ROOM AIR DISTRIBUTION MSYS4480 1 Why Air Distribution? Provide - - PDF document

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ROOM AIR DISTRIBUTION MSYS4480 1 Why Air Distribution? Provide - - PDF document

Nov 23, 2022 278 likes •499 views

1/21/2019 ROOM AIR DISTRIBUTION MSYS4480 1 Why Air Distribution? Provide Occupant Thermal Comfort Provide sufficient ventilation to meet codes Control Latent loads (Humidity) Satwinder Singh / www.tagengineering.ca 2 2 1

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ROOM AIR DISTRIBUTION

MSYS4480

Why Air Distribution?

▪ Provide Occupant Thermal Comfort ▪ Provide sufficient ventilation to meet codes ▪ Control Latent loads (Humidity)

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Comfort Criteria

For Occupied zone: ▪ Air temp maintained between 70-76°F ▪ RH maintained between 25-60% ▪ Maximum air motion in the occupied zone

▪ 50 fpm cooling ▪ 25 fpm heating ▪ 1-4 CFM per square foot

▪ Maximum temperature gradient

▪ 1-2° cooling ▪ 4° heating

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What distributes the air?

▪ Rooftop units, Fan Coil Units ▪ Mixing Boxes, VAV terminals, Water Source Heat Pumps etc ▪ Ductwork ▪ Grilles, Diffusers and other air distribution devices

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Air Diffusion / Distribution Methods

▪ Mixing Systems ▪ Displacement Ventilation ▪ Localized Ventilation (Make-Up Air)

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Mixed Ventilation

▪ Most Common

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Types of Air

▪ Primary Air:

▪ The conditioned air discharged by the supply outlet. This air provides the motive force for room air motion.

▪ Total Air:

▪ The mixture of primary air and entrained room air which is under the influence of supply

  • utlet conditions.

▪ This is commonly considered to be the air within an envelope of 50 fpm [0.25 m/s] (or greater) velocity. The temperature difference between the total air and the room air creates buoyant effects which cause cold supply air to drop and warm air to rise.

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Why Air Distribution?

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Why Air Distribution?

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Room Air Diffusion - Heating

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Space Air Distribution - THROW

THROW

  • Mass flow
  • Outlet velocity

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DROP

DROP

  • Mass flow rate
  • Bouyancy

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Terminal Velocity

Most catalog throw data is presented for isothermal conditions (i.e., supply air temperature equals room temperature). During cooling the denser supply air will shorten the horizontal throw to approximately 75% of tabulated values (multiply by 0.75), assuming a temperature differential of approximately 15 °F [7.5 °C].

Referring to the catalog page we determine the 50 fpm throw to be :

  • 0° deflection - 22 ft isothermal or (22 x .75) = 17 ft cooling
  • 22° deflection - 18 ft isothermal or (18 x .75) = 14 ft cooling
  • 45° deflection - 11 ft isothermal or (11 x .75) = 8 ft cooling

The 22° deflection provides the best coverage and would be the optimum selection.

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Terminal Velocity

Throw is the distance from the center of the

  • utlet face to a point where the velocity of the air

stream is reduced to a specified velocity. Usually ▪ 150 fpm [0.75 m/s] ▪ 100 fpm [0.50 m/s] ▪ 50 fpm [0.25 m/s] T150 [T0.75], T100 [T0.50], T50 [T0.25]

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SPREAD

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Coanda Effect

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Occupied Zone

6ft [1.8m]

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Air Distribution – Performance Factors

▪ Air Pattern ▪ Throw ▪ Drop ▪ Spread ▪ Pressure Drop ▪ Noise Level

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Air Distribution – Performance Factors – Air Patterns

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Air Distribution – Performance Factors – Air Patterns

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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.

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Air Distribution – Performance Factors – Diffusers (Group A)

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Air Distribution – Performance Factors – Pressure Drop

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Air Distribution – Performance Factors – Noise Criteria

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Air Distribution – Performance Factors – Noise Criteria

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Air Distribution – Performance Factors – Noise Criteria

Decibel(dB) is measured against a frequency and averaged into octave bands

Both tones are equally loud

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Air Distribution – Performance Factors – Noise Criteria

For HIGH Frequencies

  • 1 dB not noticeable
  • 3 dB just perceptible
  • 5 dB noticeable
  • 10 dB twice as loud
  • 20 dB four times as loud

For LOW Frequencies

  • 3 dB noticeable
  • 5 dB twice as loud
  • 10 dB four times as loud

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Air Distribution – Performance Factors – Noise Criteria

Calculating NC Level

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Air Distribution – Performance Factors – Noise Criteria

Calculating NC Level

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Air Distribution – Performance Factors – Noise Criteria

Can we add decibels? 80 dB + 76 dB 156 dB To add decibels, calculate the difference 80 dB ‐ 76 dB 6 dB

From Chart: Add 1.0 dB to higher Value

80 dB +1 dB = 81 dB

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Air Distribution – Performance Factors – Noise Criteria

Hmmmmm…….??

