Chapter 16B: FANS AND SYSTEM EFFECTS Agami Reddy (July 2016) 1. Friction loss from airflow in ducts 2. Corrections for duct roughness, altitude and temperature 3. Pressure drop in duct fittings and bends 4. Types of fans and performance characteristics 5. Fan laws 6. Duct system characteristics 7. Fan and duct system interaction 8. Obtaining variable air flow control 9. Special features and fan installation tips 10. Pressure gradient diagram 11. Duct design considerations and sizing methods 12. Flow measurement instruments HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects

Friction Loss due to Airflow in Ducts Similar charts as for water piping HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects FIGURE 16.8 (a) Round duct friction chart in IP units based on average roughness of ε = 0.0003 ft and standard air. HCB 3-Chap 16B: Fans and System Effects

If flow is specified, do not use the pressure drop chart directly! Rectangular ducts of the same cross-sectional area as round ducts have higher frictional loss. If flow is specified, do not use the pressure drop chart directly! Rectangular ducts should be as square as possible. Aspect ratios of 4:1 or less are best (if space permits), and ratios greater than 8:1 should be avoided. HCB 3-Chap 16B: Fans and System Effects

for non-circular ducts Another approach for non-circular ducts Determine equivalent round size duct D => Dh= (4 x Area)/Perimeter =(2 W.H)/(W+H) Similar to hydraulic diameter First, calculate velocity, and then use the friction chart Example: Rectangular duct 0.3 m x 0.5 m (12 “ x 19.7”) with 1.5 m3/s- -From Figure, Dh= 16” or 40.6 cm Velocity = 1.5/(0.3 x 0.5)=10 m/s Use velocity and diameter in chart (dp /L) = 3 Pa/m HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects Correction for Duct Roughness HCB 3-Chap 16B: Fans and System Effects

Correction for Altitude and Temperature- due to density change Friction loss x correction factor @ standard air Adjust the friction loss if: Temp. 40 F > T > 10 F or Altitude > 1500 ft HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects Pressure drop in Duct Fittings HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects Types of Fans HCB 3-Chap 16B: Fans and System Effects

Centrifugal Fans most popular Types of Impeller blades HCB 3-Chap 16B: Fans and System Effects

Performance Characteristics Plots of different quantities versus volume flow rate: Static pressure (b) Power consumed (c) Efficiency Figure 16.20 Forward-curved, backward-curved, and radial fan blading arrangements and performance curves. The shaded region is the suggested operating range near peak efficiency, but to the right of the peak pressure point to ensure stability. HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects Selection of Fan Size: Manufacturer Table at rated conditions temperature = 70 F and 1 atm HCB 3-Chap 16B: Fans and System Effects Outlet velocity

HCB 3-Chap 16B: Fans and System Effects Note log-log scale Different fan speeds can also be shown HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects Fan Laws Similar to pump affinity laws. Fan laws are groups of relations that predict the effect on performance when such quantities as operating speed, fan size, condition of air are changed Usefulness of fan laws lies in ability to predict changes from a base condition The laws apply only to: - Same fan - Dynamically similar fans - Constant duct system HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects Table 16.6 Summary of Fan Laws HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects Duct System Characteristics The relationship between flow rate and pressure drop of a given system with fixed L and D Note: We assume friction factor f to be constant HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects

control in HVAC systems Methods for obtaining variable air flow control in HVAC systems HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects Empirical curve-fit equations are used for modeling such control methods Figure 16.23 Part-load fan characteristics for outlet damper, inlet vane, and variable-speed control methods. HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects Special Features Ducts need to be insulated when installed in hot or cold environments (such as attics) (usually R-6 or R-8) Ducts need to have aerodynamic transitions To avoid turbulence and hence increased pressure drop. Use bends or if sharp turns are unavoidable, use turning vanes HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects Tips for ducting after fan: Velocity profiles at outlet of fan are not uniform until the air has travelled “one effective duct length” It is advisable to provide this length to avoid high pressure drop HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects Fan Installation Tips: Avoid abrupt changes in velocity and direction HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects Pressure Gradient Diagrams Pressure gradient diagram- plot of the pressure head versus pipe or duct distance This is very useful for designing piping and ducting systems and is also used for diagnosing problems associated with improper flow in existing fluid systems Figure 16.16 (a) Schematic of a pressure gradient diagram showing how the total and velocity pressures vary along the length of a pipe or duct (b) Example of pressure profile for a small commercial building with a supply fan HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects Duct Design Considerations The objective of duct design is to deliver the amount of air (at the proper condition) needed to meet the loads in each zone of a building. Duct design is constrained by many factors: -available space (often beyond the purview of the HVAC engineer) - to meet loads in a variety of zones - to meet economic criteria - to minimize operating energy subject to previous constraint - to control noise levels. HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects Proper duct design is a specialized skill and involves several years of experience. The sequence of events in duct design is as follows. preliminary system is laid out on a set of preliminary drawings including all structural members ducts are then sized based on needed air quantities in each zone and for each terminal device pressure drop calculations are made at this point, and a fan is selected. the next iteration of the design will need to account for potential flow imbalances in the original design, duct runs of excessive pressure drop, and noise problems after one or more iterations to accommodate these criteria, a set of final design drawings is prepared at least one cost estimate is necessary as part of the design process, when the design is deemed sufficiently complete Computer-aided design (CAD) of duct systems is commonplace in large and medium-size design offices; it replaces the formerly tedious, manual iterations HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects Duct systems for HVAC installations may be loosely classified into Low velocity and high velocity groups- HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects FIGURE 16.25 Recommended operating ranges for low- and high-velocity air systems; pressure drop versus flow rate HCB 3-Chap 16B: Fans and System Effects

