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Article - July 2001
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David Dougan, Ebtron
and
Len Damiano, Ebtron
Contributing Editor

Over time, we have seen the same general questions asked by engineers and control contractors about ventilation control, building pressurization and measurement technologies. Few of them require lengthy explanations, justifying their own paper. Therefore, we have consolidated some of them below in a Q&A format. We hope it helps you in your understanding of these topics.

1. Why can't I just set the outside air intake flow rates manually?

PlantPROCOREDamper blades and operators deteriorate over time and their positions can vary as a result of linkage hysteresis. Intake flow rates are difficult to measure with hand-held instruments in the field. To complicate matters, pressure changes on the intake system result from changes in wind speed and direction as well as seasonal changes in stack effect. These pressure changes can significantly increase or decrease outside airflow rates into the building. Changes in the mixed air plenum pressure will compound the problem in VAV systems. In addition, many systems will require that the outside airflow rate be continuously reset to meet changing conditions within the building. Instruments and controls must be used that can react to these changes, in real time, so that the proper dilution airflow rate can be continuously maintained, as is required in the latest proposed addenda (U) to ASHRAE 62-1999.

2. Why would the outside air requirement at the air handler change?

Dilution air requirements at the air handler can result from changes in both thermal and occupant demand at each occupied space. Thermal demand changes are significant in VAV systems where the airflow rate to each space can change independently of the occupant load. These changes result from daily and seasonal external thermal load variations. Changes in occupancy could also affect the dilution air requirement. As a result, the fraction of outdoor air required to satisfy the ventilation and thermal control requirements will differ from space to space over time. A well-conceived HVAC system must provide dilution air to accommodate these changes and provide adequate outside air to dilute both human bioeffluents and space borne contaminants.

3. Won't any effort to provide adequate dilution air increase my energy consumption?

No. The proper selection of instruments and controls will actually minimize energy consumption while meeting ventilation requirements for acceptable IAQ. This is accomplished by assuring that no more dilution air is brought into the building than is needed to satisfy each occupied space. To accomplish this, accurate and reliable control methods and instrumentation must be selected.

4. Won't additional control components actually increase our maintenance requirements?

No. Instrumentation and other control devices can be selected to minimize maintenance requirements through their greater reliability and functionality. The design solution should provide information to facility engineers and operators to help them pinpoint problems as they arise and function as a tool to lead them to fast corrective action.

5. Can I use "indirect" methods to determine my outside air intake flow rates?

No. Methods that determine outside airflow rates by taking the difference between supply and return airflow rates, or by using temperature and mass balance methods produce significant errors that result in unreliable control of outside air. These errors can be easily demonstrated with simple mathematical calculations. Other indirect methods that rely on maintaining the pressure drop across a fixed orifice (i.e. baffle, damper or louver) are subject to the difficulties of accurately measuring low pressures or the unreliability of static set-up requirements. In the case of fixed dampers, the inability of the damper blades and operators to consistently return to the same position, especially as the damper ages, renders this method inappropriate at best.

6. Can I really measure and control the low airflow rates associated with most outside air intakes?

Yes. Unlike velocity pressure-based devices or vortex shedding probes, Thermal Dispersion (TD) technology has its greatest sensitivity at low air velocities. All TD airflow-measuring stations are calibrated from still air and are well suited for measurement of airflows associated with outside air intakes (often 100 FPM or less). Microprocessor-based digital transmitters have built in dampening algorithms to filter transient wind effects that assure reliable control of minimum outdoor air.

7. Can I get accurate airflow measurements without long straight duct runs?

Yes. Thermal dispersion measures velocity by measuring the power dissipated from a heated sensor. Each sensor is independent so a true average airflow rate can be determined. Extensive testing has determined the sensor density and placement for accurate measurement in outside air intakes. Airflow can be measured accurately when sensors are placed as little as 3 inches upstream of fully open intake damper blades. Unlike other technologies, TD technology can be used with any quality control damper.

8. Will this measurement technology interface with my BAS controller?

Yes. Transmitters associated with this technology are available with standard, linear analog output signals of 0-10 VDC or 4-20mA that can be measured by all leading control providers. In addition, transmitters are also available with popular network protocols including ModBus®, LonWorks®, and N2®. There are plans to expand these serial communications options to include Siemens FLN® and BacNET® MT/SP as field-selectable protocols.

9. Can I use the airflow measuring devices already in my VAV boxes?

No. The differential pressure flow meters installed in VAV boxes are adequate for thermal comfort. However, the differential pressure transducer on the box controller does not provide sufficient accuracy to convert the velocity pressure from the VAV box pickup to velocity to measure supply flow rates, let alone for reset control using the multi-space calculation.

10. Can I reduce my energy consumption?

Yes. There are a number of ways to reduce your energy consumption that are outside of the scope of this document. CO2 or occupancy sensing methods could be used in variable occupancy spaces to reset the measured outside airflow set point for the space between a minimum value for unoccupied (base dilution ventilation only) and a maximum for fully occupied. (This should not be confused with strategies that reset intake dampers without active airflow measurement.)

Energy can also be reduced by providing reheat in critical zones and increasing the VAV box supply airflow set point, essentially lowering the critical zone outside air fraction and therefore the overall outside air requirement at the air handler. High-level filtration that removes harmful contaminants may also play a role in the reduction of outside airflow rates. Energy usage may also be reduced in some areas using energy recovery ventilators (ERV), as long as there are provisions to assure that the exhaust airflow rate does not result in negatively pressurized spaces.

11. Can I rely on my ERV to bring in more outside air than the standards require?

No. First, the energy recovered from ERVs is not 100% efficient, especially if consideration is made to provide for positive building pressure. Conditioning the outside air, even with an ERV, is not "free". Most ERV units are affected by pressure changes upstream and downstream of the unit and are susceptible to wind, stack, and mixed air plenum pressure variations. In addition, filter loading can dramatically impact ERV airflow performance. Designs using ERVs should still monitor and control airflow rates to optimize energy usage and assure minimum ventilation rates are actually maintained.

12. Do microprocessor-based TD airflow measuring stations require periodic recalibration?

No. These devices use precision components and sensors. They are fully tested prior to shipment and are factory calibrated with accuracy traceable to the NIST air speed tunnel. Independent, government test data shows no significant drift of the sensors over time. Therefore, periodic recalibration is neither required nor recommended.

13. Does this technology require periodic cleaning?

No. There are no orifices to clog. Under normal use, accumulation of dust and dirt does not affect sensor performance.

14. How long has this technology been around?

Ebtron has provided the HVAC industry with performance, quality, and service since 1984. Ebtron, Inc. was the first company to develop Thermal Dispersion technology and to produce microprocessor-based TD instruments. The company has more than 70 factory authorized sales offices throughout the United States, Canada and overseas.