1 / 38

Objectives

Objectives. Finish heat exchangers Air Distribution Systems Diffuser selection Duct design fluid dynamics review. Fin Efficiency. Assume entire fin is at fin base temperature Maximum possible heat transfer Perfect fin Efficiency is ratio of actual heat transfer to perfect case

affrica
Télécharger la présentation

Objectives

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Objectives • Finish heat exchangers • Air Distribution Systems • Diffuser selection • Duct design • fluid dynamics review

  2. Fin Efficiency • Assume entire fin is at fin base temperature • Maximum possible heat transfer • Perfect fin • Efficiency is ratio of actual heat transfer to perfect case • Non-dimensional parameter

  3. Heat exchanger performance (11.3) • NTU – absolute sizing (# of transfer units) • ε – relative sizing (effectiveness)

  4. Example problem AHU M For the problem 9 HW assignment # 2 (process in AHU) calculate: a) Effectiveness of the cooling coil b) UoAo value for the CC Inlet water temperature into CC is coil is 45ºF OA CC CC (mcp)w steam RA tc,in=45ºF Qcc=195600Btu/h tM=81ºF tCC=55ºF

  5. Summary • Calculate efficiency of extended surface • Add thermal resistances in series • If you know temperatures • Calculate R and P to get F, ε, NTU • Might be iterative • If you know ε, NTU • Calculate R,P and get F, temps

  6. Reading Assignment • Chapter 11 - From 11.1-11.7

  7. Analysis of Moist Coils • Redo fin theory • Energy balance on fin surface, water film, air Introduce Lewis Number • Digression – approximate enthalpy • Redo fin analysis for cooling/ dehumidification (t → h)

  8. Overall Heat Transfer Coefficients • Very parallel procedure to dry coil problem • U-values now influenced by condensation • See Example 11.6 for details

  9. Air Distribution System Design • Describe room distribution basics • Select diffusers • Supply and return duct sizing

  10. Forced driven air flowDiffusers Grill (side wall) diffusers Linear diffusers Vertical Horizontal one side

  11. Diffusers types Valve diffuser swirl diffusers ceiling diffuser wall or ceiling floor

  12. Diffusers Perforated ceiling diffuser Jet nozzle diffuser Round conical ceiling diffuser Square conical ceiling diffuser Wall diffuser unit Swirl diffuser Floor diffuser Auditorium diffuser Linear slot diffuser DV diffuser External louvre Smoke damper http://www.titus-hvac.com/techzone/ http://www.halton.com/halton/cms.nsf/www/diffusers

  13. Low mixing Diffusers Displacement ventilation

  14. 18.7

  15. V = maximum volumetric flow rate (m3/s, ft3/min) Qtot = total design load (W, BTU/hr) Qsen = sensible design load (W,BTU/hr) ρ = air density (kg/m3, lbm/ft3) Δt = temperature difference between supply and return air (°C, °F) Δh = enthalpy difference between supply and return air (J/kg, BTU/lbm) Diffuser Selection Procedure • Select and locate diffusers, divide airflow amongst diffusers

  16. Find Characteristic Length (L)

  17. Indicator of Air DistributionQuality • ADPI = air distribution performance index • Fraction of locations that meet criteria: • -3 °F < EDT < 2 °F or -1.5 °C < EDT < 1 °C • Where, EDT = effective draft temperature • Function of V and Δt (Eqn 18.1) • EDT=(tlocal-taverage)-M(Vlocal-Vaverage) , M=7 °C/(m/s) ADPI considers ONLY thermal comfort (not IAQ)

  18. Ideal and Reasonable Throws

  19. Select Register • Pick throw, volumetric flow from register catalog • Check noise, pressure drop

  20. Summary of Diffuser Design Procedure • Find Q sensible total for the space • Select type and number of diffusers • Find V for each diffuser • Find characteristic length • Select the diffuser from the manufacturer data

  21. Example 18.3 • Qtot = 38.4 kBTU/hr • Δh = 9.5 BTU/lbma omission in text

  22. Pressures • Static pressure • Velocity pressure • Total pressure – sum of the two above

  23. Relationship Between Static and Total Pressure

  24. Total and static pressure drops are proportional to square of velocity Plot of pressure drop vs. volumetric flow rate (or velocity) is called system characteristic Duct Design

  25. System Characteristic

  26. Electrical Resistance Analogy

  27. Frictional Losses

  28. Non-circular Ducts • Parallel concept to wetted perimeter

  29. Dynamic losses • Losses associated with • Changes in velocity • Obstructions • Bends • Fittings and transitions • Two methods • Equivalent length and loss coefficients

  30. Loss Coefficients ΔPt = CoPv,0

  31. Example 18.7 • Determine total pressure drop from 0 to 4

  32. Conversion Between Methods

  33. Reading asignement • Chapter 18 • 18.1-18.4 (including 18.4)

More Related