RE-COMMISSIONING OF VAV BOXES

Transcription

1 RE-COMMISSIONING OF VAV BOXES Authors: Cristian CUEVAS, Jean LEBRUN (ULg) Patrick LACOTE, Philippe ANDRE (FUL) September

2 Table of contents Nomenclature INTRODUCTION DESCRIPTION OF THE SYSTEM TO BE COMMISSIONED COMMISSIONING OBJECTIVES AND EVALUATION CRITERIA MEASUREMENT PROTOCOL AND PROBES LOCATION Global commissioning Experimental protocol Measured variables Individual commissioning Experimental protocol Measured variables RESULTS AND ANALYSIS OF THE MEASUREMENTS Global analysis Individual analysis Flow rate verification (C02 testing) VAV aeraulic characteristics OPPORTUNITIES FOR AUTOMIZATION CONCLUSIONS AND PRACTICAL RECOMMENDATIONS REFERENCES

3 Nomenclature P : Pressure, Pa T : temperature, C V : Volumetric flow rate, m³/s X : Concentration, ppm Subscripts AHU : Air handling unit a : Air ex : Exhaust hx : Heat exchanger rad : Radiator st : Static su : Supply w : Water 3

4 1. INTRODUCTION This paper concerns the re-commissioning of the CA-MET Building located in Namur, which was designed for a one thousand occupants. The building is divided in 11 modules representing together m 2 of gross floor area and is described in other IEA40 reports [1], [2]. The study is dealing with the verification of the performance of the VAV boxes and their control laws. 2. DESCRIPTION OF THE SYSTEM TO BE COMMISSIONED VAV principle. The air coming from the AHU is brought into the plenum of the air diffuser (A) and passes through the perforated sheet (b) towards the outlet path formed by the bellows (c) and the bellows stop (d), see Figure 1. The cross sectional area of the outlet path is varied by the changing control pressure from the control unit expanding and contracting the bellows. Figure 1 VAV diffuser in cross section 4

5 The air diffuser is essentially self-regulating. The air pressure, which is the controlling force, is itself derived from the primary air supply. This pressure acts on the bellows, which expand, and contract to regulate the rate of the airflow from the unit. The control system is presented in Figure 2; it works as explained hereafter. First, the air bleed passes through the filter that allows supplying clean air to the control system. Then the air passes through the variable orifice 1, which changes as the pressure at the controller inlet changes. The size of orifice 2, set manually by rotating a calibrated sleeve, determines the maximum airflow supplied by the unit. Finally, the thermostat keeps the room temperature constant by controlling the bleed rate across the variable orifice 3. An internal crank linked to a bi-metal strip moves the sliding plate. The bimetal strip is constantly exposed to the room airflow by means of a venturi, which creates a depression within the thermostat. Figure 2 Control schematic The VAV boxes are able to regulate the room temperature between 18 and 28 C thanks to the set point adjusting lever, which is accessible to the occupants. Figure 3 (a) shows the VAV tested during the measurement campaign and the position of the set point 5

6 adjusting lever. Figure 3 (b) shows the room temperature setting; according to the manufacturer the middle position correspond to a room temperature of 23 C. Set point adjusting l 18 C Summer 23 C 28 C Winter (a) (b) Figure 3 Position of the set point adjusting lever The control law, which is supposed to govern the operation of the VAV boxes, is shown by fig. 4. Airflow 2K 2K Max Min 20 C 28 C Summer (blue arrow) Winter (red arrow) Ambient temperature Figure 4: VAV control strategy 6

