Transcription
STATIC PRESSURE IN HVACSYSTEMSPresented by:Ross ForryDucts Unlimited Mechanical SystemsDU Services
StaticPressure StaticPressure is the potential Energy put into aDuct System by the Fan. StaticPressure is lost to Friction so the set Point mustinclude the Friction Loss Terminal requirements StaticPressure converts to Velocity Pressure at theOutlets PotentialEnergy PotentialusedEnergy is Stored energy waiting to beDEFINITIONS
VelocityPressure VelocityPressure is the Kinetic Energy of theAir moving down the Duct in a straight lineor out the Register into a space KineticEnergy Kinetic TotalEnergy is Energy that is being usedPressure TotalPressure is the sum of Static Pressureand Velocity PressureDEFINITIONS CONT.
StaticPressure is Potential Energy This is Energy that has been put intoa Duct System and is stored asPressure Think of your Duct System as aBalloonWHY IS STATIC PRESSURE IMPORTANT?
This Balloonstores all thePotentialEnergy putinto it while itwas beinginflated
VelocityPressureStaticPressureconverts toVelocityPressure atthe point ofrelease
VelocityPressure is Energy thatis being used VelocityEnergy ThinkPressure is Kineticof a Leaf BlowerWHAT IS VELOCITY PRESSURE?
The moving Air acts directly on the leaf. ThisKinetic Energy will only travel so far.
Ina closed Duct System Static Pressureand Velocity Pressure exist at the sametime, changing back and forth WithoutStatic Pressure, Velocity Pressurealone will only move the Air so farWHEN BOTH TYPES OF PRESSURE ARE USEDTOGETHER WE CAN NOW GET THE WORKACCOMPLISHED
Both types of Pressure co-exist in the Ductwork.Velocity Pressure created by the Fan is converted intoStatic Pressure allowing air movement throughout theDuct System. This Stored Energy is then converted backto Velocity Pressure at the outlet.
POTENTIAL VS. KINETIC ENERGYPOTENTIAL ENERGY, AS THE NAME IMPLIES, ISENERGY THAT HAS NOT YET BEEN USED, THUS THETERM POTENTIAL.KINETIC ENERGY IS ENERGY IN USE (OR MOTION).
Ina Forced Air System the pressure created by the Fanstarts as Kinetic Energy. A portion is converted toPotential Energy and stored for later use . ThisPotential Energy becomes the Prime Mover of Airas it converts back to Velocity Pressure or KineticEnergy. Everythingin Nature is trying to Equalize. HighPressure goes to Low Pressure creating VelocityPressure Hotalways goes to ColdDUCT STATIC
Kinetic Energy in the form ofVelocity Pressure from the FanPotential Energy converts to KineticEnergy in the form of Velocity Pressureflows to the SpacePotential Energy inthe form of StaticPressure (not yetused) along withKinetic Energy existsin the Duct at thesame time.THE ENERGY CHANGES STATE WHICHMAKES IT USEFUL ALLOWING US TO TAKEENERGY FROM A SINGLE SOURCE ANDDISTRIBUTE IT TO MANY LOCATIONS
THINK OF A DUCT SYSTEM AS ACONTROLLED LEAK
The Fan supplies Velocity and Static Pressure tothe Duct using a combination of Outside andReturn AirThe Air then “Leaks” into the space from thediffusers to Heat, Ventilate & Cool thus makingthe space HabitableThe Air introduced into the Space inconjunction with the Air Exhausted from thespace effect the Building Static Pressure. Toolittle and you draw in unwanted Outside Air.Too Much and the excess Air will hold doorsopen which can be a Breach of Security
First you must understand what Duct System you are dealing with,High, Medium or LowSO HOW MUCH STATIC PRESSURE DOYOU WANT?
Today we will focus on Medium and Low pressure Duct and themost common applications. More isn’t always better and too little will leave areas short on air As a rule of thumb you have probably used 1 ½” WC to start. Thisis relative to where it is being measured, Close to the unit, 2/3rdsout in the system or at the worst performing zone. It is important tocheck the pressure throughout the system to ensure goodperformance.MEDIUM AND LOW PRESSURE DUCTSYSTEMS
*In Multi Story Duct Systems you mayencounter pressures that don’t makesense at first glance. As Air is fillingthe duct we must generateadequate Static Pressure to ensureall the Branch Ducts have enoughStatic Pressure to convert back toVelocity Pressure at the outlets.*Depending on where the pressure isbeing sensed, you may need tohave a higher than expected SetPoint, but only checking the entiresystem will tell you what right lookslike.
