425 4 Air Distribution 2007

1ME 425 - Air Distribution & ASHRAE Outlet Selection

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ME 425Air Distribution & ASHRAE Outlet Selection

Keith E. Elder, P.E.

The Air Distribution System

The purpose of air distribution system is to create the proper combination of temperature, humidity and air velocity in the occupied zone of the conditioned room.

If not properly designed, the air distribution system has the potential to compromise the comfort conditions it was designed to maintain.


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ASHRAE Air Distribution Design Recommendations

Already defined in one lawsuit as “The Acceptable Standard of Care”

Two Basic Rules� Cooling - diffuser selection should be based on

the ratio of the diffuser’s throw to the length of the area supplied to achieve high ADPI (minimize draft)

� Heating - diffuser-to-room ∆t should not exceed 15°F to minimize stratification (minimize stratification)

Expanded Comfort Criteria

Comfort is maintained through the change in seasons when the following conditions are maintained in the space occupied zone.� Air temp maintained between 73-77°F

� RH maintained between 25-60%

� Maximum air motion in the occupied zone

� 50 fpm cooling

� 25 fpm heating

� Maximum temperature gradient

� 1-2° cooling

� 4° heating


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Successful Design Achieves:

Good ventilation effectiveness

Avoids dumping

Avoids draft

Occupied Zone

3 inches to 72 inches above the floor level.

Two feet in from the walls

O ccu piedZ on e 3"-6 '-0"

Ft.

2 F t. 2 F t.


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Definition of “Draft”

Koestel, Tuve and Reinmannn studied the effect of air motion in the 1950’s, measuring the warmth or coolness of a draft above or below a room temperature of 76°F at 30 inches above the floor at room center, with air velocity at 30 fpm. They defined “draft” as:

Any localized feeling of coolness or warmth of any portion of the body due to both air movement and temperature, with humidity and radiation considered constant.

Impact of Draft on Comfort


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Percentage of Occupants Objecting to Drafts

Effective Draft Temperature

The physiological effects on a human body due to air temperature & air motion can be described by Effective Draft Temperature (the difference in temperature between any point in the occupied zone and the control condition). E.D.T. can be quantified as:

θ = tx - tc - 0.07(Vx - 30)

Where:

• θ = effective draft temperature

� tx = local airstream temperature, °F

� tc = average room temperature, °F

� Vx = local airstream velocity, fpm


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Room Air Distribution Methods

Displacement Ventilation

Localized Ventilation

Mixing Systems

Conventional Mixing Systems


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Entrainment FlowConventional Mixing

Terminal Velocity

A targeted value of interest – usually 50, 100 or 150 fpm


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Terminology

Outlet/Inlet

Terminals

Grille

Register

Diffuser

Two Other Important Concepts

Throw

� The distance from the outlet to a point where the maximum velocity in the stream cross section has been reduced to a selected terminal velocity.

Characteristic Room Length

� The distance from the diffuser to the nearest boundary wall in the principle horizontal direction of the airflow


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Outlet Classification

Group A. Outlets mounted in or near the ceiling that discharge air

horizontally.

Group B. Outlets mounted in or near the floor that discharge air

vertically in a non-spreading jet.

Group C. Outlets mounted in or near the floor that discharge air

vertically in a spreading jet.

Group D. Outlets mounted in or near the floor that discharge air

horizontally.

Group E. Outlets mounted in or near the ceiling that project

primary air vertically.

