Monday, April 4, 2011

GRILLS, DIFFUSERS, RETURNS, FILTERS, DAMPERS

AR-461: BUILDING SCIENCE
By:
RAVINDAR KUMAR
Assistant Professor
Department of Architecture and Planning
NED University of Engineering and Technology
Karachi
LECTURE NO. 10
TOPIC:                      GRILLS, DIFFUSERS, RETURNS, FILTERS, DAMPERS

INTRODUCTION:
Grills, diffusers, returns, filters and dampers are the parts of HVAC system. In the following a brief description is given for its basic understanding.

GRILLS:
A grille or grill is an opening of several slits side by side in a wall or metal sheet or other barrier, usually to let air or water enter and/or leave but keep larger objects including people and animals in or out.[1] In heating, ventilating and air conditioning (HVAC) for room air distribution, a grille, specifically spelled with the ending e, is a class of air terminals.[2] Most HVAC grilles are used as return or exhaust air inlets to ducts, but some are used as supply air outlets. Diffusers and nozzles, are, for example, used as supply air outlets too. Registers are a type of HVAC grille that also incorporates an air damper.[3]
DIFFUSERS:[4]
Diffusers are very common in heating, ventilating, and air-conditioning systems.[5] Diffusers are used on both all-air and air-water HVAC systems, as part of room air distribution subsystems, and serve several purposes:
  • To deliver both conditioning and ventilating air
  • Evenly distribute the flow of air, in the desired directions
  • To enhance mixing of room air into the primary air being discharged
  • Often to cause the air jet(s) to attach to a ceiling or other surface, taking advantage of the Coandă effect
  • To create low-velocity air movement in the occupied portion of room
  • Accomplish the above while producing the minimum amount of noise

When possible, dampers, extractors, and other flow control devices should not be placed near diffusers' inlets (necks); either not being used at all or being placed far upstream. They have been shown to dramatically increase noise production.
For as-cataloged diffuser performance, a straight section of duct needs serve a diffuser. An elbow, or kinked flex duct, just before a diffuser often leads to poor air distribution and increased noise. Diffusers may be round, rectangular, textile or linear slot diffusers (LSDs), for example. This last type takes the form of one or several long, narrow slots (hence the name), often semi-concealed in a fixed or suspended ceiling. Occasionally, diffusers are used in reverse fashion, as air inlets or 'returns'. This is especially true for LSDs and 'perf' diffusers. But more commonly, grilles are used as return or exhaust air inlets.

RETURNS:
The returns are the air outlets or exhaust in the HVAC system. The returns are used for room air diffusion or to remove the unwanted warm or cool air. There are also the return air inlets which have very little effect on room air diffusion regardless of inlet type or location. However, return air inlets should be located a sufficient distance from the supply outlet so that short circuiting of supply air does not occur. It may also be desirable to locate the returns in the stagnant zone to remove unwanted warm or cool air. For cooling, a high sidewall or ceiling return will remove warm air from the space.

FILTERS:
A particulate air filter is a device composed of fibrous materials which removes solid particulates such as dust, pollen, mold, and bacteria from the air. Air filters are used in applications where air quality is important, notably in building ventilation systems. Some buildings, use foam, pleated paper, or spun fiberglass filter elements. Another method, air ionisers, use fibers or elements with a static electric charge, which attract dust particles.[6] An air purifier is a device which removes contaminants from the air. These devices are commonly marketed as being beneficial to allergy sufferers and asthmatics, and at reducing or eliminating second-hand tobacco smoke. Commercial grade air purifiers are manufactured as either small stand-alone units or larger units that can be affixed to an air handler unit (AHU) or to an HVAC unit found in the medical, industrial, and commercial industries and buildings.[7]

DAMPERS:[8]
A damper is a valve or plate that stops or regulates the flow of air inside a duct, chimney, VAV box, air handler, or other air handling equipment. A damper may be used to cut off central air conditioning (heating or cooling) to an unused room, or to regulate it for room-by-room temperature and climate control. Its operation can be manual or automatic. Manual dampers are turned by a handle on the outside of a duct. Automatic dampers are used to regulate airflow constantly and are operated by electric or pneumatic motors, in turn controlled by a thermostat or building automation system.

In a chimney flue, a damper closes off the flue to keep the weather (and birds and other animals) out and warm or cool air in. This is usually done in the summer, but also sometimes in the winter between uses. In some cases, the damper may also be partly closed to help control the rate of combustion. The damper may be accessible only by reaching up into the fireplace by hand or with a wood poker, or sometimes by a lever or knob that sticks down or out. On a wood burning stove or similar device, it is usually a handle on the vent duct as in an air conditioning system. Forgetting to open a damper before beginning a fire can cause serious smoke damage to the interior of a home, if not a house fire.

REFERENCES:

[1] Grille, From http://en.wikipedia.org/wiki/Grille (Retrieved April 5, 2011)
[2] Designer's Guide to Ceiling-Based Air Diffusion, ASHRAE, Inc., Atlanta, GA, USA, 2002
[3] ASHRAE Handbook: Fundamentals (SI Edition), 1997
[4] Diffuser, From http://en.wikipedia.org/wiki/Diffuser_(thermodynamics) (Retrieved April 5, 2011)
[5] Ibid 2
[6] Air Filter, From http://en.wikipedia.org/wiki/Air_filter (Retrieved April 5, 2011)
[7] Air Purifier, From  http://en.wikipedia.org/wiki/Air_purifier (Retrieved April 5, 2011)

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