7 In summer condition (figure3), the lever should be positioned toward the blue arrow, which means a set point around 20 C. If the room temperature exceeds 21 C, the VAV boxes will be wide open, in order to supply a maximum of cool air In winter condition, the lever should be moved toward the red arrow. The offices being heated by radiators, the VAV boxes are then only used to renew the air in the office. In this position, the set point is around 28 C. In that case and with a normal ambient temperature, the VAV boxes are always closed. This means the airflow rate is minimal. VAV installed in CAMET. The CAMET building is equipped with 1248 VAV boxes, which are divided in two types: Nominal flow rate Range Type [m³/h] [m³/h] The type 1 is the most used in the building and it is the VAV studied in this work. Normally, when a type 1 box is closed, there remains a minimal airflow of 68 m³/h. According to manufacturer, a minimum supply pressure of 250 Pa is necessary to guarantee a good performance of the VAV. At this pressure, the VAV covers the whole range of airflow, from the minimum (68 m³/h for type 1) to the maximum, which is fixed by the flow controller. The maximal plenum pressure is limited at 1250 Pa. 3. COMMISSIONING OBJECTIVES AND EVALUATION CRITERIA Commissioning of a VAV system can be carried out at two levels: - Global level: the objective is to check whether the operation as a whole of all the VAV boxes located in the building or in a part of it (for instance connected to a same AHU) is satisfactory - Individual level: the objective is to check individually the operation of some typical VAV boxes. The boxes to be tested are selected according to a global 7

8 analysis, to occupants complaints, or to specific observation made in some of the rooms. The properties of the VAV boxes, which deserve verifications, are: At global level: - The flow rate operating range: does the operation of the VAV boxes drive the corresponding AHU all across its operating range? - The operating pressure of the AHU: is the fan of the AHU able to maintain its nominal pressure for all configurations of the VAV boxes? At individual level: - The temperature control operation of each VAV box - The flow rate range provided by each VAV box 4. MEASUREMENT PROTOCOL AND PROBES LOCATION 4.1. Global commissioning Experimental protocol As the specific objective of this commissioning is to check whether the AHU connected to the verified VAV boxes is able to provide the full range of flow rate (from minimum to maximum), the following protocol is proposed: 1. Launch data archiving. Are concerned the variables, which are useful for the exhaust air, flow rate control (see below). 2. Switch the AHU in automatic mode, Bring the offices temperatures somewhere between the two extreme set points (typically around 24 C) and wait for almost steady state conditions. 3. Go in each office supplied by the AHU and switch each thermostat at the end of scale of summer position (lowest temperature set point). 4. Hold on the stabilization of the fan modulation. 5. Measure the static pressure in, at least, one VAV box plenum, located far away from the AHU. 8

9 If that pressure is outside the tolerances, correct the pressure set point at AHU exhaust. When the advisable pressure is reached, note the airflow rate and the corresponding pressure set point: it will be kept for all further operations. 6. Individual VAV box checking For each office: i. The thermostat being on the summer position (VAV's wide open), note: 1. the VAV noise 2. the air speed close to the outlet (typically with a portable anemometer or simply with the hand) 3. the static pressure in the plenum ii. Put the thermostat at end of scale of winter position (highest temperature set point). The VAV boxes should then reach their minimal opening. This can be checked by: 1. a reduction of VAV noise 2. a lower air speed 3. a higher static pressure in the plenum iii. If nothing is happening, we may conclude that the thermostat is badly connected or deficient. 7. Go in each office supplied by the AHU and switch the thermostat to end of scale of winter position 8. Hold on the stabilization of the fan modulation. 9. Go in each office supplied by the AHU and switch back the thermostat to its normal position, according to the season: a. If we are in winter, let the thermostat in "winter" condition. Or else, switch back the thermostat in "summer" position and ask to the occupants to adjust more accurately, according to their requirements. Note: the here-above described procedure is easier to perform in winter:the 9 th step is then irrelevant. If, the procedure should be applied in summer, then first switch the thermostat on the "winter" position in order to finish with the "summer position". 10. Process the data. By moving all the set-points of the VAV boxes on summer position and then winter position, we force the VAV to be once at maximum flow rate and once at minimal flow rate. If all VAV's local control systems are well connected, we should be able to deduce from AHU monitoring results the maximal and minimal ventilation airflow rates and to compare them with as-built values Measured variables The variables archived are: supply pressure at the exhaust of the AHU 9