Group A Outlets


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Group A High Sidewall Grilles

Group A – Ceiling Diffusers


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Neck, face, core & free area

Aneck

Vneck

Neck Area & Neck Velocity

neck

saneck

A

QV =


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Neck Velocity

Coanda Effect

The attachment of a jet flow introduced parallel to a ceiling or other surface. This “surface” effect creates a lower-pressure region above the jet stream than below it, causing the higher pressure in the room to hold the airstream to the ceiling. Conditions necessary for surface effect include:

� Angle of discharge < 40° from parallel surface

� A side wall outlet is within one foot of the ceiling

� A Floor or sill outlet is within 10 inches of a wall

� A ceiling outlet discharges along the ceiling


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Coanda Effect

Coanda Effect


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Outlet Discharge Patterns

Circular Diffuser

The downward-jet-pattern may be of benefit when the outlet is installed in a tall space and HVAC is in heating mode

The visualization shows consequences for cooling mode


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Square/Rectangular Diffusers

Square Diffuser

Still capable of establishing Coanda adherence to ceiling, but provides flexibility for distributing air in a variety of patterns


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Slot Diffuser

Slot Diffuser

Also capable of Coanda adherence, but allows directing the air downward


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Air Diffusion Performance Index

ADPI statistically relates the space conditions of local temperatures and velocities to occupant’s thermal comfort

Quantitatively, ADPI is the percentage of locations where measurements show that the effective draft temperature, θ, is between - 3 and +2°F and the air velocity is less than 70 fpm.

Isodrafts for a space with an ADPI of 94%.


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Designing to ADPI Criteria

ADPI performance can be predicted based on air outlet “throw” data referenced to room “characteristic room length”

ASHRAE Mixed Air System Outlet Selection Steps

1.Determine Air Volume Requirements & Room Size

2.Select diffuser type and location within room

3.Determine room characteristic length, L

4.Select recommended Tv/L ratio

5.Calculate throw distance by multiplying Tv/L ratio by room characteristic length

6.Locate appropriate outlet size from mfgr’s catalog.

7.Check static pressure & noise criteria


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Select diffuser type and location

0 1 2 3 4 5

CFM/SF

High Wall Grille

Ceiling Slot Diffuser

Perforated Panel

Ceiling Diffuser

Determine Characteristic Room Length

Type of Outlet Characteristic Length

High Sidewall Grille Distance to wall perpendicular to jet

Ceiling Slot Diffuser Distance to wall or midplane between

outlets

Perforated Ceiling Diffusers Distance to wall or midplane between

outlets

Circular Ceiling Diffuser Distance to closest wall or intersecting

air jet

Light Troffer Diffuser Distance to midplane between outlets,

plus distance from ceiling to top of

occupied zone


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Select Recommended Tv/L ratio

Check noise criteria

Design Guidelines for HVAC System Noise in Unoccupied Spaces

RC(N) Level Space

Private residences, apartments, condominiums 25-35

Hotels/Motels

Individual rooms, Meeting/banquet rooms 25-35

Halls, corridors, lobbies 35-45

Office buildings

Private offices and Conference rooms 25-35

Teleconference rooms 25 (max)

Open plan offices 30-40

Circulation and public lobbies 40-45

Hospitals and clinics

Private rooms, Operating rooms 25-35

Wards 30-40

Corridors, Public Areas 30-40

From 1995 ASHRAE Handbook, HVAC Applications, Chapter 43 (Abridged for this slide)


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Return Air Grilles

� Ideally located in stagnant areas

� Usually placed in ceiling� Low inlet velocities mean placement not

normally the cause of short-circuiting on well-designed systems

� Number

�Generally one per 4 supply outlets

�Minimum one per occupied room

�Air-flows under-specified 5 – 10% relative to supply for pressurization

Outlet Selection Example

Select and locate appropriate ceiling air supply diffusers for the space shown below so as to maximize comfort in the occupied space. The ceiling is 11 ft and design supply air requirement is 350 CFM.

24’

12’


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Homework

Read 05F33.1 - 33.18

Select and locate appropriate ceiling air supply diffusers on the reflected ceiling plan so as to maximize comfort in the occupied space. The ceiling is 10 feet above the floor and the design supply air requirement is 1400 CFM. The tile dimensions are 2’ x 4’

Reflected Ceiling Plan

42’

16’

Documents

425 4 Air Distribution 2007