Outdoor Airflow Control Fixed point and DCV. Presented by: David S. Dougan, President EBTRON

Transcription

1 Outdoor Airflow Control Fixed point and DCV Presented by: David S. Dougan, President EBTRON

2 OUTSIDE AIR Pressure Changes Damper Issues Intake Flow Rates (Fixed Min or DCV)

3 OUTSIDE AIR Intake Flow Rate Variations Damper Issues Pressure Variations Hysteresis Binding Deterioration Supply Fan Speed Wind Stack Effect

4 Replace Contaminants with Outdoor Air: DILUTION Hysteresis - No Wind 15% Damper Open Damper Issues 140% % of Minimum Setpoint 120% 100% 80% 60% 40% V ot Setpoint From Closed From Open Average Average 20% 0% Sample Number

5 Replace Contaminants with Outdoor Air: DILUTION Fan Speed Changes 0.3 VAV Mixed Air Pressure Effect on OA Intakes Fixed Minimum Position Intake Damper Mixed Air Pressure (in.w.g.) in.w.g. = 40% reduction Outside Airflow (% Setpoint) Mixed Air Pressure Outside Airflow Supply Airflow (% Max) Measurement for Control of Fresh Air Intakes - Solberg, Dougan & Damiano - ASHRAE Journal, January 1990

6 Replace Contaminants with Outdoor Air: DILUTION Wind & Stack Pressure Wind Pressure on Intake System mph direct to cross wind 0.2 in.w.g. = 40% reduction Pressure (in.w.g.) Direct Wind (0.7) Cross Wind (-0.6) Wind Speed (mph)

7 Replace Contaminants with Outdoor Air: DILUTION Hysteresis + 15 mph Cross Wind 15% Damper Open Wind & Stack Pressure 160% 140% % of Minimum Setpoint 120% 100% 80% 60% V ot Setpoint From Closed From Open Average Average 40% 20% 0% Sample Number

8 Wind & Stack Pressure

9 Wind & Stack Pressure

10 Wind & Stack Pressure

11 Replace Contaminants with Outdoor Air: DILUTION Wind & Stack Pressure Stack Pressure on Intake System 70 ft. (First Floor to Roof) F Temperature Drop 0.2 in.w.g. = 40% reduction Pressure (in.w.g.) Outdoor Temperature (F)

12 OUTSIDE AIR Damper Issues Fan Speed Wind & Stack Monitor Outside Airflow Control Outside Airflow

13 6. PROCEDURES 6.1 General. The Ventilation Rate Procedure, the IAQ Procedure, and/or the Natural Ventilation Procedure shall be used to meet the requirements of this section. In addition, the requirements for exhaust ventilation in Section 6.5 shall be met regardless of the method used to determine minimum outdoor airflow rates Ventilation Rate Procedure. The prescriptive design procedure presented in Section 6.2, in which outdoor air intake rates are determined based Prescriptive on space type/application, OA rates occupancy determined level, and floor by area, space shall be permitted to be used for any zone or system. type, occupancy and floor area IAQ Procedure. This performance-based design procedure (presented in Section 6.3), in which the building outdoor air intake rates and other system design parameters are based on an analysis of contaminant sources, Subjective? OA rates determined by contaminant concentration limits, and level of perceived indoor air acceptability, shall be permitted to be used for any zone or system. contaminant sources, levels & perceived IAQ

14 6.2.7 Dynamic Reset. The system may be designed to reset the outdoor air intake flow (Vot) and/or space or ventilation zone airflow (Voz) as operating conditions change Demand Control Ventilation (DCV) DCV shall be permitted as an optional means of dynamic reset The breathing zone outdoor airflow (V bz ) shall be reset in response to current occupancy and shall be no less than the building component (R a A z ) of the DCV zone.

15 The required ventilation rate per person for a given population cannot be reduced under the VRP

16 Dynamic Reset (i.e. DCV) is NOT allowed under the Indoor Air Quality Procedure

17 CO 2 DCV Outside air dampers or fans are modulated to maintain the space CO 2 level. No direct feedback on outside air intake flow rates. Damper issues, wind and stack effect and supply fan speed changes make this type of reset high risk for under- and overventilation. CO 2 levels are often considered to be a proxy for indoor air quality the lower the level the better. Not true. Space CO 2 levels rarely exceed 2,000 ppm. CO 2 does not typically become a contaminant until levels exceed 5,000 ppm.

18 Co Vo Co = Outdoor CO 2 concentration (ft 3 CO 2 /ft 3 air) Ci = Indoor CO 2 concentration (ft 3 CO 2 /ft 3 air) Vo = Outside Airflow Rate (ft 3 /min) N = CO 2 production of occupant (ft 3 CO 2 /min) P = Number of occupants N P Steady-state Mass Balance: In = Out Co Vo + N P = Ci Vo Ci Vo Can be rearranged as: N / (Ci Co) = Vo/P = OA CFM/person

19 CO 2 DCV CO 2 is used as a proxy for estimating OA CFM/person NOT a proxy for indoor air quality! There are numerous uncertainties when using CO 2 for DCV. Those uncertainties include: Lag and assumption of steady-state model validity Indoor CO 2 sensor error and placement error Outdoor CO 2 level assumption error Occupant CO 2 production rate uncertainty