VAV or Variable Air Volume Systems save energy by reducing fanHorsepower when less cooling is required. Here are some of the designassumptions: Up to 25% or more of the capacity of the VAV’s will not be used at anygiven point in time. This is known as “Diversity”. The air will automaticallyshift to where it is needed The system design allows Load Shifting by utilizing the effects of StaticPressure and velocity pressure to move the right amount of Air to thelocation where it is needed For this to work properly there must be enough Static Pressure to meetthe requirements of the worst performing Duct supplying the terminaldevicesVAV SYSTEMS
*In this exampleyou are safe byproviding .5” WC atthe worstperforming outlet.*Excessively highpressures will causeLeaks, Noise, PoorControl and wasteEnergyTYPICAL VAV BOX REQUIREMENTS
Duct Systems are broken down into Sub Systems, eachhaving their own System Curve The simple explanation is you need to know the totalresistance plus the requirements of each Terminal device Engineers calculate all of these factors when designing aMechanical System As operators we aren’t going to change the Duct Work, weadjust the Set Points based on the results needed Without sophisticated programing in a DDC system we mustcontrol to the worst performing Branch Duct to ensure it willhave enough airCALCULATING A SET POINT
BUILDING STATIC PRESSUREWhat right looks Like
Awell operated Building must be in control of theBuilding Static Pressure. Unlike Duct Static Pressures of anywhere from .5” WC to10” WC, Building Static Pressure is usually .05”WC. Negative Pressure draws un-tempered Air and dirt inthrough cracks, doors and windows and can makedoors difficult to open. Too much Positive Pressure Wastes energy and can holdDoors open which can cause Security breachesBUILDING STATIC PRESSURE
Weuse Inches of Water Column to describe the StaticPressures. This is the amount of pressure required to lifta 1 square inch column of water up 1” Thereare 27.68 Inches of Water Column in one Pound InteriorDoors should require no more that 5 poundspressure to open while Exterior Doors are typicallybetween 8.5-10 pounds according to ADA guidelines.Exterior Doors do not have a specified opening forcebut are generally set around 9 pounds.THE MATH
*2592 square inches of Surface area*One pound of force 27.68” of Water Column*9 pounds 249.12” Water Column*249.12 / 2592 Square Inches .096 Inches Water ColumnA 3070 Doorhas2592 Squareinches.Assume theDoor Closeris set at 9Lbs.*Conclusion: one tenth of an Inch of Water Column is enoughpressure to open the average Exterior Door*If the Door is secured by Magnetic Latches, the building maynot be secure due to a malfunctioning Mechanical System*Added to the Static Pressure is Velocity Pressure which is therush of Air escaping through the opening providing additionalenergy to hold a door open (the bigger the space, thegreater the effect)*If the over pressurization is great enough the air escaping upan Elevator shaft in the form of velocity Pressure can preventElevator Doors from closing.*An additional risk is someone being struck by a doorsuddenly swinging open when a card key is swiped
Findingthe most Economical operating balancebetween Space Requirements, Fan operation andDischarge Air Temperature is an art form all by itself Thereis a balance between Fan Horsepower andCompressor Horsepower Whenyou Raise the Discharge Air Temperaturesaving Compressor Horsepower you increasethe Fan Horsepower required to do a givenamount of work. There is no universal answer tothis Justunderstand that for every benefit on oneside there is a penalty on the otherOPERATING ECONOMICS
VentilationControlReset Outdoor Air SupplyFan PressureOptimizationENERGY SAVING STRATEGIES
Traditional Ventilation settings add up all the individualrequirements and calculate the amount of Outside Air to meetthe total. This method brings in the same amount of Outside Air no matter whatthe space conditions require wasting large amounts of Energy. Many buildings have reduced these settings to save Energy but mayhave Under Ventilated areasVENTILATION RESET OUTDOOR AIRCONTROL
VentilationReset monitors the amount of Outside Airtaken in by the Air Handler and the Total Ventilationrequirements of Critical Zones to minimize theintroduction of Un-Tempered Air into the system thatmust be Heated or Cooled Whena zone is at Minimum Air Flow it will requiremore Outside Air to maintain proper ventilationthen when at Maximum Air Flow. Thismethod allows for the use of unusedrecirculated Ventilation Air that does not need tobe Heated or Cooled Requiresa DDC Control system, Pressure IndependentVAV’s and an ability to Measure and Control theamount of Outside Air brought into the system
Traditionalmethods of Duct Static Control employ afixed point of monitoring and the Supply Fan willdeliver that amount of Energy whether it is needed ornot. While this is a significant savings over a ConstantVolume system we can do better SupplyFan Optimization limits the amount of Energyput into the system based on the VAV’s actual needsfurther enhancing the savings of a traditional VAVSystem Requiresa DDC Control system, Pressure IndependentVAV’s and Fan Capacity ControlSUPPLY FAN OPTIMIZATION
Minor adjustments to your HVAC System can have dramaticeffects: Tenant Comfort-the right amount of Heating, Ventilation andCooling (HVAC) Energy Usage Fan vs. Compressor Horsepower Ventilation ratios-How much Outside Air do we have toHeat or Cool Fan Horsepower needed to maintain proper Duct Static Discharge Air Temperature-Directly impacts FanHorsepowerCONCLUSIONS
THANK YOU
VAV SYSTEMS VAV or Variable Air Volume Systems save energy by reducing fan Horsepower when less cooling is required. Here are some of the design assumptions: Up to 25% or more of the capacity of the VAV's will not be used at any given point in time. This is known as "Diversity". The air will automatically shift to where it is needed
