Presentation on theme: "Heating and Air Conditioning I"— Presentation transcript:
1 Heating and Air Conditioning I Principles of Heating, Ventilating and Air ConditioningR.H. Howell, H.J. Sauer, and W.J. CoadASHRAE, 2005basic textbook/reference materialFor ME 421John P. RenieAdjunct Professor – Spring 2009
2 Chapter 9 – Duct and Pipe Sizing Duct Systems.This chapter discussed the design of systems for conveying air and water.HVAC system must also distribute conditioned air from a central equipment location to the individual spaces requiring environmental control (fans and ducts, pumps and piping)Objective of duct system design: Provide a system that efficiently transmits the required flow rate of air to each space while maintaining a proper balance between investment and operating costs (within prescribed limits of velocities, noise intensity, and space availability)When heating, cooling and ventilation loads have been established – total flow rates of air can be determined.Size of the duct system governs frictional losses and thereby the size of the fan and power required to operate the duct system.
3 Chapter 9 – Duct and Pipe Sizing Pressure ChangesResistance to airflow (in both supply and return ducts) must be overcome by mechanical energy – use of a fan.Usually incompressible flow can be assumed (standard density of lb/ft3 or 1.2 kg/m3)Total pressure is sum of static pressure and velocity pressure
4 Chapter 9 – Duct and Pipe Sizing Pressure ChangesVelocity given by (for flow rate and cross-sectional area)
5 Chapter 9 – Duct and Pipe Sizing Pressure ChangesVelocity pressure as function of velocity (standard pressure)
6 Chapter 9 – Duct and Pipe Sizing Pressure ChangesVelocity pressure as function of velocity (standard pressure)
7 Chapter 9 – Duct and Pipe Sizing Pressure ChangesIf air is not at standard conditions …Total pressure is a measure of the total available energy at a cross section.Total pressure always decreases in the direction of airflowStatic and velocity pressure are mutually convertible and either increase or decrease in the flow directionSee Figure 9-1 for total and static pressure changes in a simplified fan/duct system.
8 Chapter 9 – Duct and Pipe Sizing Pressure Changes – in a ducting system – Figure 9-1
9 Chapter 9 – Duct and Pipe Sizing Pressure ChangesConstant cross-sectional areas: total and static losses are equal – ducts (frictional) fittings (frictional and dynamic)Diverging sections: velocity pressure decreases, total pressure decreases, static pressure may increase (static regain)Converging sections: velocity pressure increases in direction of flow, total and static pressure decreaseAt exit: total pressure lass depends on the shape of the fitting and the flow characteristics – exit loss coefficients (<1, 0, or >1)When exit loss coefficient is less than one, the static pressure upstream of the exit is below the atmospheric valueTotal system resistance to airflow is noted as pt in Figure 9-1
10 Chapter 9 – Duct and Pipe Sizing Pressure ChangesStatic pressure is used as the basis for system design; total pressure determines the actual mechanical energy that must be supplied to the system.Total pressure always decreases in the direction of the flow.Static pressure can decrease and then increase in the direction of the flow even going below atmospheric.Distinction between static pressure loss and static pressure change as a result of conversion of velocity pressure.
11 Chapter 9 – Duct and Pipe Sizing Circular Equivalent of Rectangular DuctsUsually systems are size for rounded ducts – then rectangular ducts can be used upon proper conversion for proper flow ratesUp to aspect ratios of 8:1, rectangular ducts have same pressure drops and mean velocities for round ducts of the same hydraulic diameter (4 x area/perimeter)Circular equivalents given byMean velocity in a rectangular duct is less than a circular ductMultiplying or dividing by a factor in rectangular duct is same as that for a circular duct.
12 Chapter 9 – Duct and Pipe Sizing Circular Equivalent of Rectangular Ducts
13 Chapter 9 – Duct and Pipe Sizing Frictional LossesPressure drop in a straight duct is caused by surface friction (see Air Friction Chart Figure 9-2) – standard density of air flowing through clean, rund, galvanized metal ducts – beaded slip covers on 48 inch centers – all temperatures 50 – 90 oF.
14 Chapter 9 – Duct and Pipe Sizing Frictional Losses
15 Chapter 9 – Duct and Pipe Sizing Frictional Losses
16 Chapter 9 – Duct and Pipe Sizing Frictional Losses
17 Chapter 9 – Duct and Pipe Sizing Dynamic LossesEddying flow present – greater loss in total pressureFlow disturbances in fittings (entries, exits, transitions, and junctions)Fluid resistance coefficient – ratio of total pressure loss to velocity pressure at the referenced cross-section.
18 Chapter 9 – Duct and Pipe Sizing Dynamic LossesOccurs along a duct length and cannot be sepearted from frictional loss.Frictional losses considered only for long fittings
19 Chapter 9 – Duct and Pipe Sizing Dynamic Losses
20 Chapter 9 – Duct and Pipe Sizing Dynamic Losses
21 Chapter 9 – Duct and Pipe Sizing Ductwork Sectional LossesTotal pressure loss is the combination of the frictional and dynamic losses in terms of Dp. SC is the sum of the local loss coefficients within the duct section.Each fitting loss coefficient must be referenced to that section’s velocity pressure.
22 Chapter 9 – Duct and Pipe Sizing System AnalysisThe pressure drop between station 1 and station 2 can be determined for all main and branch ducts in a system (supply and return).The path with the greatest resistance to flow, usually the longest and the one with the most branches, is considered the critical path.The fan is design around satisfying the requirements of this path
23 Chapter 9 – Duct and Pipe Sizing Duct DesignRules
24 Chapter 9 – Duct and Pipe Sizing Duct DesignRules
25 Chapter 9 – Duct and Pipe Sizing Design VelocitiesOptimization – taking in cost (owning and operating)
26 Chapter 9 – Duct and Pipe Sizing Design Velocities
27 Chapter 9 – Duct and Pipe Sizing Design MethodsAcceptance of high-velocity transmission systems
28 Chapter 9 – Duct and Pipe Sizing Design Methods – in general practiceEqual Friction MethodSize a system for constant pressure loss per unit length of ductAfter sizing, plot total pressure grade line plotted and optimizedVelocity Reduction MethodSelect velocities at supply and return side of fan and work around the system, reducing the velocities – sizing the ducts accordinglyStatic Regain MethodDucts are sized so that the increase in static pressure at each take-off offsets the pressure loss of the succeeding section of the ductwork