10 Heating/cooling coil pressure drop fan modulation pressure at the supply of verified VAV box 4.2. Individual commissioning Experimental protocol In the offices concerned by verification, the following actions have to be carried out: - Loading of selected rooms with auxiliary heating system (radiators or electrical system) to generate response of VAV system - CO2 test to measure ventilation rate (injection of C02 and subsequent concentration measurement) - Test of VAV system at constant fan speed - Test of VAV system at constant exhaust pressure Measured variables In order to study the performance of the VAV boxes, some measurements were performed in two offices located in the third floor of the building. The variables measured were, see figure 5: N T amb T a, su T return T return T a, su T w, su, rad B3708 T a B3508 T a T w, su, rad T w, ex, rad T w, ex, rad Atrium Figure 5 Office schematic measurements T w, su, rad : Radiator supply temperature, C 10

11 T w, ex, rad : Radiator exhaust temperature, C T supply : Supply room temperature, C T thermostat : Air diffuser thermostat temperature, C T return : Return temperature, C T room : Room temperature, C P plenum : VAV plenum pressure, Pa Figure 6 and Figure 7 show the temperature measurements in the offices and the heating system used to load the offices. Thermostat temperature Supply temperature Return temperature Figure 6 Temperature measurements Thermocouple Figure 7 Temperature measurements and auxiliary heating system 5. RESULTS AND ANALYSIS OF THE MEASUREMENTS 11

12 5.1. Global analysis A fan exhaust set point of 400 Pa may be typically required in order to get 250 Pa in the plenum of the most distant VAV boxes, when they are wide open (the most unfavorable condition). In this condition, there is about 150 Pa of pressure drop all along the air network. Pressure measured in the plenum of 2 offices (Pa). Office B3707 Office B3708 (thermostat badly connected) Summer position Winter position Control system disconnected On summer position, when the VAV's should be fully open, we find the nominal plenum pressure (260Pa) for the boxes B3707, whereas the pressure is significantly lower for B3708. Moreover, this pressure doesn t change when the set point is moved to winter position. In this position, the pressure of B3707 increases normally, confirming that the VAV boxes are closed. We conclude that the B3708 thermostat is deficient or disconnected. Pressure in the plenum and noise measurements in 9 offices (AHU exhaust pressure: 675 Pa; thermostats in summer position) Office Pressure plenum (Pa) Noise level (db) B3708 B3508 B3509 C3513 C3713 C3512 K3358 K3361 K B3708 and B3508 are deficient. Indeed, the noise level is higher there than in the other offices and the plenum pressure is lower. The VAV boxes of the other offices are operating at maximal airflow rate, giving around 600 Pa in the plenum. On deficient VAV's, the pressure is lower because there is no more control of the maximum flow and the boxes are fully open. Here also, we can see that the local control system is either disconnected or deficient. 12

13 Pressures measured in several plena: Office C3713 C3512 B3720 K3361 K3363 K3368 Pressure plenum (Pa) As the VAV boxes are closed, these pressures at the plenum are higher than in summer conditions. Figure 8, 9 and 10 show results of an overall commissioning procedure of the VAV boxes installed in the BS module (about 80 offices) of the CAMET building. The pressure difference Htg/Clg, measured between the inlet of the heating coil and the outlet of the cooling coil allows us (after calibration), to calculate the air flow rate. Figure 8: Time schedule of VAV testing phase 13

14 Figure 9: Response of AHU to all VAVs change of set point (from summer to winter position) Figure 10: response of AHU to all VAVs change of set point (winter to summer position 14