20 CO 2 DCV (1,000 sq.ft. classroom) *Assumptions: Steady-state, N=0.0109, OA CO 2 =400ppm, no sensor error or bias 500 A constant CO 2 level (ex. 1,000 ppm) Outside Airflow (CFM) Room CO 2 Level (ppm) OA CFM Provided CO2 Setpoint Number of People 200 Would result in this outside airflow to the space*

21 CO 2 DCV (1,000 sq.ft. classroom) *Assumptions: Steady-state, N=0.0109, OA CO 2 =400ppm, no sensor error or bias 500 Outside Airflow (CFM) Over-ventilates Under-ventilates ASHRAE 62 Vbz OA CFM Provided Number of People

22 Ventilation Uncertainty: Indoor/Outdoor CO 2 Levels CO 2 DCV (1,000 sq.ft. classroom) *Assumptions: Steady-state, N=0.0109, OA CO 2 =400ppm +/-, sensor +/- 500 Ventilation Uncertainty Indoor 450 CO 2 : ± 100 ppm Outdoor 400 CO 2 : ± 50 ppm Outside Airflow (CFM) ASHRAE 62 Vbz OA CFM Provided Uncertainty Number of People

23 Metabolic Data < N actual < CO 2 Production, CFM/person Very Light Activity Office work Light Activity Moderate Activity N assumed = Physical Activity, Met Units

24 Ventilation Uncertainty: Add Physical Activity 500 CO 2 DCV (1,000 sq.ft. classroom) *Assumptions: Steady-state, N= /-, OA CO 2 =400ppm +/-, sensor +/ Outside Airflow (CFM) ASHRAE 62 Vbz OA CFM Provided Office Work Seated, Quiet Walking 2 to 3 MPH Light Work Moderate Work Heavy Work Number of People

25 Ventilation Uncertainty: Fixed Damper Limit 500 CO 2 DCV (1,000 sq.ft. classroom) *Assumptions: Steady-state, N=0.0109, OA CO 2 =400ppm +/-, sensor +/- 450 Outside Airflow (CFM) Upper limit with fixed damper? ASHRAE 62 Vbz OA CFM Provided Uncertainty Lower limit with fixed damper? Number of People

26 Without airflow measurement & control Damper Issues Fan Speed Changes Wind & Stack Pressure? 25% or more Proportional to Fan Speed Reduction 25% or more

27 Ventilation Uncertainty: Fixed Damper Limit 500 CO 2 DCV (1,000 sq.ft. classroom) *Assumptions: Steady-state, N=0.0109, OA CO 2 =400ppm +/-, sensor +/- Outside Airflow (CFM) Fixed damper airflow uncertainty from hysteresis, wind and stack effect Upper limit with fixed damper? ASHRAE 62 Vbz OA CFM Provided Uncertainty Number of People

28 Ventilation Uncertainty: Fixed Damper Limit 500 CO 2 DCV (1,000 sq.ft. classroom) *Assumptions: Steady-state, N=0.0109, OA CO 2 =400ppm +/-, sensor +/ Outside Airflow (CFM) ASHRAE 62 Vbz OA CFM Provided Uncertainty Lower limit with fixed damper? Number of People

29 Ventilation Uncertainty: Fixed Damper Limit 500 CO 2 DCV (1,000 sq.ft. classroom) *Assumptions: Steady-state, N=0.0109, OA CO 2 =400ppm +/-, sensor +/- 450 Outside Airflow (CFM) Fan Turndown Limit with fixed damper? ASHRAE 62 Vbz OA CFM Provided Uncertainty Number of People

30 Ventilation Uncertainty: AMS Limit CO 2 DCV (1,000 sq.ft. classroom) *Assumptions: Steady-state, N=0.0109, OA CO 2 =400ppm +/-, sensor +/ Outside Airflow (CFM) Upper limit with AMS max? ASHRAE 62 Vbz OA CFM Provided Uncertainty Lower limit with AMS min? Number of People

31 Co Vo Co = Outdoor CO 2 concentration (ft 3 CO 2 /ft 3 air) Ci = Indoor CO 2 concentration (ft 3 CO 2 /ft 3 air) Vo = Outside Airflow Rate (ft 3 /min) N = CO 2 production of occupant (ft 3 CO 2 /min) P = Number of occupants N P Steady-state Mass Balance: In = Out Co Vo + N P = Ci Vo Ci Vo Can be rearranged as: Vo (Ci-Co) / N = P (people!)

32 Reset OA airflow setpoint to maintain Vbz 500 CO 2 / Airflow DCV (1,000 sq.ft. classroom) *Assumptions: Steady-state, N=0.0109, OA CO 2 =400ppm +/-, sensor +/ Outside Airflow (CFM) Vo set = R p Vo meas (Ci Co)/ N + R a A z ASHRAE 62 Vbz OA CFM Provided Number of People

33 Ventilation Uncertainty: Indoor/Outdoor CO 2 Levels CO 2 / Airflow DCV (1,000 sq.ft. classroom) *Assumptions: Steady-state, N=0.0109, OA CO 2 =400ppm +/-, sensor +/- 500 Ventilation Uncertainty Indoor 450 CO 2 : ± 100 ppm Outdoor 400 CO 2 : ± 50 ppm Outside Airflow (CFM) ASHRAE 62 Vbz Number of People

34 Reset OA airflow setpoint to maintain Vbz Calculate P z =Vo meas (Ci Co)/N Calculate Vo set =R p P z + R a A z Limit Vo set within OA min and max settings Setpoint used for OA loop Is Vo set outside Vo meas ± deadband? YES Run PID Loop and update output to damper or fan