency converters up and down time ramp (not mandatory, but recommended) see Fan speed to pressure response time estimation (mandatory) see Recalculation of PID parameters (mandatory) see Fan Control Optimization 9 / 10

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