15 Figure 8 shows the development of the commissioning procedure: During the first phase (Figure 9), all VAV boxes are moving from open to closed positions and this makes the exhaust pressure to increase. Then, the AHU control makes the fan speed to decrease, in order to compensate the pressure elevation. That s what we can see on the graph: the fan modulation decreases, while the exhaust pressure is maintained to its set point (about 400Pa). The reverse situation is observed in Figure 10. The airflow rate varies from m³/h to m³/h in the first phase and from 9 582m³/h to m³/h in the second phase. This is in accordance with the as-built data. Note: fan modulations at the end of the first phase and at the beginning of the second phase are not the same, while, in both cases, the VAV thermostats are in the same winter position. This can be explained by the very strong difference between both AHU exhaust temperatures (Figure 8): In the winter (red) position during the first phase, the AHU exhaust air temperature is about 27 C, whereas, in same position, but during the second phase, the AHU exhaust temperature is about 14 C. So, during the first case, VAV s are more likely to be entire closed, because of low exhaust temperature combined with a low return temperature. The effect is a little less easy to observe when the thermostats are in summer (blue) position, probably because steady states conditions are not reached yet Individual analysis Temperature control operation. The air supplied by the VAV to the room is controlled by a thermostat, which acts according to the room temperature. So, in a normal functioning, the thermostat temperature should be the same as the room temperature. But, according to the measuring results available, this is not always the case: the thermostat temperature is highly influenced by the supply temperature and by the radiators. The measurements analyzed correspond to Wednesday 20 th (see Figure 11 and 12). Between 03:15 h and 00:05 h hot water at 80 C is supplied to the radiator, maintaining the VAV turned off. The office temperature increases from 21 C to 23 C and the thermostat temperature increases until 28 C, which is normal since hot air is stratified close to the ceiling, where the thermostat is located. At 05:00 h the AHU is turned on, supplying air at 18 C and the supply radiator temperature is decreased to 56 C. From that time, it is possible to see that the temperature measured by the thermostat is very close to the room temperature (23 C). 15

16 At 11:00 h the supply radiator temperature is again increased to 80 C and the supply temperature continues decreasing until 16 C. It disturbs the thermostat temperature, which at the beginning increases to 26 and then decreases to 21 C, which does not correspond to the room temperature (25.5 C). The VAV system sees a temperature lower than the room temperature. This one is then maintained according to a wrong set point. 100 B Temperature [ C] : : : : : : : : :00 Time 35 Tw,ex,rad Tw,su,rad Figure 11 Radiator entering and leaving water temperature B Temperature [ C] /03/ :00 0:00 3:00 6:00 9:00 12:00 15:00 18:00 21:00 Time Troom Tsupply Treturn Tthermostat Figure 12 Evolution of the room, thermostat and supply temperature 16

17 Flow rate verification (C02 testing). Two CO2 tests were performed in order to determinate the airflow rate supplied for the VAV system to the offices. The monitoring system device was used to measure the CO2 concentration, which is shown in Figure 13. As it is shown in Figure 14, one of these probes has a measuring offset. Comparing with the others devices installed in the other offices the probe installed in the office B3508 gives better results; so the values measured in the office B3708 must be corrected. Figure 13 Sensors installed in the monitored offices The first test was performed the Wednesday March 20 th at 13:20 h and the second the Thursday March 21 st at 15:10 h CO2 concentration [ppm] /03/02 00:00:00 19/03/02 12:00:00 20/03/02 00:00:00 20/03/02 12:00:00 21/03/02 00:00:00 21/03/02 12:00:00 Time CO2_B3708 CO2_B

18 Figure 14 Evolution CO2 concentrations in the offices B3508 and B3708 The air renovation can be determined using the following equation: X X X 0 0 X = 1 e τ / τ 0 where X 0 X τ τ 0 : is the initial concentration, ppm : is the final concentration, ppm : is the time, s : time constant (inverse of the air renovation), s The last equation can be expressed as: ln ( X ) = ln( X X ) n τ X 0 where n is the air renovation, 1/s. The nominal airflow rate of each VAV is 200 m 3 /h. In each office there are two VAV, so the nominal airflow rate supplied to each office must be 400 m 3 /h. In the first test the offices were heated with the radiators. The office temperatures increase up to 25 C, for an exhaust AHU temperature of 14 C, as is shown in Table 1. According to Figure 15 the VAV are not fully opened. In fact, the static pressure used to control the fan speed was not measured correctly during this test. The sensor scale was incorrect, the measured monitoring pressure was 600 Pa and in fact the real pressure was 120 Pa. So the pressure at the inlet of the VAV box was not sufficient to ensure its good performance. In the second test the offices were again heated with the radiators, but also by an additional heat load of 1500 W in B3508 and 1575 W in B3708. According to the results got in this case, the VAV boxes were fully opened, supplying the offices B3508 and B3708 with 445 m 3 /h and 485 m 3 /h respectively (see Figure 16). 18