Recommended Velocity, ft/min Maximum Velocity, ft/min TABLE 16.9 Recommended and Maximum Duct Velocities   Recommended Velocity, ft/min Maximum Velocity, ft/min Designation Residences Schools, Theaters, Public Buildings Industrial Buildings Outside air intakes* 500 800 900 1200 Filters* 250 300 350 Heating coils* 450 600 700 Air washers Suction connections 1000 1400 Fan outlets 1000–1600 1300–2000 1600–2400 1500–2000 1700–2800 Main ducts 700–900 1000–1300 1200–1800 800–1200 1100–1600 1300–2200 Branch ducts 600–900 800–1000 700–1000 800–1300 1000–1800 Branch risers 600–700 650–800 HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects Table 16.8 Different Types of Duct Sizing Methods Method Classification Basis of Duct Sizing Equal velocity Low velocity For simple systems, equal velocity in duct branches (lot of judgment needed) Equal friction Equal friction loss per unit duct length for all branches Balanced capacity Equal pressure drops from fan to outlets of all branches Static regain High velocity For large installations, same static pressure along duct length T- method Based on life cycle costing (LCC) optimization using dynamic programming HCB 3-Chap 16B: Fans and System Effects

Concept of Static Regain A loss may not show up as a decrease in static or velocity head HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects Static regain method of sizing ducts is most often used for high velocity systems with long duct runs especially in large buildings. - Initial velocity in the main duct leaving the fan is selected – range of 2500 – 4000 fpm - ducts are sized so that the air velocity in the direction of flow is reduced in such a way that the increase (or “regain”) in static pressure just balances the pressure losses in the downstream section of duct. - accomplished by progressively increasing the duct cross-section; Thus system static pressure remain about the same throughout a system. This simplifies terminal box selection and system balancing Disadvantages are very low velocities and large duct sizes at outlets, and calculation is tedious (several software programs available) Typical regain factors: 0.75-abrupt changes in area 0.95 very smooth transition HCB 3-Chap 16B: Fans and System Effects

General Recommendations on Duct Design and Layout Routes should be as direct as possible Sudden changes in direction and velocity should be avoided Turning vanes should be used whenever possible Rectangular ducts should be as square as possible. Aspects ratios greater than 8:1 should be avoided and 4:1 or less should be used whenever space permits Smooth metal construction is preferred Since calculations are approximate, a fan with some excess capacity should be selected Airflow through the fan should be 10% more than the sum of the outlet requirements to allow for leakage Dampers must be installed in all branches for balancing even if the static regain method is used. Dampers should be installed as close as possible to the main duct in order to reduce noise Nothing should be put in or through the ducts For minimum fan power and noise, the air velocity should be a slow as possible HCB 3-Chap 16B: Fans and System Effects

Flow Measurement Instruments Venturi meter Orifice plate Pitot tube Turbine flow meters HCB 3-Chap 16B: Fans and System Effects

Water and Air as Heat-Conveying Media Advantages of water vs air - more heat capacity of water than air - pipes take up less space than air ducts - pipes are easier to insulate and so higher temperatures of water for heating is possible - cheaper first cost and operating costs Disadvantages - water leakage has drastic consequences - better control of comfort with air systems HCB 3-Chap 16B: Fans and System Effects

HCB 3-Chap 16B: Fans and System Effects Outcomes Be able to solve problems of friction loss from airflow in ducts using equations and charts- correction for non-circular ducts Knowledge of how to correct pressure drop for duct roughness, altitude and temperature Be able to solve problems of pressure drop in duct fittings and bends Familiarity with the different types of fans and their differences Knowledge of performance characteristic curves of fans Familiarity with fan laws Knowledge of how to analyze interaction of duct and fan interaction Familiarity with different ways of achieving variable air flow control Familiarity with good duct and fan installation practice Understand the concept of the pressure gradient diagram for ducts Knowledge of duct design considerations and the various sizing methods Familiarity with various air flow measuring instruments HCB 3-Chap 16B: Fans and System Effects