  • Proximity to the source sources is vital
  • Properly selected diffusers shouldn’t

be heard from more than 10 feet away

  • There might be more than one

diffusers in a room, not more than 1 or 2 will be within 10 feet from an

  • ccupant

Diffuser NC values are based on a 10 dB room effect deduction in each octave band

  • Typical medium office with 8‐10 ft

high lay‐in ceiling, commercial carpet, sheetrock walls, and some

  • ffice furniture
  • 10 dB is a reasonable room effect

deduction for the critical octave bands

  • Critical octave bands are 4th (500 Hz),

5th (1000 Hz), and 6th (2000 Hz)

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Air Distribution – Performance Factors – Noise Criteria

  • Diffuser NC values are based on a 10 dB room effect deduction

in each octave band

  • Typical medium office with 8‐10 ft high lay‐in ceiling,

commercial carpet, sheetrock walls, and some office furniture

  • 10 dB is a reasonable room effect deduction for the critical
  • ctave bands
  • Critical octave bands are 4th (500 Hz), 5th (1000 Hz), and 6th

(2000 Hz)

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Air Distribution – Noise Criteria - Choosing Diffusers

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Air Distribution – Noise Criteria - EXAMPLE

A Model 520 size 6 in. x 5 in. supply grille operating at 150 cfm has been selected to supply a 10 ft x 15 ft room as shown. What is the best deflection setting of the diffuser blades if conditioned cool air is supplied? For 50 fpm throw: 0° deflection: 22 x .75= 17’ 22° deflection: 18 x .75= 14’ 45° deflection: 11 x .75= 8’ 22° deflection provides the best coverage and would be the optimum selection.

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Air Distribution – ADPI

  • ADPI: Air Diffusion Performance Index
  • Statistically related the space conditions of local or

transverse temperatures and velocities to occupants’ thermal comfort

  • ADPI >= 80 is considered acceptable
  • Effective draft temperature
  • = (tx-tc) – 0.07(Vx-30)
  • % of points where -3<= ฀฀<= +2 = ADPI
  • Velocity below 70 fpm

1. Generally, the higher the room load, the more high ADPI. 2. A value of T/L = 1.0 generally will produce an 3. Some air outlets are better than others at ach For example, a sidewall grille has a maximum the circular ceiling diffuser can achieve an AD 4. A wide T/L range allows the designer more fle air outlet for optimum ADPI. 5. Outlets with a wide T/L range are more applic they can maintain a high ADPI even when tur volume. At 20 Btu/h/ft2 [63 W/m2] a ceiling slot diffuse

  • f 20% while maintaining an ADPI of greater

condition the high sidewall grille has a turn-do approximately 50%. Light troffer diffusers hav

  • f all outlets, making them an excellent choic

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Pressure Losses

Supply air outlets produce:

  • Static pressure loss
  • Velocity pressure loss.

▪ Static pressure loss is equal to the difference between the inlet static pressure (SPi) and the room pressure (usually atmospheric). ▪ The static pressure loss is dependent on outlet geometry and/or free area and must be derived by test. ▪ Static pressure loss is directly proportional to the volume of air supplied through the outlet.

Velocity pressure loss is equal to the velocity pressure at the inlet (VPi) and the room velocity pressure (zero).

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Pressure Losses

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DEDUCTIONS

1. The square plaque and square cone diffuser are an excellent choice for acoustically sensitive applications or when high air volumes per outlet are desired. This is due to the aerodynamic cones and high free area. 2. Perforated diffusers tend to be noisier than other available models at the same air volume. This is due to the restricted free area of the perforated face and pattern deflectors in the air stream. 3. There is a fairly large variation in generated noise levels, even between various perforated diffuser types. The curved pattern controllers of the perforated curved diffuser generates less sound than the less aerodynamic neck deflectors of the perforated neck deflector diffuser. 4. Selecting outlets based on neck velocity is a poor indication of acoustic performance. 5. To ensure predictable sound levels it is essential to reference the manufacturers’ cataloged sound levels for the specified product.

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Displacement Ventilation

Supply air is introduced to the room at low level (typically via under floor plenums through diffusers) and collected at high level. The cool air disperses throughout the room until coming in contact with a hot object. It will then form a “thermal plume” which naturally rises upward in the space create convection in the occupied zone.

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Displacement Ventilation Cont…

Advantages ▪ Very good air distribution ▪ Effective in rooms with high ceilings ▪ Low energy consumption Disadvantages ▪ Expensive to install sub floors in earthquake zones such as Vancouver ▪ Effectiveness is greatly reduced when diffusers are placed improperly or too much furniture is present.

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Localized Ventilation (Make-Up Air)

Used in areas where contaminated air must be exhausted and replaced immediately Typical Applications ▪ Kitchen Hoods ▪ Commercial Dishwashers ▪ Paint Shops ▪ Wood Shops ▪ Large Locker/Change Rooms

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