19 The air temperatures were maintained at 22 C in both offices. 8 7 ln(x-xinf) y = x V = 209 m³/h y = x V = 149 m³/h Time [min] CO2_B3708 CO2_B3508 Figure 15 Airflow rate supplied to the offices ( :20) y = x V = 485 m³/h ln(x-xinf) y = x V = 445 m³/h Time [min] CO2_B3708 CO2_B3508 Figure 16 Airflow rate supplied to the offices ( :20) Table 1 Summary of the CO2 tests Test P st, P hx T exhaust, T return, a AHU V, a,b 3508 V V a,b3708 T B3508 T B

20 AHU AHU AHU VAV aeraulic characteristics. In order to observe the opening of the VAV boxes, two tests were performed: test at constant fan frequency and test at constant exhaust static pressure. The airflow rate supplied for the AHU is determined using the heat exchangers pressure drop and the model identified previously. With this information and with the temperatures measured in the offices is possible to analyze the performance of the VAV boxes installed in the modules B and C of the building. In the first test the Modules B and C were heated with the radiators and with the air supplied by the AHU. The fan frequency was kept fix at 40 Hz. According to Figure 7 the supplied temperature increases from 14 C to 40 C and the room temperature from 21 C to 27 C. The opening of the VAV increases slightly and the supplied air flow rate increases only of 8 % in 2 hour. Airflow [m3/h] :48 19:12 21:36 Test period B /03/2002 0:00 21/03/2002 2:24 21/03/2002 4: /03/2002 7:12 Temperature [ C] Time Airflow T_supplyB3708 T_roomB3708 T_returnB3708 Tthermostat Figure 17 VAV behavior 20

21 Figure 8 shows the evolution of the AHU airflow rate against the AHU return temperature for the period where the VAV was tested. It can be seen that the AHU return temperature continues increasing, while the AHU airflow rate increases. This indicates that the VAV boxes are not still fully open. As it can be seen in Figure 9 the pressure at the exhaust of the AHU decreases while the airflow rate increases. The same behavior is observed for the plenum pressure measured in the VAV boxes Airflow rate [m³/h] Treturn,AHU [ C] Figure 18 AHU airflow rate against AHU return temperature 1200 Test period Presion [Pa] :48 19:12 21:36 21/03/2002 0:00 21/03/2002 2:24 21/03/2002 4: /03/2002 7:12 Time PplenumB3708 PplenumB3508 Pexhaust,AHU Treturn,AHU Figure 19 Evolution of the pressure and return AHU temperature during the test 21

22 6. OPPORTUNITIES FOR AUTOMIZATION Automatization of such a procedure for commissioning of a VAV system entirely relies on the possibility to measure and to control the individual operation of each box. In the case presented in this report, the VAV boxes are essentially self-regulating devices: remote measurement of the thermostat temperature, remote adjustment of the set point and remote control of the VAV opening are not possible. As a consequence, automization of the procedure using that kind of device is difficult. If it were the case, the global checking would be the easiest to carry out. 7. CONCLUSIONS AND PRACTICAL RECOMMENDATIONS The methods presented in this report allow carrying out the checking if VAV boxes, both at a global level and at an individual level. In the case study presented here, they allowed to check that, as whole, the behavior of the VAV system was satisfactorily, while they identified the devices that are likely non-operating. The main recommendations would be to start from a global approach (in conjunction with a parallel verification of the Air Handling Units) to detect problems. If this global approach points out deficiencies, the analysis should carry over to a more detailed analysis of selected individual devices. The global verification can take place during normal operation of the building (passive testing), while the individual verification requires the generation of artificial (and forced) conditions (active testing). 8. REFERENCES [1] CUEVAS, C; LACÔTE, P.; LEBRUN, J. Recommissioning of an Air Handling Unit, IEA Annex 40 working document, University of Liège, May