Improved Precision Leading to Improved Energy Efficiency Edward

Improved Precision Leading to Improved Energy Efficiency Edward Decker AE 790 – Intelligent Buildings June 6, 2006

Why is Ventilation Air Needed? § The health and comfort of the building occupants § The removal of internally generated contaminants § To maintain certain pressure differentials between the indoor and outdoor environments

How to Ensure Proper Ventilation § Use a dedicated OA system – Provides 100% OA – Ensures the minimum ventilation requirements are met § Why Not? – No mixing of Airstreams § 100% of the SA must be conditioned § Increased energy use – Increased energy cost – Best to maximize return air

How is RA & OA Mixed § Dampers are modulated to allow for air to be directed – Back in to the AHU as RA – Out of the building into the atmosphere as EA § Mass Balance – The amount of EA needs to be replaced § OA is introduced to the system via an OA damper § The amount of OA must meet the minimum requirements – The first step is to quantify the required and actual amounts of SA, RA, EA, and OA

What are the Methods to Measure Ventilation Air? § Direct – Measures the airflow directly, using a devise such as an anemometer. – Require regular flow profiles for accurate readings. § Indirect – Measures other parameters that are dependant upon the airflow, as in the energy balance method – Typically for low rates that are difficult to measure

What are the Ways to Measure Ventilation Air? § Direct – Averaging Pitot-Tube Array – Electronic Thermal Anemometry – Fan Inlet – Rotating vane and propeller anemometers – Swinging vane anemometers – Vortex shedding meters – Integrated damper and measuring devises – Laser doppler anemometry (LDA) – Orifice Meters § Indirect – Enthalpy Balance – Concentration Balance

Measuring Ventilation Air § Direct Method with Pitot-tube array – Velocity must be high enough for sensor accuracy – Regular flow profile is required, and often overlooked § Spacing of a minimum of 2 D before a station and 1 D behind required § Development of flow devices – Calculate properties of fully developed flow profile § Less space required – Device to ‘trip’ the velocity profile § Achieve accurate readings § Less space required – Comfortable Methods § Current Industry Standard without inherent problems

What are the Common ways to Control Ventilation Air? § Flow monitoring using: – Hot wire anemometers – Thermistor sensors – Velocity pressure sensing (pitot-tubes) § Differential Static Pressure Control – across fixed duct elements or with respect to OA pressure § Use of commissioning data to relate system flow at various fan-operating points with OA damper actuator signal position § Volumetric Fan tracking – Measuring SA and RA and assuming that the difference is the amount of OA § CO 2 surrogate sensing § Outside Air Injection Fan

Differential Static Pressure Control § Involves maintaining a constant pressure in the mixed air plenum or other reference point in the duct. – With constant pressure in the mixed air plenum and a fixed minimum damper, the minimum OA quantity remains constant as the supply air fan modulates. § Static pressure sensors are simple, reliable, and require little space.

Modulating minimum OA damper § New product such as flow sensing built into dampers may be the best – Velocity pressure is calculated and maintained constant by modulating the minimum OA blades themselves

Mixed Air Temperature Comparison Method § Method is still being used, but is not recommended – When OA and RA are within 10 -20 °F, readings are not trustworthy § Accuracy degrades as Toa approaches Tra – Balancers sometimes use this method when there isn’t room for pitot tube traverses

Mixed Air Temperature Comparison Method § Better Sensors can Improve this Technique – Cheaper Sensors § More sensors – Redundancy in readings – More Precise Sensors § Less errors in readings § Close OA and RA temperatures can be distinguished better

Concentration Balance § Provides adequate control – Exceptions § Low occupancy § When OA represents a small fraction of SA § Problems – Only accounts for contaminants generated by people in space, but not other contaminants § i. e. -VOCs from carpets, paints, etc

Concentration Balance § Better Sensors for measurement of all contaminants – OA might be polluted § Determine the airstream with the ‘best’ quality of air § Leads to improved overall IAQ – Multiple readings with improved precision § Can measure the air requirements of a given space § Design on an individual zone basis

Concentration Balance § Design on an Individual Zone Basis – Occupied modes for each zone – Ventilation requirements for individual room – Improved IAQ – Decreased energy consumption – Decreased energy bills

Conclusion § Accurate measurement and control of outside air intake is possible – Proper installation and operation § Intelligent Design – – Improve accuracy of readings and control More precise control § Better zone conditions – Improved overall IAQ – Energy savings – Cost Savings

QUESTIONS? ?