Diffuser Performance and Selection-2 PDF

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BLDG 471 HVAC Design Air Distribution System Design Notes...

Description

1

Duct Layout, Diffuser and Return Air Grille Placement

BLDG 471

Dr. M. Zaheeruddin

Fall 2021

2

Diffuser Selection/Placement VT (terminal velocity): the value in fpm that exists at the end of the throw (T) from the diffuser. T VT Vk

T: throw (ft) Vk: outlet velocity (fpm) VT: terminal velocity (fpm) VR: room velocity (fpm)

VR Three different range of terminal velocities are normally given in catalogue: T

VR Recommended use: VT

50 ft/min

when people are located adjacent to the walls,

VT

100 ft/min

when people are not located adjacent to the walls, such as in stores and restaurants

VT

150 ft/min

when people are not at all located adjacent to the walls

A specification of 3-4-5 feet means a throw at three different velocities such as @150’

100’

50’

ADPI: Air distribution performance index ADPI is defined as the percentage of measurements taken at many locations is the occupied zone of a space that meet -3 to 2F effective draft temperature (EDT) criteria [where EDT is the difference between any point in occupied space and control condition ] 𝐸𝐷𝑇 = (𝑡𝑎 − 𝑡𝑟 ) − 𝑀(𝑉𝑥 − 𝑉𝑟 ),

where 𝑉𝑟 = 30

BLDG 471

𝑓𝑡

𝑚𝑖𝑛

, 𝑎𝑛𝑑 𝑀 𝑖𝑠 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 = 0.07 𝐹 ∙ 𝑚𝑖𝑛/𝑓𝑡

Dr. M. Zaheeruddin

Fall 2021

3

Diffuser Selection (Ceiling Diffuser) Given: -

cfm=250,

-

Load = 8000BH or load/ft2=36BH/ft2

Step1: For a known diffuser type and room size, determine the characteristic length (distance to closest wall, (Table 11-1) Step 2: select throw-to-length ratio for max ADPI (Table 11-2) Step 3: Calculate throw and select appropriate diffuser from the catalogue data. Solution:

Characteristic length: 7’ (minimum value) From Table 11-2, for 36BH/ft2 load ≈40:

BLDG 471

X50/L = 0.8

for 88% ADPI

X50/L = 0.5 – 1.5

for 80% ADPI

Dr. M. Zaheeruddin

Fall 2021

4

To achieve 50 terminal velocity: The required throw = 0.8*7 = 5.6’

for 88% ADPI

The required throw = 1.5*7 = 10.5’

for min. 80% ADPI

From catalogue (Table 11-4) for ceiling diffusers select a diffuser. Check the 10“ diffuser: 10” Ø between 220cfm/270cfm will this be adequate?

Check ΔP0 and throw:

0.027 𝑖𝑛 𝑊𝐺 0.043 𝑖𝑛 𝑊𝐺 ( 150′ 100′ 50′ ) ⟨ 7′ → 3 − 4 − 7 8′ → 3 − 5 − 8

∆𝑃0 ⟨ ∆𝑃0

Find ΔP0 for 250 cfm:

𝑄

𝑄𝑟𝑎𝑡𝑒𝑑

=(

1/2

∆𝑃0 ) ∆𝑃0 𝑟𝑎𝑡𝑒𝑑

250 2 ) ∗ 0.043 = ( ) ∗ 0.043 = 0.0341 " 𝑊𝐺 ∆𝑃0 = ( 𝑄𝑟𝑎𝑡𝑒𝑑 270 𝑄

2

𝑇 = 𝑏𝑒𝑡𝑤𝑒𝑒𝑛 7 − 8′

BLDG 471

Dr. M. Zaheeruddin

Fall 2021

5

‫ ؞‬Diffuser specifications: ▪

10ӯ ceiling (round) diffuser

▪

ΔP0 = 0.035” WG

▪

T = 7-8 ‘

▪

NC = 11 (Table 11-4)

NC in offices

25-30

NC in classroom

25-30

BLDG 471

Dr. M. Zaheeruddin

Fall 2021...